Activity -1 the Final

8/3/2019 Activity -1 the Final

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Amit R Patel, Manoj Kumar

A Group Activity

on

Degradation of Materials, Various Types of Degradations, Causes,

Consequences and Evaluation

(Erosion – Corrosion, Solid Particle Erosion, Corrosion)

Submitted to

Dr. Harpreet Singh

Assistant Professor (SMMEE)

Submitted by

Amit R. Patel Manoj Kumar

Research Scholar Research Scholar

SMMEE SMMEE




INDIAN INSTITUTE OF TECHNOLOGY ROPARRUPNAGAR-140 001 (PB)

November 2011




Definition:

According to A.W.Batchlor, Materials degradation can be defined as loss of

performance of an engineering system.

Loss of performance can relate to many parameters, e.g. loss in mechanical

strength of a structural component exposed to a corrosive medium. For the engine with

worn cylinders, wear can increase the clearance between piston ring and cylinder to

such an extent that there is no compression of combustion gases. In this case the

engine can be considered to have failed, as it will no longer be able to pull the car or

truck up the hill.

A mechanical degradation proceeds at a rate that varies with local conditions and

failure occurs if the performance declines to below the critical level. Loss of efficiency

occurs if performance declines but remains above the critical level during the servicelifetime.

Fig. Graphical definition of material degradation

Various types of degradation:-

There are three basic categories of materials degradation

Physical degradation

Chemical degradation

Biological degradation

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Physical origin refers to the effect of force, heat and radiation.

Chemical origin relates to destructive reactions between the material and chemicals that

contact it.

Biological origin includes all interactions between life forms and engineering materials.

Fig. Classification of various types of material degradation

Causes of Physical Material degradation

Heat thermal damage:All materials are vulnerable to thermal degradation even where other factors

such as corrosion or wear are absent. Increases in temperature generally lead to a

decline in hardness of materials, which is usually counterbalanced by an increase in

ductility. There are exceptions to the trend of declining hardness with temperature

where a material may show a limited rise in hardness over a narrow temperature range

but the effect is usually minor. Electrical conductivity declines for metals but is increased

for nonmetals, e.g. ceramics at elevated temperatures. Most of the changes to physical

properties are reversible, e.g. hardness except where microstructural change has

occurred.

Materials degradation at very low temperatures

A reduction in temperature inhibits the chemical reactions that cause many

materials degradation problems. Furthermore, when the temperature falls below 0°C,

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freezing of water removes sources of liquid water for electrochemical corrosion. Severe

forms of materials degradation are less likely at low temperatures than at high

temperatures. However, very low ambient and process temperatures still pose some

problems of materials degradation. Steel and many other metals become brittle at low

temperatures thus necessitating the use of special alloys that are usually based onnickel. Nickel has the advantage of maintaining toughness at low temperatures.

Nonmetals are also prone to low temperature brittleness.

Electrochemical corrosion

A basic reason why corrosion causes so much damage to machinery and

structures is that components of differing materials and shape are placed in close

contact with each other. Another reason is that pure metals are rarely used to

manufacture components; instead alloys and composite materials are usually selected.

These characteristics of mechanical construction and manufacture ensure that a

machine or structure contains many electrochemical cells even though in most cases no

electrochemical cell was deliberately designed into the system. A simple example of the

ease with which electrochemical cells are generated is to consider a steel shaft

contacting a bronze bearing when immersed in seawater. Where no electrochemical cell

is present, a metal object may last almost indefinitely. An iron column sited in India is

observed to have not significantly rusted over many hundreds of years. It is believed

that the purity of the iron used contributes to its corrosion resistance by ensuring that

electrochemical cells are not formed between the iron and any other metals or carbon

that are commonly present in iron. Alloying introduces electrochemical cells on a

microscopic scale between grains of different metallic phases.

Another factor that heightens corrosion damage is the increasing hostility of the

environments to which manufactured items are subjected. There is a continuing

demand for ever higher temperature when exposed to increasing concentrations of

acids and alkalis. Even when there is no deliberate environmental stress, environmentalpollution brings increasing amounts of sulfur compounds and other corrosive agents into

contact with manufactured materials. These pollutants combine with atmospheric

oxygen and water to generate more potent forms of electrochemical corrosion, which

can rapidly destroy equipment and structures.

Erosion-Corrosion

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According to Hodgkiess et al [10], erosion-corrosion is a form of material

degradation involving electrochemical corrosion processes and mechanical wear.

The total material loss is often significantly in excess of the sum of the

separate pure corrosion and wear processes, thus signifying important

interactions, or synergism, between the two components. High-temperaturecorrosion accompanied by erosion (E-C) caused by the impact of fly ashes and

unburnt carbon particles are the main problems to solve in steam generating

plants, especially in those regions where the component surface temperature is

above 873 K [11]. The free oxygen content in power plant boiler atmospheres is

sufficient to account for combined erosion–corrosion process, consisting of an

oxidizing gas at elevated temperature carrying erosive fly-ashes which impact

against metallic surfaces. These erodent particles can deposit or embed

themselves on the test surface in quite significant quantities, and chemical

reactions of erodent particles and oxide scales can develop on the eroding surface

[11]. Boiler steels are unable to meet these requirements for both the high

temperature strength and the high temperature corrosion resistance simultaneously

over longer periods of their usage, so protective coatings are used to counter the

latter. Coating provides a way of extending the limits of use of materials at

the upper end of their performance capabilities, by allowing the mechanical

properties of substrate materials to be maintained while protecting them against wear

or corrosion [12].

For hot corrosion and erosion-corrosion resistance at elevated temperatures,

thermal spray coatings must be hard, dense, sufficiently thick, and tightly adherent

to maintain the shielding effect for a long time. They must be resistant to elevated

temperatures, thermal cycles and chemical environment and they must also have

low internal stresses, low oxide content, and thermal expansion coefficients similar

to that of the base material. Coatings should also have small splat size and nocracks [13]. Furthermore, the coatings should have good thermal conductivity. It is

strongly believed that the nano-particle coatings could be candidate materials which

can provide most of these properties.

Among the various surface coating techniques, thermal spray processes

such as Detonation Gun Spraying (DS), Plasma Spraying (PS) and High Velocity

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Oxygen Fuel (HVOF) spraying have been under active considerations in the recent

times, whereas Cold Spray Technique is barely out of its infancy. According to

Yamada et al [14], thermal spray has emerged as an important tool of increasingly

sophisticated surface engineering technology. It is one of the many methods of

applying overlay coatings for applications ranging from protection of materials inharsh environments, to dimensional restoration of worn machine elements [14].

Cold spray has been used to produce protective coatings and

performance enhancing layers, ultrathick coatings, freeforms and near net shapes.

Cold spray is a relatively young process and still considerable R&D efforts are

needed to understand and control the process, as well as develop engineered

coatings with desired properties for specific applications. The last few years have

seen exponential growth of cold spray R&D around the globe. Considerable R&D

efforts are being undertaken at various laboratories, academic institutions, and

industries [2]. Cold spray is a solid state process and hence produces coatings with

many advantageous characteristics. Since high temperature is not involved, it is

ideally suitable for depositing temperature sensitive materials such as

nanophase and amorphous materials, oxygen sensitive materials like aluminium,

copper and titanium and phase-sensitive materials such as carbide composites.

Review of status of Research and Development in the subject

International/National status

Erosive, high temperature wear of heat exchanger tubes and other

structural materials in coal-fired boilers are recognized as being the main cause of

downtime at power- generating plants, which could account for 50-75% of their total

arrest time. Maintenance costs for replacing broken tubes in the same installations

are also very high, and can be estimated at up to 54% of the total production costs.

High temperature oxidation and erosion by the impact of fly ashes and unburned

carbon particles are the main problems to be solved in these applications. Therefore,

the development of wear and high temperature oxidation protection systems in

industrial boilers is a very important topic from both engineering and an economic

perspective [3].

Coatings provide a way of extending the limits of use of materials at the upper

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end of their performance capabilities, by allowing the mechanical properties of

the substrate materials to be maintained while protecting them against wear or

corrosion [12]. The desire for higher operating temperature, improved performance,

extended component lives, and cleaner and more fuel-efficient power

plant/processes places severe demands on the structural materials used toconstruct such a high-temperature plant. As a result, many components operating

at high temperature within such plants are coated or surface treated[4].

Porcayo-Calderon et al [5] have reported the use of protective coatings for the

superheater/re-heater components of boiler where the material severely suffers on

fireside corrosion. According to Taylor and Evans [6], a few earlier attempts have

been made on the thermal sprayed protective coatings for fossil power plants

though the thermal spray process is extensively used for gas turbine applications.Sundararajan et al [7] also advocated the need for applying thermal spray coatings

on the boiler components. From a production point of view, three methods are in

current use for depositing coatings, these being chemical vapour deposition (CVD)

from a pack, physical vapour deposition (PVD) and thermal spraying (metal

spraying). Ilavsky et al [8] have also reported that the thermally sprayed deposits

have often superior properties with potentially lower application cost or less

environmental issues as and when compared to other industrially used coatings

such as CVD, PVD, hard chromium plating.

The high resistance of high-chromium, nickel-chromium alloys to high-

temperature oxidation and corrosion makes them widely used as welded and

thermally sprayed coatings in fossil fuel-fired boilers, waste incineration boilers, and

electric furnaces. Modern thermal spray processes such as high velocity oxyfuel

(HVOF) and plasma spraying are often applied to deposit high-chromium, nickel

coatings onto the outer surface of various parts of the boilers, e.g. tubes to

prevent the penetration of hot gases, molten ashes, and liquids to the less noblecarbon steel boiler tube [9].

Erosion wear and mechanical properties of nickel- and iron-based as

well as chromium-nickel plasma sprayed coatings on carbon steel have been

studied by Hidalgo et al [10] in the simulated industrial service conditions in boilers.

These types of coatings are used as heat transfer and structural elements in boilers.

8




They have performed experiments in laboratory combustion unit which was

simulated to boiler service conditions at 400, 600 and 800°C. Coatings were

reported to have a moderate to low oxidation indexes.

Yamada et al [1] carried out studies to evaluate the high-temperature

corrosion resistance of Ni-20Cr, Ni-50Cr and Cr coated boiler tubes in actual refuse

incineration plant as well as in laboratory tests. It was observed that detonation

sprayed Ni-50Cr coating exhibited the highest corrosion resistance in laboratory test

at 873K among the detonation gun sprayed, plasma sprayed and HVOF sprayed

coatings. The detonation sprayed Ni-50Cr coated tubes performed very well for

seven years of testing in the actual plant without any problems and were expected to

have a longer life.

Recent reports emphasised that the understanding of the degradation andfailure mechanisms of high-temperature coatings in the field need to be improved,

particularly with respect to the effects of engine operation and environment on the

coating performance (e.g., thermal cycling) [11]. In general, the reaction

behaviour of protective coatings in environments of their use and their interactions

with the substrate during high-temperature performance is not well understood [9].

Ak et al [13] demonstrated the successful application of NiCr coatings on

stainless steel substrates using a high velocity oxy-fuel technique for corrosion

a p p l i c a t i o n s . Preliminary results of fabrication and microstructural

characterization of NiCr coatings were given. These coatings were characterized by

means of optical microscope, image analyzer, scanning electron microscope and X-

ray diffraction (XRD). NiCr powders with a composition of 80% Ni and 20% Cr

were deposited by HVOF spraying to give coatings that were approximately 90-

100µm thick. It has been found that the samples produced by HVOF spray process

possess porosities. XRD results revealed that the coating had Ni, CrNi2, Cr-Ni,

Fe0.93Ni0.056, Ni-Cr-Fe and Fe-Cr phases after the coating process. The

microhardness properties of the coatings strongly depended on porosity, oxide,

unmelted and semimelted particles, and inclusions and the porosity, oxide and

inclusion decreased hardness values[13].

