ISSTPapers Cover Design II · Prevention of COT Bottom Pitting Corrosion by Zinc-Prime Y. Inohara, T. Komori, K. Kyono, H. Shiomi (JFE Steel Corporation, Japan) and T. Kashiwagi (Mitsui

International Symposium

ON

SHIPBUILDING TECHNOLOGY

(ISST 2007)

- Fabrication and Coatings –

6-7 September 2007

Osaka University, Japan

PAPERS Vol.1

The Japan Society of Naval Architects and Ocean Engineers (JASNAOE)

&

The Royal Institution of Naval Architects (RINA)

International Symposium ON

SHIPBUILDING TECHNOLOGY (ISST 2007)

- Fabrication and Coatings – 6-7 September 2007

Osaka University, Japan

Organized by The Japan Society of Naval Architects and Ocean Engineers (JASNAOE)

and The Royal Institution of Naval Architects (RINA)

Supported by National Maritime Research Institute (NMRI), Japan

and LLOYD’S REGISTER ASIA

© 2007: JASNAOE-RINA JASNAOE and RINA are not, as a bobies, responsible for the opinion expressed by the individual author or speakers. Japan Society of Naval Architects and Ocean Engineers (JASNAOE) Yasaki White Building 3F 2-12-9 Shiba-Daimon, Minato-ku 105-0012 Tokyo, Japan Telephone +81 (0)3 3438 2014 THE ROYAL INSTITUTEION OF NAVAL ARCHITECTS 10 Upper Belgrave Street London SW1X 8BQ, United Kingdom Telephone: +44 (0)20 7235 4622

Shipbuilding Technology ISST 2007, Osaka, 2007

CONTENTS Development of Anti-Corrosion Steel for the Bottom Plates of Cargo Oil Tanks S. Sakashita, A. Tatsumi, H. Imamura and H. Ikeda (Kobe Steel, Japan)

1

Development of Corrosion Resistant Steel for Cargo Oil Tanks K. Kashima, Y. Tanino, S. Kubo, A. Inami and H. Miyuki (Sumitomo Metal Industries, Japan)

5

Development of New Anti-Corrosion Steel for COTs of Crude Oil Carrier S. Imai, K. Katoh, Y. Funatsu, M. Kaneko (Nippon Steel Corporation, Japan), T. Matsubara, H. Hirooka and H. Sato (Nippon Yusen Kaisha, Japan)

11

Onboard Evaluation Results of Newly Developed Anti-Corrosion Steel for COTs of VLCC and Proposal for Maximum Utilization Method S. Imai, K. Katoh, Y. Funatsu, M. Kaneko (Nippon Steel Corporation, Japan), T. Matsubara, H. Hirooka and H. Sato (Nippon Yusen Kaisha, Japan)

21

Prevention of COT Bottom Pitting Corrosion by Zinc-Prime Y. Inohara, T. Komori, K. Kyono, H. Shiomi (JFE Steel Corporation, Japan) and T. Kashiwagi (Mitsui O.S.K.Line, Japan)

29

Development of Corrosion Resistant Steel for Bottom Plate of COT Y. Inohara, T. Komori, K. Kyono, K. Ueda, S. Suzuki and H. Shiomi (JFE Steel Corporation, Japan)

33

The Third Generation Shop Primer and Japanese Shipbuilding Construction Process Y. Seki, K. Kondou and O. Harada (Chugoku Marine Paint, Japan)

37

The Development of Water Based Shop Primers M. Hindmarsh (International Paint, Japan)

45

Coating Conditions in Water Ballast Tank, Void Space and Cargo Oil Tank of Aged Ships and Required Performance Standard of Protective Coatings for New Ships T. Murakami, T. Sasaki, M. Kuwajima, M. Koori (Shipbuilders' Association of Japan), A. Takada and M. Oka (National Maritime Research Institute, Japan)

51

Pitting Corrosion on Epoxy-Coated Surface of Ship Structures T. Nakai, H. Matsushita and N. Yamamoto (Class NK, Japan)

59

Compositional Analysis of Soluble Salts in Bresle Extraction from Blocks in Newbuilding Shipyards S. S. Seo, S. M. Son, C. H. Lee and K. K. Baek (Hyundai Heavy Industries, Korea)

65

Effect of Edge Preparation Methods on Edge Retention Rate of Epoxy Coatings for Ship's Ballast Tanks S. S. Seo, K. K. Baek, C. S. Park, C. H. Lee and M. K. Chung (Hyundai Heavy Industries, Korea)

71

© 2007: JASNAOE-RINA i


Study on the Alternatives to the Secondary Surface Preparation in Protective Coatings N. Osawa (Osaka University, Japan), K. Umemoto (Kawasaki Shipbuilding Japan), Y. Nambu (Universal Shipbuilding, Japan) and T. Kuramoto (Mitsui Engineering and Shipbuilding, Japan)

77

Leaching Phenomena of Antifouling Agent from Ship Hull Paint R. Kojima, O. Miyata, T. Shibata, T. Senda (National Maritime Research Institute, Japan) and K. Shibata (Chiba Institute of Technology, Japan)

85

Space and Time Distribution of Antifouling Agent in Aquatic Environment K. Shibata (Chiba Institute of Technology, Japan), V. A. Sakkas (University of Ioannina, Greece), S. Sugasawa, Y. Yamaguchi, F. Kitamura and T. Senda (National Maritime Research Institute, Japan)

93

Acute Toxicity of Pyrithione Photodegradation Products to Some Marine Organisms T. Onduka, K. Mochida, K. Ito, A. Kakuno, K. Fujii (National Research Institute of Fisheries and Environment of Inland Sea, Japan) and H. Harino (Osaka City Institute of Public Health and Environmental Sciences, Japan)

99

Research for the Risk Assessment of Anti-Fouling System E. Yoshikawa (Chugoku Marine Paint, Japan), N. Nagai (Japan NUS), K. Namekawa (Arch Chemicals Japan), K. Shibata (Chiba Institute of Technology, Japan) and T. Senda (National Maritime Research Institute, Japan)

107

The Benefits of Foul Release Coatings I. Walker (International Paint Japan)

117

Antifouling Systems to Reduce Biocide S. Tashiro, M. Doi, Y. Kiseki and M. Ono (Chugoku Marine Paint, Japan)

121

A New Prediction Method for Deterioration of the Corrosion Protection System of the Oil Storage Barges H. Sugimoto (Shipbuilding Research Centre of Japan) and Y. Horii (Japan Oil, Gas and Metals National Corporation, Japan)

127

Matching the Coating Process to Shipyards Needs R. Kattan (Safinah Ltd., UK)

133

SI Technology and its Unique Paint Property N. Sasaki and M. Takayama (Nippon Paint Marine Coatings, Japan)

141

Corrosion Protection Regulations to Improve Ship’s Safety? T. Lohmann and D. Engel (Germanischer Lloyd, Germany)

145

Class NK's Course of Action to Protective Coating - Guidelines for Performance Standard for Protective Coating Contained in IMO Resolution MSC.215 (82) T. Matsui (Class NK, Japan)

149

© 2007: JASNAOE-RINA ii


DEVELOPMENT OF ANTI-CORROSION STEEL FOR THE BOTTOM PLATES OF CARGO OIL TANKS Shinji Sakashita, Materials Research Laboratory, Kobe Steel Ltd, JapanAkihiko Tatsumi, Materials Research Laboratory, Kobe Steel Ltd, Japan Hiroki Imamura, Kakogawa Works, Kobe Steel Ltd, Japan Hideji Ikeda, Plate Products Marketing & Technical Service Department, Kobe Steel Ltd, Japan SUMMARY For the purpose of offering the corrosion control method with high reliability, the anti-corrosion steel for the bottom plates of cargo oil tanks has been developed. The developed steel satisfies the ship's class rule and can be welded by conventional welding method. The results of the laboratory evaluation revealed that the maximum pit growth rate of the developed steel is suppressed in 1/4 - 1/5 of conventional steel. The application of the developed steel to bottom plate of COT will contribute to the improvement in the safety and the reduction in the life cycle cost of oil tankers. 1. INTRODUCTION Localized corrosion on inner bottom plates of cargo oil tanks (COT) is one of the serious corrosion problems in oil tankers, because environmental pollution may be caused by major spill of crude oil. The total number and the maximum depth of localized corrosion observed in the dry dock inspection are expected to reach several thousand and 10 mm respectively. The workload of inspection and repair of the localized corrosion in the dry dock is very big. The corrosion mechanism was scientifically clarified in the study which had been carried out by 'The Shipbuilding Research Association of Japan Panel #242 (SR242) committee' for 3 years supported by the Nippon Foundation [1, 2]. Key findings on the corrosion mechanism are as follows. The oil coats in the COT have very high insulating resistance, and localized damage to the oil coats by localized existence of water and/or physical and local removal of oil coat by crude oil washing (COW) irradiation, etc. leads to initiation of localized (pitting) corrosion. Furthermore, existence of elemental S and low pH chloride solution in the pit inside accelerates the growth of pitting corrosion. Coating has been applied to a part of tanker for the purpose of the corrosion prevention. However, the corrosion control with high reliability is desired, since the localized corrosion may be generated from a defect of coating. The material of which the localized corrosion sensitivity is small has been developed in order to heighten the reliability of the corrosion control. In this paper corrosion resistance and mechanical properties of the developed material (the anti-corrosion steel) were described.

2. APPROACH OF IMPROVEMENT OF CORROSION RESISTANCE The development of anti-corrosion steel was carried out based on the corrosion mechanism which had been clarified in the SR242 committee. The main controlling factor of the localized corrosion is elemental S which is included in sludge. Elemental S seems to accelerate the corrosion reaction of bottom plate in the defect of the oil coat. It is reported that the corrosion reaction measured by electrochemical method is remarkably promoted, when elemental S adheres to the steel surface [3]. The stable protective surface film which prevents the contact between elemental S and steel seems to be effective for the improvement of the corrosion resistance. And low pH chloride solution in the pit inside is also the controlling factor of the localized corrosion. It was found that pH at pit inside is 2 – 4 and this value is lower than pH at the outside (4 – 8) by the onboard pH measurement. It seems to accelerate the corrosion reaction of bottom plate, since hydrogen evolution reaction is promoted in the defect of the oil coat. In this case, it seems to be effective to increase pH in the inside of the pit for the improvement of the corrosion resistance. 3. TEST METHODS Simulated corrosion test methods were studied based on the corrosion mechanism. Test A (Figure 1) is for evaluating the resistance for the corrosion accelerated by elemental S in sludge. Specimen was immersed in a NaCl aqueous solution containing elemental S. As a result of chemical analysis of sludge collected from the COT bottom plate of the actual ship, concentration of elemental S was several % [1]. In actual ship, the sludge including elemental S contacts steel bottom plate in the defect of the oil coat, and localized corrosion seems to grow. In test A, the corrosion rate is greatly promoted by using higher purity (99.5%) elemental S in comparison with the actual environment.

© 2007: JASNAOE-RINA 1


Figure 2: Schematic of simulated test for pitting corrosion on COT (Test B).

FeCl3 +NaCl solution

Fixed temperature andhumidity chamber

Specimen

Bottom plate

Defect

Oil coat

Pitting

Low pH solution

Solution

Figure 1: Schematic of simulated test for pitting corrosion on COT (Test A).

Specimen Fixedtemperature

NaCl solution includingElemental S

Bottom plate

Solution Defect

Pitting

Oil coat

Elemental S

Sludge & corrosion products

Test B (Figure 2) is for evaluating the resistance for the corrosion accelerated by low pH chloride solution. Test B were carried out using fixed temperature and humidity chamber. The test solution (FeCl3 + NaCl) was dropped on specimen. The temperature and humidity of the test chamber were retained at 333 K and 95%RH respectively. In test B, the corrosion rate is greatly promoted by increasing the temperature and decreasing pH of 1 or under of test solution in comparison with the actual environment. The pit growth rate was evaluated from the corrosion depth after the corrosion test. It has been confirmed that these controlling factor of the localized corrosion promote the corrosion reaction by our laboratory electrochemical method. Therefore, these experimental methods seem to be appropriate as an evaluation method of localized corrosion of inner bottom plate of COT 4. CORROSION RESISTANCE The result of the test A and test B is shown in Figure 3 and Figure 4 respectively. The pit growth rate of the developed steel is suppressed in 1/4 - 1/5 of conventional steel. It is shown that the corrosion resistance of the developed steel is excellent for the corrosion accelerated by both elemental S and low pH chloride solution.

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Rat

io o

f pit

grow

th ra

te

Convential Developed

Figure 3: Ratio of pit growth rate for the general steel (test A).



It is known that the statistical and stochastic nature is included in localized corrosion [4]. The extreme value statistics had been introduced to analyze pit depth dispersion of oil tank base steel, pipeline steel [5] and so on. The statistical approach is important for bridging the gap between laboratory and the field (i.e. actual ship). The extrapolation of pitting data from small specimens was found to be quite useful for estimating the maximum depth of the large area structural material. It has been confirmed in SR242 committee that the pit growth rate of actual ship also follows the Gumbel distribution [1,3]. This finding indicates that the statistical approach is effective in the evaluation of the corrosion resistance of the COT bottom plates. As a result of statistical analysis of the experimental data, it was confirmed that good straight line can be seen in the Gumlel plots (Figure 5, 6). These indicate that the maximum pit growth rate obtained from both tests follows the Gumbel distribution well. The maximum pit growth rate, x, expected for the field can be estimated by the graphical method from the cross point between the line of cumulative frequency, F(x), against x and return period, T, in the Gumlel plots. T is defined as a ratio of area of specimen and area of pit on the actual ship. In this case, it is calculated that the value of T is 65.4 and 174 for test A and test B respectively. In the calculation of T, the average diameter and the number of the pit observed on COT bottom plate of VLCC was assumed to 10mm and 2000 respectively. The maximum pit growth rate expected for the whole area of actual ship is estimated to be about 1 mm/y for both tests in Figure 5, 6. Thus, the pit growth rate of the developed steel seemed to be suppressed in 1/4 of conventional steel by the statistical analysis. As a results of elemental analysis of surface film after test A, it was proven that the chloride content in the surface film (corrosion product) on the developed steel was remarkably less than that on conventional steel. This indicates that the surface film of developed steel is protective and insulates the steel from the corrosive environment including elemental S. Therefore, the

corrosion by elemental S seemed to be suppressed. Furthermore, it was found that the pH of the solution on the developed steel is higher than that of the conventional steel on the test B. The pit growth rate of the developed steel seemed to be suppressed in test B because the hydrogen evolution reaction was suppressed by the rise in pH.

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Rat

io o

f pit

grow

th ra

te

Convential Developed

Figure 4: Ratio of pit growth rate for the general steel (test B).

Figure 5: Distribution of maximum pit growth rate obtained by simulated test A.

1.010.0

50.0

90.095.0

99.099.5

Cum

ulat

ive

Freq

uenc

y,F(

%)

43210Maximum pit growth rate (mm/year)

1.52

51020

50100200

Ret

urn

Perio

d,T

Developed Conventional

Expected value for actual ship

1.010.0

50.0

90.095.0

99.099.5

Cum

ulat

ive

Freq

uenc

y,F(

%)

43210Maximum pit growth rate (mm/year)

1.52

51020

50100200

Ret

urn

Perio

d,T

DevelopedConventional

Figure 6: Distribution of maximum pit growth rate obtained by simulated test B.

Expected value for actual ship



5. MECHANICAL PROPERTIES The mechanical properties of the developed steel satisfy the AH32 specification of classes and are equivalent to or better than that of conventional steel (Table 1). FCB welded joint using a conventional welding material also has good impact properties (Figure 7). Table 1: Mechanical properties of developed steel

Developed Conventional AH32 specification

YP(MPa) 368 349 ≧315 TS(MPa) 476 481 440-590

El(%) 28 26 ≧22 vE0(J) 297 254 ≧31

YP:Yield point, TS: Tensile strength, El: Elongation, vE0: Absorbed energy at 0

6. CONCLUSIONS The anti-corrosion steel for the bottom plates of cargo oil tanks has been developed. The maximum pit growth rate of the developed steel is suppressed in 1/4 - 1/5 of conventional steel. The developed steel satisfies the ship's class rule and can be welded by conventional

to bottom plate of COT will contribute to the improvement in the safety and the reduction in the life cycle cost of oil tankers. The developed steel has b

w

elding method. The application of the developed steel

een applied to bottom plates of

. REFERENCES

. Katoh, K., Imai, S., Yasunaga, D.T., Miyuki, H.,

ki, H.,

, ‘Evaluation of Corrosion Failure by

remeValue Prediction of Maximum

. AUTHORS’ BIOGRAPHIES

hinji Sakashita is senior researcher at Materials

search

at Kakogawa

Plate Products

COTs of a oil tanker without coating in order to confirm the performance. 7 1Yamane, Y. Ohyabu, H., Kobayashi, Y., Yoshikawa, M. and Tomita, Y. ‘Study on Localized Corrosion on Cargo Oil Tank Bottom Plate of Oil Tanker’, World Marine Technology Conference, San Francisco, 2003/10. 2. Yasunaga, D.T., Katoh, K., Imai, S., MiyuYamane, Y., Ohyabu, H., Saito, M., Yoshikawa, M., Kobayashi , Y. and Tomi ta, Y., ‘Study on Cargo Oil Tank Upper Deck Corrosion of Oil Tanker’, World Marine Technology Conference, San Francisco, 2003/10. 3. M. Yoshikawa, ‘Corrosion of Cargo Oil Tanks of

Figure 7: V-notch charpy impact test results of FCB welded joints (Heat input: 130kJ/cm).

WM: Weld metal

400

300

200

100

0

Abs

orbe

d en

ergy

at 2

73K

(J)

6420-2Distance from fusion line (mm)

Developed Conventional

(WM) //

//

(≧34J)

VLCC Tankers’, Zairyo-to-Kankyo, Vol.53, pp. 388 - 395, 2004/08. 4. T. ShibataExtreme Value Statistics’, ISIJ International, Vol.31, pp. 115 – 121, 1991/02. 5. D. E. Hawn, ‘ExtPits On Pipelines’ Materials Performance, Vol. 16, No. 3, pp. 29 – 32, 1977/03. 8 SResearch Laboratory, Kobe Steel Ltd, Japan. He is in charge of the development of anti-corrosion material such as steel, titanium alloy and aluminum alloy. Akihiko Tatsumi is a researcher at Materials ReLaboratory, Kobe Steel Ltd, Japan. He is in charge of the development of anti-corrosion steel. Hiroki Imamura is senior researcherWorks, Kobe Steel Ltd, Japan. He is in charge of the development of shipbuilding steel plate. Hideji Ikeda is a section chief at Marketing & Technical Service Department, Kobe Steel Ltd, Japan. He is in charge of the marketing of shipbuilding steel.



DEVELOPMENT OF CORROSION RESISTANT STEEL FOR CARGO OIL TANKS K Kashima, Sumitomo Metal Industries, Ltd., Japan Y Tanino, Sumitomo Metal Industries, Ltd., Japan S Kubo, Sumitomo Metal Industries, Ltd., Japan A Inami, Sumitomo Metal Industries, Ltd., Japan H Miyuki, Sumitomo Metal Industries, Ltd., Japan SUMMARY Recently, ship owners require saving cost of inspection and repair due to corrosion in crude oil tanks of oil tankers. There are two types of corrosion in crude oil tank, the general corrosion at upper deck plate and the pitting corrosion at bottom plate. On the basis of the results of field examinations on several VLCCs by ‘The Shipbuilding Research Association of Japan’, simulated corrosion test methods were established and a corrosion resistant steel was developed. Laboratory test results revealed that the developed steel has twice or more corrosion resistance compared with conventional steel in corrosion environments of both upper deck and bottom. Mechanical properties and characteristics of welded joints were equivalent to conventional steel and corrosion resistance of welded joint is also equivalent to base metal. It was found that corrosion resistance is improved by the action of alloying elements in the low pH condensed water on the upper deck plate, and by the formation of protective sulphide film including alloying elements on the bottom plate. NOMENCLATURE [Symbol] [Definition] [(unit)] EPMA Electron Probe Micro Analysis YP Yielding Point (N/mm2) TS Tensile Strength (N/mm2) EL Elongation (%) E-20 Absorbed Energy at -20 °C (J) SAW Submerged Arc Welding FCB Flux Copper Backing E0 Absorbed Energy at 0 °C (J) WM Weld Metal FL Fusion Line HAZ Heat Affected Zone 1. INTRODUCTION The solution of the corrosion problem of the cargo oil tanks of oil tankers is indispensable in order to improvement of safety of the ship and the prevention of the environmental pollution. And recently, ship owners require saving cost of inspection and repair due to corrosion in cargo oil tanks. Corrosion environment in cargo oil tank consists of inert gas to prevent from exploding of oil tank, H2S originated form crude oil, liquid phase of crude oil and drain water [1]. There are two types of corrosion in crude oil tank, the general corrosion with flaky corrosion products at upper deck plate and the pitting (localized) corrosion at inner bottom plate. Typical examples of corrosion appearances at upper deck plate and inner bottom plate are shown in figure 1. From the viewpoint of the extension of lifetime of oil tanker and improvement of high reliability, countermeasures against corrosion are required. In this paper, simulated corrosion test methods were established on the basis of the corrosion mechanism, and the corrosion resistant steel was developed by the study on the effects of alloying elements on corrosion behavior of steels.

Figure 1: Corrosion appearance in cargo oil tank 2. CORROSION MECHANISMS IN CARGO OIL TANKS In vapor space of crude oil tank, inert gas containing O2 (<5 volume percent), CO2, SO2 and H2S originated from crude oil exist. According to the field examination of several VLCCs by ‘The Shipbuilding Research Association of Japan Panel #242 (SR242 committee), H2S gas was detected in high concentration in vapor space. Maximum concentration of H2S was over 0.2 volume percent at full load condition. This co-exist of O2 and H2S is very rare case on the stand point of corrosion science because of reducing property of H2S. So



corrosion environment in cargo oil tank is quite complicated and unique. Figure 2 shows the corrosion mechanism at upper deck plate. The backside of upper deck is exposed in the cyclic wet and dry conditions by the temperature change through day and night, then the pH of the condensate water become lower (approx. 2-4) in the existence of CO2 and SO2 in the inert gas. And moreover, elemental sulphur is generated by oxidation of H2S with oxygen. Corrosion product on upper deck plate mainly consists of α-FeOOH and elemental sulphur, and has layered structure of rust and elemental sulphur. According to the field examination by SR242 committee, maximum 60 weight percent of elemental sulphur was detected in corrosion product at upper deck. This indicates that the amount of corrosion product does not correspond to the corrosion loss at upper deck. It seems that existence of H2S in vapor space does not affect so much on corrosion but mainly generate elemental sulphur and increase the volume of corrosion product. These mean that the general corrosion of the upper deck plate progressed by the condensate water with a low pH [2][3]. Figure 2: Corrosion mechanism at the upper deck of cargo oil tank On the other hand, at the inner bottom of cargo oil tank drain water including high concentrated chloride ion and H2S originated from crude oil exist. As shown in figure 3 the bottom plate is covered with oil coating layer containing sludge. In general, oil coating decreases corrosion, but many defects of the oil coating caused by crude oil washing and water drops from above structure exist. Pits originate from these defects of oil coating and grow by creating the corrosion electric cell between defect (anode) and steel surface under the oil coating around defect (cathode) in severe corrosion environment with concentrated chloride ion and H2S. In this case anodic and cathodic reactions are described as follows, Anodic reaction : Fe → Fe2+ + 2e Cathodic reaction : O2 + 2H2O + 4e → 4OH-

And elemental sulphur that falls off from upper deck plate accelerates pitting corrosion by the action as an oxidizer as follows, S + 2H2O → H2S + 2OH-

It is reported that the growth of pits stops at a dry dock because prior to dock inspection inside of cargo oil tank is cleaned and pits are re-coated by new crude oil after inspection. These suggest that the pitting corrosion of the bottom plate occurred and progressed at the defect of the oil coating. Figure 3: Corrosion mechanism at the bottom of cargo oil tank 3. ESTABLISHMENT OF SIMULATED CORROSION TEST METHODS 3.1 SIMULATED CORROSION TEST FOR UPPER DECK OF CARGO OIL TANK From the corrosion mechanisms mentioned above, simulated corrosion test methods were studied to reproduce the corrosion which was observed in actual cargo oil tank of oil tanker. Upper deck plate of cargo oil tank is exposed to the corrosion environment that contains O2, CO2, SO2 in inert gas, H2S from crude oil and condensation by the temperature change. Simulated test method is shown in Figure 4. The test apparatus consists of two chambers, outer chamber corresponds to the atmosphere and inner chamber corresponds to cargo oil tank. Specimens of 25 mm X 50 mm X 4 mm were set on the upper surface of inner chamber to simulate upper deck plate. Temperature of outer chamber varied between 50°C (20 hours) and 25°C (4 hours) that is a typical temperature change at upper deck. Surface of the specimens condensed by this temperature change. Gas A (13%CO2-5%O2-0.01%SO2-bal.N2) and gas B (gas A + 0.2%H2S) were blown alternatively every 14 days to the inner chamber to simulate condition at ballast and full load. The pH of condensate water of this simulated test was about 2.7, that is similar to the pH measured in actual cargo oil tank [3]. Figure 5 shows cross section of corrosion product after simulated test for 28 days. Corrosion product consists of rust layer, mixture of rust and sulphur, and layered elemental sulphur. The structure of corrosion product



after this test was quite similar to that of cargo oil tank. And composition of corrosion products was similar to that of cargo oil tank as shown in table 1. These results suggest that the structure and the composition of corrosion product were reproduced by the laboratory corrosion test. Figure 4: Simulated corrosion test apparatus for upper deck of cargo oil tank Figure 5: Cross sectional morphology and distribution of elements by EPMA in corrosion product of specimen after simulated corrosion test for upper deck Table 1: Composition of corrosion products on the simulated test for upper deck analyzed by X-ray diffraction method (mass %)

α-FeOOH β-FeOOHCargo oil tank 37 0 Simulated test 30 0

γ-FeOOH Fe3O4 Elemental S Others

8 0 12 43 3 8 21 38

3.2 SIMULATED CORROSION TEST FOR BOTTOM OF CARGO OIL TANK As mentioned above, inner bottom plate of cargo oil tank is covered with oil coating that suppresses corrosion but many defects exist caused by crude oil washing. Pits initiate at the defects and grow rapidly in severe corrosion environment at the bottom. Figure 6 shows test apparatus for pitting corrosion at bottom. The test apparatus consists of two chambers and temperature of outer chamber was kept at 40 °C. Inner chamber corresponds to cargo oil tank and specimens of 25 mm X 50 mm X 4 mm were set in synthetic sea water (ASTM-D-1141-52) simulated drain water containing chloride ion at the bottom. Simulated inert gas (13%CO2-5%O2-0.01%SO2-bal.N2) and 0.2% H2S were blown to inner chamber continuously. According to the research by SR242 20-30 % of sludge in cargo oil tank is oil and others mainly consist of rust shown in table 2, α-FeOOH and Fe3O4. So simulated oil coating that was composed of a mixture of crude oil, α-FeOOH and Fe3O4 was coated on the specimens. To reproduce the pitting corrosion at the fixed portion of bottom plate, artificial circle defect of 5mm in diameter is induced [4]. Figure 6: Simulated corrosion test apparatus for bottom of cargo oil tank and surface appearance of a test specimen after corrosion test Appearance of specimen after pitting corrosion test for 14 days is shown in figure 6. Pit was observed at the defect of simulated oil coating and corrosion rate under oil coating is very low. This corrosion morphology is quite similar to pitting corrosion in actual cargo oil tank. Corrosion products of specimen were similar to that of the bottom in cargo oil tank as shown in table 3. Pit depth after 28 days test was about 0.9 mm, that was over 10 times larger than thickness loss of specimen without oil coating, 0.08 mm. It means that corrosion electric cell was created in this laboratory test and pitting corrosion grew by the same mechanism as cargo oil tank. These results suggest that pitting corrosion observed on actual ship was reproduced by laboratory test.



