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Preface

New materials, composites, and coatings are being developed at a rapid rate, andthere has been an increase in the substitution or replacement of one class of materialby another. More complex materials are being engineered and used in a new varietyof environments. Many materials are used in composite or coated forms to enhanceperformance. Various combinations of metals, intermetallics, ceramics, and poly-mers are becoming more common. Composites with discontinuous, dispersedphases within a matrix, and fiber and laminated reinforcements are being devel-oped. Coatings also include combinations similar to those for bulk composite mate-rials. Materials are being pushed to perform in a wider range of environmentsthan ever before. Aqueous and high-temperature environments, which may containvarying amounts of corrosive species, are commonly encountered by advancedmaterials. In other cases, the effect of environments such as water, solvents, wine,and food thought to be relatively benign must be understood.

All these developments have made it difficult to locate information on theeffects of environment on the new materials, composites, and coatings. This compre-hensive book describes such effects for a broad range of materials and environments,filling the information gap and providing a comprehensive viewpoint for the scientistor engineer interested in applying new materials to existing applications or old mate-rials to new applications.

This book would not have been possible without the many hours given by eachcontributor. Their effort and dedication are greatly appreciated. Also, the assistance ofB. H. Wardlow at PNNL in coordinating the manuscripts is greatly appreciated.

Russell H. Jonesiii

Contents

Preface iiiContributors vii

I. Metallic Alloys

1. Ferrous Alloys (Ferritic and Martensitic) 1Bruce Craig

2. Austenitic Stainless Steels 31Russell H. Jones, Stephen M. Bruemmer, Mike J. Danielson,and Bruce Craig

3. Nickel-Based Alloys for Resistance to Aqueous Corrosion 55Paul Crook

4. Nickel-Based Alloys for Resistance to High-TemperatureCorrosion 75Mark A. Harper and George Y. Lai

5. Corrosion of Copper and Its Alloys 115Andrew James Brock

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vi Contents

6. Reactive and Refractory Alloys 151Te-Lin Yau

7. Aluminum Alloys 173N. J. Henry Holroyd

8. Magnesium Alloys 253Mike J. Danielson

II. Intermetallic Alloys

9. Environmental Embrittlement of Nickel-Based and Iron-BasedIntermetallics 275Norman S. Stoloff

III. Ceramics

10. Nonoxide Ceramics 311Nathan S. Jacobson and Elizabeth J. Opila

11. Oxide Ceramics 351F. S. Pettit, G. H. Meier, and J. R. Blache`re

IV. Composites

12. Metal Matrix Composites 375Russell H. Jones

13. Ceramic Matrix Composites 391Russell H. Jones, C. H. Henager, Jr., Charles A. Lewinsohn,and Charles F. Windisch, Jr.

14. Issues in Predicting Long-Term Environmental Degradation ofFiber-Reinforced Plastics 419Aaron Barkatt

V. Metallic Glasses

15. Amorphous and Nanocrystalline Alloys 459Koji Hashimoto

Index 501

Contributors

Aaron Barkatt Department of Chemistry, The Catholic University of America,Washington, D.C.

J. R. Blache`re Materials Science and Engineering Department, University ofPittsburgh, Pittsburgh, Pennsylvania

Andrew James Brock Metals Research Laboratories, Olin Corporation, NewHaven, Connecticut

Stephen M. Bruemmer Pacific Northwest National Laboratory, Richland,Washington

Bruce Craig MetCorr, Denver, Colorado

Paul Crook Engineering and Technology, Haynes International, Kokomo, In-diana

Mike J. Danielson Pacific Northwest National Laboratory, Richland, Wash-ington

Mark A. Harper Research and Development, Special Metals Corporation,Huntington, West Virginia

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viii Contributors

Koji Hashimoto Tohoku Institute of Technology, Sendai, Japan

C. H. Henager, Jr. Pacific Northwest National Laboratory, Richland, Wash-ington

N. J. Henry Holroyd Research and Development, Luxfer Gas Cylinders, Riv-erside, California

Nathan S. Jacobson Materials Division, NASA Glenn Research Center, Cleve-land, Ohio

Russell H. Jones Pacific Northwest National Laboratory, Richland, Wash-ington

George Y. Lai Consultant, Carmel, Indiana

Charles A. Lewinsohn Pacific Northwest National Laboratory, Richland,Washington

G. H. Meier Materials Science and Engineering Department, University ofPittsburgh, Pittsburgh, Pennsylvania

Elizabeth J. Opila Department of Chemical Engineering, Cleveland State Uni-versity, Cleveland, Ohio

F. S. Pettit Materials Science and Engineering Department, University of Pitts-burgh, Pittsburgh, Pennsylvania

Norman S. Stoloff Materials Science and Engineering Department, RensselaerPolytechnic Institute, Troy, New York

Charles F. Windisch, Jr. Pacific Northwest National Laboratory, Richland,Washington

Te-Lin Yau Te-Lin Yau Consultancy, Albany, Oregon

1Ferrous Alloys(Ferritic and Martensitic)Bruce CraigMetCorr, Denver, Colorado

I. INTRODUCTION

This chapter addresses the corrosion behavior of ferrous alloys, specifically fer-ritic and martensitic irons and steels. The reason for this designation is to distin-guish these alloys from the austenitic alloys that will be discussed in a laterchapter. However, the use of the terms ferritic or martensitic is not intendedto exclude pearlitic or bainitic microstructures, but is only intended as a conve-nience. Therefore, the discussion in this chapter addresses all low-alloy ferrousmaterials and ferritic and martensitic stainless steels.