Wang and Shui [14] performed experiment testing boiler tubes using HVOF

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method and other spraying processes, and observed that all the HVOF carbide

cermet coatings showed much higher erosion resistance than 1018 steel and arc-

sprayed iron-base coatings, when eroded by bed ashes at shallow and steep impact

angles [14].

According to Karthikeyan [15] cold spray finds applications in corrosionprotection where absence of process-induced oxidation may offer improved

performance. In electrical and thermal field where the absence of process-

induced oxidation may offer improved conductivity. Aluminum and aluminum

alloy coatings are being investigated for repair/refurbishment of space shuttle solid

rocket boosters and others (aerospace), repair and retrieval of parts and plate

stocks used in aircraft structures (aircraft industry), repair/refurbishment of

casings (gas turbine), corrosion protection coatings (petrochemicals) etc. At present,

high performance layers such as high temperature oxidation resistant MCrAlY

coatings, high conductivity copper/silver coatings, phase pure biocoatings, etc., are

produced in Vacuum Plasma Spray systems or Physical Vapor Deposition systems.

These systems are extremely expensive to both install and operate. Moreover, cold

spray process may lend itself to collect the overspray and reprocess these expensive

raw materials. Hence, well founded cold spray technology will be able to compete

for a good market share of VPS/PVD coatings in turbine, power,

electronic/electrical, biotechnology and other industries. Similarly, cold spray can

produce MMC coatings and freeforms with any dissimilar materials, even with graded

properties and hence can achieve a share of the MMC market as well .

Wolfe et al [16] explored the potential of cold spray in applying Cr 3C2-25

wt.%NiCr and Cr 3C2-25wt.%Ni coatings on 4140 alloy for wear-resistant

applications. They observed that cold spray process optimization of the Cr 3C2-

based coatings resulted in increased hardness andimproved wear characteristics

with lower friction coefficients. The improvement in hardness was directly associatedwith higher particle velocities and increased densities of the Cr 3C2-based

coatings deposited on 4140 alloy at ambient temperature. Selective coatings

were evaluated using X-ray diffraction for phase analysis, optical microscopy (OM),

and scanning electron microscopy (SEM) for microstructural evaluation, and ball-

on-disk tribology experiments for friction coefficient and wear determination.

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The results obtained strongly suggested that cold spray is a versatile coating

technique capable of tailoring the hardness of Cr 3C2-based wear-resistant

coatings on temperature sensitive substrates [16].

Kim et al [17] deposited WC-12~17%Co powders with nano- and

microstructures by cold spray process using nitrogen and helium gases. Theresults showed that there is no detrimental phase transformation and/or

decarburization of WC by this method. It was also observed that nano-sized WC in

the feedstock powder is maintained in the cold sprayed coatings. The nano-sized

WC is advantageous over micro-sized WC for cold spray deposition because

higher particle velocity can be obtained with the same gas velocity. It was also

demonstrated that it is possible to fabricate the nano-structured WC-Co coating

with low porosity and high hardness (2050 HV) by cold spray deposition with

reasonable powder preheating [30].

Lima et al [18] used the cold spray process to prepare nano structured

WC-Co coatings. A 10 nm thick coating was achieved. It was concluded that the

WC-Co cold sprayed coating has a high density and microhardness when

compared to those of nanostructured feedstock. There was no significant

difference between the average grain size of the nanostructured feedstock and

coating. They concluded that it is possible to produce pure and well bonded

nanostructured WC-Co coatings via cold spray processing.

Agarwal etal [19] explored the existing knowledge of erosion, high

temperature erosion, and high-temperature Coal Gasification Atmosphere (CGA)

erosion-corrosion (EC) phenomena. Experimental results and interpretive analysis of

impingement angle effects in alloys 310 and 6B exposed to simulated CGA EC

atmosphere at 1500 °F are presented and discussed. These results clearly

demonstrate the utility of the interpretive analysis in developing better cause and

effect and mechanistic understanding of the CGA EC phenomena.Uusitalo etal. [20] carried out series of hot erosion and erosion–corrosion (E–

C) tests on thermal sprayed coatings, diffusion coatings and boiler steels using a

burner-rig type elevated temperature E–C tester in order to evaluate the possibility to

utilise thermal sprayed coatings in shielding of boiler components. Test conditions

simulated the E–C conditions in the super heater section of a circulating fluidised bed

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combustor (CFBC). Carbide containing HVOF coatings performed well in erosion

tests, as expected. Also diffusion coatings and nickel-based, high-chromium HVOF

coatings performed well. In E–C tests in presence of chlorine, nickel-based HVOF

coatings performed the best, whereas carbide containing HVOF coatings and diffusion

coatings wore away.Flores etal. [21] explored the materials and machinery components handling

corrosive slurries are exposed to erosion–corrosion (EC) processes that significantly

reduce their service lifetime. Difficulties predicting material loss in industrial

machinery, financial and security impacts due to early failure of worn components and

the necessity of managing highly aggressive slurries (e.g. oil sands), have motivated

the application of surface engineering solutions to reduce the negative effects of

erosion–corrosion processes. The erosion–corrosion resistance of nickel base and

iron base metal matrix composites (C Ni and C Fe MMCs, respectively) coatings was

studied in this paper. The behavior of the MMCs compared with their non-reinforced

matrices (NRMs); tungsten carbides (WC) particles were used to reinforce the NRMs.

The microstructures of the NRMs and MMCs were analyzed by microscopy

techniques and the effects of adding a reinforcing phase to the matrices are reported

and linked to their EC resistance. The EC resistance was assessed at different sand

contents (10 and 50 g/l) and temperatures (20 and 65 ◦C) using a submerged

impinging jet (SIJ) apparatus; the MMCs were also studied under EC at 5, 10 and 14

m/s to analyze the structural integrity of the reinforcing phase. The results shown that,

the C Ni was the more EC resistant coating under most of the experimental conditions,

at 5 m/s the EC degradation mechanism was dominated by matrix degradation with

the WC showing little damage and the CFe was negatively affected by corrosion,

especially at 65 ◦C. Interestingly, the MMCs were more susceptible to changes in

temperature whereas, for the NRMs EC resistance was more affected by the sand

content of the slurry.M.Kaur etal [22] explored the Detonation-gun (D-gun) spray technology and

found it a novel coating deposition process which is capable of achieving very high

gas and particle velocities approaching 4–5 times the speed of sound. This process

provides the possibility of producing high hardness coatings with strong adherence. In

the present study, this technique has been used to deposit Cr3C2–NiCr coating on

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T22 boiler steel. Investigations on the behaviour of this coating subjected to high-

temperature oxidation in air and oxidation–erosion in actual boiler environment at 700

± 10 °C under cyclic conditions have been carried out. The weight change technique

was used to establish the kinetics of oxidation. X-ray diffraction (XRD), field emission-

scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS) andEDS elemental mapping techniques were used to analyse the oxidation/oxidation–

erosion products. The uncoated boiler steel suffered from a catastrophic degradation

in the form of intense spalling of the scale in both the environments. The Cr3C2–NiCr

coating showed good adherence to the boiler steel during the exposures with no

tendency for spallation of its oxide scale.

V. Chawla etal. [23] explored Hot corrosion and erosion problems in coal

based power generation plants in India. The coal used in Indian power stations has

large amounts of ash (about 50%) which contain abrasive mineral species such as

hard quartz (up to 15%) which increase the erosion propensity of coal. Hot corrosion

and erosion in boilers and related components are responsible for huge losses, both

direct and indirect, in power generation. An understanding of these problems and thus

to develop suitable protective system is essential for maximizing the utilization of such

components. These problems can be prevented by either changing the material or

altering the environment or by separating the component surface from the

environment. Corrosion prevention by the use of coatings for separating material from

the environment is gaining importance in surface engineering.

C.Xu etal. [24] developed a halide-activated pack-cementation method but at

a temperature (600 °C) noticeably lower than normal, an η-Fe2Al5 coating and two δ-

Ni2Al3 coatings with and without dispersions of CeO2 nanoparticles were developed

respectively on a low-carbon steel and the steel pretreated with an electrodeposited

film of Ni or Ni–CeO2. The erosion–corrosion (E–C) performance of the three

aluminide coatings during 100 h exposure at ~600 °C in a coal-firing laboratory-scalefluidized-bed combustor (FBC) was investigated, by mounting the aluminized samples

onto a rig which maintained rotation for accelerating the relative impacting speed of

flying solid particles (mainly SiO2 bed materials). The η-Fe2Al5 and the CeO2-free δ-

Ni2Al3 coatings experienced an unacceptable recession rate. Compared to the two

CeO2-free aluminide coatings, the CeO2- dispersed δ-Ni2Al3 coating offered

13




profoundly improved E–C resistance, because the latter coating was not only

strengthened by the CeO2 dispersion and grain refinement, it also could grow a more

adherent alumina scale.

Shimizu etal [25] investigated the high temperature erosion characteristics of

two types of surface-treated SUS410 steels; overlay welding and forging of the basemetal. Two-layer overlay welding of 6 mm and forging with a 10% reduction, were

used on a base metal of SUS410, to prepare specimens. High temperature solid

particle erosion tests using a test temperature of 1173 K were performed using 1 mm

alumina particles, with impact angles between 30◦ and 90◦ and a particle velocity of

100 m/s. Erosion rates, especially at shallow angles of 30◦, were dramatically different

for all specimens. Compared with the base metal of SUS410, the erosion rate was

reduced by 50% for overlay welded material, and 30% for forged material. High

temperature hardness measurement and the observation of the eroded surface by

scan electron microscopy were undertaken to analyse the erosion behaviour. An

increase in the erosion rate of the specimen was related to a decrease in the high

temperature hardness. Although the hardness was reduced to approximately 70% at

1173 K for all specimens, this suggested that the wear resistance of the overlay

welding material was improved by restraining the plastic flow because it was harder at

high temperatures. The forged material was suggesting that the plastic flow of eroded

surface was restrained by the refinement of the microstructure and the residual stress

near the surface, which reduced erosion rate regardless of this lower hardness.

Nickel based alloys represent a significant part of overall thermal spray

business. These materials are widely used as bond coats and as top coats in

number of applications requiring combination of properties, such as good wear

resistance and corrosion resistance at the same time. There is an increasing

interest in the deposition of Ni based metallic alloys such as Ni-20%Cr, Inconel 625

for protection against corrosion. Ni based coatings are used in applications wherewear resistance combined with oxidation or hot corrosion resistance is required

[26-30].

Nickel–chromium alloys have been used as coatings to deal with

oxidizing environments at high temperature. When nickel is alloyed with

chromium, this element oxidizes to Cr 2O3, which could make it suitable for use, up

14




to about 1200°C. However, in practice its use is limited to temperatures below

800°C. The thermal sprayed 50/ 50 nickel– chromium alloy is usually recommended

as an erosion–corrosion protection for boiler tubes in power generation applications

[31].

