DEVELOPMENT OF NEW ANTI-CORROSION STEEL FOR COTS OF CRUDE OIL … · Crude Oil type Gas [vol%] The inside of the COT is washed and cleaned in advance in order to carry out the survey

Shipbuilding Technology ISST 2007, Osaka, 2007

© 2007: JASNAOE-RINA 11

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

Empty

Cargo loading ratio 93% 89% 92% 31% 0%

H2S

[vol.ppm]2790 1330 498 817 550

H2O [vol%] 4.9 3.9 5.3 2.5 3.2

O2 1.7 2.5 1.8 3.9 4.5

CO2 3.7 4.0 2.2 10.9 13.2

SOx

[vol.ppm]1.3 3.9 1.6 2.7 0.7

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%]

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

Amount of S : 14~67%,Ave. 54%(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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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(2.5y)SH(2.5y)→→

SH(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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SensorSensor

Distilled WaterDistilled Water

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

Drain HoleDrain Hole Drain HoleDrain Hole

Pits are observed at the path where water flows through

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

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SH -1 D H -1 D H -2 D H -3 D H -4 D H -5

Max

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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 ribNo shield for bottom Ribs : 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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1%

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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.

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Cumulative Frequency F

Return Period T

VLS-17years

VLD-12.4years

VLS-25years

VLS-12.5years

VLS-22.5years

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

Oil coating

Cleaned Steel Surface = No oil coat

Oil coating ,Sludge & Corrosion productsare cleaned and dried for inspection

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

New oil coat = Resetting 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

7years

5years

2.4year

: 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



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

Cumulative Frequency

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2001 DH2


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2000 -SH1Corrosion DurationDock Interval

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

H2 evolution ← →Oxygen consumption

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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. [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 rate controlling.

Figure 22 Summarized process and mechanism of the localized corrosion on COT.

The abovementioned corrosion mechanism is similar to 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 of this study, the corrosion test method 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. * 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.

2H+Fe2+

Na+

Cl- Cl-Cl-

Na+Na+

①Fe⇒ Fe2 ++2e-

②Fe2++2H2O ⇒ Fe(OH)2+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+

①Fe⇒ Fe2 ++2e-

②Fe2++2H2O ⇒ Fe(OH)2+2H+

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+

①Fe⇒ Fe2 ++2e-

②Fe2++2H2O ⇒ Fe(OH)2+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+

①Fe⇒ Fe2 ++2e-

②Fe2++2H2O ⇒ Fe(OH)2+2H+

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

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



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

← Conventional Steel

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.

Documents

DEVELOPMENT OF NEW ANTI-CORROSION STEEL FOR COTS OF CRUDE OIL … · Crude Oil type Gas [vol%] The inside of the COT is washed and cleaned in advance in order to carry out the survey