The largest group of ferrous alloys are steels which will be the emphasisin this chapter, however, cast irons, several of which can be quite corrosion resis-tant, will also be mentioned. There are tens of thousands of different steels inthe world; however, they are usually referred to in groups as a function of theirchemical composition. Thus, carbon steels (also referred to as mild steels) containlittle or no alloy elements beyond the Mn, P, S, Si, and Al needed to produce agood quality structural material. The low-alloy steels are the next group that canbe characterized by small additions of Cr, Mo, and Ni, usually in the range ofabout 0.104.0% of each element, but generally less than 5% of the total alloyingelements. Higher additions of these elements form a group of steels referred toas alloy steels. Generally, the alloying content is equal to or less than 10% (e.g.,9 Cr1 Mo steel).

The distinction between low-alloy and alloy steels is not well defined noreven well observed in practice. Often, all of these steels are lumped togetherunder the term low-alloy steel or alloy steel. As will be seen in this chapter,

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this distinction is largely unnecessary from a corrosion standpoint because alloy-ing of less than 10% for many environments is not sufficient to impart significantcorrosion resistance to steels.

In a similar vein, cast irons are used as structural or pressure-containingalloys that have little natural corrosion resistance. Additions of Cr, Ni, and Siare most often the primary means for improving corrosion resistance.

Unquestionably the most important alloying element in steels and ironsfrom a corrosion standpoint is Cr. Steels containing in excess of 11% Cr, willdisplay stainless (rust-resistant) qualities when exposed to the atmosphere; thus,this group or, more properly, family of alloys is termed stainless steels. Inthis chapter, the ferritic and martensitic stainless steels will also be discussed.

Although the number of alloys that are covered by the categories just pre-sented are myriad, the general performance is relatively easy to address. Thecorrosion performance of these ferrous alloys is of major importance not onlybecause they represent the largest tonnage of metals used by the world but be-cause they represent the benchmark from which corrosion performance of otheralloys is compared.

II. CORROSION BEHAVIORA. General CorrosionCarbon and low-alloy steels generally display active corrosion in the majority ofenvironments to which they are exposed. This means they will corrode unabatedat some corrosion rate determined by factors such as solution composition,pH, fluid velocity, presence of oxidizers, temperature, and so forth. In manyof the environments to which ferrous alloys are exposed, there is little effect orbenefit of minor alloying element additions. Figure 1 illustrates the typical pol-arization behavior for steels in many environments. The anodic curve shows ac-tive corrosion with no tendency toward passivation. The environmental factorsmentioned earlier will determine the anodic and cathodic behaviors and ulti-mately the anodic current density (i.e., the corrosion rate). Figure 2 illustratesthe effect of increasing the conductivity of the solution (produced by increasingchloride content) on the corrosion rate of carbon steel (1). Solution conductivityplays a major role in the tendency for corrosion of alloys in a specific environ-ment. For example, in many hydrocarbon environments, the conductivity andpolarizability of the solution are so low that corrosion cannot be established ormaintained. In these solutions, carbon steel is quite useful and cost-effective.Likewise, in systems containing corrosive gases (i.e., CO2, H2S, etc.), if no wateris present, there is no electrolyte for corrosion, and carbon steels are adequatefor the service.

Ferrous Alloys 3

Fig. 1 Typical polarization behavior for mild steel under active corrosion.

In those environments in which corrosion of steel follows the behavior inFig. 1, other means of mitigating corrosion must be considered. These othermeans are coatings, inhibitors, cathodic protection, or anodic protection. Theseother methods are dealt with in more detail elsewhere (2).

In some very specific environments, steels may develop a protective corro-sion product layer that essentially passivates the steel surface, reducing corrosionto an acceptable level. Although the environments for which this phenomenonoccurs are few compared to those for active corrosion, they are notable. Examplesof such environments are steels exposed to concentrated sulfuric acid, hydroflu-oric acid, and sodium hydroxide. In concentrated sulfuric acid, a soft protectiveiron sulfate corrosion product is formed that inhibits further corrosion. However,

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Fig. 2 Conductivity effects on mild steel in aqueous solutions, argon saturated, as afunction of NaCl content.

this film is not mechanically strong and is easily eroded. Thus, this film is notsuitable for exposure to high-velocity streams, yet it is beneficial from a sulfuricacid storage standpoint because carbon steel containers can be used to handlethe acid under essentially static conditions.

Other environments produce this same behavior and Fig. 3 illustrates this

Ferrous Alloys 5

Fig. 3 Passive behavior of mild steel in environments such as Na2SO4.

development of passivity in the anodic curve that reflects a decrease in the anodiccorrosion current with the formation of a passive film (1).