References:-1. Yamada, K., Tomono, Y., Morimoto, J., Sasaki, Y., Ohmori, A., (2002), Vacuum 65 (3-4) 533-540.2. J. Karthikeyan, Cold Spray Technology: International Status and USA Efforts, ASB Industries, Inc, 2004.3. Hidalgo V. H. et al., Wear 247 (2001) 214-222.4. Nicholls, J.R., JOM Jan. (2000) 28-35.5. Porcayo-Calderon, J., Gonzalez-Rodriguez, J.G., Martinez, L., J. Mater. Eng. Perform. 7 (1998) 79-87.6. Taylor, M.P., Evans, H. E., Mater. Sci. Forum 369-372 (2001) 711-717.7. Sundararajan, T., Kuroda, S., Itagaki, T., Abe F., ISIJ Int. 43 (1) (2003) 95-103.8. Illavsky, J., Pisacka, J., Chraska, P., Margandant, N., Siegmann, S., Wagner, W., Fiala, P., Barbezat,

G., Proc. 1st

Inter. Thermal Spray Conf., Montreal, Quebec, Canada, May 8-11 (2000) 449-454.9. Tuominen, J., Vuoristo, P., Mantyla, T., Ahmaniemi, S., Vihinen, J., Andersson, P.H., J. Therm. Spray

Technol. 11 (2) (2002) 233-243.10. Hidalgo, V. H., Varela, J. B., Calle, J. M. de la, Menendez, A. C., Surf. Eng. 16 (2) (2000) 137-142.11. National Materials Advisory Board, “Coatings for High-Temperature Structural Materials: Trends and

Opportunities,” National Academy Press Washington D.C.,h ttp ://www.n ap .e du /o pen b o o k /0309 0 53 8 11 /h tm l, (1996) 1-85.

12. Chatterjee, U. K., Bose, S. K., Roy, S. K., ‘Environmental Degradation of Metals,’ (2001), Pub. MarcelDekker, 270 Madison Avenue, New York.

13. Ak, N. F., Tekmen, C., Ozdemir, I., Soykan, H. S., Celik, E., Surface and Coatings Technology 173-174(2003) 1070-1073.

14. Wang, B. Q., Shui, Z. R., Journal of Materials Processing Technology 143-144 (2003) 87-92.15. Karthikeyan, J., Cold Spray Technology: International Status and USA Efforts, ASB Industries, Inc, USA

(2004).16. W o l fe , D. E., E den , T. J ., Ja roh , A. P., Journal of Thermal Spray Technology 15 (3) (2006) 400-412.17. Kim, H.J., Lee, C.H., Hwang, S.Y., Surface and Coatings Technology 191 (2-3) (2005) 335-34.18. Lima, R. S., Karthikeyan, J., Kay, C. M., Lindemann, J., Berndt, C. C., Thin Solid Films 416 (2002) 129-135.19. Agarwal S.C., Erosion-Corrosion of Materials in High- Temperature Environments: Impingement AngleEffects in Alloys 310 and 6B under Simulated Coal Gasification Atmosphere.20. Uusitalo M.A., Vuoristo P.M.J., Mäntylä T.A., Wear 252 (2002) 586–594.21. Juan. F. Flores∗, A. Neville, N. Kapur, A. Gnanavelu, Wear 271 (2011) 1331– 1340.22. Kaur Manpreet, Singh Harpreet, Prakash Satya, Surface & Coatings Technology 206 (2011) 530–541.23. Chawla Vikas, Chawla Amita, Puri D., Prakash S., Gurbuxani Prema G and Sidhu Buta Singh, Journal of Minerals & Materials Characterization & Engineering 10(4) (2011) 367-385.24. Xu C., Peng X., Zheng L., Wang F., Surface & Coatings Technology 205 (2011) 4540–4546.25. Shimizu K., Xinba Y., Ishida M., Kato T., Wear 271 (2011) 1349– 1356.26. Edris, H., McCartney, D.G., Sturgeon, A.J., J. Mater. Sci. 32 (4) (1997) 863-872.27. Knight, R., Smith, R.W., in: Berndt, C.C. (Ed.), Proc. 13

thInternational Thermal Spray Conference,

Orlando, Fl, USA (1992) 159.28. Dorfman, M.R., DeBarro, J.A., in: Ohmori, A. (Ed.), Proc. 14

thInternational Thermal Spray Conference,

Kobe, Japan (1995) 562.29. Harvey, M.D.F., Sturgeon, A. J., Blunt, F.L., Dunkerton, S.B., in: Ohmori, A. (Ed.), Proc. 14

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International Thermal Spray Conference, Kobe, Japan (1995) 531.

30. Nordmand, B., Liao, H., Landemarre, O., Coddet, C., Pagetti, J., in: Coddet, C. (Ed.), Proc. 15

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International Thermal Spray Conference, Nice, France (1998) 69.31. Sidhu, H. S., Sidhu, B. S., Prakash, S., Surface & Coatings Technology 200 (2006) 5386–5394.

Solid Particle Erosion

Solid particle erosion (or erosive wear) implies the removal of material from

component surfaces due to successive impact of hard particles travelling at substantial

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velocities. It is to be noted that solid particle erosion is different from the other forms of

erosion like liquid impact erosion, slurry erosion, cavitation erosion, etc. Material

removal due to solid particle erosion is a consequence of a series of essentially

independent but similar impact events. Thus, the contact between the hard particles and

the component surface is of a very short duration. From this point of view erosion iscompletely different from the other closely related processes like sliding wear, abrasion,

grinding and machining wherein the contact between the tool/abrasive and the

target/work-piece is continuous.

Degradation of materials due to solid particle erosion, either at room temperature

or elevated temperature, is encountered in a large variety of engineering industries as

illustrated in Table 11-12. At the same time, the erosion process has been used to

advantage in a number of situations like sand blasting of castings, shot peening of

rotating components, cutting of hard and brittle materials by abrasive jets and rock

drilling13-15. Thus, the technological and commercial significance of erosion cannot be

overlooked.

Over the years the state of the information on solid particle erosion has been

reviewed by Engel16, Preece and MacMillan17, Hutchings18, Finnie and McFadden19,

Tilly20, Ruff and Wiederhorn21, Shewmon and Sundararajan22, Sundararajan23, Kosel24

and Levy25.

Room temperature erosion

(a) Impact velocity of the erodent particle has the most dramatic effect on the erosion

rate. The erosion rate of a material (E) is usually defined as the ratio of weight loss

suffered by the eroding material to the weight of the erodent particles causing the loss.

The velocity dependence of erosion rate is characterized by the velocity exponent p

given by:

E = E0Vp

where E0 is a constant and V is impact velocity.

16




In the case of metallic materials, a detailed analysis of a large amount of experimental

data by Hutchings26 has shown that under oblique impact conditions, the mean velocity

exponent is 2.4. A similar analysis of the experimental data by Sundararajan and

Shewmon27 indicated a mean value of 2.55 for p for erosion under normal impact. The

above values of p are in contrast with the values of around 3 reported for ceramics and

values in excess of 5 observed in polymer matrix composites28.

The velocity exponent is also influenced by other parameters such as impactangle, particle size, etc. Goodwin et al.29 noted a decrease in the velocity exponent with

decreasing particle size. The velocity exponent has also been observed to be a function

of erodent particle shape30,31.

(b) Impact angle is defined as the angle between the target material and the trajectory

of the erodents. Dependence of erosion rate on the impact angle is largely determined

by the nature of target material. As shown in Fig 1a, ductile materials (like metals and

alloys) exhibit a maximum in the erosion rate at intermediate impact angles (e.g. 15°,

30°). In contrast, the maximum erosion rate of a brittle material (like glass) is usually

obtained at normal impact angle i.e. at 90° (see Fig 1a).

Among the erodent-related variables, erodent particle shape has a dramatic

influence on the erosion rate-impact angle behaviour. Cousen and Hutchings31 and

17




Reddy and Sundararajan32 noted a maximum in the erosion rate at normal impact angle,

even with ductile materials like mild steel and copper, when a spherical erodent was

used. At present there exists insufficient data regarding the effect of impact velocity,

size, friability and hardness of erodent particle on the variation of erosion rate with

impact angle.

(c) Particle size is an important variable which influences the erosion behaviour.

According to Goodwin et al29, as illustrated in Fig 1b, the erosion rate increases with an

increase in particle size up to a limiting size (50 to 100 µm) beyond which erosion rate

becomes independent of particle size. Similar observations have been made by

Montgomery and Clark33, Wood and Espenschade34, Sheldon and Finnie35, Zhou and

Bahadur 36, Yerramarredy and Bahadur 37 and Levy38. Bahadur and Badruddin39 also

observed that in the case of a 18Ni (250) maraging steel eroded with SiC, Al 2O3 and

SiO2 particles, the erosion rate of the maraging steel increased with increasing particle

size of SiC and Al2O3 while the opposite was true when SiO2 particles were used as the

erodent. A number of theories have been put forth to explain the ‘size effect’ as

observed above34-39, but no consensus has yet emerged.

18




Fig. 1 (a) Schematic diagram showing the influence of impact angle on the erosion rate

in the case of ductile and brittle material20

(b) variation of erosion rate of steel withparticle size at normal impact at different impact velocities29 (c) influence of impact

angle on erosion rate in the case of spherical and angular particles; (d) influence of flux

rate of erodent particles on the erosion rate of a 1018 stee1 48

(d) The influence of particle shape on the erosion rate has been studied by several

investigators31,32,39-41. Brown et al.40, Cousen and Hutchings31, Levy and Chik41 and

Liebhard and Levy42 observed significantly higher erosion rates in many metallic

materials when eroded with angular particles rather than spherical particles. According

to Kleis43

, the impact angle corresponding to maximum erosion rate shifted to an impactangle of 90° from 30° when the glass beads replaced crushed glass as the erodent.

Reddy and Sundararajan32 observed a maximum in the erosion rate in Cu and Cu

based alloys at normal impact angles when a non-friable, spherical steel shot was used

as the erodent. Their observation is presented in Fig 1c. In contrast, when an angular

19




SiC is used as the erodent, the same Cu and Cu alloys exhibited a ductile response

with regard to erosion behavior (Fig 1C44). Thus, as the angularity of the erodent particle

increases, the erosion rate increases and in addition, the erosion rate-impact angle

behavior shifts more towards a ductile response.

e) The influence of the hardness of the erodent particles on the erosion rate of steel has

been studied by Wellinger and Uets and Levy25. They noted that as long as the

hardness of the erodent particles was at least twice that of the target material, the

erosion rate was independent of the particle hardness. The erosion rate decreased

remarkably when the hardness of the erodent particles became comparable to that of

the target material25,44. It was also observed that the use of friable erodent particles can

result in a higher erosion rate45.

(f) Usually, the flux rate of erodent particles does not have a significant influence on the

erosion rate of metallic materials. However, at very high flux rates, due to interference

with other particles and also with the rebounding particles, the erosion rate decreases.

Such behaviour has been reported by Mills and Mason46, Montgomery and Clarke33, and

Young and Ruff 47. Anand et al48 have modelled the above interference effect due to

particle rebounding and have come to the conclusion that the erosion rate should

decrease exponentially with an increase in flux rate (Fig 1d).

(g) The effect of the various strengthening mechanisms on the erosion rate of single

phase metals and alloys has been summarized in Table 243,49-57. It is clear from Table 2

that none of the strengthening mechanisms like cold work, grain size hardening and

solid solution strengthening available for single phase materials are effective in

improving the erosion resistance of the eroding material. Figure 2a illustrates the

influence of solid solution strengthening on the erosion rate. In the case of Cu, addition

of solutes like Zn and Al increases the hardness/strength and the erosion rate44. In the

case of Ni, addition of 20 wt% of Cr increases the strength/hardness but decreases the

erosion rate56.

20




The results of the numerous investigations related to the erosion rate and

strengthening mechanism for multiphase alloys are compiled in Table 337,56,58-74 Even in

multiphase alloys, though the various strengthening mechanisms result in a substantial

improvement in material strength, the erosion rates change only marginally.

The erosion behaviour of dispersion strengthened alloys is presented in Fig.