Table 2: Examples of compositions of sludge in crude oil tanks (mass %)

Sludge Oil Solid substances

A 21.9 78.1 B 29.5 70.5

Composition of solid substances

Sludge α-FeOOH Fe3O4 others A 36 22 42 B 50 16 34

Table 3: Composition of corrosion products on the simulated test for the bottom analyzed by X-ray diffraction method (mass %)

α-FeOOH β-FeOOHCargo oil tank 16 19 Simulated test 18 19

γ-FeOOH Fe3O4 Elemental S Others

5 8 1 FeCO3, FeS6 24 11 FeCO3, FeS

In actual tank pit depth varies widely because of thickness of oil coating, size of defects and incubation time to pit initiation. And bottom plate can be investigated only in a dry dock and pit stops at a dock as mentioned in section 2, so time dependence of pit depth can not be measured in actual cargo oil tank. On the other hand, in this laboratory test method pit depth does not vary so much because all specimens have almost same thickness and defect of oil coating. And time dependence of pit depth can be measured because pitting corrosion starts at a same time on the all specimens. From that point of view this laboratory test method is suitable for comparison of pit depth among specimens. On the basis of the study for the effects of alloying elements on corrosion behavior of steels in simulated corrosion test environments, the corrosion resistant steel which contain Cu, Ni and W which improve corrosion resistance in the environments of upper deck and bottom was developed. 4. CHARACTERISTICS OF DEVELOPED STEEL 4.1 CORROSION RESISTANCE Simulated corrosion test results of the upper deck plate and the bottom plate are shown in figure 7, which show that the developed steel has about twice corrosion resistance compared with conventional steel in corrosion environments of upper deck plate for 84 days. Pitting rate of developed steel was also about a half compared with

conventional steel in the environment of bottom plate for 56 days. It was found that corrosion resistance is improved by the action of alloying elements which are effective to corrosion resistance to low pH water on the upper deck plate [5][6]. It seems that corrosion product layer containing alloying elements also suppress corrosion because corrosion loss of developed steel tended to decreases with test period. At the bottom pit depths of conventional and developed steel were comparable after 7 days test, but pit depth of developed steel decreased with test period. This indicates that the growth of pit was suppressed on developed steel. It was found that corrosion product just on steel surface of developed steel contains high concentration of S and alloying elements by cross sectional observation shown in figure 8. This sulphide film including alloying elements was observed only on the developed steel. It seems that the sulphide film formed on developed steel suppresses H2S concentration at inner corrosion product layer and permeation of chloride ions to the steel surface.

Figure 7: Simulated corrosion test results



Figure 8: Cross sectional distribution of elements by EPMA in corrosion product on developed steel after simulated corrosion test for bottom These results indicate that newly developed steel has good corrosion resistance in both upper deck and bottom that have different corrosion environments and mechanisms 4.2 MECHANICAL PROPERTIES OF DEVELOPED STEEL As shown in table 4, it was confirmed that tensile and impact properties of the developed steel were equivalent to conventional steel, and they satisfied the specification of DH36 grade of classes. And it was found that large heat input welded joint had good impact properties as shown in table 5. Furthermore, regard to corrosion resistance of the welded joint, it was equivalent to the base metal shown in figure 9. In the result of appreciation these good properties, the developed steel was certificated of classification LR, NK, ABS, and DNV. Especially, LR approved corrosion resistance of developed steel in his technical paper. Table 4: Mechanical properties of developed steel (Plate thickness: 16.5mm)

YP (N/mm2)

TS (N/mm2)

EL (%)

E-20(J)

Developed steel 432 504 24 258

DH36 spec. ≥355 490-620 ≥20 ≥34

Table 5: Impact test result of welded joint (plate thickness: 16.5mm)

Weldingprocedure SAW (3-electrode FCB method)

Notch position WM FL HAZ

1mm HAZ3mm

HAZ5mm

E0 (J) 174 132 172 224 250

Figure 9: Corrosion resistance of weld metal 5. EXPOSURE TEST RESULTS OF DEVELOPED STEEL IN ACTUAL CRUDE OIL TANKS Newly developed steel has been applied to upper deck and bottom of cargo oil tanks of actual ships. Test coupons have been also exposed. 3 test coupons exposed to vapour space for a year in 3 cargo oil tanks of 2 afra-max tankers as covers of tank cleaning holes were taken out and investigated. Figure 10 shows the results of exposure test in cargo oil tanks for 1 year. It found that corrosion rate of developed steel was about 50-60 % of conventional steel. It was confirmed that developed steel has good corrosion resistance compared with conventional steel in actual tanks. Figure 10: Results of exposure test for actual upper deck corrosion environment



6 CONCLUSIONS Countermeasures against corrosion problems in cargo oil tank from the viewpoint of material were investigated. Results obtained are as follows. • Simulated corrosion test methods for upper deck and bottom of cargo oil tank were established on the basis of corrosion mechanism, and corrosion on actual ship was reproduced by laboratory corrosion test. • New steel was developed which has good corrosion resistance in the corrosion environment of both upper deck plate and bottom plate. • Developed steel has about twice corrosion resistance compared with conventional steel. • Mechanical properties of the base metal and the characteristics of the welded joint are equivalent to conventional steel. Corrosion resistance of weld metal is equivalent to base metal. • Developed steel has good corrosion resistance after exposure test for 1 year in vapor space of actual cargo oil tanks. 7. REFERENCES 1. H.MIYUKI et al., European Federation of Corrosion Publications No.26, Advances in Corrosion Control and Materials in Oil and Gas Production 188, 1999. 2. H.YOSHIKAWA, Zairyo-to-Kankyo, 53, 388, 2004. 3. K.KASHIMA et al., Proceeding of 40th Jpn. Conf. Materials and Environments, 73, 2002. 4. K.KASHIMA et al., Proceedings of JSCE Materials and Environments 2007, 89, 2007. 5. K.KASHIMA et al., Conference proceedings The society of naval architects of Japan, 131, 2005 6. Y.TANINO et al., Asia Steel international conference-2006, 758, 2006. 8. AUTHORS’ BIOGRAPHIES Kazuyuki Kashima holds the current position of researcher at plate and structural steel research and development department, corporate research and development laboratory. He is responsible for research of corrosion mechanism and development of corrosion resistant steel for ships and bridges and other steel structures.



DEVELOPMENT OF NEW ANTI-CORROSION STEEL FOR COTS OF CRUDE OIL CARRIER Shiro Imai, Kenji Katoh, Yuji Funatsu, Michio Kaneko, Nippon Steel Corporation, JAPAN Tomoyuki Matsubara, Hideaki Hirooka, Hidehiko Sato, Nippon Yusen Kaisha, JAPAN SUMMARY

In recent years, localized corrosion of the COT bottom of a VLCC has been frequently occurring to a maximum depth of about 10 mm/2.5 years, and the danger of crude oil leakage and also increased burden on the environment due to holes forming in the COT bottom has been increasing. The ultimate aim of this research is to offer a rational method of preventing the formation of holes in the COT bottom. On the other hand, the amount of technical information for making and implementing proposals aimed at achieving the final aim is extremely small, and in this research 1) a grasp of the facts and a technical understanding concerning the phenomenon of localized corrosion of the COT bottom were obtained, and 2) countermeasures based on this technical understanding were prepared and submitted.

As a result, regarding the mechanism of corrosion, it was found that although the oil coat inside the COT has the same corrosion resistance as paint, localized corrosion occurs and progresses at the defective areas, the growth of this localized corrosion can be stopped by a dock maintenance, and also localized corrosion progresses due to a strongly acidic environment containing a high concentration of chloride ions. A method of performing a corrosion lab test that reproduced the environment inside of the localized corrosion was successfully developed.

By using this test method, the composition of new anti-corrosion steel was investigated and studied, and as a result it was succeeded to develop NSGP®-1 steel which has unprecedentedly high corrosion resistance and is extremely effective for eliminating the danger of the formation of holes in the COT bottom. This developed steel has excellent corrosion resistance and also satisfies the IACS rule, so it can be provided with the same characteristics as those of the conventional one. ¶ NOMENCLATURE VLCC: Very Large Cargo Carrier SH: Single Hull Structure DH: Double Hull Structure COT: Cargo Oil Tank COW: Crude Oil Washing 1. THE OBJECTIVES

In recent years, localized corrosion in the form of pitting shown in the Photo 1 has been occurring frequently at the COT bottom of crude oil tankers such as VLCC. This pitting is extremely deep with a maximum depth of about 10 mm/2.5years. As a result, the danger of leakage of crude oil due to holes being formed in the COT bottom and the consequent danger of increased burden on the environment have increased.

Based on this awareness, the ultimate aim of this research is to provide a rational method of preventing the formation of holes in the COT bottom. At the starting point of this research, the technical knowledge for preparing and implementing a method of achieving the ultimate aim was extremely small. For this reason, this study was divided into the following two stages. a. Obtaining a grasp of the facts and a technical understanding concerning the COT bottom localized corrosion phenomenon mainly by a thorough survey of actual carriers b. Preparing and submitting countermeasures based on the technical understanding As a result of the above activities, it was succeeded to develop steel which had excellent corrosion resistance, making it extremely effective for preventing the formation of holes in the COT bottom. Details are set out below.

Photo 1 Observed typical localized corrosion on COT.

2. SURVEY OF ACTUAL CARRIERS As mentioned above, in order to obtain a grasp of the

facts and a technical understanding of the localized corrosion phenomenon concerning which there were many unclear technical points, a field survey of about 10 actual carriers was carried out.

2.1 CORROSION ENVIRONMENT OF ACTUAL CARRIER

The corrosion phenomenon is a result of the relationship between the environment and the material. In this study, the material is limited to one type of material condition stipulated in the ship class standard. Consequently, obtaining a grasp of the facts and acquiring a technical understanding concerning the environment conditions are basically important.

Table 1 shows the results of an analysis of stagnant water sampled from the COT bottom. Sampled water has been kept out of contact to atmosphere until succeeding



chemical analysis procedure. It became clear that water, which is indispensable for the occurrence and progression of corrosion, existed, and it was salt water with a high NaCl concentration of about 10%. Mg was not detected, and it was judged that this water was not sea water but brine resulting from the oil well.

Table 2 shows the results of analysis of the composition of the gas sampled from the COT gas phase. From this it was clear that O2, CO2, and H2S exist in significant amounts in the COT atmosphere as cathode reaction substances that can trigger the corrosion reaction.

On the other hand, H2S observed in COT vapour space was not observed in brine as shown in Table 1.

Table 1 Typical composition of water on COT bottom [1]. COT No Na T-Fe Fe3+ Cl- SO4

2- Mg pH H2SDH-1 13600 2 2 42500 14 ND 7.0 NDDH-3 1S 40000 42 11 48000 1470 ND 7.2 NDDH-3 2P 40000 2.5 1 54000 1350 ND 7.5 ND

Table 2 Typical composition of gas inside of COT [1]. COT No. ３Ｓ４Ｃ４Ｓ５Ｃ５Ｓ

A B C DD

Cargo loading ratio 93% 89% 92% 31%

[vol.ppm]2790 1330 498 817

vol%] 4.9 3.9 5.3 3.2

1.7 2.5 3.9 4.5

3.7 4.0 10.9 13.2

SOx

[vol.ppm]1.3 3.9 1.6 2.7

N2 32.9 45.0 25.7 62.0 69.5

CxHy 54.9 42.4 62.2 15.0 4.4CO 0.0 0.0 0.0 0.0 0.0

Crude Oil type

Gas[vol%]

Empty

0%

H2S 550

H2O [ 2.5

O2 1.8

CO2 2.2

0.7

The inside of the COT is washed and cleaned in

advance in order to carry out the survey. Even in this condition, adhesion of residual crude oil to the walls of the COT was found, as shown in Photo 2. As is already known, the adhesion of oil to the surface of the steel plate provides corrosion resistance, so attention should be paid to the results of this observation.

Photo 2 Observed typical condition inside of COT.

Analysis result of the dropped flake

(Cross-sectional EPMA)

Main composition: Large amount of solid S(Mixture of S and rust)

60wt%60wt%of S!of S!60wt%60wt%of S!of S!

S

Fe

O

Ave. 54%Amount of S : 14~67%,(by another analysis)

Figure 1 Flake observed on COT bottom [1].

On the inside of the COT, the existence of a flakey

substance called sludge was seen at the site. These flakes form on the ceiling of the COT, then separate and drop off. As indicated in the analysis example of Figure 1, these flakes are not caused by corrosion of iron but rather consist mainly of solid S. To add a few more words by way of precaution, one may obtain the impression that the large amount of flakes generated is due to severe corrosion of the COT ceiling, however, this is quite incorrect. It has already been established that the generation of flakes has absolutely nothing to do with corrosion . Also, it has been found in recent years that the adhesion of solid S to the iron boundary face increases the cathode reaction, resulting in an increase in the corrosion rate [1].

2.2 ACTUAL CONDITION SURVEY CONCERNING LOCALIZED CORROSION

Corrosion of the COT bottom appears as localized pitting. This pitting does not occur in a fixed area, but rather is dispersed in various areas. In other words, many unclear points remained previously regarding the basic points such as the areas where localized corrosion occurs and the corrosion rate. An actual carrier field survey was carried out concerning localized corrosion from such a viewpoint.

Photo 3 is an example of pitting observed on the perpendicular face inside the COT. These pits were observed at a position of about 10 cm from the COT bottom. It became clear that pitting occurs not only on the COT bottom but also on the perpendicular face near the COT bottom.



Photo 3 Observed pits on vertical wall in COT.

As shown in Photo 1 and Photo 3, the shape of the pits

is hemispherical. From the results of detailed shape measurement, it was clarified that the depth and the opening diameter bore a constant proportional relationship to each other, as shown in Figure 2.

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Dia

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VLS-F

Figure 2 Half spherical shape of observed pits in COT[1]. Photo 4 shows an example of the appearance of the

surface after matters adhering to the vicinity of the pitted part have been blasted away. As is clear from the photograph, only the pits have corroded hemispherically, and there is no uniform corrosion on other parts than pits.

Photo 4 Non-corrosion surface around pits.

The results of observing an area of localized corrosion

are shown in Photo 5. Photo 5A is an example of the area where pitting on the COT bottom of an SH tanker has been repaired. It can be seen that pitting has occurred along the Longi. drain hole. Photo 5B shows the results of observation near a point directly beneath the cargo pipe inside the DH tanker COT. It can be seen from the photograph that pitting has occurred along the bottom line of the pipe.

←←Oil coatingOil coating

↑↑Drain holeDrain hole

↑↑Drain holeDrain hole

(A) SH

Laser Pointer

(B) DH Photo 5 Typical location where pits observed [1].

Figure 3 shows the results of a comparison of the

frequency of pitting concerning SH and DH. It can be seen that the frequency of pitting is higher for DH compared to SH.



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SH1 SH2 DH#1COT

SH1 SH2 DH#2COT

SH1 SH2 DH#3COT

SH1 SH2 DH#4COT

SH1 SH2 DH#5COT

Coun

tSH2 is successive inspection following SH1

SH(5y)↓

SH(2.5y)SH(2.5y)→→

←DH(2.5y)

Figure 3 Observed pits frequency on COT of SH and DH[1].

On the other hand, Figure 4 shows the results of

comparing the maximum pitting corrosion depths observed on SH and DH. As is clear from the figure, no significant difference was observed between the two.

Figure 4 Observed max. depth of pits on COT of SH and DH. 2.3 EXISTENCE OF OIL COAT AND AFFECTOR

As mentioned previously, residual crude oil was found adhering to the inside of the COT. The surface of the steel plate to which the oil adhered was not corroded. This is based on the environmental insulating effect as in the case of paint. Accordingly, the environmental insulating resistance of the oil coat that exists throughout the inside of the COT was measured.

Figure 5 is an example of the measurement results for the oil coat surface resistance. From the figure, it is clear that the environmental insulating resistance of the oil coat increases to the same level as that of tar epoxy painted areas. Consequently, it was judged that the stable adherence of oil coat accounts for the non-corroded areas outside the pitted areas, as shown in Photo 4.

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Oil Coat Tar Epoxy

Resis

tanc

e kΩ

･ c

Oil coat T/E paint

SensorSensorRSTRST®®

Measurement of resistance

Figure 5 Measured insulating resistance of oil coat and tar epoxy coating onboard [1].

In this way, it became clear that the oil coat has the same corrosion prevention effect as paint. Generally, with paint, when the insulating resistance falls due to the permeation of water and reduction of the film thickness, the corrosion resistance also falls. From the same viewpoint, the environmental insulating performance of the oil coat and the effect of water permeation and the reduction of film thickness were tested in the COT of an actual carrier.

Figure 6 shows the results of measuring the insulating resistance before and after the exposure to water. It is clear that the insulating resistance of the oil coat, that is, the corrosion resistance performance, falls significantly as a result of exposure to water. As shown in Table 1, a significant amount of water exists at the COT bottom. In addition, the area shown in Photo 5 where pitting has occurred is the area where water stagnates, flows and drips. Putting these results together, it is judged that reduction of the partial environmental insulating resistance of the oil coat is one cause of the partial corrosion. Figure 7 shows the relationship between the water flow and the area where pitting occurs. Pitting is observed at the area where water flows.

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Befor AfterWetting effect

Resis

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kΩ･ c

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SensorSensor

Distilled WaterDistilled Water

Figure 6 Change of oil coat insulating resistance by wetting[1].

Drain HoleDrain Hole Drain HoleDrain Hole

Pits path water flowsare observed at the where through

Figure 7 Location of observed pits around drain holes [2].



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Befor AfterRemoving effect(Scraping)

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tance

kΩ･ c

m2

020406080

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Befor 1min 2minRemoving effect(by ethanol)

Resis

tance

kΩ･ c

m2

Figure 8 Change of oil coat resistance by removal [1].

In this way, the oil coat exhibits corrosion resistance

and deterioration behavior akin to those of paint. Because the environmental insulating performance of paint depends upon the paint thickness, a study concerning the effect of thickness reduction of the oil coat was performed as well using actual carriers. The results are shown in Figure 8.

The thickness of the oil coat was reduced by removal with a scraper and also by wiping away with cotton wool moistened with ethanol. As shown in Figure 8, it was found that reducing the thickness of the oil coat markedly reduced the insulating resistance.

One method that is effective in reducing the film thickness of the oil coat is COW. Inside a general COT there are several COW units. For this reason, there are areas that are not exposed to any COW at all, areas that are exposed to one COW, and areas that are exposed to two or more COW. Accordingly, an observation and survey of the corrosion situation and the environmental insulating resistance of the oil coat at these areas in an actual carrier were carried out. The results are shown in Figure 9.

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No COW 1COW 2COW

Number of COW

Res

ista

nce

kΩ･ c

!!COW#1

COW#2

Apparent corrosion!

Figure 9 Change of oil coat resistance with number of COW[1].

As can be seen in Figure 9, the environmental insulating resistance of the oil coat falls as the number of COW exposures increases. Also, it was found that at the positions where there is no COW exposure at all, there was complete corrosion resistance, whereas at positions where there are multiple COW exposures, there was unmistakable corrosion accompanied by rusting. In areas between these where there was a single COW exposure, areas that were protected by the oil coat and areas where rust occurred due to the localized corrosion existed

simultaneously, and it was judged that the localized corrosion occurred and progressed in these areas. In this way, it was clear that the COW exposure condition greatly affected the localized corrosion behavior at the COT bottom, and also the construction used to mask the COW had a large effect on rust prevention.COW.

Inner Bottom

Outer Bottom Outer Bottom

D/H S/HBottom Longi.Inner Bottom

Hidden ribNo shield for bottom Ribs : Shield for bottom

Inner Bottom

Outer Bottom Outer Bottom

D/H S/HBottom Longi.Inner Bottom

Hidden ribRibs : Shield for bottomNo shield for bottom

Figure 10 Difference in structure between SH and DH[1].

As shown in Figure 3, the frequency of localized pitting corrosion at the COT bottom differs greatly depending upon whether the carrier is SH or DH. Figure 10 shows a pattern diagram of the construction of the COT bottom of both the SH and DH. Based on the above results, in the case of SH, Longi. is located inside the COT bottom and acts as a COW mask structure, thus protecting the oil coat at the COT bottom. As a result, the fact that the frequency of localized corrosion of SH is lower than that of DH can be explained, together with the technical reason.

The above information is summarized as follows. 1) Exposure to water and physical damage are the causes of oil coat defects. Exposure to water occurs due to stagnant brine at the COT bottom, and physical damage occurs due to COW. 2) The environmental insulating resistance of the oil coat defect area falls. Localized corrosion occurs and progresses under the condition that stagnant brine at the COT bottom, which is indispensable for the corrosion reaction, coexists in this area. 3) The vicinity of the localized corrosion area is protected by an oil coat without defect, and the corrosion does not occur and progresse. 4) A DH-COT structure which does not have a structure that masks exposure to COW causes the frequency of localized corrosion occurrence to increase. 3. RATE-THEORETICAL UNDERSTANDING OF THE LOCALIZED CORROSION PHENOMENON AND CORROSION MECHANISM 3.1 BEHAVIOR OF LOCALIZED CORROSION RATE IN AN ACTUAL CARRIER

Generally, the rate of localized corrosion is not constant but rather variable. Figure 11 shows the results of performing extreme value statistical analysis of the corrosion rate obtained by measuring the maximum corrosion depth at each COT of actual carriers, in order to obtain a grasp of the corrosion rate behavior including the degree of the random variation. The corrosion rate of Figure 11 was calculated by dividing the corrosion depth by the ship age.

From Figure.11, it can be seen that the rate of localized corrosion at the COT bottom is a phenomenon which



conforms to the extreme value statistical behavior. The corrosion rate distribution based on ship age indicates the individual distribution condition for each carriers, and the statistical behavior varies greatly. This means that the corrosion mechanism varies greatly with each carrier. However, because the contents of the COT are common to crude oil and a similar pit profile can be seen as shown in Figure 2 even if the carrier is different, it is inconceivable that the corrosion mechanism varies greatly from one carrier to another.

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3.332.521.671.431.251.111.05

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99%

98%

97%96%95%

90%

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70%60%50%40%30%20%10%5%

1%

0 1 2 3 4

Corrosion Rate mm/year

Cumulative Frequency F

Return Period T

VLS-1

VLD-1

VLS-2

Figure 11 Extreme statistic analysis result on the assumption that the corrosion duration is ship-age. Figure 12 shows the results of ongoing measurement for the same carrier when it is in dock regarding the occurrence of corrosion.is the area in which pits of at least 4 mm occurred when the carrier was in the first inspection dock. Here, some kind of repair was carried out.is the area where pits of at least 2 mm occurred when the carrier was in the first inspection dock. The carrier left the dock without any repair being carried out. is the area in which pits of at least 4 mm were observed when the carrier was in the second inspection dock, which is the next dock. Here, some kind of repair was carried out.

In this figure, there is no correlation between the position of and . From this result, it is considered that the unrepaired pits do not grow after the carrier has been docked, that is, the growth of pits stops during the dock inspection.

Figure 12 Change of pitting location in plural inspection [1].

Based on this knowledge, the corrosion rate was calculated with the corrosion depth divided by the repair interval. Figure 13 shows the results of performing the same statistical analysis as that of Figure 11.

Looking at these results, the statistical distribution of the pitting localized corrosion rate was roughly the same for all three carriers that were analyzed. This was interpreted to mean that the corrosion phenomenon occurred by means of the same corrosion mechanism. In other words, it was judged appropriate to consider the COT bottom localized corrosion duration as the dock interval. 7years

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97%96%95%

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1%

0 1 2 3 4


Cumulative Frequency F

Return Period T

VLS-17years

VLD-12.4years

VLS-25years

VLS-12.5years

5years VLS-2

2.5years

2.4year

Figure 13 Extreme statistic analysis result on the assumption that the corrosion duration is dock interval.

The reason for the occurrence of this situation is shown in Figure 14. Before the inspection at the dock, pits exist at the COT bottom. At the dock inspection, the inside of the COT is washed and cleaned. During the survey, a large quantity of residual solid matter was removed from the COTs that had been subjected to a field survey, and the COTs were then dried. After the completion of the inspection at the dock, transportation of crude oil is commenced again. The loading of the carrier with new crude oil results in the formation of a new oil coat, as shown in the diagram. Also, the existing pits are covered with a particularly thick oil coat, insulating it from the corrosive atmosphere. As a result, after the repair is carried out at the dock, the growth of the existing pitted area stops. (1) Under servicing condition

(2) Dock inspection

(3) Re-Start of service

(4) Nuclear of new pit

Sludge & Corrosion products

,Sludge & Corrosion products

Oil coating

Cleaned Steel Surface = No oil coat

Oil coatingare cleaned and dried for inspection

Cleaned and dried pits are re-coated by new crude oil

New oil coat = Resetting

: Pits over 4mm at 1st inspection (repaired) : Pits less than 2mm at 1st inspection (NOT repaired)

: Pits over 4mm at 2nd inspection (repaired)

Old pits() did not grow!New pits() appear at different points.

1C : Pits over 4mm at 1st inspection (repaired)

: Pits less than 2mm at 1st inspection (NOT repaired) : Pits over 4mm at 2nd inspection (repaired)

Old pits() did not grow!New pits() appear at different points.

1C

insulating condition

defect

New defect in oil coat =Nucleation of pitting

New defect : by COW, Water …..

Figure 14 Mechanism of pit termination at dock inspection [1].

Figure 15 shows the results of calculating the corrosion

rate for all of the carriers surveyed in this research, based on the assumption that the corrosion duration was the dock interval, and carrying out an extreme value analysis. From the figure, it can be seen that, regardless of whether



the carrier uses an SH or DH structure, a roughly single statistical corrosion rate behavior occurs.

Cumulative Frequency

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2001 DH22001 DH2

Figure 15 Extreme statistic analysis result of all the investigate ships on the assumption that the corrosion duration is dock interval. 3.2 SURVEY OF MICRO CORROSION ENVIRONMENT IN THE PITTING AREA OF ACTUAL CARRIERS

Regarding the macro corrosion environment at the COT bottom, as mentioned in Section 2 a highly concentrated NaCl solution is stagnant at the COT bottom and also O2, CO2, and H2S exist in significant quantities as cathode reaction substances that can trigger the corrosion reaction.

pH inside of pit (Bottom Plate Cut Sample)

pH inside and outside of pit (Onboard measurement)

Figure 16 Onboard pH measurements results at inside and outside pits.

Generally, the localized corrosion area is under

corrosion environment conditions that differ from those of the surrounding areas. Particularly, the pH is the most basic and important parameter for understanding the corrosion phenomenon. Accordingly, the pH inside and outside the pits of the COT bottom of an actual carrier were evaluated. The results are shown in Figure 16. It was found that the pH of the inside of the pits was between about 1.5 and 2 which is extremely low, and the pH in the vicinity was between 5 and 10, that is, between neutral and mildly alkaline.

3.3 LAB. STUDY OF A REPRODUCTION OF THE LOCALIZED CORROSION

Next, a lab. test of reproducing the COT bottom localized corrosion was performed. Figure 17 shows an outline of the test method. The surface of an ordinary steel test piece was coated with crude oil to a thickness of about 1 mm, and then the crude oil was removed from part of the coated surface with a cotton bud. In this condition, the test piece was immersed in a 10% solution of NaCl, and left under conditions that reproduce the COT gas atmosphere. After a 3-month corrosion period, the test piece was withdrawn and the corrosion condition was evaluated.