Great care must be taken in applying this method of passivity, however,because many factors in actual service can eliminate or degrade this protectivefilm, causing significant corrosion to occur. Velocity changes, temperature in-creases, the presence of impurities (i.e., chlorides), and concentration changescan produce high-corrosion rates instead. A useful example of this change in

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corrosion rate is carbon steel in 90% sulfuric acid at room temperature. Understatic conditions, the corrosion rate is about 0.5 mm/year. However, at a concen-tration of less than 50% H2SO4, the corrosion rate exceeds 5 mm/year. Thus, thestability of the passive film is an important factor in the choice of any materialfor a specific environment and that choice may be suitable only over a narrowrange of conditions. As will be discussed in later chapters, the stability of thepassive layer on nickel-based alloys, titanium alloys, and other materials is muchgreater than for steels, thus the reason these alloys are more resistant to corrosiveenvironments. It is the great stability of the air-formed oxide on ferritic and mar-tensitic stainless steels that produce their stainless quality when exposed to theatmosphere. Yet, this passive film is not stable in all environments and care mustbe taken in their application, as the oxide is particularly susceptible to attack byhalides.

B. Localized CorrosionIn addition to the uniform or general corrosion of ferrous alloys, there are numer-ous forms of localized corrosion that can cause failure of these alloys. Pittingcorrosion is a highly localized attack of the metal, creating pits of varying depth,width, and number. Pitting may often lead to complete perforation of the metalwith little or no general corrosion of the surface. This can be a considerableproblem in steels and is one of the most common causes of failure for stainlesssteels. At this time it is impossible to predict the remaining life of a pitted struc-ture; thus, pitting remains one of the leading causes of failure for ferrous alloys.

Crevice corrosion is similar to pitting corrosion in its localized nature butis associated with crevices. Stainless steels and some nickel-based alloys are par-ticularly susceptible to this form of corrosion; however, steels are less susceptibleto this form of attack, except in aerated environments.

Intergranular corrosion is the preferential corrosion of grain boundaries in ametal caused by prior thermal treatments and related to specific alloy chemistries,especially in stainless steels and nickel alloys. Corrosion of this type is rare incarbon and alloy steels but can be a problem in ferritic and martensitic stainlesssteels.

Dealloying is the selective removal of one element (usually the least noble)from an alloy by the corrosive environment. Also referred to as selective leachingor dezincification, denickelification, and so forth, designating the element re-moved. Steels are not generally attacked by this mechanism, nor are ferritic ormartensitic stainless steels. However, some cast irons, especially gray iron, arequite susceptible to dealloying. For gray cast iron, the graphite flakes are cathodicto the surrounding ferritic matrix. Thus, the ferrite is selectively corroded away,leaving a mechanically weak graphite structure.

Corrosion fatigue is the initiation and extension of cracks by the combinedaction of an alternating stress and a corrosive environment. The introduction of

Ferrous Alloys 7

a corrosion environment often eliminates the fatigue limit of a ferrous alloy,creating a finite life regardless of applied stress level. It is currently impossibleto predict the corrosion fatigue life of an alloy because of the difficulty in distin-guishing the contributing effects of the corrosion portion and the mechanicalportion of corrosion fatigue.

Galvanic corrosion is the accelerated corrosion of the least noble metalwhen coupled to one or more other metals. The more noble metals are protectedfrom corrosion by this action. This form of attack is one of the most commoncauses of corrosion for all of the ferrous alloys, especially carbon and alloy steels.More detail on this type of corrosion is provided later in this chapter.

Many forms of flow-assisted corrosion are often included under the termerosioncorrosion such as cavitation, impingement, and corrosionerosion.All of these types of attack are the result of accelerated corrosion due to flow ofsolids, liquids, or gases, and the ferrous alloys are very susceptible to this formof attack. Therefore, ferrous alloys are quite limited for applications where acorrosive fluid, even one that is mildly corrosive, is combined with rapid flow.

Environmental cracking is the initiation and propagation of cracks by thecombined action of a corrosive environment and a tensile stress. Typically, underanodic conditions, this form of attack is most often referred to as stress-corrosioncracking (SCC). Generally, susceptibility to cracking increases with increasingtemperature, but not every alloy cracks in every environment. This form of corro-sion causes significant damage to steels and stainless steels.

Another form of cracking is strictly related to hydrogen absorption intoferrous alloys and the resultant cracking. In aqueous environments and in contrastto SCC, this occurs under cathodic conditions. There are numerous forms ofdamage associated with hydrogen, which are contained under the collective termhydrogen damage (HD). For hydrogen embrittlement and hydrogen-stresscracking, tensile stress and hydrogen atoms are necessary to cause failure. How-ever, contrary to SCC, susceptibility is greatest near room temperature. Otherterms and forms are hydrogen-induced cracking (HIC), blistering, sulfide-stresscracking (SSC), hydrogen stress-corrosion cracking, hydriding, and hydrogen at-tack. There are many other terms too numerous to mention. As with SCC, thisis a major problem in steels and martensitic stainless steels.

Although all of these corrosion mechanisms are of some concern for ferrousalloys, the three most problematic and often observed forms are pitting, galvaniccorrosion, and environmental cracking (this term is frequently used to encompassall forms of SCC and HD). Therefore, these three forms will be discussed ingreater detail as they relate to ferritic and martensitic steels.