2b56.70-72 it is clear that the erosion rates of the dispersion strengthened alloys are

generally higher compared to the same alloy without the dispersion. The erosion

behaviour of alloys based on intermetallics, presented in Fig. 2c, indicates that the

erosion rates of various intermetallics are comparable in spite of their varying crystal

structure and melting point. However, erosion rates of inter metallics appear to be lower

than that of the base material (see Fig. 2c for comparison of the erosion rates of Ti 3Al

based alloys with Ti).

A large number of investigators have characterized the erosion behaviour of a

variety of quenched and tempered steels 51, 58-60. One such work is that due to McCabe

21




et al59. These authors studied the erosion behaviour of a plain carbon steel having two

different carbon contents and a variety of microstructures. Among the various

microstructures, spherodized steel exhibited the maximum erosion resistance while the

martensite microstructure showed minimum erosion resistance. Pearlite and tempered

martensite structure had intermediate erosion rates. The steel having higher carboncontent had a lower erosion resistance. Both steels, irrespective of their microstructure,

exhibited a ductile erosion behaviour. The only exception was the steels with martensite

microstructure since they showed brittle erosion response especially in the higher

impact velocity erosion tests. Balan et al” characterized the erosion behaviour of grey,

malleable and nodular cast irons having a variety of microstructures. Their observations

clearly indicated that the nodular cast iron exhibited the lowest erosion rates while grey

cast iron had the highest erosion rates. The malleable cast iron exhibited intermediate

erosion rates. In terms of the matrix microstructure, a spherodized matrix generally

exhibited a lower erosion rate as compared to cast irons having pearlitic or tempered

martensitic matrix microstructures.

22




Fig. 2 (a) Influence of solid solution strengthening on the erosion rate of Ni-base and

Cu-base alloys (b) bar diagram illustrating the effect of dispersion strengthening on the

erosion rates of various Ni-base, iron-base and Al-base alloys56,68,70,71

23




Fig. 2 (c) Bar diagrams illustrating the erosion rates of Ti base, iron-base and Ni-base

inter metallics61, 72-74

Elevated temperature erosion

(a) The variation of erosion rate with temperature for a number of metals and alloys is

presented in Figs 3a and 3b 75,76. The erosion data presented in these figures pertain

to high impact velocities and mostly with angular particles. The observed temperature

dependence of erosion rate can be conveniently classified under three groups. In the

first group, the erosion rate initially decreases with increasing temperature, reaches a

minimum and then starts increasing with increasing temperature. Materials such as 5Cr-

24




OSMo, 17-4PH, 41OSS, Alloy 800, Ti-6Al-4V and tungsten, etc., belong to this group.

The second group comprises metals like Ta, lead (oblique impact) and alloys like

310SS (oblique impact) 1018 steel, 1100 aluminium (normal impact) which exhibit a

temperature independent erosion rate up to a critical temperature followed by an

increasing erosion rate with increasing temperature. Finally, the group III materials show

an ever increasing erosion rate with increasing temperature. Into 600, carbon steel,12Cr-lMo-V steel and 2.25Cr-1Mo steel, lead and 2024 Al are some typical examples in

this group.

(b) At present there exists insufficient data relating to erosion rate and impact angle at

different temperatures. However, most of the metallic exhibit a ductile behaviour, i.e. a

maximum erosion rate at oblique impact angles (10◦ to 30◦). The universality of such an

observation is shown in Figs 3c and 3d, wherein the data from a large number of

investigations have been presented77-83. Levy84 obtained a higher erosion rate at normal

impact than at oblique impact for 9Cr-1Mo steel at 850°C using rounded Al2O3 (130 µm)

erodent observations are presented in Fig 3e. However, at a low impact velocity of 20m/s, a maximum in the erosion rate occurred at oblique impact angles. Observations

due to Chang et al85 (Fig 3f) indicated that the peak erosion rate of Co at a test

temperature of 780°C occurred at an impact angle of 60” when impacted with 20 km

angular alumina particles at the impact velocities of 70 and 140 m/s. However, when the

erosion test was carried at 600°C (V=140m/s) the erosion rate peaked at an impact

angle of 30°C (Fig 3f). Thus, there apparently exists conflicting observations regarding

the erosion rate-impact angle behavior.

25




Table 3 Effect of various strengthening mechanisms on room temperature

erosion of multiple phase material

26




*Generally valid for erosion tests carried out with angular erodent particles.

Fig. 3 (a) Variation of erosion rate with test temperature in the case of 2.25 Cr-1 Mo, 5

Cr-0.5 Mo, 1018 steels and 304, 310, 410 and 17-4 PH stainless steels75

(b) effect of test temperature on the erosion rate of a number of pure metals (Ta, W, Pb) and alloys(Ti-6Al-4V, 2024 Al, 410 SS) at two impact angles (30 ◦ and 90°) 75

However such observations can be rationalized the basis of erosion-oxidation

interaction mechanisms as will be demonstrated in a later section.

(c) The velocity exponent (p) obtained by various investigators is plotted in the velocity-

temperature space in Fig 4a. The value of velocity exponent covers a wide range from

27




0.9 to 2.88. The velocity exponent decreases with an increase in erosion test

temperature for 304SS to values as low as 0.9 at low impact velocities. At relatively

higher impact velocities, p appears to lie in the range 2 to 3. Levy and Man 86-87 obtained

velocity exponents for erosion of 9Cr-1Mo steel at 650°C using angular SiC particles.

According to them, the velocity exponent initially remained constant and then increased

with an increase of erodent size especially at low impact velocities. However, at highimpact velocities p went through a minimum at an intermediate size of the erodent.

(d) Tabakoff and Vitta80 carried out erosion tests on Inco 600 alloy using quartz particles

in the size range of 70 µm to 800 µm. Their results, presented in Fig 4b, indicate that

the erosion rate increased marginally with the increase of particle size. Zhou and

Bahadur 82 investigated the effect of particle size of SIC on the erosion rate of 304SS at

650°C (impact angle: 30◦; impact velocity: 65 m/s). Their results (Fig 4c) indicate a

particle size independent erosion rate beyond a particle size of 40 µm. Levy and

coworkers86-88 however, noted an increase of erosion rate with an increase of particle

size for a 9Cr-1Mo steel eroded at a temperature of 650°C and also for a 1018 steeleroded at 450°C.

(e) There is very limited data available on the influence of particle shape on elevated

temperature erosion. Levy et al84, 86-87, 89 investigated the erosion rate of number of Cr

containing steels at 850°C with angular SiC and spherical Al203 as erodent particles. A

typical result valid for a 9Cr-1Mo steel, is presented in Fig. 4d. the erosion rate is

considerably higher when SiC is used as the erodent. In addition, the velocity

dependence of erosion rate is itself influenced by the type of erodent used.

(f) Zhou and Bahadur 90 have investigated the influence of particle flux rate on the

erosion of 304 and 430 SS over a large temperature range. their results shown in Fig 5aindicate that upto a temperature of about 500 ◦ C, a lower flux rate actually resulted in a

substantially higher erosion rate. As will be shown subsequently, the sudden

manifestation of a particle flux rate dependent erosion behavior beyond 500 ◦ C can be

related to the mechanisms of interaction between erosion and oxidation.

(g) The influence of eroding material properties on the erosion behavior of metallic

materials at elevated temperatures has been investigated only to a very limited extent.

In addition, the interpretation of the available data is also complicated by the fact that

the behavior of oxide scale under erosion conditions need to be considered in addition

to the behavior to the behavior of metallic material per se. The fact that the variousstrengthening mechanisms discussed in relation to erosion at room temperature

becomes less important or even unimportant at elevated temperatures leads to further

difficulties with respect to the analysis of the experimental erosion data.

28




Fig 3. Continued (d) influence of impact angle and test temperature on the erosion

behavior of (i) 304 SS (ii) 410 SS and 316 SS 77,79,80

References

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[2]. Bitter J.G.A. Weur 1963, 6, 169

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[3]. Lehrke W.D. and Nonnen F.A. 1st Int. Conf. on Protection of Pipes, BHRA. Durham. UK 1975. paper G2

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[11]. Sage W. and Tilly G.P. Aerorzaut. .I. 1969. 73, 429

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[22]. Shewmon P. and Sundararajan G. Ann. Rev. Mater. Sri.1983, 13, 301

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[24]. Kosel T.H. Metals Handbook. Vol. 18. ASTM International, The Materials Information Society, 1992, 199

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Ohio, USA, 1995

[26]. Hutchings I.M. Proc. Conf. on Corrosion/Erosion of Coal Conversion System Materials (Ed. Levy A. V.)

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[29]. Goodwin J.E., Sage W. and Tilly G.P. Proc. Inst. of Mech. Engg. 1969-70, 184, 279

[30]. Ives L.K. .I. Eng. Mater. Tech. Trans. ASME 1977, 99, 126

[31]. Cousens A.K. and Hutchings I.M. Proc. 6th Int. Conf. on Erosion by Liquid and Solid impact (Eds. Field J.E.

and Corney N.S.) Cavelzdish Laboratory, Cambridge University, September 1983, Cambridge, UK

[32]. Reddy A.V. and Sundararajan G. Wear 1986, 111, 313

[33]. Montgomery J.E. and Clarke J.M. SAE Summer Meetings, New York. 1962, paper 538A

[34]. Wood C.D. and Espenschade P.W. SAE Summer Meetings, New, York, 1964, paper 880A

[35]. Sheldon G.L. and Finnie I. Trans. ASME 1966, 88B, 387

[36]. Zhou J.R. and Bahadur S. Wear 1989, 132, 235

[37]. Yerramareddy S. and Bahadur S. Wear 1991, 142, 253

[38]. Levy A.W., Wang B.Q. and Geng G.Q. Mater. Sci. Engg. 1989, A121, 603

[39]. Bahadur S. and Badruddin R. Wear 1990, 138, 189

[40]. Brown R., Kosco S. and Zun E.J. Wear 1983, 88, 181

[41]. Levy A.V. and Chik P. Wear 1983. 89, 151

[42]. Liebhard M. and Levy A.V. Proc. Int. Conf on Wear of Materials, (Eds. Ludema K.C. and Bayer R.G.) ASME

1991,245

[43]. Kleis I. Wear 1966. 13, 199

[44]. Manish Roy, Tirupataiah Y. and Sundararajan G. Mater. Sci. Engg. 1993. A165, 51

[45]. Ives L.K. and Ruff A.W. ASTM STP 664, (Ed. Adler W.A.) ASTM, Philadelphia, PA, 1979, 5

[46]. Mills D. and Mason J.S. Proc. 6th Int. Conf. on Erosion by Liquid and Solid impact, (Eds. Field J.E. and

Corney N.S.) Cavendish Laboratory, Cambridge University, September 1983, Cambridge, UK

[47]. Young J.P. and Ruff A.W. J. Engg. Mater. Tech., Trans. ASME 1977. 99, 121

[48]. Anand K., Hovis S.K., Conrad H. and Scattergood R.O.Wear 1987, 118, 243

[49]. Finnie I. Wear 1960, 3, 87

[50]. Kleis I. Z. Werksto~ech. 1983. 15, 49

[51]. Soderberg S., Hogmark H., Engman U. and Swahn H. Tribol. Inter. December 1981, 333

[52]. Goretta K.C., Arroyo R.C., Wu C.T. and Routbort J.L.Wear 1991, 147, 145

[53]. Foley T. and Levy A.V. Wear 1983. 91, 45

[54]. Behrendt A. Proc. 3rd Ini. ConJ: 517 Rain Erosion and Allied Phenomenon, (Eds. Fvall A.A. and King R.B.)