Paint Crude Oilwith defect

ImmersionNaCl 10wt%

Pick Up the specimenCOT gas Environment

Defect↓

Figure 17 Corrosion test procedure with crude oil painting with defect.

Figure 18 shows the appearance of the test piece from which matter adhering to the surface was removed immediately after the test piece was withdrawn, and also the result of measuring the pH in the vicinity of the localized corrosion area. As can be seen from the figure, localized corrosion occurred even in a simulated environment in the lab. The pH level of the localized corrosion area was about 1, that is, strongly acidic, and in the vicinity of the localized corrosion area was alkaline.

Figure 18 Observed surface conditions after the test



Figure 19 shows the appearance and the maximum. corrosion depth after the corrosion test in which the size of the area from which crude oil was removed (= oil coat defect) in the abovementioned test was changed.

As is clear from the figure, the localized corrosion depth varied even when the size of the oil coat defect was the same. In addition, as shown in the figure, when the size of the oil coat defect was small, the corrosion depth decreased proportionally, which conformed to the relationship between the depth and the opening diameter measured in an actual carrier.

If the localized corrosion phenomenon is caused by a localized corrosion mechanism due to a so-called macro-cell as in the case of corrosion of stainless steel, and the oil coat defect is small, the cathode/anode ratio will increase, and the localized corrosion depth will also increase. However, as is clear from these results and the survey of Figure 20, no effect of the cathode/anode ratio could be seen either in an actual carrier or in the lab study, and it was judged that this localized corrosion phenomenon was due to the corrosion mechanism whereby the cathode area was not clearly separated from the anode. A similar corrosion phenomenon is the blistering/corrosion phenomenon in the painting defect area.

Depth : 0.76mm Depth : 1.02mm

Depth : 0.65mm Depth : 0.35mm

Figure 19 Observed pits after the test with varied size of defect.

As shown in Figure 20, the shape of the localized corrosion is similar to a constant depth/opening ratio, and it can be seen that this localized corrosion is a phenomenon which advances isotropically. In other words, it is reasonable to assume that the corrosion in a localized corrosion area is extremely active.

Figure 21 shows the result of quantitating the change of the corrosion rate with respect to pH in a lab using a simulated NaCl 10 wt% 30°C brine environment. At a pH level of about 3 or less, the cathode reaction is an extremely fast hydrogen ion reduction reaction, while in this pH region the corrosion rate is determined mainly by the iron dissolving anodic reaction. The survey results shown in Figure 16 shows the fact that the pH in the localized corrosion area in an actual carrier was no more than 1.5, that is, strongly acidic even when dilution by

the water used for measurement is taken into account. From these results as well, it was judged that the corrosion reaction in the localized corrosion area is a hydrogen generating type anodic dominant phenomenon in a strongly acidic environment.

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Labo

Figure 20 Relationship between diameter/depth ratio onboard and those observed in the simulated test.

H2 evolution ← →Oxygen consumption

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Corr

osi

on R

ate m

m/ye

ar

NaCl 10wt.%30

Figure 21 Corrosion rate of conventional steels in simulated brine solutions (10%NaCl with pH changed). 3.4 CORROSION MECHANISM AND DETERMINATION OF THE METHOD OF PERFORMING AN EVALUATION LAB. TEST

Based on the above study results, the mechanism of the reaction in the pitting localized corrosion area shown in Figure 22 is derived. By classifying the oil coat area into the oil coat sound area and the oil coat defect area, the corrosion process can be described as follows. [Oil coat sound area] A. The oil coat sound part has the same environmental insulating effect as that of paint, and thus protects the COT bottom. B. Along with the passage of time, water and oxygen permeate the oil coat, and the cathode reaction which forms a pair with the anode reaction in the localized



corrosion area advances. From the pH level in this area, it is judged that this is an oxygen reduction reaction.

Beaker

SpecimenTest solution

Notice

NaCl 10 mass%

pH 0.85 Adjust by HCl

Amount 20cc/.cm2 or more

Change Every 24 to 48 hoursAir open

30

Size 40mm x 50mm x t 　t: less than 4mml

Surface #600 emery paper Follow JIS G0591

repeat n=3 or more

144hours

Dipping specimen Follow JIS G0591Immersion

Gas

Temperature

Condition

Solution

Specimen

DurationBeaker


Beaker


Notice

NaCl 10 mass%

pH 0.85 Adjust by HCl

Amount 20cc/.cm2 or more

Change Every 24 to 48 hoursAir open

30

Size 40mm x 50mm x t 　t: less than 4mml

Surface #600 emery paper Follow JIS G0591

repeat n=3 or more

144hours

Dipping specimen Follow JIS G0591Immersion

Gas

Temperature

Condition

Solution

Specimen

Duration

[Oil coat defect area] C. In the oil coat defect area, highly concentrated NaCl, and oxygen, which is the cathode reaction substances, disperse and the corrosion reaction ① Fe→Fe2+ + 2e-

starts. D. After Fe2+ is generated in the corrosion reaction ①Fe→Fe2+ + 2e-, the H+ concentration in the localized corrosion area increases as a result of the next hydrolytic disassociation reaction ② Fe2+ + 2H2O → Fe (OH)2 + 2H+, and reduction of the pH value commences. E. Cl- in the brine disperses in the substances generated by the corrosion, counterbalances the H+ concentration in the localized corrosion area, and satisfies the electrical neutral conditions. F. D→E above are repeated, and the measured NaCl concentration in the brine is 10 wt%, which is extremely high, so the H+ concentration continues to increase until it counterbalances that of Cl-, which concentrate in the localized corrosion area. As a result, the environment in the localized corrosion area becomes an active strongly acidic corrosion atmosphere at a pH level of 1 or less, and the corrosion reaction advances by anode reaction

Figure 22 Summar

rate controlling.

ized process and mechanism of the localized

The abovementioned corrosion mechanism is similar to

of this study, the corrosion test met

corrosion on COT.

that of blister formation and corrosion beneath a painted surface, but it is characterized by the fact that oil coat damage exists at the beginning of corrosion, and the chloride concentration in the atmosphere which can freely disperse to the corrosion boundary face is extremely high, which causes the atmosphere in the localized corrosion area to become strongly acidic. In this way, the mechanism of localized corrosion at the COT bottom is entirely different from the mechanism of pitting in stainless steel.

Based on the results hod which reproduces the localized corrosion

environment under the conditions indicated in Figure 23 was determined.

Figure 23 The corrosion test condition for simulating localized corrosion environments. 4. STUDY OF CORROSION COUNTERMEASURES BASED ON KNOWLEDGE OBTAINED FROM A SURVEY AND ANALYSIS

Based on knowledge obtained from the above survey and analysis, the following countermeasures and issues were submitted. * Anti-corrosion materials: The reduction of the danger of holes being formed in the localized corrosion area due to the improved corrosion resistance in a strongly acidic atmosphere coexisting with a high chloride concentration is an issue. It is necessary to simultaneously satisfy characteristics that do not impede constructability, utilization characteristics and economic efficiency, while improving corrosion resistance.

2H+Fe2+

Na+

Cl- Cl-Cl-

Na+Na+

①

* Highly corrosion resistant paint: Obtaining stable long-term environmental insulating performance due to a paint film is an issue. Concretely, there is the acquisition of painting quality, the durability of the paint film (resistance to dissolving in oil, resistance to damage, and so on), and so on. With conventional painting, there are cases in which localized corrosion occurs as shown in the photograph below, so it cannot be said that this countermeasure is an adequate one.

4.8mm depth4.8mm depth5.1mm depth5.1mm depth Photo 6 Typical example of pits on painted COTs [3].

* Zn primer painting: An exposure test for validating the effectiveness in an actual carrier was performed. The results are shown in Figure 24. As shown in the figure, no reduction of the maximum pit depth was found [1].

In addition, it is considered that protection of the COT bottom oil coat, reduction of brine stagnation, complete elimination of brine from crude oil, and so on, are important points concerning the preparation of countermeasures.

Fe⇒ Fe +2e②Fe2++2H2O 2

2 + -

⇒ Fe(OH) +2H+

Cl- Cl-Cl-

Cl-

Na+

H+Cl-

e-e

H+H2

H2 H+

crude oil

saline water(eq. 8%NaCl)

bottom plate

sludge oroil coat

O2,CO2,H2S,…

2H+Fe2+

Na+

Cl- Cl-Cl-

Na+Na+

H+①Fe⇒ Fe2 ++2e-

②Fe2++2H2O ⇒ Fe(OH)2+2

Cl- Cl-Cl-

Cl-

Na+

H+ Cl-

e-e

H+H2

2OH-1/2O2+H2O

crude oil

brine ( ～10%NaCl)

bottom plate

O2,CO2,H2S,…

H2O H2OH2O

H2OH2O

H2OH2O

Corrosion Products

Oil Coat

2H+Fe2+

Na+

Cl- Cl-Cl-

Na+Na+

① 2 + -

⇒ Fe(OH) +2H+Fe⇒ Fe +2e

②Fe2++2H2O 2

Cl- Cl-Cl-

Cl-

Na+

H+Cl-

e-e

H+H2

H2 H+

crude oil

saline water(eq. 8%NaCl)

bottom plate

sludge oroil coat

O2,CO2,H2S,…

2H+Fe2+

Na+

Cl- Cl-Cl-

Na+Na+

H+①Fe⇒ Fe2 ++2e-

②Fe2++2H2O ⇒ Fe(OH)2+2

Cl- Cl-Cl-

Cl-

Na+

H+ Cl-

e-e

H+H2

2OH-1/2O2+H2O

crude oil

brine ( ～10%NaCl)

bottom plate

O2,CO2,H2S,…

H2O H2OH2O

H2OH2O

H2OH2O

Corrosion Products

Oil Coat



0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

4C-MS 4C-TM 3C-MS 3C-TM 3P-MS 3P-TM

Max

.Cor

rosi

on

Dep

th m

m

As Polished

As Rolled

As ShotPrimer Painted

Ave.:1.80mm

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

4C-MS 4C-TM 3C-MS 3C-TM 3P-MS 3P-TM

Max

.Cor

rosi

on

Dep

th m

m

As Polished

As Rolled

As ShotPrimer Painted

Ave.:1.80mm

Figure 24 Onboard corrosion test results on various surface conditions including Zn primer painting[1]. 5. DEVELOPMENT OF ANTI-CORROSION STEEL AND CORROSION RESISTANCE CHARACTERISTICS

Using the abovementioned test method, an investigation and study of a new anti-corrosion steel composition system were performed.

The results are shown in Figure 25. By using the newly developed steel, it was succeeded to greatly reduce the corrosion rate compared to that of conventional steel in a highly concentrated acidic atmosphere which reproduced the localized corrosion environment of a crude oil tanker.

The composition of this corrosion resistance steel is shown in Table 3. This composition system was realized by means of new discoveries and the use of the latest corrosion resistance alloy design technology. It satisfies the IACS rule, has excellent corrosion resistance, and has the same characteristics as those of conventional steel.

Table 3 Typical composition of NSGP®-1 C Si Mn P S Al Ti Ceq

NSGP-1 0.124 0.331AH32 0.140 0.20 1.09 0.018 0.006 0.031 0.014 0.322

IACS Standard ≦0.18 ≦0.5 0.9～1.6 ≦0.035 ≦0.035 ≧0.02 ≦0.02 ≦0.36

meets IACS Standard (including all ally elements)

0

1

2

3

4

5

6

8

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5pH

Corro

sion R

ate m

m/ye

ar

← Conventional Steel

NaCl 10wt.%30

Air OpenStagnant

n=3

NSGP-1

0

1

2

3

4

5

6

8

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5pH

Corro

sion R

ate m

m/ye

ar


NaCl 10wt.%30

Air OpenStagnant

n=3

NSGP-1

Figure 25improved anti-corrosion property of the NSGP®-1

6. CONCLUSION

The following measures were adopted in order to eliminate the danger of crude oil leakage and consequent increased burden on the environment due to the localized corrosion and the formation of holes in the COT bottom of a VLCC, which have been occurring frequently in recent years.

1) Obtaining a grasp of the facts and a technical understanding concerning the phenomenon of localized corrosion of the COT bottom 2) Preparing and submitting countermeasures based on technical understanding

As a result , the following knowledge and conclusions were obtained. 1. An oil coat exists inside the COT, providing the same degree of corrosion prevention as that of paint. Regarding the mechanism of corrosion, it was clarified that localized corrosion occurs and progresses in its defect area, the growth of this localized corrosion stops due to dock cleaning, and localized corrosion progresses as a result of a strongly acidic environment containing highly concentrated chloride ions. A corrosion lab test method that reproduced the environment inside the localized corrosion was successfully developed. 2. By using this test method, a new search and study of the composition of anti-corrosion steel was performed, leading to the successful development of NSGP®-1 steel which has excellent corrosion resistance unobtainable with conventional steel and is extremely useful for eliminating the danger of holes forming in the COT bottom. In addition to having an excellent corrosion resistance, this newly developed steel satisfies the IACS rule, and has the same characteristics as those of conventional steel. 7. REFERENCES 1. K. Katoh, S. Imai, D.T. Yasunaga, H. Miyuki, Y.

Yamane, H. Ohyabu, Y. Kobayashi, M. Yoshikawa and Y. Tomita : " Study on Localized Corrosion on Cargo Oil Tank Bottom Plate of Oil Tanker ", World Maritime Technology Conference, San Francisco, Oct. 2003.

2. Ship Research 242 Report: “Study on new type corrosion of crude oil tankers”,March 2002.

3. Ship Research 242 Report: “Study on new type corrosion of crude oil tankers”,March 2003.

8. AUTHORS’ BIOGRAPHIES Shiro Imai holds the current position of general manager of technical group in plate division. He is responsible for the technological issues of all the steel plates products of the company. Kenji Katoh (Dr. Eng.) holds the current position of leader of anti-corrosion steel group of Steel Research Lab. He is responsible for R&D of the anti-corrosion materials. Yuji Funatsu holds the current position of senior manager of technical group in plate division. He is responsible for technical issues of steel plates for ships. Michio Kaneko (Dr. Eng.) holds the current position of chief researcher of anti-corrosion steel group of Steel Research Lab. He is responsible for R&D of the anti-corrosion materials. Tomoyuki Matsubara holds the current position of Corporate Officer and General Manager of Technical Group. He is responsible for all the new building and the technical issues of the company. Hideaki Hirooka holds the current position of Manager of Technical Group. He is Naval Architect and responsible for technical issues of new and existing vessels especially for hull structure. Hidehiko Sato holds the current position of Manager of Technical Group. He is Naval Architect and responsible for technical issues of new and existing vessels especially for hull structure.



ONBOARD EVALUATION RESULTS OF NEWLY DEVELOPED ANTI-CORROSION STEEL FOR COTS OF VLCC AND PROPOSAL FOR MAXIMUM UTILIZATION METHOD Shiro Imai, Kenji Katoh, Yuji Funatsu, Michio Kaneko, Nippon Steel Corporation, JAPAN Tomoyuki Matsubara, Hideaki Hirooka, Hidehiko Sato, Nippon Yusen Kaisha, JAPAN SUMMARY The objective of this study is to develop a practically and technically reasonable counter measure to the risk on leakage

of crude oil by corrosion perforation of the COT bottom plate. To make clear the validity, the newly developed anti-corrosion steel NSGP®-1 has been applied to all bottom plates of COTs of a newly built double hull VLCC tanker. Prior to the application, properties not only on excellent anti-corrosion but also good mechanical properties of the steel

have been evaluated in cooperation with the ship builder. At last the ship building work has been completed with no problem, showing good workability during construction. Detailed corrosion investigation of the COT built with newly developed anti-corrosion steel was carried out at the first

dock after 2year 3 month after the launching. No localized corrosion which needs to be repaired was observed, and she docked out without repair for corrosion damages. As a result, technological validity of the scientific understanding of the corrosion phenomena and development policy on anti-corrosion steel in this study was confirmed. Based on this onboard evaluation result, the authors quantitatively investigated corrosion environment and phenomena

of localized corrosion on bottom plate in detail. As a result, corrosion test method to evaluate and define anti-corrosion steel for COT bottom plate of crude oil carrier is proposed, and anti-corrosion property of the newly developed steel was scientifically clarified. Considering the recent finding that pit growth stops at dock cleaning, a new model on maximum utilization of the anti-

corrosion steel for best corrosion life cycle design of COT has been proposed. As a result, newly developed anti-corrosion steel capability to be corrosion repair free during whole ship life is derived. ¶ NOMENCLATURE VLCC: Very Large Cargo Carrier SH: Single Hull Structure DH: Double Hull Structure COT: Cargo Oil Tank COW: Crude Oil Washing 1. Objective

The objective of the present study is to develop an anti-corrosion steel and thereby provide a technically reasonable measure to avoid the danger of perforation of the bottom plate of a cargo oil tank (COT).

Photo 1 Newly Built VLCC ‘TAKAMINE’ applying NSGP®-1.

In the previous paper, the authors reported the

development of an anti-corrosion steel for the COT bottom plates of a VLCC. To evaluate not only the corrosion resistance of the developed steel but also its mechanical and application properties as required for field shipbuilding work, it was used for all the COT bottom plates of a newly constructed VLCC shown in

Photo 1. The localized corrosion of COT bottom plates results

from the combined conditions of oil coats and bottom plate water, and, statistically, the rate of its progress fluctuates as seen in Figure 1. In consideration of this, the developed steel was used for all the COT bottom plates of the ship to evaluate its corrosion-resistance performance comprehensively and statistically.


F

2533.3

50

100

0 1 2 3 4


Return Period T

-

2000 -DH1

2000 -

2001 -DH4

Supposed corrosion period

2-2.5years


F

1999 -SH1

2001 -DH3

2000 -SH1

1%

5%10%20%30%40%50%60%

70%

80%

90%

95%96%97%

98%

99%

1.01

1.051.111.251.431.6722.5

3.33

5

10

20

1%

5%10%20%30%40%50%60%

70%

80%

90%

95%96%97%

98%

99%

1.01

1.051.111.251.431.6722.5

3.33

5

15202533.3

50

100

0 1 2 3 4


Return Period T

2000 -DH1

2000

-

SH2

2001 -DH4

1999 -SH1

2001 -DH3


2-2.5years

2001 DH2


F

2533.3

50

100

0 1 2 3 4


Return Period T

-

2000 -DH1

2000 -

2001 -DH4

Supposed corrosion period

2-2.5years


F

1999 -SH1

2001 -DH3

2000 -SH1

1%

5%10%20%30%40%50%60%

70%

80%

90%

95%96%97%

98%

99%

1.01

1.051.111.251.431.6722.5

3.33

5

10

20

1%

5%10%20%30%40%50%60%

70%

80%

90%

95%96%97%

98%

99%

1.01

1.051.111.251.431.6722.5

3.33

5

15202533.3

50

100

0 1 2 3 4


Return Period T

2000 -DH1

2000

-

SH2

2001 -DH4

Corrosion DurationDock Interval

2-2.5years

2000 -SH1

2001 -DH3

1999 -SH1

2001 DH22001 DH2

Figure 1 Statistical variation of maximum localized corrosion rate on COT bottom plate[1].

For constructing a highly reliable ship, steel material must have a wide variety of properties such as the basic mechanical properties for structural steels and good workability in field use as well as the corrosion resistance of the base metal and weld joints in the ballast environment on the back side of COT bottom plates. Before the construction of the VLCC, the authors evaluated these properties of the developed steel with the help of Nagasaki Shipyard of Mitsubishi Heavy Industries, who constructed it. The present report describes the results of the evaluation



of the developed steel, and based on the result, proposes a definition of corrosion-resistant steel for achieving the objective of the present study and guidelines for optimum anti-corrosion design of a ship in the whole service life. 2. MECHANICAL PROPERTIES AND SEAWORTHINESS OF DEVELOPED ANTI-CORROSION STEEL

Table 1 shows a typical chemical composition of the developed steel, NSGP®-1. This chemical composition, which was worked out based on new discoveries and the latest technologies of corrosion-resistant alloy design, meets the IACS rules, and exhibits the same mechanical properties and application performance as those of conventional shipbuilding steels, in addition to excellent corrosion resistance.

As stated in the previous report, the developed steel demonstrates a markedly lower corrosion rate than that of conventional steels in an environment of chlorides in high concentrations and strong acids simulating the localized corrosion environment to which the COT bottom plates of a crude-oil carrier are exposed.

Table 1 Typical chemical composition of developed NSGP®-1.

C Si Mn P S Al Ti CeqNSGP-1 0.124 0.331

AH32 0.140 0.20 1.09 0.018 0.006 0.031 0.014 0.322IACS Standard ≦0.18 ≦0.5 0.9～1.6 ≦0.035 ≦0.035 ≧0.02 ≦0.02 ≦0.36

meets IACS Standard (including all ally elements)

0

1

2

3

4

5

6

8

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5pH

Corro

sion R

ate m

m/ye

ar


NaCl 10wt30

Air OpenStagnant

n=3

.%

NSGP-1

0

1

2

3

4

5

6

8

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5pH

Corro

sion R

ate m

m/ye

ar


NaCl 10wt.%30

Air OpenStagnant

n=3

NSGP-1

Figure 2 Anti-corrosion property of the NSGP®-1 under simulated localized corrosion environment at COT bottom.

As Figure 2 shows, the corrosion rate of NSGP®-1 in a strongly acidic and corrosive environment having a pH of 1.0 or lower is as small as approximately 0.6 mm/y;

the photos of the steel surfaces after corrosion test clearly show the corrosion resistance of NSGP®-1 far superior to that of conventional steel.

Table 2 Welding Conditions. Welding Rod US36(4.8mmφ)

Flux PF152E

Backing plate FAB-1 Metal powder PR2

Grove shape 50˚ (25˚ each side) root gap :2 mm

Pass

conditions

1000 A, 35 V, electrode speed: 35

cm/min, heat input: 102.3 kJ/cm

Developed NSGP®-1

NSGP-1 ｜ Weld Metal ｜NSGP-1(0.072) (0.089) (0.071) (Corrosion Loss in mm)

１００μｍ１００μｍ

Developed NSGP®-1

Conventional

Weld Metal Conventional Conventional

0102030405060708090

100

NSGP-1 Weld Metal NSGP-1

Relat

ive C

orro

sion R

atio t

o Con

venti

onal

0102030405060708090

100

Figure 3 Anti-corrosion property of the NSGP®-1 including weld under simulated localized corrosion environment at COT bottom. (pH=0.8 , 336hrs).

Conventional Developed NSGP®-1

Conventional Weld Metal Conventional0

102030405060708090

100


Relat

ive C

orro

sion R

atio t

o Con

venti

onal

0102030405060708090

100


Relat

ive C

orro

sion R

atio t

o Con

venti

onal

0102030405060708090

100

0102030405060708090

100

Conventional Weld Metal ConventionalConventional Weld Metal Conventional



Needless to say, COT bottom plates are constructed by welding, and therefore, an anti-corrosion steel for such applications must exhibit good corrosion resistance in weld joints as well.

Figure 3 shows the result of a corrosion test of welded specimens of NSGP®-1 under the same conditions as those for Figure 2. The welding of the specimens was done under the conditions of Table 2. The graphs and photos clearly show that the developed steel exhibits excellent corrosion resistance at both the base metal and weld joints.

Since the COTs of the latest VLCCs are constructed in double hull (DH) structure, the reverse side of COT bottom plates is exposed to a corrosive environment of a water ballast tank (WBT). This means that anti-corrosion steel used for COT bottom plates must have the same properties as those of conventional steels in a WBT environment.

Figure 4 compares NSGP®-1 with a conventional steel in terms of the corrosion rate and appearances of welded specimens after immersion in artificial seawater. The welding was done under the same conditions of Table 2.

From these figures, it is clear that the corrosion resistance of NSGP®-1 in a WBT environment is, in either the base metal or weld joint, the same as or better than that of conventional steels.

0

0.05

0.1

0.15

0.2

0.25

Matrix Weld

COrro

sion R

ate m

m/ye

ar

ConventionalNSGP-1

Artificial Sea Water40

Figure 4 Corrosion property of the NSGP®-1 including weld under WBT environment.

The evaluation results of the mechanical properties and

field use performance of NSGP®-1 are shown in Figures 5 to 7.

Figure 5 shows the result of Y-groove cracking test. It was manually welding. The graph clearly shows that the weldability of NSGP®-1 is excellent, totally free from cracking.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100

Pre-heat Temperature

Crac

k Rat

io %

SurfaceCross sectionRoot

No Crack

0

50

100

150

200

250

300

350

0 20 40 60

Bead Length (mm)

Maxim

um H

ardn

ess(

Hv 1

0kgf

)

Figure 5 Y-groove cracking test result.

Figure 6 shows the result of fatigue property test of the

base metal and weld joints. The welding condition of Butt weld Joint is shown in the table of Figure 7. Fillet Weld T-joint was done by CO2 welding. As is clear from the graphs, the fatigue properties of NSGP®-1 are the same as or better than those of conventional steels.

100

1000

Stre

ss R

egion

, Δσ(

N/mm

2 )

NS-GP1

YP32,36HT(5 Joints)

Butt Weld Joint (Flat Position)

100

1000

10000 100000 1000000 10000000Number of Cycles to Failure

Stre

ss R

egion

, Δσ(

N/mm

2 )

NS-GP1YP36HT(3 Joints)

Base Metal

10

100

1000

10000 100000 1000000 10000000Number of Cycles to Failure

Stre

ss R

egion

, Δσ(

N/mm

2 )

NS-GP1

YP32,36HT(5 Joints)

Fillet Weld T-joint

Conventional

Developed NSGP®-1

Figure 6 Fatigue property test result.

Figure 7 shows the result of Charpy impact test of various types of weld joints. The welding conditions is also shown in the Figure here. As is clear from the graphs, NSGP®-1 stably satisfies the requirements under relevant standards, demonstrating sufficiently high toughness of weld joints.



Figure 7 Toughness property test result. 3. Results of Application to VLCC 3.1. Application

Figure 8 shows the portions of the TAKAMINE to which NSGP®-1 was applied. It was used for all the COT bottom plates, and in the cases especially of six tanks of Nos. 3 and 4 COTs at the center of the hull, the steel was used without protective paint coating.

NO Coat : 6COT

Figure 8 Applied area of NSGP®-1 without coating.

As had been expected from the evaluation results of the workability of NSGP®-1 prior to the shipbuilding, the steel caused no problems whatsoever in the field work, and the ship construction went as smoothly as that of any ordinary vessels. 3.2 Result of Corrosion Resistance Evaluation 3.2.1 Evaluation Results at first Dock Inspection

At a first dock inspection of the TAKAMINE after 2 years and 3 months of service, the state of corrosion of the COT bottom plates of NSGP®-1 without coating was examined in detail.

It has to be noted here that the COT insides of the TAKAMINE underwent crude oil washing (COW) after each trip; this is a very unfavourable condition with respect to the occurrence and progress of the localized corrosion of the bottom plates because, as stated in the previous report in detail, COW adversely affects the environmental insulation effects of the oil coat, which protects the internal surfaces of COTs from corrosion.

Inspectors allocated at every 1.5 m in the width direction of a COT visually inspected the corrosion condition of the entire bottom plate in the length direction removing the oil with a scraper. The inspection was repeated three times.

0 0 0 0 0 00

200

400

600

800

1000

1200

1400

3P 3C 3S 4P 4C 4SCOT Position

Pits

Coun

t ove

r 4mm

Dep

th

At the 1st dock : 2.25 years after building

Figure 9 Observed pit count over 4mm depth on each COT Bottom.