1. Pitting CorrosionPitting corrosion is one of the most common and most insidious types of corrosionattack on steels. Pitting may rapidly produce perforation of a metal or may take

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many years to develop. Currently, there are no methods to accurately predict thepropagation rates of pits and, therefore, no valid means to estimate the remaininglife of a structure or component once pitting has initiated. There has been somesuccess in modeling pitting as a stochastic process, but, as yet, there is not anaccepted methodology. Because of this inability to predict pitting and remainingservice life, the primary focus in materials selection for a specific environmentis to choose a material that is either immune to a particular environment or atleast highly resistant to pitting in the first place. This can be a difficult task be-cause often it is not the major component of the service environment that inducespitting but rather the small concentration of some impurity that does. A goodexample of this is shown in Table 1, where increasing the Cl2 content of H2SO4requires a corresponding increase in Cr to the steel to resist pit initiation (3). Asdiscussed earlier, steels are generally resistant to concentrated H2SO4, but theintroduction of small amounts of Cl2 makes the solution particularly corrosive.This same effect is observed for steels exposed to seawater. Seawater itself isnot very aggressive to ferrous alloys; however, it is the introduction of dissolvedoxygen that causes seawater to become corrosive, producing severe pitting attack.Figure 4 illustrates the dramatic effect of oxygen, in only the parts-per-billion(ppb) range, on the corrosion of steel (4).

The mechanism of pitting is well understood in a general sense. Pit initia-tion begins with the very localized breakdown of the passive film, leading to theformation of a small pit bottom that acts as the anode and the remainder of thepassive surface as the cathode. Thus, there is a large driving force to continuedevelopment and propagation of the pit. However, the nature of pitting is a self-sustained autocatalytic process that continues pit propagation. During the propa-gation process, the solution in the pit bottom becomes and remains very acidic,further enhancing propagation. Moreover, the potential difference between thesteel surface and the pit bottom acts as a driving force for propagation. During

Table 1 Minimum Concentration ofCl2 Necessary for Pit Initiation in 1NH2SO4 Solution

Alloy Cl2 (normality)Fe 0.0003Fe5.6 Cr 0.017Fe11.6 Cr 0.06918.6 Cr9.9 NiFe 0.120.0 CrFe 0.124.5 CrFe 1.029.4 CrFe 1.0

Ferrous Alloys 9

Fig. 4 Effect of oxygen concentration on corrosion of mild steel in Pacific Ocean water.

this period, the original steel surface, which has not begun pitting, is effectivelyprotected from further corrosion by the resulting cathodic polarization.

The difficulty in predicting the remaining life of a structure during pittingcorrosion is due to continual pit initiation, propagation, repassivation, and re-propagation. Not all pits in the same structure propagate at the same rate andpropagation is not linear but rather an exponential function.

It is generally recognized that pitting will initiate at microstructural discon-tinuities on the steel surface. These discontinuities can be grain boundaries,second-phase particles, and so forth, but they are most often sulfide inclusions.This latter feature is most commonly the origin for pits in stainless steels. There-fore, it is quite predictable that resulfurized steels, especially the resulfurizedstainless steels, will suffer pitting corrosion in an environment long before andunder less severe conditions than the lower sulfur version of the same steel. Forexample, AISI 416 stainless steel, which contains 0.15% S minimum, compared

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to its counterpart, AISI 410, which contains 0.030% S maximum, is highly sus-ceptible to pitting corrosion and may pit in environments where AISI 410 doesnot.

It is impossible to list all the environments in which steels pit because ofthe numerous factors involved and the great variety of possible combinations ofchemicals. Moreover, great care must be taken when selecting an alloy for acertain application in not simply scanning the large number of corrosion datareferences and databases for alloys with low corrosion rates, as most of theseresources do not present pitting data, but rather provide only uniform corrosionrates that can be very misleading. However, that said, it is often the case thatsteels and, for that matter, many other alloys have a great tendency to pit inenvironments that contain chlorides or, more generally, halides. Although chlo-rides are by far the most prevalent species in many environments bromides, io-dides and fluorides can also induce pitting.

Therefore, the presence of halides in a process stream should be a signalthat pitting must be considered in the choice of alloys. Yet, the absence of thesespecies does not necessarily eliminate the possibility of pitting. An example ofpitting in the absence of halides is corrosion from CO2 gas dissolved in water.This condition produces carbonic acid that can lead to pitting corrosion of carbonand low-alloy steels. Of course, the situation becomes more complex as a functionof temperature and the introduction of chlorides. Figure 5 shows the envelopeof applicability of AISI Type 420 stainless steel (also referred to as 13 Cr) to acombined environment of CO2 and chlorides as a function of temperature in theabsence of oxygen (5). Within this envelope, no pitting occurs and corrosion isminimal but uniform. However, the introduction of small concentrations (ppb)of oxygen creates a severe pitting attack of the 13 Cr even at ambient temperature,thereby eliminating the use of this alloy.

Thus, prior experience or laboratory testing is often necessary to confirmthat a particular alloy will not be susceptible to pitting in a specific environment.

2. Environmental CrackingAs previously indicated, environmental cracking (EC) is a general term that en-compasses all forms of cracking that are induced or accelerated by the serviceenvironment. The two principal categories within this form of corrosion that arepertinent to this discussion on ferrous alloys are SCC and HD. An in-depth reviewof these types of cracking and their mechanisms can be found elsewhere (6). Itis simplest and consistent with much of the literature to discuss SCC in termsof an active path corrosion coupled with a tensile stress (often referred to asanodic cracking) and HD as all those forms of cracking that depend on hydro-gen assistance (often referred to as cathodic cracking in aqueous environments,

Ferrous Alloys 11

Fig. 5 The corrosion resistance of 13 Cr (Type 420) stainless steel in CO2/NaCl environ-ments in the absence of O2.

hydrogen-stress cracking, HIC, blistering, and hydrogen embrittlement, to namea few).