Royal Aircraft Establishment, UK, 1970

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[56]. Manish Roy, Tirupataiah Y. and Sundararajan G. Mater. Sci. Technol. 1995, 11. 791

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[57]. Trilok Singh and Sundararajan G. Wear 1991, 145, 77

[58]. Finnie I., Wolak J. and Kabil Y. J. Mater. 1967, 2, 682

[59]. McCabe L.P., Sargent G. and Conrad H. Wear 1985. 105,257

[60]. Levy A.V. and Jahanmir S. In Corrosion-Erosion Behaviour of Materials, (Ed. Natesan K.) Met. Sot. AIME.

Philadelphia, 1979, 177

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Hyderabad,India, 1995

[62]. Balan K.P., Reddy A.V., Joshi V. and Sundararajan G.Wear 1991, 145, 283

[63]. Emiliani M. and Brown R. Wear 1984, 94, 323

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[65]. Salik J. and Buckley D.H. Proc. Int. Conj Wear of Materials, (Eds. Rhee SK., Ruff A.W. and Ludema K.C.)

ASME, New York, 1987, 592

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[67]. Rao D.R.K., Venkataraman B., Asundi M.K. and Sundararajan G. Surf. Coat. Technol. 1993, 58, 85

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Materials, (Eds. Santhnnam A.T. and Linga Murthy K.) ASM, Materials Park, ohio,1987,23

[71]. Liebhard M., Levy A.V. and Verpoort C. Proc. Corrosion-Erosion-Wear of Materials at Elevated

Temperature, (Ed. Levy A.V.) Berkeley, California, 1991, 381

[72]. Manish Roy, Ranganath S., Venkataraman B., Subramanyam J. and Sundararajan G. Unpublished work,Defence Metallurgical Research Laboratory. Hyderabad, India. 1990

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Aluminide. Report MTL 89-68 (J), Canada Centre for Mineral and Energy Technology. CANMET, 1981

[75]. Levy A.V., Yan J. and Patterson J. Proc. Int. Conf. on Wear of Materials, (Ed. Ludema K.) ASME, New York,

1985, 708

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V.) Berkeley, California, 1991, 13-l

1. CORROSION

The corrosion processes follow the basic laws of thermodynamics. Corrosion is

an electrochemical process. Under controlled conditions it can be measured, repeated,

and predicted. Since it is governed by reactions on an atomic level, corrosion processes

can act on isolated regions, uniform surface areas, or result in subsurface microscopic

damage. Complicate these forms of corrosion with further subdivisions, add just basic

environmental variables such as pH, temperature, and stress, and the predictability of

corrosion begins to suffer rapidly.

2. FORMS OF CORROSION

There are nine basic forms of corrosion that metallic materials may be subject to:

1. Uniform corrosion

2. Intergranular corrosion

3. Galvanic corrosion

4. Crevice corrosion5. Pitting

6. Erosion corrosion

7. Stress corrosion cracking

8. Biological corrosion

9. Selective leaching

33




2.1 Uniform Corrosion

Although other forms of attack must be considered in special circumstances,

uniform attack is one form most commonly confronting the user of metals and alloys.

Uniform or general corrosion, which is the simplest form of corrosion, is an even rate of

metal loss over the exposed surface. It is generally thought of as metal loss due to

chemical attack or dissolution of the metallic component into metallic ions. In high-

temperature situations, uniform metal loss is usually preceded by its combination with

another element rather than its oxidation to a metallic ion. Combination with oxygen to

form metallic oxides, or scale, results in the loss of material in its useful engineering

form; scale ultimately flakes off to return to nature. Fig. 1 shows how corrosion attacks

the cover of a tank.

Fig. 1 Cover of tank attacked by uniform corrosion

A metal resists corrosion by forming a passive film on the surface. This film is

naturally formed when the metal is exposed to the air for a period of time. It can also be

formed more quickly by chemical treatment. For example, nitric acid, if applied to

austenitic stainless steel, will form this protective film. Such a film is actually a form of

corrosion, but once formed it prevents further degradation of the metal, provided that

the film remains intact. It does not provide an overall resistance to corrosion because it

may be subject to chemical attack. The immunity of the film to attack is a function of the

film composition, temperature, and the aggressiveness of the chemical. Examples of

such films are the patina formed on copper, the rusting of iron, the tarnishing of silver,

the fogging of nickel, and the high-temperature oxidation of metals.

34




There are two theories regarding the formation of these films. The first theory

states that the film formed is a metal oxide or other reaction compound. This is known

as the oxide film theory. The second theory states that oxygen is adsorbed on the

surface, forming a chemisorbed film. However, all chemisorbed films react over a period

of time with the underlying metal to form metal oxides. Oxide films are formed at roomtemperature. Metal oxides can be classified as network formers, intermediates, or

modifiers. This division can be related to thin oxide films on metals. The metals that fall

into network-forming or intermediate classes tend to grow protective oxides that support

anion or mixed anion/cation movement. The network formers are noncrystalline,

whereas the intermediates tend to be microcystalline at low temperatures.

Uniform corrosion of steel under the high pressure of CO2 environment is very

critical problem faced on shore and also off shore at oil wells. Crolet et.al. [2] throw light

on analyzing the various effects of CO2 on corrosion in general, based on this work a

policy for fighting corrosion in offshore wells can be made, not only that it helps in

precisely predicting the mechanisms of local CO2 attack, and a method of predicting the

risks of CO2 corrosion in wells as well. Under this work the current knowledge of CO2

corrosion, starting with descriptions of the various effects of CO 2 on corrosion in general

and the diversity of the definitions of corrosivity is discussed. Methods for predicting the

risks of CO2 corrosion are proposed as outcome of this work.

Nesic [3] also worked on a predictive model was developed for uniform carbon

dioxide (CO2) corrosion, based on modeling of individual electrochemical reactions in a

water-CO2 system. The model takes into account the electrochemical reactions of

hydrogen ion (H+) reduction, carbonic acid (H2CO3) reduction, direct water reduction,

oxygen reduction, and anodic dissolution of iron. The required electrochemical

parameters (e.g., exchange current densities and Tafel slopes) for different reactions

were determined from experiments conducted in glass cells. The corrosion process wasmonitored using polarization resistance, potentiodynamic sweep, electrochemical

impedance, and weight-loss measurements. The model was calibrated for two mild

steels over a range of parameters: temperature (t) = 20 °C to 80 °C, pH = 3 to 6, partial

pressure of CO2 (P-CO2) = 0 bar to 1 bar (0 kPa to 100 kPa), and omega = 0 rpm to

5,000 rpm (v(p) = 0 m/s to 2.5 m/s). The model was applicable for uniform corrosion

35




with no protective films present. Performance of the model was validated by comparing

predictions to results from independent loop experiments. Predictions also were

compared to those of other CO2 corrosion prediction models. Compared to the previous

largely empirical models, the model gave a clearer picture of the corrosion mechanisms

by considering the effects of pH, temperature, and solution flow rate on the participatinganodic and cathodic reactions.

The effect of inert atmosphere on the corrosion is studied by Jin Huang et. al.[4].

Here uniform CO2 corrosion of mild steel under inert solid deposits is studied. The effect

of an inert solid deposit on uniform CO2 corrosion of mild steel is modeled based on a

mechanistic electrochemical CO2 corrosion model. Laboratory testing has shown that

the dominant factors introduced by the inert solids deposit are related to surface

coverage, where both anodic and cathodic reaction rates are decreased because of

less active surface area being exposed. The inert solid deposits also create a mass

transfer barrier for corrosive species which limits the rate of the cathodic reactions. An

existing mechanistic electrochemical model was modified to account for these effects

and was capable of capturing the features of uniform CO2 corrosion of mild steel under

inert solid deposits.

The iron is protected from the corrosion environment by a thin oxide film 1–4 mm

in thickness with a composition of (Fe2O3)0.5/ Fe3O4. Jiabin Han, et al. [5] applied SEM,

XRD and GIXRD (grazing incidence XRD) were applied to characterized and identify

the protective/passive film. It is noted that under the usual mode of data collection in

XRD technique involves symmetric theta/2theta geometry wherein the angle of

incidence and angle of diffraction increases continuously during the data collection. In

such a mode the beam can penetrate deeper into the sample compared to GIXRD with

a fixed small incidence angle. These small and large incidence angles for XRD and

GIXRD are catalogued by the critical incidence angle. Both the schemes are explainedherewith with a basic schematic as Figure 2 & 3.

36




Fig 2. Scheme for XRD with larger incidence angle

Fig 3. Scheme for XRD with grazing incidence angel (GIXRD)

The electrochemical corrosion processes of iron in a borate solution have been

investigated by, Jing Li [6] with the help of in situ electrochemical atomic force

microscopy (ECAFM). A freshly polished iron surface is passivated electrochemically in

a borate solution to give a film which is different from the oxide layer which forms on the

iron surface in air. Study revealed that most areas of the passive film are uniform, but

some defects still exist in the film which allows localized corrosion to occur. In situ

ECAFM are used to examine these defects directly and to observe the initial corrosion

processes in which ferric oxide particles are both formed and reduced by cyclic

voltammetry.

The passive film on nickel can be formed quite readily in contrast to the formation

of the passive film on iron. Corrosive surface on nickel was obtained by localized

37




corrosion of Ni(111) surfaces passivated in a chloride-free sulphuric acid solution and

subsequently exposed to chloride (0.05 M) are reported by In situ scanning tunnelling

microscopy measurements, by V. Mairice, et al. [7]. The result evident the deterministic

role of the structure of the passivated surface on the dissolution in the passive state,

independent of the addition of chloride in the electrolyte. The atomic lattice of thepassive film formed in the absence of chloride remained intact even after the addition of

chloride. At the investigated potential of +0.85 VSHE (0.05 V below the stable pitting

potential), the dissolution proceeds by a two-dimensional step flow process that is

dependent on the step orientation, the steps oriented along the closed-packed

directions dissolving much less rapidly than the steps oriented along no well-defined

directions of the oxide lattice. The dissolution rate of the oxide lattice is around 6 NiO

‘molecules’ s−1, independent of the presence of chloride. The comparison with the rate

of dissolution that can be estimated from the measured electrochemical current density

in the passive state suggests that the dissolution of the oxide lattice of the passive film

is not the major anodic reaction in the passive state.

Most of stainless steels are used under the ambient atmosphere including indoor

and outdoor environments, but not in aqueous solution. However, certain researchers,

[8] have studied passive films that are formed in electrolytic solutions. As commonly

recognized, the excellent protection ability of stainless steel derives from the highly Cr

enriched passive film which is formed as a result of selective dissolution of Fe into the

balk solution. On the other hand, the passive films formed under atmospheric conditions

do not necessarily exhibit Cr enrichment, because the amount of the solution on a

stainless steel as an adsorbed thin water layer is not sufficient for selective dissolution

of Fe. Therefore, the modification of passive films may occur as tiny mass transfer

between hydroxide layer and oxide layer of the passive films, and/or occasional replace

of the adsorbed thin water layer. In the present work, in order to discuss atmospheric

corrosion, passive films on stainless steels formed under humid atmospheric

environments were characterized using X-ray photoelectron spectroscopy.

Most of passive films formed for various period of exposure up to 3 months in

humid environments with/without a wet and dry sequence, and without solution supply

during exposure has the cation content equivalent to substrate steel, and exhibits

38




almost no change with time. This is reasonable because mass transfer is limited to

among the most surface of the steel, passive film and adsorbed water layer. Slight

increase in Cr content in the inner oxide later was recognized. During the exposure, Fe+

+ in oxide layer may transfer into outer hydroxide layer. On the other hand, gradual

changes in cation content with time was observed for the specimen on which smallamount of pure water or especially salt solution was supplied periodically during

exposure. Supply of chloride stimulates Cr enrichment, because formation of salt or

complex may accelerate transfer of Fe++ into solution layer.