Figure 9 shows the result of the inspection. No pits more than 4 mm in depth, which had to be repaired, were found. According to the authors’ investigation of an ordinary VLCC, the number of pits from 4 to roughly 10mm in depth, requiring repair, was at least tens per COT, the number being as large as several hundreds in some cases. The above inspection result of the TAKAMINE made it clear that NSGP®-1 was effective in protecting the COT bottom plates from localized corrosion that required repair work.

Photo 2 shows typical examples of observed condition in portions around drain holes of the TAKAMINE in comparison with those of another vessel of conventional steels. Pits requiring repair work occur in substantially all such portions made of conventional steels, but no corrosion pits were observed in these portions of the TAKAMINE.

In addition, Photo 3 shows another example of observed condition of the TAKAMINE along a weld joint of a COT bottom plate. No pitting corrosions were found near weld joints of the TAKAMINE’s COTs, like in COTs of conventional steels.

The dock inspection confirmed that the ship was totally free from other types of corrosion as well as problems such as cracking due to poor mechanical properties of the steel.

With the above inspection results, the TAKAMINE left the dock without any repair work at all inside the COTs. Thus, NSGP®-1 proved effective in avoiding the danger of corrosion perforation of COT bottom plates to a level where repair work of localized corrosion, which is a usual work item in periodical dock inspection of an ordinary VLCC, is absolutely unnecessary. It has to be noted that this result was obtained in spite of COW after every trip, an exceptionally tough condition from the viewpoint of corrosion prevention. This corroborates the excellent corrosion resistance of NSGP®-1 when applied to VLCC.

The above also evidences the technical correctness of the development policy of NSGPP

®-1 to decrease the danger of corrosion perforation of COT bottom plates due to localized corrosion discussed in the previous report.

0

50

100

150

200

250

300

WeldMetal

Fusion Line

HAZ 1mm

HAZ 3mm

HAZ 5mm

Notch Position

Abso

rbed

Ene

rgy a

t 20

vE20

(J)

: Center : 1mm below surface

Standard ≧34J

0

50

100

150

200

250

300

WeldMetal

Fusion Line

HAZ 1mm

HAZ 3mm

HAZ 5mm

Notch Position

Abso

rbed

Ene

rgy a

t 20

vE20

(J)


Standard ≧34J

0

50

100

150

200

250

300

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Fusion Line

HAZ 1mm

HAZ 3mm

HAZ 5mm

Notch Position

Abso

rbed

Ene

rgy a

t 20

vE20

(J)


Standard ≧34J

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HAZ 5mm

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Abso

rbed

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rgy a

t 20

vE20

(J)


Standard ≧34J

FAB, L direction

Welding Method Shape of groove Heat Input(kJ/cm)

Flux(PFI-52E) Backing Metal(FAB-1)

Welding Material

Table Welding Condition

125Wire(US-36ƒ³6.4)FAB V

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300

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HAZ 5mm

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rgy a

t 20

vE20

(J)


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HAZ 3mm

HAZ 5mm

Notch Position

Abso

rbed

Ene

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t 20

vE20

(J)


Standard ≧34J

0

50

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vE20

(J)


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vE20

(J)


Standard ≧34J

0

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(J)


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Welding Material





Welding Material





--

Photo 2 Observed condition around drain holes.

Weld Line : No Pit

Photo 3 Observed condition around weld line.

3.2.2 Result of Detailed Quantification of Pitting Corrosion from Technical Viewpoint

It was technically difficult to quantitatively evaluate the effect of the use of anti-corrosion steel for a real VLCC based only on the results described above. Furthermore, the study results at the laboratory study stage indicated that theoretically the steel then being developed would significantly decrease the rate of corrosion in the local-corrosion environment of COT bottom plates, but it would not totally prevent it from occurring. In consideration of the above, the authors conducted another detailed inspection of the COTs focusing on shallower pitting corrosions, 2 to 4 mm in depth, which had not been practiced in usual dock inspection. The above depth range was adopted because it was practically difficult to detect a corrosion depression 2 mm or less deep.

The inspection proved very difficult because of the small corrosion depth to detect and evaluate. The result is shown in Figure 10; the maximum depth of pitting corrosions found was less than 3 mm and the number of their occurrence was very small, 30 or less per COT.

Photo 4 shows an example of the pitting corrosions thus found. The size (diameter and depth) of the pit was so much smaller than that of the pitting corrosions of

conventional steels that it was very close to the lower detection limit. NSGP® -1

7 7 7 20 4 270

200

400

600

800

1000

1200

1400

3P 3C 3S 4P 4C 4SCOT Position

Pits

Coun

t ove

r 2mm

Dep

th

At the 1st dock : 2.25 years after building

Figure 10 Observed pit small count being over 2mm and less than 3mm depth on COT Bottom.

Photo 4 Only very shallow pits with small counts were observed on NSGP®-1 compared to pits on conventional steel. 4. Discussion on NSGP®-1 Application to VLCC and Proposal of Optimum Method for Using Anti-corrosion Steel 4.1 Quantitative Analysis of Results of NSGP®-1 Application to Real VLCC

As stated earlier, theoretically, the developed steel was only to significantly decrease the rate of corrosion in the localized corrosion environment of COT bottom plates rather than to totally prevent it, and for this reason, it was necessary to well understand its corrosion rate behavior to make the most of its technical advantages.

Figure 11 shows the result of a extreme statistical analysis of localized corrosion rate estimated from the depth data of pits 3 mm or less in depth accumulated over a very long period. The graph shows that, as was

Developed NSGP®-1

Conventional

Conventional

NSGP® -1

NSGP® -1

Conventional

Conventional

NSGP® -1

20mm20mm

20mm20mm20mm

Conventional

NSGP® -1

20mm20mm

20mm20mm20mm20mm NSGP® -1

Conventional



conventionally known, the distribution of the rate of localized corrosion of NSGP®-1 agrees with the extreme statistical distribution. The gradient of the regression curve that expresses the statistical distribution of the corrosion rate regarding an anti-corrosion steel is nearly equal to that regarding a conventional steel. It follows, therefore, that the corrosion phenomenon that has been occurring in the TAKAMINE is presumably identical to that which occurs in a VLCC of conventional steels. In addition, the graph indicates that the expected maximum localized corrosion rate of the developed steel applied to all the 15 COTs of the TAKAMINE will not surpass 4 mm in 2.5 years (= 1.6 mm/y), and no repair work will be required until next dock inspection.

Figure 11 Estimated max. pit depth for 15 COTs with NSGP®-1 by Statistical analysis. 4.2 Evaluation Method and Definition of Anti-corrosion Steel

As stated in the previous report in detail, investigations of actual VLCCs and laboratory simulation tests have made it clear that the pH inside a pit of localized corrosion is as low (acidic) as about 1.5 or less. This value of pH, however, is the one measured after wetting the corrosion pits, namely diluting the environmental ingredients, and is not necessarily accurate. This means that to define the environmental condition for evaluation of corrosion resistance of a steel, it is necessary to estimate the pH value precisely.

It is generally difficult to measure the pH value inside a localized corrosion accurately. On the other hand, it is known that the corrosion rate of ordinary carbon steel is strongly correlated with pH, and it is possible to estimate the pH inside a corrosion pit from the maximum rate of localized corrosion obtained so far through investigations of actual VLCCs. Figure 12 shows the maximum rate of localized corrosion measured in the past and the calculation result of the pH-dependence of the corrosion rate of ordinary steels. The figure indicates that the value of pH inside the corrosion pit that shows the highest corrosion rate among those occurring in a ship is estimated at 0.85 to 1.16.

Ship Estimated pH

0.85

A 1.11B 1.16C 0.93DE 1.07F 0.94

Ship Max depthmm/2.5years

Max Corrosion Ratemm/year

A 7.50 3.00B 7.06 2.82C 9.51 3.80D 10.57 4.23E 7.90 3.16F 9.40 3.76

A B

CD

EF

0

1

2

3

4

5

6

7

8

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Corro

sion R

ate m

m/ye

ar

pH

↓ Conventional Steel

Laboratory Evaluation

30

Air OpenStagnant

n=3

NaCl 10wt.%

Figure 12 Quantitative estimation result on pH inside of pits.

The objective of the present development study is, as

initially stated, to decrease the danger of perforation corrosion of COT bottom plates. Judging from the result of the above pH estimation, the corrosion-resistant properties of anti-corrosion steel should be evaluated under the condition most likely to cause corrosion, namely at a pH of 0.85 or lower with a NaCl concentration of 10 mass % at 30˚C.

Estimated max. pit depth for 15 COTs : Less than 4mmNS-GP1 shown excellent corrosion resistance compared to conventiona l steels

1%

5%10%20%30%40%50%60%70%

80%

95%96%97%

98%

99% 100

50352520

←15COT10

5

32.521.61.41.21.11.051.01

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5


Cumu

lative

Fre

quen

cy F

Retur

enPe

riod T

No Repair

On the other hand, when anti-corrosion steel is applied to all the 15 COTs of a VLCC, the 50%-cumulative probability figure of the extreme statistics in Figure 13 indicates that the average corrosion rate must be 0.9mm/y in order that the maximum corrosion depth is less than 4 mm in 2.5 years (= 1.6 mm/y) so that no

repair work is not required.

igure 13 Criterion for anti-corrosion steel with which

This means that a steel can be used as an anti-corrosion st

he corrosion rate of NSGP -1

Fmaximum corrosion depth less than 4mm.

eel for COT bottom plates if, as described above, its average corrosion rate calculated in terms of weight loss is 0.9 mm/y or less under the laboratory test condition of a pH value of 0.85 or lower

On the other hand, since t ®

in the laboratory test to evaluate its corrosion resistance was roughly 0.6 mm/y under pH changing from 0.6 to 1.0 as seen in Figure 14, the developed steel is fully entitled for use as an anti-corrosion steel for COT bottom plates.

Repair by Paint Repair by Weld

Conventional Steels

IMPROVEMENT

→over 4mm/2.5years→over 4mm/2.5years

1%

5%10%20%30%40%50%60%70%

80%

95%96%97%

98%

99% 100

50352520

←15COT10

5

32.521.61.41.21.11.051.01

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5


Cumu

lative

Fre

quen

cy F

Retur

enPe

riod T

No Repair Repair by Paint Repair by Weld

Conventional Steels

IMPROVEMENT


90%90%

NSGP®-1

1%

5%10%20%30%40%50%60%70%

80%

95%96%97%

98%

99% 100

50352520

←15COT10

5

32.521.61.41.21.11.051.01

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5


Cumu

lative

Fre

quen

cy F

Retur

enPe

riod T

Estimated max. pit depth for 15 COTs : Less than 4mmNS-GP1 shown excellent corrosion resistance compared to conventiona l steels


Conventional Steels

IMPROVEMENT


1%

5%10%20%30%40%50%60%70%

80%

95%96%97%

98%

99% 100

50352520

←15COT10

5

32.521.61.41.21.11.051.01

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5


Cumu

lative

Fre

quen

cy F

Retur

enPe

riod T


Conventional Steels


IMPROVEMENT

90%90%

NSGP®-1

99%

98%

97%96%95%

90%

70%60%50%40%30%20%10%

5%

1% 1.01

1.051.11.21.41.6

←2 Average λ2.53

5

10←15 COT20253550

100

0 0.5 1 1.5 2 2.5 3Corrosion Rate mm/year

Cumu

lative

Fre

quen

cy F

Retur

n Per

iod T

0.9mm/y

Criteria forCriteria forAnti-corrosion Steel

Max Corrosion rate for4mm depth for 2.5years in 15COT

Criterion : less than 0.9mm/year in laboratory test

Need to repair

← Corresponds toLaboratorytest result

← Corresponds toOnboardMax. Depth

80%

NSGP® -1

99%

98%

97%96%95%

90%

70%60%50%40%30%20%10%

5%

1% 1.01

1.051.11.21.41.6

←2 Average λ2.53

5

10←15 COT20253550

100

0 0.5 1 1.5 2 2.5 3Corrosion Rate mm/year

Cumu

lative

Fre

quen

cy F

Retur

n Per

iod T

0.9mm/y

Criteria forCriteria forAnti-corrosion Steel

Max Corrosion rate for4mm depth for 2.5years in 15COT

Criterion : less than 0.9mm/year in laboratory test

← Corresponds toLaboratorytest result

← Corresponds toOnboardMax. Depth80%

NSGP® -1

Need to repair



y

CriteriaCriteriaLess than Less than 0.9mm/y0.9mm/y

= Anti= Anti--corrosion Steelcorrosion Steel

0

1

2

3

4

5

6

7

8

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5pH

Corro

sion

Rate

mm/

year

← Conventional Steel Laboratory Evaluation

NaCl 10wt.%30

Air OpenStagnant

n=3

OnboardEnvironment

NS-GP1LaboratoryEvaluation

Figure 14 Variation of corrosion rate of the NSGP®-1 with pH. 4.3 Proposal of Optimum Application Method of Anti-Corrosion Steel

As stated in the previous report in detail, the growth of localized corrosion of COT bottom plates is considered to halt upon cleaning of the COT inside at a dock inspection.

On the other hand, if the depth of pitting corrosion found at a dock inspection is 4 mm or less, the danger of cargo oil leakage due to corrosion perforation of COT bottom plates is negligibly small and the repair of the attacked portion is considered unnecessary.

When an anti-corrosion steel such as NSGP®-1 is used for COT bottom plates and if the ship undergoes dock inspection at an interval of around 2.5 years, then the maximum corrosion depth of all the 15 COTs will not exceed 4 mm as stated earlier, and as Figure 15 shows, the repair work of localized corrosion of the bottom plates will not be required throughout its whole service life. In other words, it is possible to achieve the object of the present development, namely to avoid the danger of cargo oil leakage due to corrosion perforation of COT bottom plates with minimum maintenance loads, by applying anti-corrosion steel to COT bottom plates and setting the interval of the periodical dock inspection at 2.5 years.

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35Ship Age / years

Corro

sion D

epth

/ mm

No need for maintenance during a whole ship life with NS-GP1

Observed Results

No ne

ed to

Re

pair

Pit Growth Terminatesat Dock inspection

[Found by SR242]

Observed Max. Corrosion Depthwith NS-GP1

Less than 4mm

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35Ship Age / years

Corro

sion D

epth

/ mm

No need for maintenance during a whole ship life with NS-GP1

Observed Results

No ne

ed to

Re

pair

Pit Growth Terminatesat Dock inspection

[Found by SR242]

Observed Max. Corrosion Depthwith NS-GP1

Less than 4mm

Figure 15 Maximum utilization method of anti-corrosion steel through life time of a ship. 5. CONCLUSION

There have been some cases of oil leakage from VLCCs due to perforation of COT bottom plates owing to localized corrosion over the last years. To provide a rational means to prevent such danger and avoid

unnecessary environmental loads, the authors developed a new anti-corrosion steel, NSGP®-1, applied it to the COTs of an actual VLCC, and examined its anti-corrosion performance comprehensively and statistically. As a result, the following findings and conclusions were obtained: 1. Prior to the application of the developed steel to an

actual VLCC, its corrosion resistance and other properties were evaluated, and its mechanical and application properties were found substantially the same as those of conventionally used steels. As a result, the developed steel posed no problem in its application to an actual VLCC.

2. After 2 years and 3 months of its commissioning, the VLCC for which NSGP®-1 was used underwent a first dock inspection, where no localized corrosion whatsoever requiring repair was found to have occurred to the six COT bottom plates to which the steel was applied without protective paint coating. This means that, through application to an actual VLCC, the developed steel proved effective in achieving the following unprecedented results: to decrease the danger of COT bottom plate perforation to the degree where the repair work of their localized corrosion, which is a routine work item with ships of conventional steels, is not necessary; and that this is realized even under a condition more likely to cause corrosion than usual cases, with COW being conducted after each voyage.

3. Based on the result of the inspection and examination of shallower pits of the COT bottom plates of NSGP®-1, the authors proposed test methods regarding corrosion-resistant steel, an approach to quantitatively defining a corrosion-rate condition that an anti-corrosion steel should satisfy and a specific corrosion-rate condition for such steel. In addition, the authors also proposed that the interval between two dock inspections of a VLCC be roughly 2.5 years to make the most of the excellent performance of corrosion-resistant steel. This will ensure that the danger of COT bottom plate perforation due to localized corrosion is effectively avoided and the repair work of localized corrosion becomes unnecessary all through the service life of a VLCC.

6. Acknowledgement

The authors would like to express their sincere gratitude for the substantial assistance of Nagasaki Shipyard and Nagasaki Research & Development Center of Mitsubishi Heavy Industries extended to them for the evaluation of the material properties of the developed steel in relation to its applicability to a real ship. 7. REFERENCES 1. K. Katoh, S. Imai, D.T. Yasunaga, H. Miyuki, Y.

Yamane, H. Ohyabu, Y. Kobayashi, M. Yoshikawa and Y. Tomita : " Study on Localized Corrosion on Cargo Oil Tank Bottom Plate of Oil Tanker ", World Maritime Technology Conference, San Francisco, Oct. 2003.



8. AUTHORS’ BIOGRAPHIES Shiro Imai holds the current position of general manager of technical group in plate division. He is responsible for the technological issues of all the steel plates products of the company. Kenji Katoh (Dr. Eng.) holds the current position of leader of anti-corrosion steel group of Steel Research Lab. He is responsible for R&D of the anti-corrosion materials. Yuji Funatsu holds the current position of senior manager of technical group in plate division. He is responsible for technical issues of steel plates for ships. Michio Kaneko (Dr. Eng.) holds the current position of chief researcher of anti- corrosion steel group of Steel Research Lab. He is responsible for R&D of the anti-corrosion materials. Tomoyuki Matsubara holds the current position of Corporate Officer and General Manager of Technical Group. He is responsible for all the new building and the technical issues of the company. Hideaki Hirooka holds the current position of Manager of Technical Group. He is Naval Architect and responsible for technical issues of new and existing vessels especially for hull structure. Hidehiko Sato holds the current position of Manager of Technical Group. He is Naval Architect and responsible for technical issues of new and existing vessels especially for hull structure.



PREVENTION OF COT BOTTOM PITTING CORROSION BY ZINC-PRIMER Y Inohara, JFE steel corporation, Japan T Komori, JFE steel corporation, Japan K Kyono, JFE steel corporation, Japan H Shiomi, JFE steel corporation, Japan T Kashiwagi, Mitsui O. S. K. Lines, Ltd., Japan SUMMARY On the inner bottom of COT (Cargo Oil Tank) of crude oil tankers, a lot of bowl shaped pitting corrosion of 4 millimeters or more depth occurs, and the time and the cost to repair them in the dock every 2.5 years are the large load to the tanker owners. The relationship between the number of pitting corrosion needed to repair and the zinc-primer application of the inner bottom plate of COT was investigated. As a result, it was discovered that the number of pitting corrosion occurred on the inner bottom painted the zinc-primer was clearly lower than the number of pitting corrosion occurred on the non-painted inner bottom plate. The inner bottom plate samples cut out from COT of the tanker under operation, and analyzed. It was confirmed that zinc or zinc chemical compound remained in the rust on the inner bottom plate surface after 5 years operation. ¶ 1. INTRODUCTION Recently, the corrosion problem of the ship attracts a lot of attention. The corrosion of ship not only loses the safety of operation but also increases an economical load along with the repair. For example, on the inner bottom of COT (Cargo Oil Tank) of crude oil tankers, a lot of bowl shaped pitting corrosion of 4 millimeters or more depth occurs [1], and the time and the cost to repair them in the dock every 2.5 years are the large load to the tanker owners. Figure 1 shows the shape of pitting corrosion in COT.

Pitting Corrosion

20 mm

Pitting Corrosion

20 mm

Figure 1: Pitting corrosion in COT. The relationship between the number of pitting corrosion needed to repair and the zinc-primer application of inner bottom plate of COT was investigated. As a result, it was discovered that the number of pitting corrosion occurred on the inner bottom painted zinc-primer was clearly lower than the number of pitting corrosion occurred on the non-painted inner bottom plate. The investigation results of the number of pitting corrosion in some oil tankers are shown as follows. And

the mechanism of prevention of pitting corrosion supposed from an analytical result of the sample cut out from the inner bottom plate of COT of the tanker under operation was reported. 2. FIELD EXAMINATION In the dock every 2.5 years, pitting corrosion of 4 millimeters or more depth is inspected, marked and repaired. 6 tankers are chosen (5 VLCC and 1 Suez-max tanker, the inner bottom plate surface of 2 VLCC is no-paint and that of the rest of tankers is painted zinc-primer), and the number of pitting corrosion of 4 millimeters or more depth is counted (as to Suez-max tanker, 3 millimeters or more depth). Table 1 shows the number of pitting corrosion in need of repair. The number of pitting corrosion occurred on the inner bottom painted zinc-primer was clearly lower than the number of pitting corrosion occurred on the non-painted inner bottom plate. In the first dock (there is no influence of repairing), the number of pitting corrosion occurred on the inner bottom painted zinc-primer was from one-fifteenth to one-thirtieth as compared with the number of pitting corrosion occurred on the non-painted inner bottom plate. It is presumed that the zinc-primer has caused some effects for a decrease in the number of pitting corrosion. Table 1: The number of pitting corrosion in need of repair. (Pitting Depth : more than 4mm, Suez-max F : more than 3mm)

Ship age (years) Inner bottom Tanker 2.5 5 7.5 10 12.5

VLCC A 1323 2356 1082 No paint VLCC B 1246 2919 1756 1138 VLCC C 49 213 VLCC D 44 61 VLCC E 88 39

Paintedzinc

-primerSuez-max F - - - 47 684



0

5

10

15

20

25

30

VLCC D (2.5years)

VLCC D (5years)

VLCC E (5years)

Zn

(g/m

2 )

0

100

200

300

400

500

600

Fe (g

/m2 )

ZnFe

Fe

Zn

3. ANALYSIS OF INNER BOTTOM PLATE 3.1 CUTTING OUT INNER BOTTOM PLATE Samples of the inner bottom plate were cut out from 2.5 years old VLCC D, 5 years old VLCC D and 5 years old VLCC E. These VLCC were painted zinc-primer on the inner bottom plate of COT. As cutting out location, the areas included no pitting corrosion in need of repair were chosen. Because these areas were thought that the possibility of showing the effect of zinc-primer was high.

Rust

Metal

Zn

Fe

O

S

Rust

Metal

Zn

Fe

O

S

3.2 RESULTS OF ANALYSIS 3.2 (a) Chemical analysis by ICP 2 or 3 specimens, about 30 x 30 millimeters area, were cut out from every inner bottom plate samples, and the whole rusts of every specimen were dissolved in hydrochloric acid with inhibiter. Then the quantity of Fe and Zn in the each rust was analyzed by ICP (Inductively Coupled Plasma) atomic emission spectrometer. Figure 2 shows the results of chemical analysis. All of the 8 rust specimens contained Zn. It was confirmed that zinc or zinc chemical compound remained on the inner bottom plate surface after 5 years operation. Table 1 shows that the number of pitting corrosion occurred on the inner bottom painted zinc-primer was still lower than the number of pitting corrosion occurred on the non-painted inner bottom plate in the dock after 5 years operations. After 5 years operations, it is supposed that the existence of zinc or zinc chemical compound decreased the occurrence and growth of pitting corrosion. Figure 2: The quantity of Zn and Fe present in rust. 3.2 (b) Analysis by EPMA Specimens for EPMA (Electron Probe Microanalyzer) analysis, also, were cut out from every inner bottom plate

samples. Sectional rust layer of every specimen was analyzed by EPMA, and the distributions of elements were mapped. Figure 3 shows the COMP image and the distributions of elements (Zn, Fe, O, S) on the inner bottom plate of COT of 5 years old VLCC D. Zn existed in the rust layer with Fe, O and S. Figure 3: Results of EPMA analysis. (COMP image, Zn, Fe, O and S distribution) 3.2 (c) Analysis by XRD The rust specimens for XRD (X-ray Diffraction) analysis were scraped off the surface of every inner bottom plate samples. There is a thickness in the rust layer, so the upper layer rust and the lower layer rust were sorted out and analyzed. All elements contained in the rust specimens were analyzed by EDX (Energy Dispersive X-ray) fluorescence spectrometer first, and then the chemical compounds contained in the rust specimens was analyzed by XRD. By EDX analysis, elements, Fe, Zn, Si, S, Cl, C, O, were detected in all specimens, and elements, Ca and Na, were detected in a part of specimens. Table 2 shows major chemical compounds found by XRD analysis. Main peaks of XRD analysis show some iron oxide and iron carbonate only. The plain peaks of



zinc or the zinc chemical compounds could not be found. This shows the possibility of existence as amorphous that is not crystalline in addition to a little the amount in zinc or zinc chemical compounds. Table 2: Results of XRD analysis.

Tanker Rust layer Major chemical compounds

Upper

alpha-FeOOH, beta-FeOOH, gamma-FeOOH, Fe3O4

VLCC D (5 years)

Lower alpha -FeOOH, beta -FeOOH, gamma -FeOOH

Upper

alpha -FeOOH, beta -FeOOH, gamma -FeOOH, Fe3O4, FeCO3VLCC E

(5 years)

Lower

alpha -FeOOH, beta -FeOOH, gamma -FeOOH, Fe3O4, FeCO3

3.3 EFFECT OF ZINC It was confirmed that zinc or zinc chemical compound remained in the rust on the inner bottom plate surface after 5 years operation. By XRD analysis, the existence form of zinc could not be identified. However, by EPMA analysis, the distribution of Fe, O, S and Zn was observed at almost the same position. The possibility of existence of ZnSO4, Fe0.85-xZnxO and etc. as a chemical compound bonding zinc is presumed. In the zinc coated steel sheet, it is known to have the effect of anti-corrosion even when zinc included in rust as chemical compounds such as zinc oxides, and to stop the form of iron oxide in not the crystalline but the dense amorphous [2]. In this case, the possibility that the rust of inner bottom plate of COT became denser by the existence of zinc, and corrosion resistance improved is presumed. 4. CONCLUSIONS The field examination and analysis of the inner bottom plate of tankers were carried out, and following results were obtained. • Painting the zinc-primer on the inner bottom plate of

COT is effective to the decrease of the number of pitting corrosion in need of repair, and decreases the number of pitting corrosion from one-fifteenth to one-thirtieth in the first dock inspection.

• Zinc in the zinc-primer stays in the iron oxide on the inner bottom plate of COT after oxidation, and keeps

giving the effect to corrosion resistance. This effect continues for several years at least.

5. REFERENCES 1. Ship Research Panel 242, ‘Study on Cargo Oil Tank Corrosion of Oil Tanker’, Ship Research Summary Report No.431, Tokyo, JSRA, 2002 2. S Fujita, H Kajiyama, M Yamashita, ‘Corrosion mechanism of zinc coated steel sheet inside the lapped portion’, CAMP-ISIJ, Vol. 9, P. 1283, 1996 6. AUTHORS’ BIOGRAPHIES Yasuto Inohara holds the current position of senior researcher at Corrosion Protection Research Department, Steel Research Laboratory, JFE Steel Corporation. He is responsible for development of corrosion resistant steel.