In many environments where steels are susceptible to SCC, a frequent pre-cursor to crack initiation is pitting corrosion. In these environments when stresses,either applied or residual, are relatively low, pitting ultimately produces failure.However, as the stress level increases, SCC can become the controlling mode offailure. Figure 6 illustrates this sequence of events (7). The important feature ofSCC is that cracking initiates and propagates at a subcritical level below thescale of macroscopic flaws that would be considered critical from a linear elastic-fracture mechanics (LEFM) standpoint. Therefore, LEFM by itself cannot beused to predict the likelihood of EC.

One of the most significant factors affecting EC is the strength level of thesteel. High-strength steels are very susceptible in a variety of environments andthis susceptibility is a function of the yield strength. Figure 7 shows that manysteels fail in a simple marine environment at ambient temperature when the yieldstrength exceeds about 180 ksi, regardless of alloy composition (8). However,below 150 ksi yield strength, cracking does not occur in this environment. Figure

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Fig. 6 Proposed sequence of crack initiation, coalescence, and growth for steels under-going subcritical cracking in aqueous environments.

7 should not be construed to mean that ferrous alloys do not crack at all below150 ksi, because quite low-strength steels are very susceptible to EC, just indifferent environments. Moreover, the cracking of these high-strength steels inseawater is thought to involve HD. Thus, it is convenient to further discuss theEC of ferrous alloys in two groups; high-strength steels (.150 ksi) and low-strength steels (#150 ksi).

a. Environmental Cracking of Low-Strength Steels Low-strength steels(#150 ksi yield strength) are quite susceptible to EC in certain specific environ-ments. The yield strength of the steel in this strength range is not particularlysignificant to the susceptibility to EC as it is for higher-strength steels. Rather,other factors such as applied stress, steel composition, pH, solution composition,potential, and temperature are much more critical. Increasing applied (or residual)stress and increasing temperature enhances the SCC of low-strength steels asdoes decreasing pH. Small concentrations of trace or impurity elements in thealloy can have a profound effect on SCC of steels.

Some of the more common environments known to cause SCC of low-strength steels are liquid NH3, CO2/CO, carbonate/bicarbonate, hydroxide, nitrate

Ferrous Alloys 13

Fig. 7 Stress-corrosion behavior of steels exposed to marine atmosphere.

solutions, and amine solutions. Generally, as the concentration of the solutionincreases, the susceptibility to SCC increases. Table 2 shows the effect of increas-ing nitrate concentration on the threshold stress of a plastically deformed lowcarbonmanganese steel (9). The threshold stress (that stress below which crack-ing does not occur) decreases with increasing concentration of nitrate and is par-tially dependent on the specific cation associated with the nitrate anion.

Similar behavior has been observed for OH solutions and sufficient datahave been gathered to develop the useful engineering diagram shown in Fig. 8

Table 2 Threshold Stress Values (ksi) for Mild Steel inBoiling Nitrate Solutions of Various Concentrations

Solution concentration

Nitrate 8N 4N 2.5N 1N

NH4NO3 2.2 3.4 7.8 13.4Ca(NO3)2 5.6 7.8 13.4 25.8LiNO3 5.6 9.0 21.3 (2N ) 25.8KNO3 6.7 4.5 15.7 26.9NaNO3 9.0 9.5 24.7 29.2

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Fig. 8 Temperature and concentration limits for stress-corrosion cracking susceptibilityof carbon steels in caustic soda (NaOH).

(10). Area C can also be handled successfully with austenitic stainless steels.This diagram illustrates the important effect of residual welding stresses on SCCand the ability to extend the range of applicability of steels in OH simply byreducing the residual stresses.

An area of great concern that has recently received increased attention isthe SCC of low-strength pipeline steels. The external SCC of pipeline steels hasoccurred in two distinct environments. Early failures were in soil environmentsthat produced solutions of carbonate/bicarbonate with a pH of about 9.5 on theoutside of the pipe, causing intergranular SCC. Figure 9 shows the intergranularSCC of a low-strength pipeline steel that failed in the high-pH environment. Morerecently, transgranular SCC has been found to be the cause of several pipelinefailures. The pH in this latter case frequently falls in the range of 68. Many of

Ferrous Alloys 15

Fig. 9 Intergranular fracture of a low-strength pipeline steel from SCC. Magnification:1003.

the pipeline failures occurred in pipelines that are more than 20 years old thathave yield strengths around 52,000 psi. This long incubation time for crack initia-tion and propagation is typical for low-strength steels and in sharp contrast tothe often rapid initiation and fracture of high-strength steels. Yet, it would bemisleading to assume that SCC of low-strength steels is always a slow process.Figure 10 shows that the crack growth rate in many low-strength steels is a strongfunction of the solution composition and is directly related to the bare surfacecurrent density (11). This current density, in combination with straining at thecrack tip, is the driving force for cracking and is frequently referred to as activepath or anodic cracking. It is generally believed, though not entirely agreed, thatSCC progresses by the rupture of the oxide film at the crack tip, thereby providinga bare surface for the peak current to advance the crack tip a certain distance

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Fig. 10 Crack velocities and peak current densities at the same potentials for a varietyof systems and alloys.

before the oxide re-forms and the crack arrests. These events may occur overmany cycles or just a few. However, it is important to recognize the differencein this mode of cracking versus that due to cathodic cracking, where hydrogenis the primary agent to assist cracking.