Study of protective film is very relevant for the protection of steel for the

application of reinforcement corrosion [9]. Reinforcement corrosion has become the

most serious cause of premature deterioration of concrete structures. Especially, for

structures exposed to high environmental aggressiveness and with a long design

service life, it is necessary to provide additional preventive measures. The use of

corrosion resistant reinforcement materials, such as stainless steel, is an important

technical approach which offers many advantages as a preventive measure, such as,

reduced maintenance and increased durability. With the increased application of

stainless steel reinforcement, the attention to the research in this area is increased. A

fundamental area is the development of new stainless steel alloys, such as high

manganese alloys, with suitable mechanical properties and high corrosion resistance.

Besides its stated importance in the development of new alloys, the corrosion

resistance characterization is also essential to allow a thorough selection, of the distinct

stainless steel alloys available, for a given application. The structure of the passive films

is considered to be a crucial factor in localized corrosion events, the determination of its

properties being an important tool for evaluating the electrochemical behavior of passive

metallic alloys.

The structure of the passive films in this case has been studied by capacitancemeasurements using the Mott-Schottky approach. The studied passive films were

formed under distinct conditions: with and without pre-polarization, both in alkaline

solutions with and without chlorides. The stainless steel passive films have shown a bi-

layer structure of two space charge depletion layers (n- and p-type semiconductors).

Generally, the passive films formed on low nickel alloys have higher donor doping

39




densities than those of the Fe-Cr-Ni based alloys. The doping densities in chloride

containing solution are also higher than those in alkaline solution.

2.2 Intergranular Corrosion

Intergranular corrosion is a localized form of corrosion. It is a preferential attackon the grain boundary phases or the zones immediately adjacent to them. Little or no

attack is observed on the main body of the grain. This results in the loss of strength and

ductility. The attack is often rapid, penetrating deeply into the metal and causing failure.

Typical corrosion under this category for aluminum and carbon steel is as shown in

Figure 4.

Figure 4 Exfoliation of aluminum and carbon steel

Exfoliation is a form of intergranular corrosion. It manifests itself by lifting up the

surface grains of a metal by the force of expanding corrosion products occurring at the

grain boundaries just below the surface. It is visible evidence of intergranular corrosion

and most often seen on extruded sections where grain thickness is less than in rolled

forms. This form of corrosion is common on aluminum, and it may occur on carbon

steel.

Intergranular corrosion (IGC) can also be defined as the phenomena in which the

corrosion rate of the grain boundary is higher than that of the bulk grain body. Austenitic

stainless steels can become “sensitized” to this type of corrosion when the bulk metal

40




experiences a temperature excursion above that necessary to make chromium carbides

dissolve.[10] As the metal cools, chromium carbide preferentially precipitates at the

grain boundaries causing the local Cr concentration adjacent to the grain boundary to

decrease relative to the Cr concentration in the bulk of the grain (Figure 5). This relative

drop in Cr concentration adjacent to the grain boundary is called sensitization or segregation. A single loop electrochemical potentiokinetic reactivation (SLEPR) method,

originally developed for the nuclear industry and now an ASTM Standard, will be used

to quantitatively measure the DOS of material in each array. A modification of this test

can be made to show that IGC could occur in sensitized stainless steel adjacent to pits

that grow by the acid-pitting mechanism.

Figure – 5 Relative Cr concentrations near a sensitized Grain Boundary

2.3 Galvanic Corrosion

This form of corrosion is sometimes referred to as dissimilar metal corrosion, and

is found in unusual places, often causing professionals the most headaches. Galvanic

corrosion is often experienced in older homes where modern copper piping is

connected to the older existing carbon steel lines. The coupling of the carbon steel to

the copper causes the carbon steel to corrode. The galvanic series of metals provides

details of how galvanic current will flow between two metals and which metal willcorrode when they are in contact or near each other and an electrolyte is present (e.g.,

water). Figure 6 gives the corrosion because of dissimilar metal.

41




Fig. 6 corrosion because of dissimilar metal corrosion caused by a stainless steel

screw causing galvanic corrosion of aluminum

2.4 Crevice Corrosion

Crevice corrosion is a localized type of corrosion occurring within or adjacent to

narrow gaps or openings formed by metal-to-metal-to-nonmetal contact. It results from

local differences in oxygen concentrations, associated deposits on the metal surface,

gaskets, lap joints, or crevices under a bolt or around rivet heads where small amounts

of liquid can collect and become stagnant. Crevice corrosion may take place on any

metal and in any corrosive environment. However, metals like aluminum and stainless

steels that depend on their surface oxide film for corrosion resistance are particularly

prone to crevice corrosion, especially in environments such as seawater that containchloride ions. The gap defining a crevice is usually large enough for the entrapment of a

liquid but too small to permit flow of the liquid. The width is on the order of a few

thousandths of an inch, but not exceeding 3.18 mm. The material responsible for

forming the crevice need not be metallic. Wood, plastic, rubber, glass, concrete,

asbestos, wax, and living organisms have been reported to cause crevice corrosion.

After the attack begins within the crevice, its progress is very rapid. It is frequently more

intense in chloride environments. Prevention can be accomplished by proper design

and operating procedures. Figure 7 shows the basic mechanism for the crevice

corrosion and its industrial occurrence.

42




Fig. 7 Crevice corrosion basic mechanism and its occurrence

Crevice corrosion is very relevant for the sea boat sailing in the sea water[11]. In

order to understand the effect crevice corrosion tests were carried out in order to

evaluate the performance of some conventional and high alloy stainless steels in

Arabian Gulf seawater at 25°C and 50°C using specimens having three different surface

conditions namely as received wheel ground and 180 grit SiC ground. Immersion tests

of 150-180 days duration and accelerated tests were employed to investigate the

crevice corrosion behavior. Immersion test results show that crevice corrosion of 3127

hMo, Remanit 4565, 654 SMO, and Monit 44635 initiated as surficial corrosion with

virtually no measurable depth of attack; 254 SMO, Duplex 2205, Remanit 4575, 904L

and 317L corroded on l-2 sites attack. Crevice corrosion attack was most predominant

at 50°C in 180 grit SiC finished surfaces and there was no evidence of crevice corrosion

attack in as received samples at room temperature. A method based on Oldfield and

Sutton9 mathematical modelling of corrosion in chloride media has been applied for

determining critical crevice solution pH (CCSpH), an important parameter which

delineates the transition between passive and active states. The CCSpH appears to be

a linear function of PREN (PREN = %Cr + 3.3x%Mo + 16x%N) indicating the strong

influence of Cr, MO and N additions on the crevice corrosion characteristics of steels.Figure 8 gives the PREn v/s CCSpH for various materials stated. A linear fit is observed

for the various materials.

43




Fig. 8 Plot of PREN v/s CCSpH value for steels in natural sea water and NaCL

solution

The simplest method for preventing crevice corrosion is reducing crevices in the

design of the structure When it is not possible to get rid of crevices, improving drainage

and sealing of edges or keeping crevices as open as possible and therefore preventing

entrance of moisture is the best protective action A protection method called “hot wax

dip” is commonly used in automotive industry.[12] In this method faying surfaces that

will make crevices are usually painted before assembly. In aerospace industry sealing

the faying surfaces with a polysulfide is known to be an effective method for preventing

crevice corrosion. Cathodic protection could be an effective method against crevice

corrosion, but anodic protection is often improper as shown in Figure 9. Another

common protection method is using alloys which are less vulnerable to crevice

corrosion. Addition of inhibiting substances to bulk solution is also a protection method.

Application of passivating compounds such as chromate and nitrate is well practiced to

prevent crevice corrosion. Overlaying susceptible areas with an alloy which is more

resistant to crevice corrosion is another protective measure.

44




Fig 9: Mechanism of active and passive corrosion and its corresponding Anodic

and Cathodic reactions in crevice corrosion

2.5 Pitting Corrosion

Pitting corrosion is in itself a corrosion mechanism, but it is also a form of

corrosion often associated with other types of corrosion mechanisms. It is characterized

by a highly localized loss of metal. In the extreme case, it appears as a deep, tiny hole

in an otherwise unaffected surface. The initiation of a pit is associated with the

breakdown of the protective film on the metal surface. The depth of the pit eventually

leads to a thorough perforation or a massive undercut in the thickness of the metal part.

The width of the pit may increase with time, but not to the extent to which the depth

increases. Most often, the pit opening remains covered with the corrosion product,

making it difficult to detect during inspection. This, along with a negligible loss in weight

or absence of apparent reduction in the overall wall thickness, gives little evidence as to

the extent of the damage. Pitting may result in the perforation of a water pipe, making it

unusable even though a relatively small percentage of the total metal has been lost due

to rusting.

Nickel-aluminium bronze (NAB) is widely used in marine applications because of

its high toughness and erosion-corrosion resistance. NAB is used for high performance

propellers and seawater handling systems – seawater valves and heat exchangers.

Pitting and crevice corrosion for copper-based alloys is often attributed to a metal-ion

45




concentration cell. Areas exposed to high copper-ion concentrations are considered to

act as cathodic sites (with copper deposition sometimes observed).[13]

Figure 10 Pitting corrosion occurrences at propeller at marine applications

NAB may encounter corrosion related problems under in-service conditions. NAB

can encounter variability in corrosion performance worldwide, i.e. different local

environments. NAB components in naval vessels can be affected by their operational

cycles (open / dock type seawater) – long periods in the dock compared with

commercial vessels. Corrosion problems can lead to expensive repairs to NAB

propellers and seawater intakes. Propeller replacement costs is exorbitantly high where

as dry dock costs are also significant component. For this a study by Balic et al. is

carried out to a detailed study[13]. Since NAB is known to be susceptible to localised

corrosion, e.g. crevice corrosion rates as high as 0.7 to 1.0 mm.y–1 have been reported

- compared with 0.25 mm y–1 for type 304 stainless steel. NAB is prone to selective

phase corrosion (SPC), and this SPC may initiate pitting corrosion.

The study concludes the following:

• On exposure the copper-rich α-phase was initially (during the first 6 months) susceptible

to corrosion leaving the unattacked κ-phases to create an adherent skeletal lattice.

• At localised sites, SPC occurred and due to the continuous nature of the κIII-phase this

resulted in the accumulation of corrosion products / deposits at these locations.

46




• The formation of a micro-environment beneath the deposit developed into a ‘pitting’ type

phenomenon after prolonged exposures (within 15 months).

• Overall, the pitting mechanism is characterised by very wide although relatively shallow

corrosion features.

2.6 Erosion Corrosion

The term “erosion” applies to deterioration due to mechanical force. When the

factors contributing to erosion accelerate the rate of corrosion of a metal, the attack is

called “erosion corrosion.” Erosion corrosion is usually caused by an aqueous or

gaseous corrodent flowing over the metal surface or impinging on it. The mechanical

deterioration may be aggravated by the presence of a corrodent, as in the case of

fretting or corrosive wear. The attack takes the form of grooves, i.e., scooped-out

rounded areas, horseshoe-shaped depressions, gullies, or waves, all of which often

show directionality. At times, attack may be an assembly of pits. Ultimate perforation

due to thinning or progression of pits, and rupture due to failure of the thinned wall to

resist the internal fluid pressure are common. All equipment exposed to flowing fluid is

subject to erosion corrosion, but piping systems and heat exchangers are the most

commonly affected. Erosion corrosion is affected by velocity, turbulence, impingement,

presence of suspended solids, temperature, and prevailing cavitation conditions. The

acceleration of attack is due to the distribution or removal of the protective surface film

by mechanical forces exposing fresh metal surfaces that are anodic to the uneroded

neighboring film. A hard, dense adherent and continuous film, such as on stainless

steel, is more resistant than a soft brittle film, as that on lead. The nature of the

protective film depends largely on the corrosive itself.