DEVELOPMENT OF CORROSION RESISTANT STEEL FOR BOTTOM PLATE OF COT Y Inohara, JFE steel corporation, Japan T Komori, JFE steel corporation, Japan K Kyono, JFE steel corporation, Japan K Ueda, JFE steel corporation, Japan S Suzuki, JFE steel corporation, Japan H Shiomi, JFE steel corporation, Japan SUMMARY Recently, the corrosion problem in the ship is paid to attention. Especially, the pitting corrosion occurred on the inner bottom plate of COT (Cargo Oil Tank) of crude oil tanker to need the inspection and the repair of every dock is one of the big problems. The inner bottom plate is being covered with the ‘oil-coat’ that is the crude oil element and protected from corrosion. But, the pitting corrosion occurs, and grows up by the ‘oil-coat’ defect part become a local anode site. This phenomenon was reproduced in the laboratory, and the laboratory pitting corrosion test method was established. And the pitting corrosion decrease effect of the zinc-primer on which it reported before is used, the low alloy corrosion resistant steel to which the number of pitting corrosion was greatly decreased by using it together with the zinc-primer was developed. When this corrosion resistant steel is applied to a tanker, the number of pitting corrosion for which the repair at the dock is necessary can be decreased sharply at a low level. ¶ 1. INTRODUCTION Recently, the corrosion problem in the ship is paid to attention. Especially, the pitting corrosion occurred on the inner bottom plate of COT (Cargo Oil Tank) of crude oil tanker to need the inspection and the repair of every dock is one of the big problems. Ex. : 13%CO2, 5%O2, 5%H2O,

0.2%H2S, 0.01%SOx, Bal. N2

H2S

Vapor space

Sludge

Crude oil

Oil coat

Salt water

Ex. : 13%CO2, 5%O2, 5%H2O, 0.2%H2S, 0.01%SOx, Bal. N2

H2S

Vapor space

Sludge

Crude oil

Oil coat

Salt water

In Japan, for three years from 1999, field examination was carried out to make the corrosion phenomenon of COT of cargo oil tanker clear. As a result, the actual corrosion environments in COT were clarified. Because of the investigation result, it was presumed that the pitting corrosion on the inner bottom plate occurred and grew up under the environment not uniform, such as a defect of ‘oil coat’, localized salt water or etc., and existence of oxidizer, such as iron oxide, iron sulfide, elemental sulfur or etc [1]. This phenomenon was reproduced in the laboratory, and the laboratory pitting corrosion test method was established. And the pitting corrosion decrease effect of the zinc-primer on which it reported before is used, the low alloy corrosion resistant steel to which the number of pitting corrosion was greatly decreased by using it together with the zinc-primer was developed. 2. PITTING CORROSION 2.1 ACTUAL ENVIRONMENT Figure 1 shows the cross section of the crude oil tanker. In operation, the gas part of COT is always filled with ‘inert gas’ for the explosion-proof. ‘Inert gas’ is the exhaust gas of the low oxygen concentration. It is composed of CO2, O2, SO2, N2 and etc. The inner bottom plate surface of COT was covered with ‘oil coat’ composed of heavy ingredients of crude oil, piled up ‘sludge’ composed of rust and solid in crude oil, and collected the high concentration salt water separated

from crude oil. In addition, it is presumed that the elements of ‘inert gas’ and H2S volatilized in crude oil merge in this salt water. This environment is very severe for corrosion of conventional steel [1]. Figure 1: Cross section of crude oil tanker. 2.2 MECHANISM OF PITTING CORROSION GENERATION AND GROWTH Usually, the inner bottom plate covered with ‘oil coat’ has corrosion protection. However, piling up of the ‘sludge’, COW (Crude Oil Washing) and etc. cause the defect of ‘oil coat’. If the low protective point such as the defect of ‘oil coat’ occurs on the inner bottom plate, the pitting corrosion is generated in this point. In the salt water, the low protective point becomes anode site, the ‘oil coat’ and the ‘sludge’ become cathode site. In this area, macro-cell is formed and pitting corrosion grows up [1]. As a result of field examination, it was presumed that the ‘sludge’ is piling up various solid such as solid in the crude oil, iron rust and sulfur generated in COT and etc. Figure 2 shows the image of pitting corrosion growth.




Sludge (cathode)Pitting (anode)

Seawater

Steel plate

Oil coat

Sludge (cathode)Pitting (anode)

Seawater

Steel plate

Oil coat

Seawater

Steel plate

Oil coat

Salt water (10%NaCl)

Gas (Simulated inert gas + H2S)

Specimen Oil coat

(313K)

Salt water (10%NaCl)

Gas (Simulated inert gas + H2S)

Specimen Oil coat

(313K)

Figure 2: Image of pitting growth. 3. PITTING CORROSION TEST METHOD Figure 3 shows the laboratory pitting corrosion test method. Test solution was 10% NaCl solution, and it was saturated with 13%CO2-5%O2-0.01%SO2-0.2%H2S-bal.N2 gas. The temperature of the solution was maintained 313K with a double cell. Specimen size was 75 x 50 x 4 millimeters. Surface of specimen was covered with the seal tape expect one test surface. Test surface of specimen was coated with the crude oil residue gathered from COT, and at the center of test surface, no coated area (diameter: 5mm) imitated the defect of ‘oil coat’ was made. The specimens were soaked upward in the solution. After these specimens were soaked from 28 for 36 days, the shape of the pitting corrosion that occurred on the surface was measured, and each specimen was evaluated by each maximum pitting corrosion depth. The shape (ratio of average diameter and depth) of the pitting corrosion that occurred by this test was corresponding to the shape of the pitting corrosion observed on the inner bottom plate of COT well.

Figure 3: Simulated pitting corrosion test for COT. 4. CHARACTTERISTICS OF DEVELOPED STEEL 4.1 EFFECT OF DECREASE OF PITTING CORROSION OF ZINC-PRIMER As a result of field examination, the number of pitting corrosion occurred on the inner bottom painted zinc-primer was clearly lower than the number of pitting corrosion occurred on the non-painted inner bottom plate. In the first dock, the number of pitting corrosion occurred on the inner bottom painted zinc-primer was from one-fifteenth to one-thirtieth as compared with the number of pitting corrosion occurred on the non-painted inner bottom plate. Various addition elements that strengthened the effect of the zinc-primer of the pitting corrosion decrease were examined, and the corrosion resistant steel was developed. 4.2 PITTING CORROSION RESISTANCE The zinc-primer painted specimen of conventional steel and that of developed steel were prepared, and they were evaluated by the above-mentioned pitting corrosion test method. Figure 4 shows the result of pitting corrosion test. The maximum pitting corrosion depth of the developed steel decreased by about 35% compared with that of conventional steel. When this result is applied to the distribution of the depth of the pitting corrosion measured by field examination of the zinc-primer specification tanker, the number of pitting corrosion in need of repair of developed steel that painted the zinc-primer is provisionally calculated that it is possible to decrease to one third or less of the conventional steel painted the zinc-primer.


0.0

0.5

1.0

1.5

Conventional Steel Developed Steel

Max

. Pitt

ing

Dep

th (m

m)

a decrease ofabout 35%

with Zinc-primer, Test period: 36days Figure 4: Maximum pitting depth of conventional steel and developed steel. 4.3 MECHANICAL PROPERTIES Table 1 shows an example of the typical mechanical properties of base metal of the developed steel. The developed steel was satisfied with the specification of 32D grade of IACS. Table 2 shows the results of tensile test and Charpy V notch impact test of the welded joint of the developed steel. The welded joint of the developed steel was satisfied with the specification of 32D grade of IACS, too. The mechanical properties of developed steel are equal to them of conventional steel, and in building of the tanker, the welding and construction performance similar to conventional steel are usually possible. Table 1: Mechanical properties of developed steel.

Charpy Impact

Test at 253K

Grade YS (N/mm2)

TS (N/mm2)

EL (%)

Energy (J)Developed

steel 399 485 31 326

IACS, 32D > 315 440/590 > 18 > 31

Table 2: Mechanical properties of welded joint. Charpy

Impact Test at 273K Grade TS

(N/mm2) Notch

positionEnergy

(J) WM 106 FL 149

HAZ 1mm 247 HAZ 3mm 273

Developed steel

(FCB welding method, Heat input: 108(kJ/cm))

515

HAZ 5mm 317 IACS, 32D > 440 - > 34

5. CONCLUSIONS • The pitting corrosion test method that was able to

simulate the pitting corrosion that occurred on the inner bottom plate of COT was established.

• The corrosion resistant steel that strengthened the effect of the zinc-primer of the pitting corrosion decrease was developed. The maximum pitting corrosion depth has decreased by about 35% compared with conventional steel.

• The developed steel has mechanical properties and construction performance equal with conventional steel as steel plate for shipbuilding.

6. REFERENCES 1. Ship Research Panel 242, ‘Study on Cargo Oil Tank Corrosion of Oil Tanker’, Ship Research Summary Report No.431, Tokyo, JSRA, 2002 7. AUTHORS’ BIOGRAPHIES Yasuto Inohara holds the current position of senior researcher at Corrosion Protection Research Department, Steel Research Laboratory, JFE Steel Corporation. He is responsible for development of corrosion resistant steel.





The Third Generation Shop Primer and Japanese Shipbuilding Construction Process

Yasuyuki SEKI, Katsumi KONDOU and Osamu HARADA

Chugoku Marine Paints, 1-7 Meijishinkai Otake Hiroshima, 739-0652 Japan

ABSTRACT

Inorganic zinc silicate primer shows excellent adhesion property to a steel surface and to over coat such as epoxy paints, because hydroxyl functional group of silicate derived by hydrolysis while curing process.

The adhesion property of inorganic zinc silicate primer is retarded by white rust, salt deposit and other contaminants. Secondary surface preparations are required to remove such contaminants, which causes delay of construction process. Deduction of white rust and retention of anti-corrosive performances, not only improvement of weldability, cutting and heat resistance, had been requested for inorganic zinc silicate shop primer.

The Third Generation Shop Primer has developed to meet the construction process of Japanese shipyards where shop primer is retained without blasting off, and has widely adopted by Japanese shipyards. The Third Generation Shop Primer keep intact surface while construction process and could be over coated with epoxy main coat, to provide enough performance to comply with ANNEX-1 test of PSPC requirements.

1. INTRODUCTION

In 1950’s, shop primer was developed as a long durable etching primer based on polyvinyl butyral resin. In 1960’s, epoxy zinc-rich primer and epoxy non-zinc primer based on epoxy resin was developed as more durable shop primer.

In 1970’s, inorganic high-zinc paint was developed to improve in heat-resistance and weldability, which provided less heat damage than before, and reduced second surface treatment work. We call this “the First Generation Shop Primer”.

In the latter half of the 1970's, low-zinc shop primer was launched into a market. We call this “the Second Generation Shop Primer”. The Second Generation Shop Primer was widely accepted to ship building market as consequence of the expansion of ship-building market.

In 2000, development had been continued to introduce high-heat-resistant low-zinc shop primer which could provide welding speed of 600-800mm/min (Twin-Single welding) and high welding speed of over 1,200mm/min (Twin-Tandem welding). We call this “the Third Generation Shop Primer”.

2. PERFORMANCE OF THE FIRST, SECOND AND THIRD GENERATION SHOP PRIMER

2-1.Comparison of each paint composition

Specific features of the Third Generation Shop Primer are introduced by the use of weldabilitive pigments and zinc protector. Paint composition of each generation and characteristic are shown in Table-1.

Table-1. Example of sample composition1st 2nd 3rd

Zinc powder(solids cont.)

Silica powder (*1)Talc (*2)

Fluorite powder (*3)Wax Acrylic Bentonite

Bentonite - -Organic resin - Add -Zinc protector - - Add

*1: Silica powder (ignition loss: 2.07% 300oC, 3.87% 500oC, 4.75% 800oC)*2: Talc (ignition loss: 1.27% 300oC, 1.60% 500oC, 8.97% 800oC)*3: Fluorite powder (ignition loss: 0.10% 300oC, 1.58% 500oC, 1.66% 800oC)*4: Weldabilitive pigment (ignition loss: 0.88% 300oC, 1.02% 500oC, 1.13% 800oC)

generation

PasteSilica powder (*1)Pigments (*4)

Additives

hydrolysis of TES-40Base

65%wt 55%wt 50%wt



2-2. Designing

Both Reduction of second surface preparation and high-speed weldability is essential to achieve improvement of productivity at shipbuilding construction process.

2.2.1. Studies to achieved to reduce second surface preparation

Zinc generates white rust easily when zinc is exposed to a corrosion atmosphere (e.g. moisture, wet, electrolyte, and electric current) and an oxidation atmosphere (e.g. high temperature, and oxygen). (Figure-1 upper path)

Zn

Znhydrolysis

Zncorrosion factor

Zinc protection

Zn White rust

Zn２+ Zn２+ Zn２+ Zn２+

Zn２+Zn２+

Excessive corrosion of zinc

Contraction of silicate

Formation ofsilicate film

Zn２+Zn ２+ Zn２+Zn２+ Zn２+Zn２+ Zn２+Zn２+

Zn２+２+

the 3rd Generation

Figure-1 Generation of white rust model

To prevent the generation of white rust, we combined zinc protector as the composition of the Third Generation Shop Primer. The mechanism is not clearly, but it looks the zinc protector cover zinc surface and even if zinc melts by welding heat, zinc will not be oxidized (Figure-1 lower path). If zinc is not treated by zinc protector, zinc is oxidized and needle crystals (zinc oxide) are generated on the surface of zinc particle. (Photo-1)

1st and 2nd 3rd

Photo-1. The surface of zinc after heating

Zinc particle in an inorganic zinc shop primer is covered consecutive films of the ethyl silicate binder just after coated, but this film shrinks with progress of hardening and the surface of zinc particle crop out form the surface of shop primer.



If the film is exposed to a corrosion atmosphere, the zinc particle begins to dissolve and electric current is generated. This electric current is caused by ionization potential between zinc and iron, which works to protect iron from corrosion, but excess current accelerate the generation of white rust. By this reason, ionization potential could be used as an indicator of anti-corrosive property and white rust.

We have prepared test panels coated by each generation shop primers at average dry film thickness 15μm, and dried at room temperature (23oC) for seven days. One of the test panels were kept at room temperature and others were heated to 300, 500 and 800oC. The panel was immersed into salt water (3% NaClaq) and ionization potential was measured. The panel of First, Second and Third Generation Shop primer which kept at room temperature, respectively shows

potential 990, 960 and 960mV at 0.1hrs, 880, 890 and 840mV at 24hrs, 790, 770 and 760mV at 48hrs. The Third Generation Shop primer showed lower potential compared to the First and Second Generation, which generation of white rust will be controlled (Figure-2, 3, 4). The panel heated at 300, 500 and 800 oC showed same tendency as the panel kept at room temperature.

Figure-2. Protective potential of 1st generation

600

650

700

750

800

850

900

950

1000

1050

0.1 0.5 1 2 4 8 24 48 72 168(hours)

(-m

V)

23300500800

Figure-3. Protective potential of 2nd generation

600

650

700

750

800

850

900

950

1000

1050

0.1 0.5 1 2 4 8 24 48 72 168(hours)

(-m

V)

23300500800

Figure-4. Protective potential of 3rd generation

600

650

700

750

800

850

900

950

1000

1050

0.1 0.5 1 2 4 8 24 48 72 168(hours)

(-m

V)

23300500800

The anti-corrosive property and anti-white rust property of each shop primer was tested by the exposure test with combination of natural weathering and water spray. The anti-corrosive property is evaluated based on ASTM D610-01, and anti-white rust property is evaluated based on ASTM D610-01 using our own criteria1. All shop primer has outstanding anti-corrosive property when kept at room temperature (23oC). The Third Generation Shop Primer, of which

1 Anti-white rust grade: White rust generates to 10: less than or equal to 1%; 9: greater than 1% and up to 5%; 8: greater than 5% and up to 10%; 7: greater than 10% and up to 15%; 6: greater than 15% and up to 20%; 5: greater than 20% and up to 30%



zinc consumption controlled, has outstanding anti-corrosive property and anti-white rust property even after heated at 300, 500, or 800oC. On the other hand, the First and Second Generation Shop Primer generate large amount of white-rust. White-rust on the steel surface of the First and Second Generation Shop Primer flow out as time passes, white-rust is improved, but anti-corrosive property is decreased (Figure-5 to 10).

The weldabilitive pigments and zinc protector provided to the Third Generation Shop Primer has properties to control the generation of zinc ion excessively by forming insoluble film by reacting with zinc ion.

Figure-5. Anti-corrosive property of1st

6

7

8

9

10

7days 15days 30days 60days 90days(days)

(eva

luat

ion)

23300500800

Figure-6. Anti-white rust property of1st

456789

10


(eva

luat

ion)

23 300500 800

Figure-7. Anti-corrosive property of

2nd

6

7

8

9

10


(eva

luat

ion)

23300500800

Figure-8. Anti-white rust property of2nd

456789

10


(eva

luat

ion)

23 300500 800

Figure-9. Anti-corosive property of

3rd

6

7

8

9

10


(eva

luat

ion)

23300500800

Figure-10. Anti-white rust property of3rd

456789

10

7days 15days 30days 60days 90days

(days)

(eva

luat

ion)

23300500800

The test result of the Third Generation Shop Primer tested the exposure test with combination of natural weathering

and water spray shows good anti-white rust property as expected by the effective protective potential test. When steel panels are piled up and water penetrates into the interspaces of panel, much electric currents are provided

by electric crevice corrosion, and generate much white-rust. We prepared panel coated by the First, Second, and Third Generation Shop primer and dried at room temperature



(23oC) for seven days. Two peaces of test panel were piled and raped with a vinyl sheet with 50ml water, them exposed to incubator and keep at 50oC for seven days. After testing, the Third Generation Shop primer showed much less white rust than the First and Second Generation Shop primer (Photo-2).

1st 2nd 3rd

Photo-2. Comparison of white rust by accelerated test

2.2.2. Studies to achieve high-speed weldability

(a) Reduction of organic components

Organic compounds in film induce welding defects, because of it is gasificated by heat at welding. The organic compounds provided in the inorganic shop primer are mainly additives to improve storage stability to adjust settling of the pigment. The organic compounds in the Third Generation Shop Primer are reduced as much as possible. (b) The reduction of crystalline water in pigments

Pigments used for paints normally adsorb atmospheric water. Some pigments include crystalline water. The water adsorbed to the pigment becomes estranged when exceeded the boiling point, and does not affect weldability seriously. Crystalline water becomes estranged at 400oC or higher temperature. Crystalline water causes gasification while welding and cause welding defect.

Crystalline water in the pigment was measured using differential thermal analysis. The pigments used for the Third Generation Shop Primer shows ignition loss at 800oC with about 1.1 % but Silica, Talc and Fluorite which used for the First and Second Generation shows ignition loss with 4.7%, 9.0% and 1.7% respectively (Table-1).

We tested the Weldability of the First, Second and Third Generation Shop primer, using CO2 arc welding. Weldability

of all generations was good when dry film thickness is thin. When dry film thickness is thick, the Third Generation Shop Primer shows good weldability, but the First Generation Shop Primer occurred to a lot of welding defects at welding speed 800mm/min. The Second Generation Shop Primer increased welding defects regardless of welding speed that resulted from organic resin of paint film (Table-3).

Table-3. Weldability test

15 25 15 25 15 25 15 25 15 25 15 25Second 0 5 0 2 0 0 0 2 0 3 0 0

bead 0 27 0 20 0 0 0 31 0 56 0 03.3 5.3 3.6 5.1 2.1 2.2 4.4 5.9 4.3 6.1 2.4 2.6

* Welding condition: Welding position: horizontal fillet weld, Welding method: CO2 gas shielded automatic arc welding, Welding direction: ahead method, Welding wire: KOBELCO, MX Z-200, Φ=1.2mm, Welding speed: 600mm/min, 800mm/min, Distance between torches: 100mm, Degree of torch: 45o, forward 5o, Wire extension: 25mm, Root gap: zero, route gap zero make tack welding with three points at end face and centre of first bead while pressurized, Leg of a fillet weld: 6±1mm

3rd 1st600 800

* Blow hole (%) = (Blow hole's width×height)/(test-piece's width: 500mm×height: 4mm)×100

2nd 3rdGeneration

Blow hole (%)

Pit (number)Groove (mm)

Welding speed (mm/min.)Dry flm thickness (μm)

1st 2nd



2.2.3. Over coatability We prepared the test panels blasted at Sa2.5 and coated by the Third Generation Shop Primer with the average dry

film thickness. 15μm and dried at room temperature (23oC) for seven days. One of the test panels was kept at room temperature and other panels were heated to 300, 500 and 800oC. Epoxy main coat is coated over the each test panel with the average dry film thickness. 300μm. The test panel was exposed to salt spray (JIS K 5600-7-1), artificial sea-water immersion tests (JIS K 5600-6-1), artificial sea-water immersion test with cathodic protection and humidity chamber test (JIS K 5600-7-2). After exposure, the panel was dried for a few minutes and its appearance, knife test, rust at cut area, and blister was checked. Also the pull-off strength is measured using MOTOFUJI’s pull-gauge.

The Painting system using the Third Generation shop primed showed good results. (Table-4)

Table-4. Over coatability of "3rd Generation Shop Primer"+"Epoxy paint" after 2months

* Paint system: 3rd S/P (15μm) + Epoxy paint (300μm), 2.3mm-thick test panel* Evaluation is based on ASTM D610-01 for rust, ASTM D714-87 for blister* Cathodic protection: Zn (Φ=15mm/test panel×4)* Failure area of pull-off test: all test panels have cohesion break of Epoxy paint (100%).

no blister

Knife cuttingAppearance

Blister (general)Knife cuttingAppearanceno peelno change

no change

Resistance to 3% NaCl aq immersion with cathodic protection at 40oC

Blister (general)Rust width of cut areaKnife cuttingAppearance

Blister (general)Rust width of cut area

no blisterno change no peel

Knife cuttingAppearanceno change no peel sometimes rust

no blister-

no rust no blister

Rust width of cut area

Rust width of cut area Blister (general)

no rust no blister3.9Mpa

Test Item

Epoxy paint only

Epoxy paint only

Epoxy paint only3rd S/P + Epoxy paint

3rd S/P + Epoxy paint

3rd S/P + Epoxy paint

Test Item

Test Item

- no blister

no change no peel no rust no blister

no peelno change

no blisterno rust

3rd S/P + Epoxy paint no change no peelEpoxy paint only

3.5Mpa4.0MPa

Pull-off test

4.0Mpa(JIS K 5600-7-2)

(JIS K 5600-6-1)

(JIS K 5600-7-1)

4.3Mpa4.6Mpa

Pull-off test

Pull-off test

4.5Mpa4.3Mpa

Resistance to contenuous condensation at 50oC ( >95%RH)

Resistance to 3% NaCl aq immersion at 40oC

Resistance to contenuous salt srpay at 35oC (5% NaCl aq)

Pull-off test

no peel

Test Itemno change no peel sometimes rust

We have arranged 8 test panel coated by shop primer and heated with gradation at 23, 300, 500, 800oC. Four of the panel is coated by epoxy main coat. The pull-off strength of the shop primer heated and epoxy main coat coated on heated shop primer are measured using MOTOFUJI’s pull-gauge.

The First and Second Generation Shop Primer heated below 500oC showed cohesive failure in shop primer layer, but that heated at 800oC showed interface detachment with the substrate and shop primer. The Third Generation Shop Primer showed cohesive failure in shop primer layer even heated up to 800oC.

Over coatability (to epoxy coat) of the Third Generation Shop Primer heated at 800oC is better than that of the First and Second Generation Shop Primer, because of the detachment was caused cohesive brake down and detachment between glue and dolly. The First and Second Generation Shop Primer showed interface detachment between shop primer and the substrate at 800oC (Table-5, Figure-11, 12).



Table-5. Comparison in over coatability of each generation by pull off test Heat temp. coating 1st generation 2nd generation 3rd generation

single film 3.4MPa, 100%B 3.3MPa, 100%B 5.0MPa, 100%Bovercoat 7.0MPa, 60%D, 40%E 5.6MPa, 70%B, 30%D 7.0MPa, 80%D, 20%E

single film 3.9MPa, 100%B 4.0MPa, 100%B 4.7MPa, 100%Bovercoat 7.1MPa, 75%D, 25%E 6.1MPa, 30%B, 40%D, 30%D 7.2MPa, 90%D, 10%E

single film 4.0MPa, 100%B 3.7MPa, 100%B 5.0MPa, 100%Bovercoat 7.2MPa, 90%D, 10%E 7.0MPa, 95%D, 5%E 7.0MPa, 60%D, 40%E

single film 3.5MPa, 100%A 3.5MPa, 100%A 4.0MPa, 100%Bovercoat 5.6MPa, 15%A, 85%D 5.8MPa, 25%A, 75%D 6.1MPa, 60%D, 40%E

[nature of failure area] A: Interface detachment between steel plate and shop primer, B: Cohesion break of shop primer, C: Interface detachment between shop primer and epoxy paint, D: Cohesion break of epoxy primer, E: Interface detachment between glue and dolly

23

300

500

800

Figure-6. Tensile stress of single film after heat

2.5

3.5

4.5

5.5

1st 2nd 3rdgeneration

stre

ngth

(MPa

)

23300500800

Figure-11 Tensile stress of single film after heat Figure-7. Tensile stress of overcoated film after-heat

4.5

5.5

6.5

7.5

1st 2nd 3rd none primergeneration

stre

ngth

(MPa

)Figure-12 Tensile stress of overcoated file after

23300500800

From these results, the Third Generation Shop Primer shows better resistance property to ignition damages than the First and Second Generation shop primer.

3. CONCLUSION

The Third Generation Shop primer shows improved anti-white rust property and anti-corrosive property compared to the First and Second Generation Shop primer. It is also proved that the Third Generation Shop primer shows good weldability comply with high speed welding.

Painting system including intact inorganic zinc paints has adopted than over 20 years. The Third Generation Shop Primer has developed to meet the construction process of Japanese shipyards, and has widely adopted. The Third Generation Shop Primer keeps intact surface while construction process, controlling white rust and rust, and provide over coatability to epoxy main coat. The Third Generation shop primer could comply with ANNEX-1 test of PSPC requirements.






THE DEVELOPMENT OF WATER BASED SHOP PRIMERS M W Hindmarsh, International Paint Ltd, UK SUMMARY Shop primers are used in the shipbuilding process to protect steel from corrosion during the build cycle. The industry standard shop primer type is now solvent based organic zinc silicate technology. These products are fully compatible with the ship building process and depending on the zinc levels, offer the yard a blend of good anticorrosive performance without a compromise on welding speeds and steel cutting performance. The shipbuilding community is under pressure to reduce emissions including Volatile Organic Compounds (VOC’s). Solvent based shop primers can contain a very large proportion of solvent (~650g/l) that has historically been released into the atmosphere. Even before the proposed solvent emission legislation in Europe and the US, International Paint were proactive and recognised the opportunity for the development of a low, ideally zero, VOC shop primer which retained the performance characteristics of current solvent based zinc silicates. This paper describes the development of the latest water based shop primers and includes an assessment of options available to shipyards for reducing solvent emissions. 1. INTRODUCTION Shop primers are used in the shipbuilding process to protect steel from corrosion during the build cycle. Shipyards throughout the world have different corrosion control requirements since they build in a range of different climatic conditions and adopt different build cycles. Most shipyards now use shop primers to protect the steel through the fabrication process up to block construction. The blocks are then painted with the full anticorrosive system. The degree of secondary surface preparation before block painting depends on many factors, including the Owners specification, the standard yard practise, the condition of the shop primed steel and the expectations for the performance of the finished system. Shop primers must not significantly interfere with the normal shipbuilding practises of the yard nor slow down productivity. Minimal secondary surface preparation, maintaining weld speeds and maintaining cutting characteristics are all pre-requisites of a good user friendly shop primer. With the world now focussed on environmental issues, the shipbuilding community is under pressure to reduce emissions including Volatile Organic Compounds. Shop primers by design are typically very low volume solids systems (25-36%) that are applied at very low dry film thickness (15-18um). Solvent based shop primers can contain a very large proportion of solvent (~650g/l) that has historically been released into the atmosphere. VOC’s are some of the emissions that can harm the environment whether it is an effect on the ozone layer or a negative impact on global warming.