In general, martensitic stainless steels are used at higher strengths thanferritic stainless steels because the former can be strengthened by heat treatmentand the latter cannot. Therefore, the martensitic stainless steels are discussedunder the high-strength section. Although ferritic stainless steels are generallymore resistant to SCC than austenitic stainless steels, especially in chloride solu-tions, they are not entirely immune. Small additions of Ni and plastic deformationcan each increase the tendency for SCC in chloride environments.

Hydrogen damage of low-strength steels typically occurs in steels that haveyield strengths less than 100 ksi. As with the SCC of low-strength steels, theyield strength is not an important factor in HD. Moreover, residual and/or appliedstresses have little effect.

Ferrous Alloys 17

The primary cracking modes are stepwise cracking (SWC, also referred toas HIC) and blistering (HB) or blister cracking. Both types of cracking are theresult of relatively high hydrogen input fugacities compared to the HD of high-strength steels and are often found together in the same steel. Both SWC andHB are considered to occur by the classical hydrogen-pressure mechanism. Ac-cording to this mechanism, hydrogen atoms enter the steel and combine at discon-tinuities (i.e., nonmetallic inclusions) to form molecular hydrogen, which is toolarge of a molecule to diffuse back out of the steel. The molecules continue toaccumulate, increasing the local hydrogen pressure until a crack or blister forms.Figure 11 shows an example of SWC in a low-strength steel exposed to H2S.Hydrogen damage of steels occurs over the entire strength range of typical engi-neering applications. Figure 12 shows that regardless of the strength level of thesteel, some form of hydrogen cracking may occur and the only distinction is inthe morphology of cracking (12).

b. Environmental Cracking of High-Strength Steels The EC of high-strength steels (.150 ksi) is highly dependent on strength, and in many environ-ments, it is difficult to distinguish between the more classical HD and SCC mech-

Fig. 11 Stepwise cracking from hydrogen in a low-strength steel exposed to H2S. Magni-fication: 253.

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Fig. 12 Critical hydrogen concentration in steel for cracking as a function of yieldstrength and the morphology of cracking.

anisms. On the other hand, EC from caustic solutions are not obviously hydrogenrelated, but the crack propagation rate is many times faster than for low-strengthsteels in caustic.

Seawater and brackish waters do not typically produce EC failures of low-strength steels, but they do produce the EC of high-strength steels, as demon-strated earlier in Fig. 7. Again, this is most likely a HD mechanism.

From an engineered materials sense, the actual mechanism is not as impor-tant as the fact that high-strength steels are so susceptible to EC, and the resultingcrack propagation rate so high that catastrophic failure in many otherwise benignenvironments can easily occur. Because of this high risk of steel failure withincreasing strength beyond 150 ksi, it is common practice to select other alloysand materials that have a greater overall corrosion resistance for high-strengthapplications. These materials and their performance are dealt with in the remain-der of this book.

Ferrous Alloys 19

As an example of the behavior of high-strength steels in some of theseenvironments, Fig. 13 shows the crack propagation rate of various alloy steels(13). It is apparent that above 150 ksi yield strength, the crack growth rates areso high that strength level becomes meaningless. Moreover, the speed at whichthe crack propagates is too rapid for detection in actual service, often leading toa catastrophic failure.

Martensitic stainless steels are generally resistant to chloride SCC whenheat treated to yield strengths less than about 100 ksi. However, above this yieldstrength, they become increasingly susceptible to EC in seawater and H2S. Bothof these environments are known to produce hydrogen so that the failure of high-strength martensitic stainless steels in these cases is probably a HD mechanism.

When selecting an alloy or material for a specific application, it is commonpractice to first ensure that EC will not be a potential problem in service. Oncethis form of degradation is eliminated, the select material can be further evaluatedfor resistance to other less catastrophic forms of attack.

Fig. 13 Comparison of stress-corrosion crack velocities in maraging and low-alloysteels.

20 Craig

C. Galvanic CorrosionGalvanic corrosion is one of the most common yet least recognized corrosionproblems for ferrous alloys. When two or more dissimilar metals are intimatelyconnected and placed in a solution, current will flow between them because ofthe potential difference of the metals. The metal with the least resistance to corro-sion (active metal) in the particular environment will become the anode, and themore corrosion-resistant metal (noble metal) will become the cathode. Corrosionof the anode will usually be more severe than if that metal was alone in thesame solution, whereas the cathode will achieve a degree of protection from theenvironmentsometimes to the extent that corrosion is completely stopped onthe cathodic metal. These effects can be measured and have been done for couplesof metals in seawater at 25 C. Table 3 provides a relative ranking of metals andalloys regarding their resistance to corrosion in seawater (14). The greater thedistance between two metals in the table, the greater their potential difference andthe higher the probability that the active metal will suffer accelerated corrosion.