Erosion-corrosion is often accelerated at pipe elbows, tube constriction, and anywhere

fluid flows are altered and there is an increase in turbulence. Other aspects that

increase erosion corrosion are the corrosivity of the flowing corrodant, a two-phase flow,

such as steam and water, or a flow in which suspended solids are flowing with the fluid

[14], as shown in Figure 11.

47




Figure 11: Erosion-corrosion of brass condenser tubing showing individual

teardrop shaped pits with under cutting in the downstream direction.

In 1982, Heitmann and Kastner published the results of erosion corrosion

experiments of KWU Siemens. Some years after this publication KWU-Siemens has

processed these experimental results in a model and made the software programme

WATCHEC which is applied in many power stations over the world. [15]

With the help of the calculation model influence of various factors can be

estimated rather well. Cr and Mo were incorporated in the correlation formula of KWU

Siemens. Other elements as Cu and C were neglected. From the extensive erosion-

corrosion experiments of KEMA it was proven that Cu had a 40% more erosion-

corrosion resistant effect than Cr. The relative erosion-corrosion resistance of C-steels

and 15Mo3 was expressed in the Cr equivalence formula (Cr-equivalent = Cr +1.4 Cu +

0.3 Mo - 0.3 C > 0.09).

By comparing the Cr equivalence of steels from failures and non-failures it

appeared that steels from failures generally had a Cr-equivalence less than 0.09.

48




Fig 12 Steam blanketing at the inner side and erosion corrosion at the outer side

of the bellow having Cr - 0.005, Cu - 0.02, Mo - 0.005, C - 0.12, and Cr-eq.for this

combination is 0.0015.

The dominant factor for the erosion and corrosion is water chemistry, water

velocity, and chemical composition of the steel the suspended iron oxides. Huijbregts

[16] worked on the relationship between the chemical composition of steel and erosion

corrosion resistance. This relation-ship has been neglected in the past because water

chemistry and velocity were thought to be the dominant factors. Especially in steam-

water circuits may cause trouble in both conventional and nuclear power stations. This

happens rather often; e.g., corrosion in conventional boiler evaporators, corrosion in

PWR steam generators and deposition of iron oxides on fuel elements in nuclear

reactors. In view of these problems, it is recommended that the iron content in steam

water circuits be kept as low as possible. This iron comes from erosion-corrosion, the

major sources of which are water separators, wet steam pipes, preheaters, and

evaporators. Erosion-corrosion depends on water chemistry, water velocity, and

chemical composition of the steel.

The resistance against erosion-corrosion in wet steam was determined for 58

steels in a laboratory test. Minute quantities of chromium, copper, and molybdenum in-

crease the resistance against erosion-corrosion. The erosion-corrosion resistance (R)

can be calculated by the regression equation R = 0.61 + 2.43 Cr + 1.64 Cu + 0.3 Mo.

49




Steels from in-service erosion-corrosion failures showed low resistance against erosion-

corrosion in the laboratory test. No failures were found in steels with a calculated

resistance value R of more than 1.0. Figure 13 shows schematic of experimental set-up

for the same.

Figure 13 - Design of the erosion-corrosion loop.

Power companies provided various erosion-corroded specimens. They came

from steam sieves, a spindle, switch levers in feed water pumps, and a control valve

housing. All the corroded steels had low Cr, Cu, and Mo content.

Case 1. In the experimental boiler, erosion-corrosion occurred in a steam pressure

drop vessel in which steam expands and a water steam jet hits the vessel wall. After 10

years of operation, there was a leakage. The vessel wall had a protecting steel plate

opposite the steam inlet tube. The steel plate (10 mm thick) and the vessel wall (5 mm

thick) were corroded and leakage occurred. The resistance R calculated for the steel

plate was 0.90.

Case 2. A number of steam sieves suffered erosion-corrosion. Chemical analyses

showed that three different steel heats were applied. The calculated resistances were

rather low (0.70).

50




Case 3. A spindle from a condensate effluent valve of a low pressure preheater was

coated only partly with a 9% chromium steel. On top of the spindle. This coating was

lacking. Erosion-corrosion was observed there. Again low Cr. Cu, and Mo values were

found, and the resistance value R amounted to 0.75.

Case 4. The switch lever of a supply water pump had been attacked severely by

erosion-corrosion. Steel No. 58 appeared to have suffered a high weight loss in the

laboratory test, i.e. number of specimens 116 mg. The operation time of this lever was

10,000 hours. Another lever from an identical pump at the same station after 40,000

hours showed less corrosion. The Cr, Cu, and Mo content of this steel is slightly higher.

The calculated R values of the two levers amounted to 0.65 and 0.73, respectively.

Case 5. There was heavy erosion-corrosion in control valve housing, resulting in

leakage. This steel No. 57 suffered a high weight loss in the erosion-corrosion test, 107

mg. Cr, Cu, and Mo content was low, and the resistance value R was 0.67.

Case 6. A pipe behind the BWR water separator was constructed by welding bent

steel plates The wet steam pipe after the pre-water separator. The pipe was welded of

bent steel plates. One plate had been corroded severely (R = 0.67). The adjacent non-

corroded plates had higher resist-ances (R = 0.89).

Case 7: Steel part from the PWR water separator as shown in Figure 14. One steel

plate had suffered erosion-corrosion, and much oxide was deposited on the other. The

resistance value of the erosion-corroded steel was much lower than that of the

noncorroded steel (0.65 to 1.20).

51




Fig - 14 Steel part from the PWR water separator

2.7 Stress Corrosion Cracking (SCC)

SCC is defined as the delayed failure of alloys by cracking when exposed to

certain environments in the presence of static tensile stress. The importance of a

conjoint action of corrosion and stress is reflected in the definition; an alternate

application of stress and corrosive environment will not produce SCC. The stress level

at which the failure occurs is well below the stress required for a mechanical failure in

the absence of corrosion. The minimum stress below which SCC will occur is called the

threshold stress, but this may be as low as 10% of the yield stress in some systems.Corrosion alone in the absence of stress does not cause SCC. SCC occurs at points of

stress. Usually the metal or alloy is virtually free of corrosion over most of its surface,

yet fine cracks penetrate through the surface at the points of stress. Depending on the

alloy system and corrodent combination, the cracking can be intergranular or

transgranular. The rate of propagation can vary greatly and is affected by stress levels,

temperature, and concentration of the corrodent.

A simplified stress corrosion test fixture with a round tensile specimen installed isillustrated on Figure 15 in a simulated corrosion environment.[17] A masking material is

applied to the test fixture to ensure that the specimen alone is exposed to the corrosive

environment. The tensile specimen is stressed to a desired level (typically 25, 50, 75 or

90 percent yield strength). The specimen is then submerged in a 3.5 percent NaCl

alternate immersion bath, in a 5 percent NaCl salt spray (fog), or in a 90-95 percent

52




relative humidity test. Test duration is typically three months for low alloy steels and

aluminum alloys, and six months for stainless steel. The resistance of metals to SCC is

always less when tension is applied in a transverse direction. It is least for the short

transverse direction. Stress corrosion is aggravated when tensile stresses due to

assembly have been applied in the short transverse direction. Table 1 lists typicalmaterials and environments that may cause stress corrosion.

Fig. 15 Stress Corrosion Test Setup for the SRB Forward Separation Bolt

Table 1 Environments That May Cause Stress Corrosion of Metals and Alloys

53




The stress corrosion cracking (SCC) of stainless steel is one of the biggest

problems for maintaining atomic power and chemical plants and occurring widely.

However the mechanism has not been solved because of difficulty in observing

hydrogen movement. In order to solve this problem, the author has developed a new

SCC test method that enables the super Kelvin force microscope (SKFM) and theKelvin force microscope (KFM) observations. By using this test method, the crack tip

deformation and surface potential distribution on SUS316L stainless steels were

observed by SKFM and KFM. The existence of hydrogen-induced martensite was

examined by the magnetic force microscope (MFM) observations. The results showed

that a less noble potential region existed near the crack tip. MFM and KFM observation

showed hydrogen induced martensite existed at the less noble potential region.

Repeated SKFM observations revealed that the crack is formed by the movement of

hydrogen-induced martensite.[18]

Fasteners are susceptible to many forms of embrittlement, with stress corrosion

cracking generally considered to be the most complex failure mechanism. Stress

corrosion cracking is a type of localized corrosion characterized by fine cracks (as seen

54




in the two photographs in Figure 16) that propagate quite rapidly leading to failure of the

component and potentially the associated structure.[19].

Fig. 16 Typical appearance of stress corrosion cracking failure.

Fig.17 End cap application of pipelock

Pipelocks and the Mechanical Stress Improvement Process (MSIP) have been

applied in BWRplants. Pipelocks restore the 'integrity of the weldments with identified

cracks. MSIP removes residual tensile stresses from weldments, thus, preventing

55




initiation of cracks or retarding growth of pre-existing flaws 1n piping systems. MSIP

was applied for various geometries of weldments including nozzle-to-safe-end Joints.

Extensive verification has been carried out by United States Nuclear Regulatory

Commission EPRI and the Argonne National Laboratory. Basic concepts and practical

application of MSIP and Pipelocks are presented[20]. The Pipelock is a mechanicaldevice which is designed to prevent pipe break under the assumption of a throughwall

IGSCC crack around the entire circumference of the pipe weld. It is simple and easy to

apply as shown in the Fig. 17 where shell end of a small shell and tube type heat

exchanger is shown. The arrangement is additionally applied over the pipe. At the time

of the inspection it can be removed quickly without stopping of the power plant. This

eliminates inter granular stress corrosion cracking.

2.8 Biological Corrosion

Corrosive conditions can be developed by living microorganisms as a result of

their influence on anodic and cathodic reactions. This metabolic activity can directly or

indirectly cause deterioration of a metal by the corrosion process. This activity can

1. Produce a corrosive environment

2. Create electrolytic cells on the metal surface

3. Alter the resistance of surface films

4. Have an influence on the rate of anodic or cathodic reaction

5. Alter the environmental composition

Because this form of corrosion gives the appearance of pitting, it is first

necessary to diagnose the presence of bacteria. This is also referred to as microbial

corrosion.

The term microorganism covers a wide variety of life forms, including bacteria,

blue-green cyanobacteria, algae, lichens, fungi, and protozoa. All microorganisms may

be involved in the biodeterioration of metals. Pure cultures never occur under natural

56




conditions; rather, mixed cultures prevail. Of the mixed cultures, only a few actually

become actively involved in the process of corrosion. The other organisms support the

active ones by adjusting the environmental conditions to support their growth. For

example, in the case of metal corrosion caused by sulfate-reducing bacteria (SRB), the

accompanying organisms remove oxygen and produce simple carbon compounds, suchas acetic acid and/or lactic acid, as nutrients for SRB.

Bacteria are the smallest living organisms on this planet. Some can only live with

and others without oxygen. Some can adapt to changing conditions and live either

aerobically or anaerobically. There is a wide diversity with regard to their metabolisms.

They are classified as to their source of metabolic energy as given in Table 2:

Table - 2 Energy Source and its classification

These six terms may be combined to easily describe the nutritional requirements

of a bacterium. For example, if energy is derived from inorganic hydrogen donators and

biomass is derived from organic molecules, they are called microtrophs

(chemolithoorganotrophs).

Microorganisms can directly or indirectly affect the integrity of many materials

used in industrial systems. Most metals, including iron, copper, nickel, aluminum, and

their alloys, are more or less susceptible to damage. Only titanium and its alloys appear

to be generally resistant. [21]

Viable microorganisms can be found over a surprisingly wide range of

temperature, pressure, salinity, and pH. In the 1950s, pioneering work by Zobell isolated

sulfate-reducing bacteria (SRB) that grew at 104°C (219°F) and pressures of 1000 bar

from oil-bearing geological formations deep underground. Microbial communities exist in

57




environments as diverse as subzero snowfields to deep ocean thermal vents.