2. DEVELOPMENT OF WATER BASED SHOP PRIMERS

2.1 TRADITIONAL SHOP PRIMERS Traditional solvent based shop primers have been around since the 1970’s. Many generic types have been used including poly vinyl butyryl (PVB), acrylic and iron oxide epoxy systems. However, as shipbuilding developed and production increased, these early organic based products were not able to meet the demands from the modern shipyard in relation to welding and cutting of the shop primed steel. The industry standard shop primer type is now solvent based inorganic zinc silicate technology. These products are more fully compatible with the ship building process and depending on the zinc levels, offer the yard a blend of good anticorrosive performance without a compromise on welding speeds and steel cutting performance. Shop primers are supplied with low volume solids (VS) to aid application and film thickness control but this low VS leads to a consequential VOC of around 650g/l. Zinc silicate shop primers contain a high zinc metal pigment volume concentration (PVC) and are normally formulated above the critical pigment volume concentration (CPVC). This means the coating is porous when first applied and the zinc particles are in direct contact with the steel substrate. The zinc, having a lower position in the electrochemical series than steel, protects the steel sacrificially. As the zinc metal slowly corrodes, the resulting zinc corrosion products (zinc hydroxide, zinc carbonate etc) cause a reduction in the porosity of the film and the coating acts by reducing diffusion of corrosive species (e.g. H2O, O2) to the steel in a classical ‘barrier’ mechanism.



The modern solvent based inorganic zinc silicate shop primer offers the advantages over other shop primer types of;

• Fast drying and handling • Good corrosion resistance • Reduced secondary surface preparation • Compatibility with typical shipyard welding

and cutting processes

2.2 1st GENERATION WATER BASED ZINC

SILICATE COATINGS Water based zinc silicate coatings have actually been around since the 1930’s though they were not used in the marine shipbuilding business. Early high build (high film thickness) high zinc containing products were used for above ground pipe protection in countries like Australia. Development of water based zinc containing products based on alkali silicate binder technology continued through the 1950’s and 1960’s however these products produced high levels of water soluble salts during the cure process. Certain post treatment processes could be adopted to address this but these treatments added cost to the process and were not generally accepted. The high level of salts is due to the commercial binders having an SiO2:M2O mole ratio of ≤4:1. The high level of the alkali cation resulted in water soluble salts such as potassium hydroxide or potassium carbonate (if potassium was the alkali cation used). Later developments of this technology managed to achieve higher SiO2:M2O mole ratio’s of up to 5.2:1, but these still produced significant levels of water soluble salts. High levels of water soluble salts on a shop primed surface cause major problems when the shop primer is top coated with an anticorrosive system. The retained water soluble salts accelerate diffusion of water though the anticorrosive by a process known as osmosis. The resulting anticorrosive system suffers from osmotic blistering and eventual breakdown as the blisters rupture and corrosion occurs. 2.3 2nd GENERATION WATER BASED SHOP

PRIMERS In the mid 1990’s even before the proposed solvent emission legislation in Europe and the US, International paint were proactive and recognised an opportunity for the development of a low, ideally zero, VOC shop primer which retained the performance characteristics of current solvent based zinc silicates. To meet the solvent emission regulations shipyards have two options: i) to invest in abatement equipment and continue to use solvent based shop primers or ii) switch to a new low VOC shop primer. The relatively

high installation and operational cost of abatement systems makes option ii) more attractive. In order to meet the performance requirements of modern shipyards it was recognised that any new coating would need to be based on an inorganic binder system. A wide range of experimental coatings based on novel inorganic binders were investigated and from this research a new waterbased shop primer Interplate Zero was developed and commercialised by International Paint in 2004. Interplate Zero is a two pack product. Pack 1 comprises a proprietary alumina modified silica sol binder together with a number of additives while Pack 2 is a dry blend of zinc and other extender pigments. Silica sols have a much higher SiO2:Na2O mole ratio (normally ≥ 40:1) than conventional alkali silicate binders and therefore coatings based on silica sols are not susceptible to osmotic blistering problems. The use of sols with a low particle size (≤ 20nm) improves the film properties and the alumina modification increases the pot life of the mixed paint. The technology is protected by a number of patents. 2.4 SHOP PRIMER PERFORMANCE The performance of shop primers can be assessed in a number of ways, each with a relevance to a specific customer requirement.

a) Preparation and Application Shop primers are normally applied by an automated process on a ‘shop primer line’. Steel plates are fed onto rollers where they are treated and shop primed using the following sequence

i) Air knife cleaning – To remove stockyard debris such as snow or water from steel at the start of the automated shop primer process.

ii) Pre-heating – Purpose is to heat the steel prior to blasting to: a. Dry steel plate b. Loosen millscale c. Reduce drying time of the applied

shop primer iii) Shot blasting – To clean the steel

removing millscale and corrosion and to provide the steel with a good profile for coating application so that maximum performance can be achieved from the shop primer and subsequent high performance coating schemes

iv) Paint application v) Volatile cabinet – Where the volatile

species (solvent or water) evaporates. The length of this cabinet and the speed of



the line dictates the time allowed for the shop primer to dry

In the volatile cabinet, the shop primed steel plate is moved on a knife edge conveyor so that contact with the freshly applied shop primer and hence mechanical damage is kept to a minimum. Once the steel plate leaves the volatiles cabinet the plate is transferred to rollers. At this stage the shop primer must be dry enough so that the roller action on the underside of the plate does not damage the curing shop primer. Normally it takes approximately 2 – 3 minutes to get to this stage, depending on line speed and cabinet length. 2 – 3 minutes is the time that a shop primer, whether solvent based or water based, has to dry. b) Weldability The weldability of the shop primer is a measure of how much the shop primer affects the welding procedures used in the ship building yard. There are many welding techniques such as manual, semi-automatic, submerged arc welding etc and there are many types of welding consumables such as metal core wire and flux cored wire. It is not the intention of this paper to discuss specific combinations. However, in testing of the ‘weldability’ of new shop primer technologies it is important to use standard industry accepted shop primers as reference in any test so that direct comparisons can be made. Steel plate coated with organic shop primer is difficult to weld without removing the coating first. The organic material degrades when heated and produces gases that get trapped within the molten metal of the weld causing welding defects such as; Blow holes Worm holes Embittlement Inorganic coatings such as zinc silicates are easier to weld at yard production speeds and also produce less fume during the process. Standard independent tests are carried out on shop primed steel to demonstrate that at acceptable welding speeds the integrity of the weld is not compromised. c) Cuttability Shop primed steel plate, as well as being welded, also needs to be cut into the shapes required for the complex design structure of the modern ship. Again, the coating on the steel plate may have a negative affect on how easily the steel can be cut, whether it is simply an oxy acetylene cut or a complex submerged plasma cut. d) Heat resistance

The whole ship construction process involves heat whether it is from welding or cutting. When a steel profile is welded to a flat plate to form a structure such as a double bottom, the heat used to form the weld is transferred through the steel plate. Temperatures may reach 800oC on the reverse of the weld. If the shop primer is not heat resistant to this temperature, the shop primer will degrade and lose it’s adhesion and anticorrosive properties. This then leads to corrosion of the steel at these heat affected zones and subsequently leads to an increase in the secondary surface preparation requirements. A well formulated solvent based zinc silicate shop primer is designed to withstand temperatures of 800oC so that on exposure to the elements the shop primer continues to protect. Water based shop primers also have to demonstrate the same heat resistance. e) Overcoating compatability The overcoatability of a shop primer is very important for the shipyard. If the shop primer is not overcoatable with the specified anticorrosive system then the shop primer must be completely removed during the secondary surface preparation process. This adds to shipyard costs. Overcoating of the 1st generation alkali silicate water based shop primers was not possible for critical immersed areas since the retained water soluble zinc salts caused osmotic blistering of the anticorrosive system. However, the development of the 2nd generation silica sol water based shop primers allowed direct overcoating of the anticorrosive system even on critical immersed areas since the water soluble salt levels are significantly reduced. In addition to the need for overcoatability, so allowing reduced secondary surface preparation, the IMO have recently agreed a performance standard for the protective coating (PSPC) of dedicated water ballast tanks and dry spaces of bulk carriers. In this standard, the requirement is for the shop primer to be removed unless it has been independently tested and approved in accordance with a specific anticorrosive test outlined in the standard. The test outlined follows the test that has been used for many years in the marine industry. The test was devised by Marintek and has latterly been carried out by DNV. Interplate Zero has a full DNV test B1 approval and is therefore compliant to the requirements of the IMO PSPC. f) Anticorrosive performance Anticorrosive performance is a measure of how long the applied shop primer can protect the steel from corrosion. The shop primer must be able to protect the steel under varying climatic conditions for the duration of the build cycle. This obviously differs from one newbuilding location to the next. The anticorrosive properties of a zinc silicate or zinc silica sol shop primer are largely determined by the level of zinc. The



higher the level, the better the anticorrosive properties. However, the level of zinc also has an effect on the weldability of the shop primer and the welding speeds possible. Film thickness has a similar effect. When heated during the direct welding process, the zinc vapourises and this can lead to welds with reduced integrity. A compromise between weldability and anticorrosive performance has been accepted by the shipbuilding industry. Water based shop primers have to be able to match the expected properties of solvent based zinc silicate shop primers in terms of anticorrosive properties, as well as the welding, cutting and heat resistance properties, figure 1. 2.5 WATER BASED SHOP PRIMERS: COST

BENEFIT ANALYSIS The shipbuilding industry is now under increasing pressure to reduce emissions. In Europe for example, the solvent emissions directive 1999/13/EC states that by 31st October 2007 a facility that uses 5 or more tonnes of solvent per year including paint, thinners, cleaners etc, must have total emissions of less that 273g/Kg of paint used. A typical shop primer will have a calculated SED VOC value of between 411 and 464g/Kg, well outside the legislative requirement for the facility as a whole. If 17% of a European shipyard total paint volume is shop primer, then it can be shown that this has a high influence on the total emissions from the yard ( >30% of the emissions). Shipyards have 2 possibilities if they wish to reduce the emissions from shop primers;

1. Fit abatement equipement to the shop primer line

2. Change from solvent based shop primer to water based shop primer

Abatement is where solvent emissions are burned in a thermal oxidiation plant. Typical installation costs of abatement are €500,000 with associated running costs of €75,000 per annum and increasing as the cost of energy rises. In addition to the cost, the thermal oxidation process also produces CO2 and SOx both of which are also being studied carefully for their impact on the environment. In Cost Benefit Analysis studies it has been shown that adopting a water based shop primer is a more cost effective way of reducing emissions than installing solvent abatement equipment. 2.6 WATER BASED SHOP PRIMERS: ECO-

EFFICIENCY ANALYSIS

Eco-Efficiency Analysis (EEA) assesses the cost structure and ecological impact of competing products and processes from a value chain perspective. Cost is fairly easy to assess since most of the associated costs are known, however, the ecological impact assessment is more challenging and requires a rigorous assessment of six ecological dimensions: • Energy use • Material use • Toxicity • Hazard potential • Emissions to air, water and land • Land use or biodiversity A water based zinc containing silica sol shop primer (Interplate Zero) was compared to competing solvent based zinc silicate shop primers in an Eco-Efficiency Analysis. An ecological footprint was calculated using the known factors of the two technologies. The eco-efficiency analysis was a relative measure comparing products, processes, investments etc. It was not a method that predicted the absolute sustainability of a product or process. The combination of cost impact and environmental impact allows a competitive comparison of products and processes. The 5 steps of the analysis are given in Figure 2. For the study into water based zinc containing silica sol shop primers, the main focus for comparing the competing technologies was in the value chain proposition. It describes the costs and ecological footprint from the beginning of life (supplier phase) to the end of life (end of user life phase) of the product. This way all costs and ecological effects that accompany the product through its life cycle can be described and analysed. The value chain for shop primers is given in Figure 3. The ecological footprint of both water based shop primer and solvent based shop primer (with and without abatement) was calculated. The method is complex and is outside the scope of this paper but the analysis concluded that water based shop primer technology was more eco-efficienct than competing solvent based shop primer technology, even when solvent abatement was utilised, Figure 4. Further analysis to complete the study looked at the sustainability dimension of the market (the customers) and had the purpose of showing the sensitivity to sustainability of the market. This section of the study looked at technological improvement potential and information about external pressures from NGO’s etc, but again the details of this are outside the scope of this paper.



2.7 FIGURES Generic Type VOC Protection Weldability Cutability Heat g / lt months solid wire ResistanceZinc Ethyl Silicate 609 18 Fair Fair N/A Weldable and Heat resistant 628 6 Excellent Excellent Very GoodZinc Silicate High speed Weldable and Heat 636 >12 Very Good Excellent Excellent Resistant Zinc Silicate Water based zinc coating based on silica sol binder (Interplate Zero) 0 >6 Very Good Excellent Excellent

Figure 1: Comparison of properties for water based and solvent based shop primers

Eco-efficiency analysis in five steps

Define functional unit

Identify competing alternatives

Define the value chain

Collect and assess economic and ecological data

Conduct eco-efficiency analysis and draw conclusions on best, averageand worst alternatives

Figure 2: The five steps of Eco Efficiency Analysis

Figure 3: Value chain analysis for shop primers

Raw materials

(resin)

Raw materials

(crosslinker)

Raw materials

(pigments)

Coating

(spraying of shop primer)

OEM (welding

& cutting)

Use

(overcoating)

Disposal

(removal of shop primer)

Manufacturing

(making of shop primer)

No significant differences in costs or ecological footprint



Key

1. Solvent based weldable and heat resistant zinc silicate 2. Solvent based high speed weldable and heat resistant zinc silicate 3. Water based zinc silicate coating based on silica sol binder (Interplate Zero) 4. Competitor solvent based weldable and heat resistant zinc silicate

Figure 4: Eco-Efficiency Analysis model for competing technologies with abatement costs included. 3 CONCLUSIONS The development of 2nd generation water based shop primers has allowed shipyards to conform to environmental legislation without the investment in expensive solvent abatement equipment. Interplate Zero water based shop primer is equal to that of traditional solvent based zinc silicate shop primers in terms of shipyard processes, productivity and anticorrosion properties. 4 ACKNOWLEDGMENTS

i) Dr Paul Jackson and Paul Hutchinson, International Paint, Felling, UK

ii) Triple Value Strategy Consultants, The Hague, The Netherlands for their help with the Eco Efficiency Analysis study

5 AUTHORS BIOGRAPHY Michael Hindmarsh is a Business Development Manager for International Paint. He joined International Paint in 1982 and spent 10 years in the product development laboratories (including 6 years as a corrosion engineer) before moving to a Business Development Role. In his current capacity he is responsible for Newbuilding and Military business. He has an Hons. degree in Chemistry.

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COATING CONDITIONS IN WATER BALLAST TANK, VOID SPACE AND CARGO OIL TANK OF AGED SHIPS AND REQUIRED PERFORMANCE STANDARD OF PROTECTIVE COATINGS FOR NEW SHIPS Takanobu Murakami, Shin Kurushima Dockyard Co., Ltd. Takayuki Sasaki, IHI Marine United Inc. Masahiro Kuwajima, Mitsui Engineering & Shipbuilding Co., Ltd. Masanobu Koori, Mitsubishi Heavy Industries, Ltd.

SUMMARY PSPC (Performance Standard for Protective Coatings) for water ballast tanks and double hull spaces of bulk carrier was adopted at MSC82 as resolution MSC.215(82) in last December, and protective coatings for void spaces was discussed at DE50 in March this year. Furthermore, protective coatings for cargo oil tanks have been discussed at JWG (Joint Working Group) of IACS, and the proposal of JWG will be submitted to IMO. We, members of SAJ (the Shipbuilders’ Association of Japan) participated in all discussions and introduced the actual situation of ship’s coating through the results of several investigations. In this paper, we would introduce the coating conditions of the ships aged more than 10 years in water ballast tanks, void spaces and cargo oil tanks with some photos, and express our considerations regarding the relation between tank coating at new building stage and the coating conditions of aged ships. 1. INTRODUCTION

In these couple of years, coating standard including surface preparation and coating application has been discussed for the purpose of enhancing the safety of ships. It has been expanding from double hull space of bulk carrier to water ballast tanks and void spaces of all kinds of ships, and further to cargo tanks of oil tankers.

Discussion in IMO on the coating standard for water ballast tanks was started based on TSCF15 (15 years specification issued by Tanker Structure Co-operative Forum). During the discussions at IMO meetings and in corresponding group, we have repeatedly express our view on the effectiveness and practicability of the standard. With regard to the expansion of the standard to void spaces and cargo oil tanks, NMRI (National Maritime Research Institute) and SAJ carried out the inspection of ships to clarify the actual coating conditions of aged ships in order to know the necessary specification and application of coating for 15 years target life. In this connection, we would show our considerations on the necessary standard for each spaces of ship, i.e. ballast water tanks, void spaces and cargo oil tanks. 2. NEW REGULATIONS FOR PAINTING 2.1 WATER BALLAST TANKS As for water ballast tanks, new regulation was adopted as resolution MSC.215(82) in last December.

The summary of the resolution is as follows; a) The resolution shall apply to ships of not less than 500

gross tonnage: for which the building contract is placed on or after 1 July 2008; or in the absence of a building contract, the keels of which are laid or which are at a similar stage of construction on or after 1 January 2009; or the delivery of which is on or 1 July 2012.

b) The resolution shall apply to all dedicated seawater ballast tanks arranged in ships and double side skin spaces arranged in bulk carriers of 150 m in length and upwards.

c) CTF (Coating Technical File) shall be kept on board. d) Inspection work shall be done by qualified inspectors

certified to FROSIO LevelⅢ or NACE Level 2 or equivalent.

e) Epoxy type multi-coat system with stripe coats shall be applied.

f) Coating system shall pass pre-qualification test. g) NDFT (nominal dry film thickness) of 320μm with

90/10 rule shall be applied. h) Free edges shall be treated to rounded radius of 2 mm,

or by three pass grinding or equivalent process. i) Damaged shop primer and welds shall be blasted to Sa

21/2. j) After erection, butts shall be treated to St 3 or better or

Sa 2 where practicable. Small damages up to 2% of total area shall be treated to St 3, and contiguous damages over 25 m2 or over 2% of the total area of the tank shall be treated to Sa 21/2 respectively.

k) Requirements of surface cleanliness such as oil contamination, dust, water soluble salts are prescribed



Guideline for implementation of MSC.215(82) is being discussed in IACS/JWG for practical and consistent application. 2.2 VOID SPACES PSPC for void space was discussed at DE50 in March this year. As the output of DE50, the draft resolution will be submitted to MSC83. While the meeting in DE50, we proposed the practical standard based on the inspection results for void spaces of aged ships. The summary of the draft resolution is as follows; a) The draft resolution shall apply to bulk carriers and

oil tankers as recommendation basis until the SOLAS amendments to make it mandatory will be adopted.

b) Definition of void spaces is clarified. c) Epoxy based paint and NDFT of 200μm with 90/10

rule shall be applied. d) Free Edges shall be treated by one pass grinding or

equivalent. e) Damaged shop primer shall be treated to Sa 2 or St 3. f) After erection, butts shall be treated to St 3 or better

or Sa 2 where practicable. g) One stripe coat shall be applied to thermally cut free

edges and small holes only.

h) Requirements for dust quantity and soluble salts limit are relaxed compared with those for water ballast tanks.

2.3 CARGO OIL TANKS European countries and shipowners’ and oil companies’ associations submitted their proposal of new SOLAS regulation to MSC82 which make PSPC for cargo oil tanks mandatory. In this connection, Japan submitted the proposal of using corrosion resistance steel in the cargo oil tanks as an alternative. DE50 instructed the correspondence group to develop the draft new SOLAS regulation regarding cargo oil tank protection for the discussion at DE51. The development of PSPC for cargo oil tanks will be further considered at DE51. 3. REPORT ON THE COATING CONDITION OF ACTUAL SHIPS 3.1 INSPECTED SHIPS In order to investigate appropriate coating standard, three (3) aged ships were inspected. Outline of the inspected ships are shown on Table 1.

Table 1. Inspected ships Ship A Ship B Ship C Type of Ship Tanker

(VLCC) Tanker

(VLCC) Bulk Carrier

(PANAMAX) Age of Ship 15 years old 12 years old 11 years old Shipyard Japanese Japanese Japanese Operator Japanese company Japanese company Japanese company Major Sea Route East Asia – Middle East East Asia – Middle East East Asia – Oceania



3.2 WATER BALLAST TANKS 3.2.1 COATING SPECIFICATIONS IN WATER

BALLAST TANKS

Coating specifications in water ballast tanks of inspected three (3) ships are shown on Table 2.

Table 2. Coating Specifications in Water Ballast Tanks Ship A Ship B Ship C Type of Paint Tar Epoxy paint Tar Epoxy Paint Tar Epoxy paint Dry Film Thickness 200μm

(1spray) 200μm (1spray)

220μm (1spray)

Stripe coat Not applied Holes and narrow spaces Not applied Primary surface preparation Sa2.5 (shot blast)

IZP Sa2.5 (shot blast)


IZP Free edge Removed burrs 1 pass grinding 1 pass grinding Steel

condition Weld bead No treatment No treatment No treatment Surface treatment St 3 St 3 Sa2.5 (Partially St3) 3.2.2 OVERVIEW IN THE WATER BALLAST

TANKS Ship A) Photo-A-W-1

Photo-A-W-2

Photo-A-W-3

These photos show deck head areas under upper deck, and these areas are considered to be in severest condition for corrosion. There were some areas of which condition was worse than of these photos and deck plates of such areas had cut off for renewal work.



Ship B) Photo B-W-1

Photo B-W-2

Photo-B-W-3

These photos show middle areas in height. Some rust was initiated from edges, welds, small fittings and small holes.

Deck head areas of this ship had already recoated at previous docking and kept good condition. Ship C) Photo C-W-1

Photo C-W-2

These photos show middle areas in height. Rust was initiated from edges and welds and spread to surrounding. 3.3 VOID SPACES 3.3.1 COATING SPECIFICATIONS IN VOID

SPACES Coating specifications in void spaces of inspected three (3) ships are shown on Table 3.



Table 3. Coating Specifications in Void spaces Ship A Ship B Ship C Coating type Alkyd based Tar epoxy Surface-tolerant epoxy Dry film thickness 70μm

(2 spray) 125μm (1spray)

100μm (1spray)

Stripe coat Not applied Not applied Not applied Primary surface treatment Sa 2.5 (Shot blast)

IZP Sa 2.5 (Shot blast)

IZP Sa 2.5 (Shot blast)

IZP Free edge No treatment No treatment No treatment Weld bead No treatment No treatment No treatment

Steel condition

Weld spatter Loose spatter to be removed by scraper

Loose spatter to be removed by scraper

Loose spatter to be removed by scraper

Grade St 2 (By disk sander and/or

power brush)

Between St 2 and St 3 (By disk sander and/or

power brush)

Loose rust to be brushed off.

Surface treatment

Treated area Damaged shop primer, welds and rusted areas

Damaged shop primer, welds and rusted areas

Damaged shop primer, welds and rusted areas

Water soluble salt Removed to the extent invisible to the naked eye. Oil contamination Removed. The traces may be visible. 3.3.2 OVERVIEW IN THE VOID SPACES Ship A) Photo-A-V-1

Photo-A-V-2

Photo-A-V-3

Almost all areas were in quite good condition. Slight rust was found on the welds in Photo-A-V-2 and at the edge of manhole in Photo-A-V-3, but did not spread further.



Ship B) Photo-B-V-1

Photo-B-V-2

Photo-B-V-3

All areas including edges and welds were in perfect condition. Photo-B-V-3 shows a part where main coating was not applied and shop-primer was exposed. Slight rust was found on the shop primer damaged during construction, but good condition was kept where shop-primer was intact even without main coatings.

Ship C) Photo C-V-1

Photo-C-V-2

Photo-C-V-3

Almost all areas were in quite good condition. There were small mechanical damages at the edge of opening in Photo-C-V-2, but the rust did not spread further. Some rusted areas caused by mechanical damages were appeared on the floor in Photo-C-V-3, but the rust also did not spread further.



3.4 CARGO TANKS 3.4.1 COATING SPECIFICATIONS IN CARGO

TANKS

Coating specifications in cargo oil tank (slop tank) of inspected two (2) ships are shown on Table 4. Slop tanks were inspected because ordinary cargo tanks were not coated.

Table 4. Coating Specification in Cargo tanks Ship A Ship B Ship C Type of Paint Tar Epoxy paint Tar Epoxy Paint - Dry Film Thickness 200μm

(1spary) 200μm (1spray)

-

Stripe coat Not applied Holes and narrow spaces - Primary surface preparation Sa2.5 (shot blast)


IZP -

Free edge Removed burrs 1 pass grinding - Steel condition Weld bead No treatment No treatment - Surface treatment St 3 St 3 - 3.4.2 OVERVIEW IN THE CARGO TANKS Ship A) Photo A-C-1

Photo A-C-2

Photo-A-C-3

These photos show rusted areas of deck head under upper deck.A lot of rust was found but the condition was much better than in water ballast tanks of this ship, and plate thickness loss was quite small. Ship B) Photo B-C-1



Photo B-C-2

Photo B-C-3

Overall coating was still in good condition. Coating damages were observed mainly on upper deck plate and on fillet welds of upper deck longitudinal.

4. CONCLUSION From the results of the inspection, it is considered as follows; 4.1.1 WATER BALLAST TANKS ・ Deck head area under upper deck seems to be in quite

severe environmental condition, while other regions are comparatively in mild condition.

・ In general, rust is initiated from edges and welds, and good treatment on edges and welds will prevent rust initiating.

・ Apparently, coating specifications of the ships for water ballast tanks at new building don’t satisfy 15 years coating life.

4.1.2 VOID SPACES ・ Although all inspected ships had been in use for 10 to

15 years, coatings in void spaces were still almost perfect condition.

・ Very small local corrosions were observed in the areas prone to be damaged mechanically. (estimated to be smaller than 0.1% of the total area)

・ Coating specifications of the ships in the void spaces at new building are sufficient for 15 years coating life.

4.1.3 CARGO TANKS ・ Coatings in the cargo tanks were comparatively in

better condition than those in water ballast tanks. ・ Though some corrosions were observed in deck head

areas, plate thickness loss was quite small, while recoating or plate renewal was necessary for some of deck head areas in water ballast tanks of the same ship.

・ Therefore, PSPC for cargo tanks could be relaxed compared with those required by PSPC for water ballast tanks.