Note that some alloys and metals are listed twice in the table: once withthe word active following and once with the word passive. Some metalsand alloys become essentially immune to corrosion in certain environments be-

Table 3 Galvanic Series of Some Commercial Metals and Alloys in Seawater

Active or anodic Yellow brassMagnesium Admiralty brassMagnesium alloys Red brassZinc CopperGalvanized steel Silicon bronzeAluminum 1100 7080 Cupro nickelAluminum 2024 GbronzeMild Steel Silver solderWrought Iron Nickel (passive)Cast Iron Nickel alloy 600 (passive)13% Chromium stainless steel 13% Chromium stainless steel

Type 410 (active) Type 410 (passive)18-8 Stainless steel 18-8 Stainless Steel

Type 304 (active) Type 304 (passive)Leadtin solders SilverLeadTinMuntz metalManganese bronze GraphiteNaval brass GoldNickel (active) PlatinumNickel Alloy 600 (active) Noble or cathodic

Ferrous Alloys 21

cause of the formation of a surface film so thin that it is impossible to see withthe naked eye or even with an optical microscope. The stability of these films isparamount to the enhanced corrosion resistance of these alloys. Moreover, corro-sion films represent the controlling factor in almost all corrosion (15). The addi-tion of chromium to iron can produce a passive alloy (18-8 stainless steel) ofconsiderable corrosion resistance compared to the original iron. However, in anenvironment in which the passive film is not functional, the active surface be-comes far less noble, as indicated in Table 3.

A more active metal in the series will corrode at the expense of a noblerone. Thus, coupling zinc to steel will cause the zinc to corrode and will protectthe steel. This is the reason for galvanizing steel; when pinholes in the galvanizingoccur, the steel underneath, once exposed to the environment, will be protectedby the zinc. This is also the basis for cathodic protection. Sacrificial anodes aremade of metals or alloys that are more active than steel, which allows for theconsumption of the anode and the protection of the steel structure. However, ifsteel is coupled to copper, the distance on the chart is large, so in this case, steelwill be the anode and have a greater tendency to corrode.

The galvanic series is useful for approximating the behavior of coupledalloys; however, care must be used in its application. Several parameters affectgalvanic corrosion and, as such, may affect the actual behavior of a couple inservice. Two important factors in galvanic corrosion are the temperature and therelative area of the metals. Increasing temperature in some cases may cause areversal in the anodecathode relationship. This reversal has been responsiblefor failures of galvanized systems or systems protected with zinc sacrificialanodes. These effects point to the need to measure the potential of a couple,especially in cathodic protection, in the actual environment prior to its applica-tion. It must always be borne in mind that the ranking of alloys in Table 3 isstrictly true only for seawater and that extending it to other environments mayresult in some changes in the position of metals and alloys in the series.

The other factor, the ratio of the area of the anode to the area of cathode,is of considerable importance. If the anode area is smaller than the cathode area,the corrosion rate may be increased many orders of magnitude as a function ofthis ratio. However, if the anode area is greater than the cathode area, corrosionof the anode will be less than for a 1:1 anode/cathode ratio.

III. EFFECTS OF ALLOYING ELEMENTS

Small additions of alloying elements to ferrous alloys generally do not signifi-cantly improve their corrosion resistance. As stated earlier, at least 11% Cr isneeded to ensure that a steel becomes stainless and thus possesses a certain degreeof corrosion resistance. One important exception to this behavior is the class of

22 Craig

Fig. 14 Corrosion rate of steels in wet CO2 as a function of the chromium content ofthe alloy.

steels referred to as weathering steels. Small additions (,0.5%) of elements suchas Cr, Ni, Cu, and P greatly enhance these steels resistance to rusting in theatmosphere. This is accomplished through the production of a tight tenaciousrust that forms on the steel after exposure to its environment. Hence, furthercorrosion is stifled. However, for most other service environments, the smallvariations in alloying or tramp elements are not sufficient to increase the corrosionresistance of a steel. Figure 14 shows the benefit of increasing the Cr content onsteels exposed to water containing a high concentration of dissolved CO2 (16).This behavior is typical for steels exposed to many acids as well as other environ-ments. Thus, Cr is an important alloying element for a steels resistance to acid

Ferrous Alloys 23

attack. A similar benefit is observed for cast irons exposed to nitric and hydro-chloric acid by alloying with either Si or Ni in excess of about 5%.

IV. EFFECTS OF HEAT TREATMENT

Carbon and low-alloy steels and the martensitic stainless steels can be and areheat treated to enhance certain mechanical properties and, as a result, developdifferent microstructures. Various combinations of ferrite, pearlite, bainite, andmartensite may be present in a particular steel, depending on its thermal history.By and large, heat treatment and the subsequent phases formed are not a signifi-cant factor in the corrosion of steels. However, there are important exceptions.As previously discussed, environmental cracking is highly dependent on steelstrength and, to some degree, its microstructure. Likewise, in certain corrosiveenvironments, a distinction can be observed in corrosion rate as a function ofheat treatment. One important example is shown in Fig. 15, for Type 420 stainlesssteel in wet CO2 (17). In actual practice, heat treatment/microstructure is not alarge concern, except for environmental cracking.