Halophiles evolved to live at extreme salinities turn pink the evaporation pans used to

win salt from seawater. Sulfur-oxidizing bacteria create very acidic conditions (pH < 1)

by producing sulfuric acid as an end product of their metabolism, while other

microorganisms survive the opposite end of the pH scale. Given these examples, itshould not be surprising that microorganisms have been implicated in the accelerated

corrosion and cracking of a correspondingly wide range of industrial systems. For

example, the involvement of thermophilic SRB in the severe intergranular pitting of 304L

stainless steel condenser tubes in a geothermal electrical power plant operating at

100°C (210°F) has been reported. In another example, microbiological activity and

chloride concentrated under scale deposits were blamed for the wormhole pitting of

carbon steel piping used to transport a slurry of magnesium hydroxide and alumina at

pH 10.5.

Whatever the environmental conditions, microorganisms need water, a source of

energy to drive their metabolism, and nutrients to provide essential building materials

(carbon, nitrogen, phosphorus, trace metals, etc.) for cell renewal and growth. An

understanding of these factors can sometimes help in failure investigations. Energy may

be derived from sunlight through photosynthesis or from chemical reactions. The

importance of photosynthetic metabolism is limited in the context of this article to above-

ground facilities or submerged structures that receive sunlight. For closed systems and

buried facilities, microbial metabolism is based on energy derived from oxidation

reduction (redox) reactions. Under aerobic conditions, reduction of oxygen to water

complements the metabolic oxidation of organic nutrients to carbon dioxide. Under

anaerobic conditions, electron acceptors other than oxygen can be used. Figure 1

illustrates the range of pH and redox potential where anaerobic forms of microbial

metabolism tend to be found. Whatever the metabolism, electrochemical reactions

catalyzed by enzymes provide energy for cell growth. Many of these reactions are not

important under abiotic conditions, because they are kinetically slow in the absence of

organisms. By promoting these reactions, microbes produce metabolites and conditions

not found under abiotic conditions. In some cases, electrons released by the oxidation

of metals are used directly in microbial metabolism. In other cases, it is the chemicals

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and conditions created by microbial activity that promote MIC. Secondary effects can

also be important. These include such things as the biodegradation of lubricants and

protective coatings designed to prevent wear or corrosion in an operating system, or the

alteration of flow regimes and heat-transfer coefficients due to the biological fouling of

metal surfaces. Given the potential impact of MIC on a wide range of industrialoperations, it is not surprising that microbiological effects are of significant concern in

failure analysis and prevention. Microbially induced corrosion problems afflict water-

handling operations and manufacturing processes in oil and gas production, pipelining,

refining, petrochemical synthesis, power production, fermentation, waste water

treatment, drinking water supply, pulp and paper making, and other industrial sectors.

Microbially induced corrosion is also a concern whenever metals are exposed directly to

the environment in applications including marine or buried piping, storage tanks, ships,

nuclear waste containers, pilings, marine platforms, and so on.

Corrosion Mechanisms Involving SRB. Perhaps the best-known mechanism of

MIC involves corrosion cells generated and sustained on steel surfaces by the action of

anaerobic SRB. Evidence for direct MIC based on other electron acceptors is limited.

Microorganisms can influence the corrosion process by a number of less direct

mechanisms. Depolarization Mechanisms. Buildup of hydrogen on the cathodic surface

can stifle the corrosion process through cathodic polarization. Microorganisms with

hydrogenase enzymes are able to use hydrogen and have been widely cited as

accelerating anaerobic corrosion through cathodic depolarization. Even though this

concept has been challenged, commercial kits for hydrogenase activity are available for

assessing MIC in practical applications. Metabolites, such as organic acids produced by

acid-producing bacteria (APB), may alleviate anodic polarization. Organic acids can

form soluble chemical complexes with metal ions released by the corrosion process,

reducing the buildup of Mn on anodic surfaces.

Considerable evidence indicates that bacteria and other organisms frequently

initiate or accelerate corrosion of metal- Microorganisms accelerate corrosion by

producing hydrogen sulfide or acids, both of which are highly corrosive to iron and steel.

Thiobacills thioOxidans, the bacteria which convert sulfides or free sulfur to sulfuric acid,

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may cause the extremely rapid corrosion of steel waterfront structures in the intertidal

zone; but this has not been established by experimental evidence.[22]

Experiments were undertaken at the U. S. Naval Civil Engineering Laboratory to

ascertain if the presence of microorganisms is necessary for corrosion to occur. It was

shown that, in aerated sea water, iron corrodes fairly rapidly whether or not

microorganisms are present; but that in sea water from which oxygen is excluded, iron

rusts very slowly unless sulfate-reducing bacteria or their metabolic by-product,

hydrogen sulfide, is present. To induce rapid anaerobic corrosion, the bacteria must be

supplied with carbohydrates or other nutrients. Anaerobic conditions and bacterial

nutrients might both be found in the layer of slime that accumulates on the surfaces of

structures placed in the ocean. Another experimental finding at the U. S. Naval Civil

Engineering Laboratory was that the carbonic anhydrase inhibitor, acetazolamidet is an

effective inhibitor of sea water corrosion.

2.9 Selective Leaching

When one element of a solid alloy is removed by corrosion, the process is known

as selective leaching, dealloying, or dezincification. The most common example is the

removal of zinc from brass alloys that contain more than 15% zinc. When the zinc

corrodes preferentially, a porous residue of copper and corrosion products remains. The

corroded part often retains its original shape and may appear undamaged except for

surface tarnish. However, its tensile strength, and particularly its ductility, are seriously

reduced. Dezincification of brasses takes place in either localized areas on the metal

surface, called “plug type,” or uniformly over the surface, called “layer type.” Low-zinc

alloys favor plug-type attack while layer-type attack is more prevalent in high-zinc alloys.

The nature of the environment seems to have a greater effect in determining the type of

attack. Uniform attack takes place in slightly acidic water, low in salt content and at

room temperature. Plug-type attack is favored in neutral or alkaline water, high in salt

content and above room temperature. Crevice conditions under a deposit of scale or

salt tend to aggravate the condition. A plug of dezincified brass may fall out, leaving a

hole, whereas water pipe having layer-type dezincification may split open.

Conditions that favor selective leaching are:

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1. High temperatures

2. Stagnant solutions, especially if acidic

3. Porous inorganic scale formation

Corrective measures that may be taken include:

Use of a more resistant alloy. This is the more practical approach. Red brass, with less

than 15% zinc, is almost immune. Cupronickels provide a better substitute in severely

corrosive atmospheres.

2. Periodic removal of scales and deposits from the inside surfaces of pipelines.

3. Removal of stagnation of corrosives, particularly acidic ones.

4. Use of cathodic protection.

REFERENCES:

[1]. P.A. Schweitzer. 2004. Corrosion Resistance Tables, Vols. 1–4, 5th ed., New York: Marcel Dekker.

[2]. Crolet, Jean-Louis, Bonis, M.R., Elf Aquitaine, Prediction of the Risks Of CO2 Corrosion in Oil andGas Wells, Journal Society of Petroleum Engineers, Vol. 6, Number 4, pp 449-453, Nov 1991.

[3]. S. Nesic, J. Postlethwaite, and S. Olsen, An Electrochemical Model for Prediction of Corrosion of Mild Steel in Aqueous Carbon Dioxide Solutions, NACE International Journal of corrosion, Volume52, Number 04, April, 1996.

[4]. Jin Huang et. al. Prediction of Uniform CO2 Corrosion of Mild Steel Under Inert Solid Deposits,NACE International conference on Corrosion 2011, March 13 - 17, 2011 , Houston, Texas.

[5].Jiabin Han, David Young, and Srdjan Nešić, Characterization of the passive film on mild steel inCO2 environments, 17th international corrosion congress, Paper no. 2511.

[6]. Jing Li, Dale J. Meier, An AFM study of the properties of passive films on iron surfaces, elsivier Journal of Electroanalytical Chemistry 454 (1998) 53–58.

[7]. V. Maurice, L. H. Klein, P. Marcus, Atomic-scale investigation of the localized corrosion of passivated nickel surfaces, Surface and Interface Analysis, Volume 34, Issue 1, pages 139–143, August 2002.

[8]. Shinji Fujimoto, and Rock-Hoon Jung, Passive Films Formed on Austenitic Stainless Steels under Humid Atmospheric Environments, ICE transaction.

[9]. M. J. Correia, M. M. Salta, I. T. E. Fonseca, Corrosion Resistance and Passive Film Characteristicsof Austenitic Stainless Steel Alloys in Alkaline Solution, ICE transaction.

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http://onlinelibrary.wiley.com/doi/10.1002/sia.v34:1/issuetoc

http://onlinelibrary.wiley.com/doi/10.1002/sia.v34:1/issuetoc




[10]. C. S. Wichman, J. R. Scully, Investigation of Intergranular Corrosion Propagation Promoted byInteractions between Sensitized Grain Boundaries using Closely Packed Electrode Arrays.

[11]. Anees U. Malik, Nadeem A. Siddiqi and Ismaeel N. Andijani, crevice corrosion performance of some commercial stainless steels in arabian gulf seawater, Issued as Technical Report No. SWCC(RDC)-28 in August, 1993

[12]. Navid Rashidi, Seyed Alavi-Soltani, Ramazan Asmatulu, Crevice Corrosion Theory, Mechanismsand Prevention Methods, Proceedings of the 3rd Annual GRASP Symposium, Wichita StateUniversity, 2007, pp 215-216

[13]. R.C. Barik, J.A. Wharton, R.J.K. Wood, K.R. Stokes, Nickel-aluminium bronze pitting corrosion inseawater, Environment and Mitigation

[14]. [f] Jones, Denny A., Principles and Prevention of Corrosion, 2nd Edition, New Jersey: Prentice Hall,

Inc., 1996.

[15]. Excel programme for calculation of FAC (Flow Assisted Corrosion = Erosion Corrosion) on the

basis of the publication of Kastner and Riedle (1986), 65 FAC-Calculation model engels

www.hbscc.nl.

[16]. [e] W. M. M. Huijbregts, Erosion-corrosion of carbon steel in wet steam, Materials Performance

October, 1984, pp 39-45.

[17]. Controlling stress corrosion cracking in aerospace applications, NASA, Marshall space flight center,Document Practice No. PD-ED-1227, pp 1 of 5.

[18]. Fastener Failures Due to Stress Corrosion Cracking, Daniel H. Herring, “The Heat Treat Doctor”,President The Herring Group, Inc., Fastener Technology International/August 2010.

[19]. Hiroyuki Masuda, Stress Corrosion Crack Growth Mechanism on SUS316L Stainless Steel, TheOpen Corrosion Journal, 2009, 2, 204-210.

[20]. J. S. POROWSKI, W. J. O'DONNELL, M. L. BADLANI, E. J. HAMPTON, Advanced Methods of Improving Resistance to Stress Corrosion Cracking in BWR Piping Systems, pressure vessel andthe Piping conference, Pittsburg, Pennsylvania-June: 19-23. 1988

[21]. Thomas R. Jack, NOVA Chemicals Ltd., Biological Corrosion Failures, ASM Handbook Volume 11:Failure Analysis and Prevention (#06072G).

[22]. Harold P. Vind, Ph. D., and Mary Jane Noonan, biological corrosion at naval shore facilities,technical note n-831, u. s. naval civil engineering laboratory Port Hueneme, California, 20 July 1966

http://www.hbscc.nl/



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