PITTING CORROSION ON EPOXY-COATED SURFACE OF SHIP STRUCTURES T Nakai, H Matsushita and N Yamamoto, Nippon Kaiji Kyokai (ClassNK), Japan SUMMARY It should be noted that corrosion patterns may change when new coating systems are introduced because corrosion patterns highly depends on the coating types of structural members (e.g., no protective coatings, oil coatings, tar epoxy coatings, etc.). As for the hold frames of bulk carriers, it was made mandatory in 1992 to apply epoxy coating or equivalent. When there were oil coatings or no protective coatings on hold frames, general corrosion was observed. After hold frames of bulk carriers carrying coal and iron ore came to have tar epoxy paints, the typical corrosion pattern changes to pitting corrosion. The present paper deals with corrosion observed in structural members of cargo holds of bulk carriers carrying coal and iron ore. Firstly, the corrosion pattern observed in non-coated structural members is briefly explained. Secondly, the corrosion pattern observed in structural members with tar epoxy coating is presented in detail. Thirdly, progress rate of corrosion in structural members with different coating systems is briefly described. It can be said that applying tar epoxy coating is a very effective measure to protect the structural members from deterioration due to corrosion. However, the evaluation of residual thickness and/or residual strength became difficult because the corroded surfaces of the structural members with tar epoxy coatings have large unevenness due to pitting corrosion. Such a corrosion pattern was not expected when the coating system was made mandatory. 1. INTRODUCTION It is well known that corrosion is one of the dominant life-limiting factors of ships because hull structural members are exposed to corrosive environment after commissioning and ageing effect such as thickness diminution due to corrosion may be unavoidable. It is recognized that one of the main measures to protect hull structural members from deterioration due to corrosion is to apply protective coatings. There were bulk carrier losses in the late eighties and early nineties with considerable loss of human life. One of the main causes for the losses was severe corrosion of the hold frames of cargo holds. For the purpose of protecting the hold frames from deterioration due to corrosion, it was made mandatory in 1992 to apply epoxy coating or equivalent to hold frames in way of cargo holds of bulk carriers. Introducing the coating system, the Enhanced Survey Program (ESP) and retroactive requirements for existing bulk carriers (so called ‘Bulk

Carrier Safety’) have helped to improve the safety of bulk carriers. To ensure the structural integrity of ships, it is of crucial importance to understand the corrosion process, estimate the corrosion rate and evaluate the effect of corrosion wastage not only on overall strength but also on local strength accurately. It should be noted that corrosion patterns may change when new coating systems are introduced because corrosion patterns highly depends on the coating types of structural members (e.g., no protective coatings, oil coatings, tar epoxy coating, etc.). Therefore, the corrosion pattern observed in the target structural members should be accurately evaluated.

Figure 1: Schematic View of Cargo Hold of Bulk Carrier

hold frame loading conditionof coal

loading conditionof iron ore

Figure 2: Actual Corroded Hold Frame of 13-Year-Old Bulk Carrier (No Protective Coatings at Construction)



The corrosion pattern observed in hold frames of cargo holds of bulk carriers changed completely after it was made mandatory to apply epoxy coating or equivalent. The present paper focuses on the corrosion patterns observed in structural members of cargo holds of bulk carriers which carries exclusively carry coal and iron ore. Firstly, the corrosion pattern observed in non-coated structural members is briefly explained. Secondly, the corrosion pattern observed in structural members with tar epoxy coating is presented in detail. Thirdly, progress rate of corrosion in structural members with different coating systems is briefly described.

2. CORROSION PATTERN IN NON-COATED STRUCTURAL MEMBERS Figure 1 shows a schematic view of cargo hold of bulk carrier and Figure 2 gives the example of actually corroded hold frames in way of cargo holds of a 13-year-old bulk carrier. The hold frames were not coated at construction. In this case, unevenness of the corroded surfaces of the web plates is small and this type of corrosion is categorized as general (uniform) corrosion. 3. CORROSION PATTERN IN EPOXY COATED STRUCTURAL MEMBERS An example of the actual corroded web plates (approximately 500mm×700mm) of hold frames taken from a 13-year-old bulk carrier is shown in Figure 3. The bulk carrier is different from the one mentioned in the previous chapter and its hold frames were coated with tar epoxy paints at construction. Figure 3(a) shows the web plate before sand-blasting and many heavy rust blisters can be seen on the plate surface. Figure 3(b) represents the same web plate after sand-blasting to remove the heavy rust blisters covering pitting corrosion. It can be seen that progressive pitting corrosion is prevailing on the plate surface. Pitted regions of the web plate are shown in Figure 3(c) in black shade. Figure 4 shows a cross-sectional view of corrosion pit with rust blister. The rust blisters are very hard and it is difficult to remove these blisters even with hammers. Figure 5 gives the relationship between pit diameter and depth. It can be seen that the ratio of diameter to depth is in the range between 8-1 and 10-1, and the diameter of corrosion pit might become up to 50mm [1]. Figure 6 gives examples of corroded webs after sand-blasting taken from the 13-

(a) before Sand-Blasting

(b) after Sand-Blasting

(c) Corroded Part Shown in Black Shade

Figure 3: Actual Corroded Hold Frame of 13-Year-Old Bulk Carrier (Tar Epoxy Coatings at Construction)

Figure 4: Cross-Sectional View of Corrosion Pit with Rust Blister (Tar Epoxy Coatings at Construction)

1 2 3 4

10

20

30

40

0Pit Depth (mm)

Pit D

iam

eter

(mm

) Ratio of diameter to depth 10 to 1 8 to 1

BC-A(14years)BC-B(12years)BC-C(20years)BC-D(13years)

Figure 5: Relationship between Pit Diameter and Depth



(a) DOP = 19%

(b) DOP = 35%

Protective Coating Steel Plate

Mechanical Damage to the coating

Rust Blister

Corrosion Pit

Figure 7: Mechanism of Generation of Pitting Corrosion

year-old bulk carrier with a wide variety of DOP (Degree Of Pitting intensity) defined as ratio of the pitted surface area to the entire surface area. Although the webs in Figure 6 were taken from the same bulk carrier, this figure demonstrates well the progress of pitting corrosion. Generation and progress of pitting corrosion could be explained as follows (see Fig.7)

(1) Mechanical damage to the protective coating occurs due to the scratch of cargo.

(c) DOP = 60%

(d) DOP = 87%

(2) Corrosion process starts at the damaged parts of the protective coating.

(3) This leads to pitting corrosion. (4) In the early stage of corrosion, each corrosion pit

exists independently. (5) Then, the number of corrosion pits increases, and

each corrosion pit develops, and some of them start to overlap.

(6) Some parts of the plate surface remain uncorroded in this stage.

(e) DOP = 100%

Figure 6: Examples of Corroded Surface Conditions of Webs of Hold Frames (80mm×200mm, 13-Year-Old Bulk Carrier, Tar Epoxy Coatings at Construction)

(7) When the number of corrosion pits increases further and each corrosion pit develops further, they form a very uneven surface all over the plate.

(8) In the later stages of corrosion, unevenness of the plate surface due to pitting corrosion becomes smaller with the progress of corrosion.

In order to investigate the geometrical characteristics of corroded surface conditions of pitted hold frames in more detail, surface unevenness of these actual corroded web and face plates has been investigated using laser displacement sensors. Figure 8 show the relationship between statistics of corroded condition and DOP and Figure 9 depicts the relationship between statistics of corroded condition and average thickness diminution for the webs and face plates taken from the 13-year-old bulk carrier, where measured area of the webs is 80mm×200mm (see Figure 6) and that of the face plates in 50mm×200mm. It can be seen that average thickness diminution and standard deviation of thickness diminution vary within small scatter bands, and DOP reaches 100% when the average thickness diminution on



one m. A trend is

OSION

ure kness iminution behavior of structural members in cargo holds

0 that thickness diminution of structural

he present paper describes the corrosion pattern ural members of cargo

olds of bulk carriers. It can be said that tar epoxy paint

side exceeds approximately 2mobserved that a form of corrosion changes from pitting corrosion to general corrosion with further progress of corrosion after DOP reaches 100%. The probability distribution of diminution varies exponential to normal one depending on DOP and thickness diminution [2] and it is considered that the probability distribution of diminution follows normal distribution when a form of corrosion changes almost completely to general corrosion with the progress of corrosion. 4. PROGRESS RATE OF CORR Fig 10 shows an example of average thicdof bulk carriers with different coating types. The average thickness diminution behavior shown in this figure is obtained by applying the probabilistic corrosion model

members with tar epoxy coatings is smaller than those with oil coatings. It is clear from this figure that the average amount of corrosion is significantly reduced by applying tar epoxy coatings and applying tar epoxy coatings is a very effective measure to protect structural members from deterioration due to corrosion. Applying epoxy coatings or equivalent to the hold frames is mandatory at present and there are no bulk carriers whose hold frames have oil coatings or no coating. In the case of structural members with oil coatings or no protective coatings, the typical corrosion pattern is considered to be general corrosion which uniformly reduces the thickness and the evaluation of residual thickness and/or residual strength is relatively easy, because the thickness diminution can be directly compared with the allowable diminution level and residual strength calculations can be performed by excluding the uniform thickness diminution. On the other hand in the case of structural members with tar epoxy paints, pitting corrosion occurs as previously mentioned and this makes it difficult to evaluate the residual thickness and/or residual strength. From such a point of view, visual assessment of pitting corrosion observed in structural members of cargo holds has been developed [3]. 5. CONCLUSIONS

[4] to the thickness measurement records. It can be seen from Fig. 1

Tobserved in tar epoxy coated structhis a very effective measure to protect structural members from deterioration due to corrosion. However, it has been revealed that the typical corrosion pattern for the tar-epoxy coated structural members of cargo holds of bulk carriers carrying coal and iron ore is pitting corrosion and this makes it difficult to evaluate residual thickness and/or residual strength. Such a situation was not expected at the time the coating systems were applied. It

0 20 40 60 80 100-8

-6

-4

-2

0

2

Degree of Pitting Intensity DOP (%)

Thic

knes

s D

imin

utio

n (O

ne S

ide)

(mm

)

Measurement results Ave. (web) Ave. (face) Std.dev (web) Std.dev (face)

Max.depth (web) Max.depth (face) Min. cross section ave. (web) Min. cross section ave. (face)

Figure 8: Relationship between Statistics of CorrodedCondition and DOP

-5 -4 -3 -2 -1 0-8

-6

-4

-2

0

2

Average Diminution (mm)

Thic

knes

s D

imin

utio

n (O

ne S

ide)

(mm

)

Measurement results

Std. dev. (web) Std.dev (face)

Max.depth (web) Max.depth (face) Min. cross section ave. (web) Min. cross section ave. (face)

Figure 9: Relationship between Statistics of CorrodedCondition and Average Diminution

5 10 15 20 25

1

2

3

4

5

0Ship Age (years)

Ave

rage

Thi

ckne

ss D

imin

utio

n (m

m)

Probabilistic Corrosion ModelStructural Members in Cargo Hold

Oil CoatingsAt present, there are no bulk carrierswhose hold frames have oil coatings

Tar Epoxy Coatings

Figure 10: Thickness Diminution of Structural Members with Different Coating Types (Bulk Carriers, DWT > 50,000 ton)



should be noted that corrosion patterns may change when new coating systems are introduced. It can be said from this lesson that monitoring the relation between the corrosive environment and actual state of corrosion is also important when the new coating systems are introduced. 6. REFERENCES 1. Nakai, T., Matsushita, H., Yamamoto, N. and Arai, H.,

ocal strength of hold ames of bulk carriers (1st report)’, Marine Structures,

lates Subjected to Uni-axial

l Strength - In the

ilistic Approach’, Transactions of

atsuro Nakai holds the current position of researcher at lassNK).

e is a member of material and equipment section of the

at Research Institute of Nippon Kaiji Kyokai lassNK). He is a member of material and equipment

stitute of Nippon Kaiji Kyokai lassNK). He is the head of material and equipment

‘Effect of pitting corrosion on lfr17(5), 403-432., 2004. 2. Nakai, T. and Yamamoto, N., ‘Pitting Corrosion - Probabilistic Modeling and Its Effect on the Ultimate Strength of Steel PCompression’, Proceedings of the 10th International Conference on Applications of Statistics and Probability in Civil Engineering (ICASP10), 2007. 3. Nakai, T., Matsushita, H. and Yamamoto, N. ‘Visual Assessment of Corroded Condition of Plates with Pitting Corrosion Taking into Account ResiduaCase of Webs of Hold Frames of Bulk Carriers’, Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering (OMAE2007), 2007. 4. Yamamoto, N. and Ikegami, K. ‘A Study on the Degradation of Coating and Corrosion of Ship’s Hull Based on the Probabthe ASME, Journal of Offshore Mechanics and Arctic Engineering, 120, 121-128., 1998 7. AUTHORS’ BIOGRAPHIES TResearch Institute of Nippon Kaiji Kyokai (CHinstitute. Hisao Matsushita holds the current position of senior researcher(Csection of the institute Norio Yamamoto holds the current position of principal researcher at Research In(Csection of the institute.





Compositional Analysis of Soluble Salts in Bresle Extraction from Blocks in Newbuilding Shipyards

S. S. Seo*, S. M. Son**, C. H. Lee** and K. K. Baek**

*) Hull Design Dept., Hyundai Heavy Industries Co. Ltd. 1 Jeonha-Dong, Ulsan, Korea

**) Protective Coatings & Corrosion Res. Dept., R&D Center, Hyundai Heavy Industries Co.Ltd. 1 Jeonha-Dong, Ulsan, Korea

e-mail: [email protected]

ABSTRACT

Soluble salts, especially NaCl, on the substrate shall cause premature coating failure, promoting osmotic blistering of the coating and underfilm corrosion of the steel. The salt measurement method specified by IMO’s PSPC rule, ISO standard 8502-6 & 8502-9, inherits the shortcoming that the measured value is the amount of Total Dissolved Salt(TDS), and is neither of NaCl nor of chloride ion. In this study, an Ion Chromatography(IC) measurements, which measures specific ion concentration in a solution, is employed to measure exact amount of NaCl in the Bresle extraction water from several blocks in a newbuilding

shipyard. By using conductometric methods, 30 / ~ 70 / TDS was detected for sweep blasted blocks, but NaCl contents of the same extracted solutions from the same blocks, as confirmed by the IC,

were measured to be around 20 /. Comparison of these two methods confirmed that actual content of NaCl was below 60% of TDS, due to the existence of many other anions in the extracted solution,

suggesting that ~ 80 / TDS corresponds to 50 / NaCl.

1. INTRODUCTION

Soluble salts, especially NaCl, left upon the substrate prior to coating application shall cause premature coating failure, since the presence of salts at the steel/paint interface can promote osmotic blistering of the coating and underfilm corrosion of the steel [1]-[3]. The current technology, however, still lacks full confidence regarding salt(s) level determination under the field condition, which includes wet extraction followed by ionic/conductiometric measurement of the salt(s). The method specified by IMO’s PSPC regulation (adopted by Res. MSC.215(82) in SOLAS Reg. II-1/3-2 and XII/6.3), ISO standard 8502-6 & 8502-9, inherits the same shortcoming that the measured value is the level of Total Dissolved Salt(TDS), and is neither of NaCl nor of chloride ion. Conductivity measurement, as prescribed in ISO 8502-9, cannot determine the concentration of specific ions which are dissolved in the extracted solution, thus, only detects the TDS level as whole. To measure exact amount of NaCl or chloride concentration, it is necessary to employ an Ion Chromatography(IC) method, which measures specific ion concentration in a solution, of which result is summarized herewith [6].

© 2007: JASNAOE-RINA- 65

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2. EXPERIMENTAL CONDITION

Extraction or sampling of soluble salt were carried out on two(2) blocks in a ship yard, which had been sweep blasted for secondary surface preparation. 3 points for each block, total of 6 points, were measured following ISO 8502-6 & 8502-9 method with sampling patches and a conductivity meter (ElcometerTM, Elcometer 138 Bresle KitTM). The same, extracted solutions were used for measurement with an Ion Chromatography (DIONEXTM, DX300 Model), of which equipment are shown in Fig. 1 and Fig. 2, respectively.

Fig.1 Bresle method using patch extraction (ISO 8502-6)

For ISO 8502-6 & 8502-9 method, the TDS in extracted solution were evaluated by employing the

following conversion equation;

--------------------------- Eq.1 E=(0.5)·S·V/A

where, E is surface concentration of total chloride in /, S is conductivity measured in /, A is measurement area in , V is volume of extracted solution in , 0.5 is the conversion factor, which depends on the chemical composition of the dissolved salts, which can vary between 0.54~0.96, and the conversion factor was selected to be 0.5 in this study [4].



Fig.2 Ion Chromatography and detector(DIONEXTM, DX300 Model)

3. RESULT AND DISCUSSION

As summarized in Table 1, about 3.5% of salts are existed in natural seawater and the portion of NaCl in the salts is about 85%. IC measurements of the extracted solutions from six measuring points were carried out for major elements such as Cl-, NO3

-, SO42- , which are reported to affect the coating’s

performance, compared to the normal, chemical composition of seawater, as summarized in Table 1 and Table 2, respectively.

By using conductiometric methods, 30 mg/m2 ~ 70 mg/m2 TDS was detected for sweep blasted blocks, but NaCl contents of the same extracted solutions from the same all blocks, as confirmed by the IC, were measured to be around 20 mg/m2. Comparison of these two methods revealed that actual content of NaCl was below 60% of TDS (Fig. 3 and Table 3), due to the existence of many other anions in the extracted solution. Therefore, the maximum allowable limit of salts in the IMO’s PSPC guideline shall be designated to be 80 mg/m2, since 83.3 mg/m2 TDS corresponds to 50 mg/m2 NaCl (Fig. 4 & Fig. 5).

Table 1. Major ion composition of natural seawater

Cl- Na+ SO42- Mg2+ Ca2+ K+ etc. Total

g/ℓ(‰) 18.98 10.55 2.65 1.26 0.4 0.38 0.26 34.48

wt% 55.04 30.59 7.69 3.66 1.16 1.10 0.76 100

NaCl/TDS 85.63% 14.37% 100%

Table 2. Composition of extracted solutions (A~F) analyzed by IC

PPM(mg/l)

A B C D E F

Cl- 1.0 1.0 1.1 1.3 1.1 0.9

NO3- 0.1 0 0.1 0.1 0.1 0.1

PO43- - - - - - -

SO42- 0.4 0.4 1.0 0.5 0.4 0.4



Ion

conc

entr

atio

n(

/ℓ)

Sample No. Fig.3 Ion composition of extracted solutions (analyzed with IC)

Table 3. Content of Sodium chloride in Total Dissolved Salt

Methods Conductivity meter Ion Chromatography

Measured Converted to Measured Converted to

Conductivity TDS Cl- Cl- NaCl

Units / / /ℓ /ℓ /

NaCl/TDS

(%)

A 12.1 72.6 3.6 1.0 20.2 27.8

B 5.7 34.2 1.7 1.0 20.2 59.1

C 7.4 44.4 2.2 1.1 22.2 50.0

D 7.4 44.4 2.2 1.3 26.3 59.2

E 6.2 37.2 1.84 1.1 22.2 59.7

F 5.1 30.6 1.52 0.9 18.2 59.5



Salt

Conc

. (

/)

Fig.4 Comparison of salt content analyzed with Bresle and IC

NaC

l/TD

S (%

)

Fig.5 Portion of Sodium Chloride in TDS

REFERENCES

[1] M. Morcillo and J. Simancas, “Effects of soluble salts on coating life in atmospheric services,” J. Protective Coatings & Lings, pp. 40-52.

[2] B. P. Alblas, et al., “A literature review”, J. PCE(February), pp. 16-25, 1997 [3] K. K. Baek, et al., “Effect of Blasted Surface contaminants on Coating Performance,” NACE,

CORROSION 2006, Paper No.06014, San Diego, USA, 2006 [4] http: // www.globe.gov/tctg [5] SSPC, Guide 15, “Determining Equipment Surface Contamination from Conductivity” [6] http://www.chemistry.nmsu.edu; Ion chromatography is used for analysis of aqueous samples in

parts-per-million (ppm) quantities of common anions (such as fluoride, chloride, nitrite, nitrate, and

sulfate and common cations like lithium, sodium, ammonium, and potassium) using conductivity

detectors. Ion chromatography is the only technique that can provide quantitative analysis of anions at the ppb level. This technique is used to determine ions in liquids and ionic contamination of waters.





Effect of Edge Preparation Methods on Edge Retention Rate of Epoxy Coatings for Ship’s Ballast Tanks

S. S. Seo*, K. K. Baek**, C. S. Park**, C. H. Lee** and M. K. Chung**

*) Hull Design Dept., Hyundai Heavy Industries Co. Ltd. 1 Jeonha-Dong, Ulsan, Korea

**) Protective Coatings & Corrosion Res. Dept., R&D Center, Hyundai Heavy Industries Co. Ltd. 1 Jeonha-Dong, Ulsan, Korea

e-mail: [email protected]

ABSTRACT

To avoid insufficient coating film thickness at certain areas, such as corners, edges and weld seams in the ballast tanks, mechanical grinding of the edge area is required since stripe coatings applied on a smoother edge profile will retain liquid paints longer than the sharper shaped edge does. For this purpose, “3-Pass or 2-R” edge grinding treatment prior to secondary surface preparation is specified by IMO’s PSPC rule. However, the coating application experience revealed that the actual coating thickness at these areas tend to be thicker than flat areas, which makes those areas vulnerable to coating cracking during service. Therefore, it is important to come up with proper edge preparation method to ensure achieving reasonable coating thickness at the edge areas. Examination of disk grinding-induced burrs at the edges showed that most of excessive burrs were removed during the ISO Sa 2½ blasting stage before coating. This result combined with measurements of the coating thickness at the edges showed that the widely practiced RC grinding(one pass

grinding + disk papering for burr removal) with stripe coating for 2 coats (avg. 300μm total D.F.T.) epoxy system would provide enough coating thickness at the edges with Edge Retention Ratio(ERR) reaching over 1.0 value.

1. INTRODUCTION

The protective coating for certain areas of the ballast tanks such as corners, edges and weld seams need to be re-coated or stripe coated to avoid coating defects such as insufficient film thickness, resultant corrosion of steel, etc. A common approach for solving these problems is mechanical grinding of the edge area to provide a smoother edge profile, which will lead to better retention of liquid paint than the sharper shaped edge does. For this purpose, the IMO’s PSPC rule (adopted by Res. MSC.215(82) in SOLAS Reg. II-1/3-2 and XII/6.3) calls for “3-Pass or 2-R” edge grinding treatment prior to secondary surface preparation based on the understanding that the rounder edge would better retain the coating. However, according to the actual coating application experience, the negative effect of this requirement is that the final coatings at these areas tend to be thicker than flat areas, making such areas more vulnerable to coating cracks during service. Therefore, it is considered important to select a proper edge preparation method to ensure reasonable coating thickness at the edges. Thus, in this paper, practical edge preparation methods are discussed based on the several recent studies employing the cross sectional measurement of the coating thickness at the edges [1]~[4].


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2. COMPARATIVE STUDY ON EDGE PREPARATION

2.1 Experimental Preparation

Steel panels (300mm×100mm×12mm thickness) or angle bars (25mm×25mm(90o)×3mm thk.× 4mm L) with various edge curvature were prepared [3], [4]. Edge curvature conditions were categorized as follows:

sharp (as flame cut) 1C (1-Pass grinding) 2C or RC (equivalent to 3-Pass grinding) 2R (2mm radius edge)

For each steel panel (or angle bar) with specific curvature, coating (2 coats, avg. 300μm total D.F.T.) was applied by airless spray with 60% S.V.R. epoxy coating materials after ISO Sa 2½ blasting. To measure the coating thickness at the edges, cross-sections of coatings were observed using an optical microscope and Scanning Electron Microscope(SEM). Edge Retention Ratio (E.R.R.) of coating for each edge curvature was calculated with the following equation:

E.R.R.(%) = (D.F.T. at the edge) / (D.F.T. at the flat surface)

2.2 Results

Examination of disk-grinding-induced burrs at the edges showed that most of excessive burrs are removed during the ISO Sa 2½ blasting stage before the coating is applied, and that the areas of residual burrs accounts for 0.59~0.72% of total edges of blocks to be painted, as shown in Fig.1 [1]. This result strongly indicates that ISO Sa 2½ blasting, as a secondary surface preparation method for blocks to be painted, will either remove or minimize disk-grinding-induced burrs at the edges, thus eliminating the needs for extra edge grinding for burr-removal purpose as shown in Fig. 1.

Additionally, E.R.R. measurements result showed that all edge-ground panels/angles displayed improved edge retention tendency after ISO Sa 2½ blasting [2]~[4]. The measurement of the coating thickness at the edges, as shown in Fig.2~Fig.4, clearly showed that E.R.R. of epoxy coating in the area

which had undergone 1C edge grinding without the stripe coating (2 coats, avg. 300μm total D.F.T.) were 0.8~1.0 [3], [4]. These results were believed to be due to the combined effect of ISO Sa 2½ blasting and 2-layer coating practice. In other words, the sharp edges are rounded during ISO Sa 2½ blasting stage and become buried in the first coat, leading to the secondary and final coating being applied on much rounder edges.



Fig.1 Variation of bevel angle after sharp edge grinding and blast cleaning [

Fig.2 Edge retention on various E/G steel angles vs. coating thickness(D.F.T. flat) [3]

Fig.3 E.R.R. (%) with coating thickness (2 coats, avg. 300μm D.F.T.) with 1 C and other E/G [3]

2]



Fig.4 Variation of coatings thickness (2 coats, avg. 300μm D.F.T.) with 1 C and other E/G [4]

Table 1 shows the differences in edge coverage rate for the single coated (primer coating) and 2-layer coated (

Table 1 Edge coverage rate for the 1-la er coated and 2-layer coated films [4]

Flame Cut

C

RC

SVR = 60%SVR = 60%

primer and top coating) on the zero (flame cut) and 1mm roundness(1R) edge, yielding a higher ratio for the 2-layer coated edge than a single coated edge. This difference was also attributed to the addition of extra roundness, provided by a primer coating, in the 2-layer coating.

y

Dry film thickness Dry film thickness Edge coverage (μm, Flat) (μm, Edge) Coatings & Edge

1st c coat 1st c oat 1st coat Roundness

oat 2nd oat 2nd c 2nd coat Flame cutting 179.4 165.8 60.1 75.9 0.34 0.46

R=1mm 173.8 136.2 61.9 119.4 0.36 0.88

R=2mm 174.7 153.7 141.9 152.0 0.81 0.99

80% SVR

R=3mm 158.0 142 141.4 159.0 0.90 1.20 Flame cutting 153.5 1 22.4 64.0 133.8 0.42 1.13

R=1mm 133.7 155.1 137.8 152.7 1.03 0.99

R=2mm 146.8 180.5 165.1 160.2 1.13 0.97

60% SVR

R=3mm 148.7 116.7 161.2 155.8 1.08 1.24

Another important aspect regarding on this subject is the possible coating breakdown at the edges

due to hull stress, vibration and water flow induced wear even under 100% edge retention condition, as shown in Fig. 5. This rather new observation emphasizes the importance of measures against coating



cracking, considering that less flexible, light colored epoxy coating system has already replaced tar epoxy coating in order to comply with the Enhanced Survey Program(ESP) for W.B.T. section of ships. Considering that additional stripe coating(s) is applied to edges, the chance of edges having overly (not under) thick coating becomes greater, increasing the possibility of mud-type cracks at such areas also [5].

iioonn

Fig.5 EFM analysis of fluid status around T-bar in W.B.T., showing much higher wall shear

he most favorable way to avoid overly thick coating, therefore, is to maintain proper balance

betwee

. CONCLUSIONS

Examination of disk grinding-induced burrs at the edges showed that most of excessive burrs were removed

se overly t

REFERENCES

S. K. Chi et al., “Surface Preparation Effect on Edge’s Coating Performance”, Presented at Annual

stress and impact load concentration around the edge [3]

T

n edge preparation and stripe coating to ensure sufficient edge retention of coating and at

the same time, avoid overly thick edge coating.

3

during the ISO Sa 2½ blasting stage before coating. This result combined with measurements of the coating thickness at the edges showed that the widely practiced RC grinding(one pass grinding + disk

papering for burr removal) with stripe coating for 2 coats (avg. 300μm total D.F.T.) epoxy system would provide enough coating thickness at the edges with Edge Retention Ratio(ERR) reaching over 1.0 value.

Excessive edge preparation combined with multiple stripe coating would, on the other hand, cauhick coating, which are more susceptible to premature cracking. Thus, it is critical to maintain

proper balance between edge preparation and stripe coating to ensure sufficient edge retention of coating without risking overly thick coating on those areas.

[1]Workshop of Protective Coatings & Corrosion Engineers for Korean Shipbuilding Industries, Geoje, July, 2004.



[2] mov et al., “Corrosion Protection of the Members (Elements) of Steel Structures with

[3] J Symposium

[4] M oatings on Sharp Edges in Ship's

[5] E hy

I. V. KharlaDifferent Bevel Angles and Radii of Curvature of the Edges”, Izvesiya VUZ, No. 8, 1976. . T. Yun et al., “A Critical Study on Edge Retention of Protective Coatings for a Ship Hull”,Paper No.5016 presented at NACE CORROSION/05, Houston, 2005. . K. Chung et al., “Approaches for Achieving Successful High Build C

Ballast Tanks”, Symposium Paper No.3010 presented at NACE CORROSION/03, San Diego, 2003. . H. Song, M. K. Chung, C. H. Lee, S. K. Lee, H. I. Lee, C. S. Park, C. S. Shin, and K. K. Baek, “WWe Do Have Cracks in Epoxy Coatings for Water Ballast Tanks?”, Paper No.0615 presented at NACE CORROSION/06, San Diego, 2006



STUDY ON THE ALTERNATIVES TO THE SECONDARY SURFACE PREPARATION IN PROTECTIVE COATINGS Naoki OSAWA, Osaka University, Japan Koichiro UMEMOTO, Kawasaki Shipbuilding, Ltd., Japan Yukinori NAMBU, Universal Shipbuilding, Ltd., Japan Tatsuya KURAMOTO, Mitsui Engineering & Shipbuilding, Ltd., Japan SUMMARY Anti-corrosive performance of a protective coat applied on blowholes dressed out by puttying is investigated. Edge retention behaviours and anti-corrosive performances of a ferromagnetic pigment (FMP) paint system applied to the steel plates with various edge geometries are investigated. The effectiveness of puttying as an alternative to repair welding for blowhole dressing, and stripe coating with FMP paint as an alternative to mechanical grinding of sharp edges are discussed. As results, the followings are found: (1) The protective performance of a top coat applied to a weld bead with blowholes which is dressed out by 100% solid epoxy / polyamide putty is better than or equivalent to that for the case where blowholes are dressed out by repair-welding. Puttying by 100% solid epoxy / polyamide putty is an effective alternative to repair welding for blowhole dressing. (2) A thick area of coating along sharp edge is created when FMP paint system is applied as a top coat. Edge retention behaviour is improved when the bevel angle decreases. The protective performance of a specimen with sharp edge coated by FMP paint system is higher than or equivalent to that of specimens with edge preparation coated by an ordinary paint system. FMP paint is an attractive alternative to mechanical grinding of edge because it can eliminate the need for a great amount of labour. 1. INTRODUCTION Ship structures often come with complex geometric configurations, having large surface and highly stressed areas, such as corners, edges, and weld seams areas. Coating defects such as insufficient film thickness are often observed, resulting early coating failure and corrosion in these areas. It is well known that secondary surface preparation (SSP) is effective in preventing these coating defects. It is mandated to apply grinding weld beads contaminant and mechanical grinding of sharp edges by IMO/PSPC [1]. The SSP defined in IMO/PSPC requires finishing weld beads in accordance with ISO8501-3 grade P2. In this standard, it is required that 'surface pores shall be sufficiently open to allow penetration of paint or dressed out'. This requires that blowholes are removed or filled up. Usually, blowholes are dressed out by repair welding, but contingent works, such as surface re-preparation and removal of dust, are associated with this process. Blowholes can be dressed out by puttying up. If adequate protective coating performance is obtained when blowholes are dressed out by puttying, we can save the manpower substantially. In this paper, anti-corrosive performance of a protective coat applied to blowholes dressed out by puttying is investigated by adhesion measurements, immersion tests in NaCl solution, and salt spray cabinet tests. The effectiveness of puttying as an alternative to repair welding for blowhole dressing is discussed by comparing the protective performance for the cases with blowholes dressed out by puttying and that for the cases without blowholes.

Mechanical grinding of edges will retain paints of liquid phase for a longer duration than the sharper shaped edge does and improve edge retention behaviour. However, it takes a great amount of labour to mechanically grind all edges to be painted in ships. Another possible approach is to select coating materials with better edge formation ability. Much effort has been made on developing coating material with better edge retention behaviour [2-5]. However, a coating system, which makes the film thickness at sharp edge become larger than that of grinded edge, has not been developed yet, and no alternative to mechanical grinding is allowed in IMO/PSPC. It is known that magnetic flux on sharp edge becomes larger than that on flat surfaces when magnetic field is applied to a steel panel. If ferromagnetic pigment is mixed into a coating paint, this magnetic field causes the force that draws the ferromagnetic particles in the paint toward the edge. If this force overcomes the effect of surface tension, it is anticipated that a thick area of coating along edge is created. If edge retention behaviour is improved drastically and adequate protective coating performance is obtained by using a ferromagnetic pigment (FMP) paint, we can cut off the amount of manual labour required for edge preparation. In this paper, edge retention behaviours and anti-corrosive performances of a FMP paint system applied to the steel plates with various edge geometries are investigated. The effectiveness of stripe coating with FMP paint as an alternative to mechanical grinding of sharp edges is discussed by comparing the edge retention behaviours and protective performances of ordinary and FMP paint systems.



2. METHODLOGIES 2.1 ANTI-CORROSIVE PERFORMANCE MEASUREMENTS The anti-corrosive performances of coating systems are tested by spray cabinet tests according to JIS K5600-7-1 (equivalent to ISO7235, 5% NaCl solution, 35ºC) and immersion tests according to JIS K5600-6-2 (equivalent to ISO2812-2) in 3% NaCl solution. In these tests, the dimension and number of blisters on every test specimen are monitored according to the ASTM D-714. The presence of corrosion spots on every specimen is also monitored according to ASTM D-610. In the rating of corrosion data, stain made by the rust which forms outside the monitored region is not treated as corrosion damage. 2.2 ADHESION MEASUREMENTS The adhesive strength of the coating film is examined by knife-cut test nearly equivalent to JIS K5400-8.5.3. In the test, cutting of cruciform pattern is performed, and the peeling is checked by putting the knife edge to the intersection of the cutting lines. Tape peeling is not applied in this test. The peeling of the coating film is rated according to JIS K5400-8.5.3. 3. EFFECTIVENESS OF BLOWHOLE DRESSING BY PUTTYING 3.1 EXPERIMENTAL T-weld joints shown in Figure 1 are used as the test specimens. The sizes of main and attached plates (length, width and thickness) are 50mm x 100mm x 8mm and 50mm x 50mm x 8mm. The joints are made of mild steel. The total number of the specimens is 20. The leg length of the fillet weld ranges from 5mm to 8mm, and the flank angle of the weld ranges from 135 deg. to 145 deg. For all specimens, the shape of the weld bead meets ISO8501-3/P2 requirements except that there are blowholes in some specimens. There are blowholes in 16 of 20 specimens. The maximum diameter of blowhole is about 3mm. The specimens are blasted with steel grits to near white metal finish (ISO 8501-1, Conditions SIS Sa 2 1/2). Figure 2 shows the examples of blowholes in the weld bead before and after blasting. Blowholes are puttied up using 100% solid epoxy / polyamide putty (Chugoku Marine Paints, BUNDET PUTTY [1]). The drying time of this putty at 20˚C is 3 hours for surface dry, and 8 hours for hard dry, and the drying shrinkage is negligible because the solid volume ratio (SVR) is 100%. This putty is applied to the blowholes so that the putty surface is flushed with the bead surface using spatulas.

For the specimens with blowholes, the specimens are coated with anticorrosive paint after the blowholes are puttied up. The time between puttying and coating application, Tp, is 0 hours (apply paint immediately after puttying), 3 hours (apply paint when the putty surface gets dried) and 8 hours (apply paint after the whole putty gets hardened). For the specimens without blowholes, the specimens are coated just after blasting. The applied paint systems are tar epoxy resin system (Chugoku Marine Paints, BISCON HB-200 [7]) and modified epoxy resin system (Chugoku Marine Paints, NOVA-2000 [8]). The top coat is applied by airless spray after the touch-up coat is applied to the weld bead with a brush. After the top coat is applied, the painted specimens are dried for more than 24 hours at room temperature (about 20˚C). Dry film thickness ranges from 173μm to 442μm. Additional touch-up coat is not applied to the specimen’s end faces. Figure 3 shows an example of the weld bead surface after the dressing by puttying, and Figure 4 shows an example of the weld bead surface after the application of the top coat. The back faces of 4 of 16 specimens with blowholes are heated by LP gas flame so that the maximum back face temperature is above 600˚C. This gas heating causes the reverse side burn damage of the top coat on the weld beads as shown in Figure 5. These burn damages are repaired by grinding and repainting. Test conditions of blowhole dressing tests are identified by presence or absence of blowholes, putty drying time (Tp=0, 3, or 8 hours), type of paint system (tar epoxy or modified epoxy), presence or absence of burn damage. The conditions are identified by the names described in Table 1. Four specimens for each test condition are prepared. Dry film thicknesses measured are summarized in Table 1. Duplicate specimens of each test condition are subjected to the immersion test in NaCl solution for 300 days. Duplicates of these specimens are tested in a salt spray cabinet for 1000 hours. In these tests, the dimension and number of blisters as well as the presence of corrosion spots on the weld beads of every painted specimen are monitored. The immersion and spray tests are performed, and the blistering / corrosion data are measured according to the procedures described in Sec. 2.1. In the rating of corrosion data, stain made by the rust which forms on the specimen’s end faces is not treated as corrosion damage. After the immersion and spray tests, the adhesion data on the weld bead is measured by the knife-cut test described in Sec. 2.2. The knife-cut test is performed at the location where a blowhole is puttied-up for the specimens with blowholes (CASE A1, A2, A3, B1, B2, B3, D1 and D2 in Table 1), and at a distance of 20mm



from the specimen end for the specimens without blowholes (CASE A4 and B4 in Table 1). 3.2. EXPERIMENTAL RESULTS AND DISCUSSION Table 2 lists blistering / corrosion data obtained for the welded joint specimens with / without blowholes subjected to the immersion tests. The results of the knife-cut tests are also shown in this table. The ratings for each test condition shown in this table are the mean values of the duplicate specimens. It is shown that all specimens show neither corrosion products nor coating defects on the weld beads during the periodic visual inspections. It is also shown that no signs of the adhesion loss are recognized for all specimens. Table 3 lists blistering / corrosion data for the welded joint specimens subjected to the salt spray cabinet tests. The results of the knife-cut tests are also shown. The ratings for each test condition are the mean values of the duplicate specimens. It is shown that all specimens show neither corrosion products nor coating defects on the weld beads during the periodic visual inspections except that the rust which forms on the specimen's end faces stain the coating film on the weld beads. It is also shown that no signs of the adhesion loss are recognized for all specimens. The above results can be summarized as follows: 1) Under the conditions chosen, the performance of the anticorrosive tar-epoxy or modified-epoxy top coats applied to the weld bead, on which blowholes are dressed out using 100% solid epoxy / polyamide putty, is almost equivalent to that for the cases where there is no blowhole on the weld bead. 2) Under the conditions chosen, for the weld joint specimens with putty-upped blowholes, loss of the anticorrosive performance of the top coat coming from the shortening of the putty drying time is not recognized. 3) Under the conditions chosen, loss of the anticorrosive performance of the top coat applied to the weld bead with putty-upped blowholes is not recognized when reverse side burn damage occurs. It is considered that the protective performance of the top coat on the weld bead without blowholes is equivalent or better than that for the weld bead dressed out by repair welding. The above results suggest that the performance for the puttying case is better than or equivalent to that for the repair-welded case. Contingent works needed for the repair-welded cases, such as surface re-preparation and removal of dust, is unnecessary in the puttying cases. It can be said that dressing by 100% solid epoxy / polyamide putty is an effective alternative to repair welding.

4. PROTECTIVE PERFORMANCE OF FERROMAGNETIC PIGMENT (FMP) PAINT 4.1 EXPERIMENTAL A KA32 steel plate with thickness 12mm is blasted with steel grits to a commercial finish (ISO 8501-1, Conditions SIS Sa 2), then coated with inorganic zinc rich primer. Dry film thickness of the primer coat is 15μm. Test specimens are created by cutting rectangular boards (150mm x 70mm x 12mm) by a laser cutting system from this steel plate. They are prepared to evaluate edge retention ratio (ERR %). Edge preparation is performed by milling machine or grinder. The following four different types of edge geometry shown in Figure 6 are prepared to evaluate each specimen's edge retention ability: 1) GEOM 90M: sharp edge, prepared by milling machine, no burr; 2) GEOM 45M: 1C (the inclination of the sloped face is 45 deg), prepared by milling machine, no burr; 3) GEOM 45G: 1C (the inclination of the sloped face is 45 deg), prepared by grinder, with burr; 4) GEOM 20G: 1C (the inclination of the sloped face is 20 deg), prepared by grinder, with burr; GEOM 45G is prepared in order to investigate the effect of burr on ERR. GEOM 20G is typical edge geometry created by hand grinding. After edge preparation, test specimens are coated with anticorrosive paint systems. The primer is not removed before the application of the top coat. The applied systems are modified epoxy resin system (Chugoku Marine Paints, NOVA-2000 [8]) and ferromagnetic pigment (FMP) paint system. The FMP paint system is developed by Chugoku Marine Paint, Ltd. This paint is created by adding ferromagnetic pigments to a modified epoxy resin-based system. The top coat is applied by airless spray following the protocol illustrated in Figure 7. In this protocol, the paint is delivered by 2 pathes spray in the direction perpendicular to the cutting surface, 2 pathes in the direction perpendicular to the plate face, and 1 path in the oblique direction. After the top coat is applied, additional touch-up coat is applied by brush on the specimen surface within 20mm from the plate ends. The painted specimens are dried for more than 24 hours at room temperature (about 28˚C). When FMP paint is applied, the test specimen is attached to a plate-like permanent magnet as shown in Figure 8. Size of the magnet is 160mm x 80mm x 25.4mm, and the nominal surface magnetic flux density is 140 x 10-3 Wb/m2. The measured surface magnetic fluxes on the test specimen are shown in this figure. It is shown that the flux density in the vicinity of a free edge becomes larger than that on the flat surfaces, and the flux density



on the cutting surface tends to become larger than that on the plate face. Test conditions of edge retention behaviour tests are identified by edge geometry (GEOM 90M, 45M, 45G and 20G) and type of paint system (modified epoxy or FMP). The conditions are identified by the names described in Table 4. Triplicate specimens for each test condition are prepared. The cross-sections of the first specimen of each test condition are ground using a grindstone, and the coating thickness is measured at the edge and neighbouring flat surfaces using a light microscope (KEYENCE VH-800) of 100 magnifications. The coating thicknesses are measured on the sections at distances of 50mm, 75mm and 100mm from the plate end. For these sections, ERR is calculated by the equation below:

DFT(edge)ERR (%) 100DFT(flat surface)

= × . (1)

The second specimen of each test condition is subjected to an immersion test in NaCl solution for 270 days, and the third one to a salt spray cabinet test for 1000 hours. In these tests, the presence of corrosion spots on every painted specimen is monitored. The immersion and spray tests are performed, and the corrosion data are measured according to the procedures described in Sec. 2.1. 4.2 EXPERIMENTAL RESULTS AND DISCUSSION 4.2.1 Edge retention behaviour Figure 9 gives an example of cross-section views for each test condition. Edge retention measurements are listed in Table 5. In this table, 'upper edge' is the intersection of cutting surface and sloped surface, and 'lower edge' is that of plate surface and sloped surface. ERR in this table is an average value of the measurements evaluated on three sections. In the calculation of Eq. (1), DFT(flat surface) is DFT on the cutting surface for the upper edge, and that on the plate surface for the lower edge. For the sharp edge (GEOM 90M), DFT(flat surface) is the mean value of DFTs on the cutting and plate surfaces. These results show the followings: 1) DFT on the cutting surface tends to be larger than that on the plate surface for all test conditions. The difference in DFTs on cutting and plate surfaces for FMP paint system is much larger than that for the ordinary (without ferromagnetic particles) system. 2) DFTs at the edges and neighbouring flat surfaces for FMP system is much larger than those of the ordinary system, the edge geometry being equal. 3) For the ordinary paint system, DFTs at the edges are smaller than those on the flat surfaces. ERR is less than or nearly equal to 100% for all edge geometries. The

smaller the bevel angle, the smaller ERR, and the minimum ERR is less than 30%. 4) For FMP paint system, DFTs at the edges are larger than those on the flat surfaces. ERR is larger than 100% for all edge geometries. The smaller the bevel angle, the larger ERR. The maximum ERR exceeds 240%. In the application procedure described in Sec. 4.1, the delivery of the paint per unit area on the cutting surface tends to be larger than that on the plate surface. It is supposed that this unevenness causes the difference in DFTs on cutting and plate surfaces. In Figure 8, it is shown that the magnetic flux density on the cutting surface is larger than that on the plate face. For this reason, the difference in DFTs for FMP paint systems becomes larger than that for the ordinary system. For an ordinary paint system, a thin area of coating along the edge is created. The main reason for difficulty in achieving a proper film thickness along the edge area is known to be the surface tension of the paint [9]. It is known that magnetic flux on the edge becomes larger than that on the flat surfaces when magnetic field is applied to a steel panel. This causes the force that draws the ferromagnetic particles in FMP paint toward the edge. If this force overcomes the effect of surface tension, a thick area of coating along the edge is created. As opposed to the cases for the ordinary system, a thick area of coating along the edge is created for FMP system. This means that the effect of surface tension can be overcome by electomagnetic force when magnetic flux concentration to the edge is comparable to that observed in Figure 8. The degree of magnetic flux concentration becomes higher when the bevel angle decreases. This gives a reasonable explanation of the observed relation between ERR and bevel angle for FMP system. 4.2.2 Immersion test in NaCl solution and salt spray cabinet test Table 6 and Table 7 list the corrosion data obtained in the immersion tests and salt spray cabinet tests. These results and edge retention behaviour described in Sec. 4.2.1 can be summarized as follows: 1) Most of the specimens coated with the ordinary paint system fail by corrosion before the end of the tests, while most of the specimens coated with FMP paint system show no corrosion products during the periodic visual inspections. 2) When a specimen coated with FMP paint system displays corrosion during a test, the specimen with the same edge geometry and coated with the ordinary system also fails, and its corrosion proceeds faster. 3) Corrosion tends to be displayed more readily when DFT and ERR become smaller, the paint system being equal.



These results indicate that not only the improved edge retention behaviour but also the high protective effectiveness is developed by using FMP paint system. 4.2.3 Effectiveness of FMP paint system as an alternative to mechanical grinding of edges Table 6 and Table 7 show that the protective performance of a specimen with sharp edge coated by FMP paint system (90M-M) is higher than or equivalent to that of specimens with edge preparation coated by the ordinary paint system (45M-N, 45G-N and 20G-N). FMP paint is an attractive alternative to mechanical grinding of edge because it can eliminate the need for a great amount of labour to mechanically grind all edges to be painted in ships. However, this paint system has following problems to be solved: 1) A technique, that enables us to apply magnetic field to a member in a ship hull, has not been established. 2) Colour of the ferromagnetic pigment is limited to blackish colours. 3) Protective performance may be deteriorated when iron fillings are attracted and it enters the coating film. It is hoped that efforts will be made to solve these problems, and FMP paint system is put to practical use. 5. CONCLUSIONS Anti-corrosive performance of a protective coat applied on blowholes dressed out by puttying is investigated by adhesion measurements, immersion tests in NaCl solution, and salt spray cabinet tests. Edge retention behaviours and anti-corrosive performances of a ferromagnetic pigment (FMP) paint system applied to the steel plates with various edge geometries are investigated. The effectiveness of puttying as an alternative to repair welding for dressing blowholes, and stripe coating with FMP paint as an alternative to mechanical grinding of sharp edges is discussed. As results, the followings are found: (1) The protective performance of a top coat applied on a weld bead with blowholes which is dressed out by 100% solid epoxy / polyamide putty is better than or equivalent to that for the case where blowholes are dressed out by repair-welding. Puttying by 100% solid epoxy / polyamide putty is an effective alternative to repair welding for dressing blowholes. (2) A thick area of coating along sharp edge is created when FMP paint system is applied as a top coat. Edge retention behaviour is improved when the bevel angle

decreases. The protective performance of a specimen with sharp edge coated by FMP paint system is higher than or equivalent to that of specimens with edge preparation coated by ordinary paint system. FMP paint is an attractive alternative to mechanical grinding of edge because it can eliminate the need for a great amount of labour to grind edges. 6. ACKNOWLEDGEMENTS This research was carried out as a part of the research program of Research Committee on Revision of Steel Ship Manufacture Method of Japan Society of Naval Architects and Ocean Engineers (JASNAOE). The immersion tests and salt spray cabinet tests are carried out at Chugoku Marine Paint, Ltd. Technical Centre. The authors gratefully acknowledge to Dr. Satoru Furukawa (Chugoku Marine Paint, Ltd.) for his cooperation in the experiments. The authors would like to express to Prof. Yasumitsu Tomita (Kinki Polytechnic College-Kyoto), Mr. Kazuhiko Kumagawa (Sasebo Heavy Industries, Ltd.), Mr. Fukumi Hamaya and Mr. Tomoki Sunayama (Mitsubishi Heavy Industries, Ltd.) our gratitude for their cooperation. 7. REFERENCES 1. IMO Resolution MSC.215(82), "Performance Standard for Protective Coatings" (IMO/PSPC), 2006. 2. DePaiva, M. P. and Martins, J., "An Edge Retentive Coating Solution based on a Tolerant Solvent-free Epoxy System", Proc. SSPC Conference, November, 2000. 3. Guan, S.W., Liu D., Moreno, M. and Garneau, R., "100% Solids Rigid Polyurethane Coatings Technology for Corrosion Protection of Ballast Tanks", Proc. CORRSION2004, Paper 04029, 2004. 4. Chung, M.K., Park C.S., Lee C.H. and Baek K.K., "Approaches for Achieving Successful High Build Coatings on Sharp Edges in Ship's Ballast Tanks", Proc. CORROSION2003, Paper 03010, 2003. 5. Yun, J.T., Kwon, T.K., Kang, T.S. and Kim, K.L., "A Critical Study on Edge Retention of Protective Coatings for a Ship Hull", Proc. CORROSION2005, Paper 05016, 2005. 6. Chugoku Marine Paints, Ltd., ‘BONDET PUTTY’, Technical Sheet. 7. Chugoku Marine Paints, Ltd., ‘BISCON HB-200’, Technical Sheet. 8. Chugoku Marine Paints, Ltd., ‘NOVA-2000’, Technical Sheet. 9. Sandor, L.W., "The Effect of Edge Preparation on Coating Life", U.S. DOT document, May 1983.



Figure 1: A T-weld joint specimen.

(a) As weld

(b) After blasting

Figure 2: Blowholes on the weld beads of a T-weld joint specimen.

Figure 3: Weld bead surface after the blowhole dressing by puttying.

Figure 4: Weld bead surface after the application of the top coat.

Figure 5: Reverse side burn damage caused by gas heating. Table 1: Test conditions of blowhole dressing tests. Name Num.

of blow-holes

Tp [Hr]

paint system reverse side burn damage

DFT on weld bead [μm]

A1 4~13 8 tar epoxy NO 347~442A2 1~2 3 tar epoxy NO 201~315A3 5~14 0 tar epoxy NO 235~370A4 NO - tar epoxy NO 177~274B1 1~5 8 modified epoxy NO 214~416B2 2~4 3 modified epoxy NO 261~333B3 1~9 0 modified epoxy NO 173~317B4 NO - modified epoxy NO 224~330D1 1~11 8 tar epoxy YES 295~367D2 1~8 8 modified epoxy YES 321~370

Table 2: Blistering (ASTM D-714) / corrosion (ASTM D-610) results as a function of the exposure time in the immersion test in 3.0% NaCl solution of the weld joint specimens.

Exposure time (days) Name30 90 180 300

knife peeling test results

A1 10/10 10/10 10/10 10/10 100/100 A2 10/10 10/10 10/10 10/10 100/100 A3 10/10 10/10 10/10 10/10 100/100 A4 10/10 10/10 10/10 10/10 100/100 B1 10/10 10/10 10/10 10/10 100/100 B2 10/10 10/10 10/10 10/10 100/100 B3 10/10 10/10 10/10 10/10 100/100 B4 10/10 10/10 10/10 10/10 100/100 D1 10/10 10/10 10/10 10/10 100/100 D2 10/10 10/10 10/10 10/10 100/100

Table 3: Blistering (ASTM D-714) / corrosion (ASTM D-610) results as a function of the exposure time in the salt splay cabinet (5.0% NaCl solution) of the weld joint specimens.

Name Exposure time = 1000 Hr knife-peeling test results

A1 10/10 100/100 A2 10/10 100/100 A3 10/10a 100/100 A4 10/10 100/100 B1 10/10 100/100 B2 10/10 100/100 B3 10/10 a 100/100 B4 10/10 a 100/100 D1 10/10 100/100 D2 10/10 100/100 A1 10/10 100/100

a Rust on the specimen's end faces stain the coating film of the weld bead.



Figure 6: Schematic diagram of edge geometry to evaluate the edge retention behaviour.

Figure 7: Application protocol for top coat in edge retention behaviour tests.

Figure 8: A test specimen attached to a plate-like permanent magnet and magnet flux distribution on the specimen surface.

Table 4: Test conditions of edge retention behaviour tests.

Name Edge geometry Paint system 90M-N 90M modified epoxy 45M-N 45M modified epoxy 45G-N 45G modified epoxy 20G-N 20G modified epoxy 90M-M 90M FMP paint 45M-M 45M FMP paint 45G-M 45G FMP paint 20G-M 20G FMP paint

(a) 90M-N

) 45M-M

(g) 45G-M Figure 9: Optical microgra ion behaviour.

ating systems

(b) 45M-N

(c) 45G-N (d) 20G-N

(e) 90M-M (f

(h) 20G-M

phs showing edge retent

Table 5: Edg easurementsand FMP co .

e retention m of modified-epoxy

DFT at the

edges ERR(%) Name DFT

on the cutting surface

DFT on the plate face

wer Upper edge

Lower edge

Upper edge

Loedge

90M-N 187.9 115.1 2% 68.5 45.45M-N 150.7 120.1 %41.8 85.4 27.7% 71.145G-N 185.9 128.5 53.6 97.3 28.8% 75. %720G-N 19 26.7 103.4 13.8.1 107.9 5% 95.8%90M-M 444.8 134 703.5 243.1% 45M-M 316 218.7 338.4 278 107.1% 127.1%45G-M 336.6 135.6 339 199.1 100.7% 146.8%20G-M 304.3 169 329.2 188.9 108.2% 111.8%

Unit of DFT: μm



Table 6: io TM 610 lt fu

er m .0 aC tio the n ou s.

Exp m

Corros n (AS D- ) resu s as a nctionof imm sion ti e in 3 % N l solu n in edgeretentio behavi r test

osure ti e (days)Name 30 60 90 180 270 30

90M-N 10 8 7 7 6 1045M-N 10 10 8 8 8 1045G-N 10 7 6 5 5 1020G-N 10 10 8 8 8 1090M-M 10 10 8 8 8 1045M-M 10 10 10 10 10 1045G-M 10 10 10 10 10 1020G-M 10 10 10 8 7 10

Table 7: Corrosion (ASTM D-610) results as a function of the exposure time in the salt spray cabinet in the edge retention behaviour tests.

Exposure time hours) Name 500 1000

90M-N 10 8 45M-N 10 8 45G-N 7 6 20G-N 10 10 90M-M 10 10 45M-M 10 10 45G-M 10 10 20G-M 10 10


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