V. EFFECTS OF SOLUTION VELOCITY

In general, the corrosion rate of ferrous alloys increases with increasing velocity.This can be readily explained by the increased mass transport of ferrous ionsacross the fluid boundary layer that is established under flowing conditions com-

Fig. 15 Effect of the tempering on the yield strength and on the corrosion resistanceof quenched-and-tempered 13% Cr stainless steel.

24 Craig

bined with the enhanced transport of corrodents to the metal surface across thisboundary layer. Of course, the picture is actually more complex when the Helm-holtz double layer and the presence of a corrosion product film are included.However, regardless of these issues, at some critical velocity these layers areessentially stripped away and bare metal is continually exposed to the fluidstream. At this point, one of two entirely divergent phenomenon may occur. Theerosioncorrosion rate becomes extremely high due to the loss of the rate-determining mass transport across all of these layers or the erosioncorrosionrate becomes much lower as a result of the inability of the corrodent to reachthe bare metal surface and have sufficient time to react. Both of these phenomenaare observed for ferrous alloys. Figure 16 illustrates the former case for carbonsteel in distilled water (18). At different pHs, the corrosion product formed canbe resistant to erosioncorrosion (pH , 5 and 9), thus minimizing the erosioncorrosion rate, or the corrosion film is unstable (pH , 4, pH 68), leading tohigh erosioncorrosion rates. The introduction of solid particles such as sandsignificantly lowers the erosioncorrosion threshold, making the selection ofalloys resistant to erosioncorrosion much more difficult. In the absence of solidparticles, it has been found that erosioncorrosion resistance is strongly a func-tion of the nature of the oxide. Therefore, more corrosion-resistant alloys suchas stainless steels, nickel-based alloys, and titanium alloys have greater erosioncorrosion resistance than steels, even if the alloy is much softer, because of theirtighter more resilient oxides.

Fig. 16 Effect of pH of distilled water on erosion corrosion of steel at 50 C and 12m/s flow rate.

Ferrous Alloys 25

VI. GENERAL APPLICABILITY OF FERROUS ALLOYS

Every service environment is different and care should be taken in trying to gener-alize the performance of ferrous alloys, especially carbon and low alloys, in com-mon environments. However, several important characteristics are worth empha-sizing.

In almost every circumstance, the presence of dissolved oxygen in solutioncauses an increased corrosion rate for steels and cast irons. However, for ferriticand martensitic stainless steels, the presence of oxygen is not as critical and oftenoxygen in only the ppb range is necessary to maintain the oxide film. Moreover,under total anaerobic conditions at sufficiently high temperatures, oxygen is avail-able from dissociation of the water molecule. The problem for steels and castiron is due to the fact that oxygen is a very effective cathodic depolarizer; thatis, it stimulates the cathodic reaction, and because the anodic and cathodic reac-tions are interdependent, it produces a net increase in the corrosion rate.

Likewise, steels are not resistant to corrosion in acidic pH environmentsor even in many neutral pH environments, especially in the presence of dissolvedoxygen. It is for this reason that steels are most often painted (coated) or cathodi-cally protected to ensure a satisfactory service life. Exposure to the atmosphereis often sufficient to cause corrosion of ferrous alloys to the extent they becomeeither unserviceable or, more typically, aesthetically unpleasing. Moisture, tem-perature, periods of wet and dry, the presence of chlorides (coastal locations), andindustrial pollutants (oxides of sulfur and nitrogen) all contribute to atmosphericcorrosion of steels and cast iron.

Atmospheric corrosion requires a critical moisture content in the atmo-sphere and its rate generally increases when the humidity exceeds this criticalvalue (i.e., approximately 60%). Figure 17 illustrates this phenomenon for corro-sion of many alloys, including steels, as a function of relative humidity (RH).

A form of atmospheric corrosion, wet corrosion, is considered to occurwhen actual water layers or pools form on the surface of the metal, often fromdew, rain, or sea spray. This can be a very complex state because a thin layerof water can act to dissolve a high concentration of gases from the atmosphere,causing a concentrated solution at the metal surface, which produces a corre-spondingly high, short-term corrosion rate that produces a locally high metal ionconcentration, resulting in an oxide that stifles further corrosion or, if the corro-sion product is soluble, continued localized attack.

Temperature can have many secondary effects on atmospheric corrosionbesides the primary effect of increasing reaction rate. Temperature influences therelative humidity, dew point, and time of wetness; all important factors in them-selves on atmospheric corrosion.

Contaminants, essentially airborne in nature, can profoundly affect atmo-spheric corrosion. Agents such as gases, like SO2, that can selectively absorb on

26 Craig

Fig. 17 Corrosion of ferrous alloys as a function of relative humidity.

metal surfaces which act as a catalyst to form SO3 and thus H2SO4 in moistenvironments or particulates such as dust that can cause local cells to form byaiding in the absorption of water and chlorides can accelerate corrosion of alloysthat normally would be resistant to atmospheric corrosion in a relatively cleanenvironment.

Climatic conditions have a variable effect on corrosion rate. For example,in some regions, winter exposure may be more severe than summer if fuels areused during cold spells that increase combustion products in the air such as NOxand SO2. Conversely, if these fuels are not used in the region, then summer maybe worse due to the higher metal temperatures.

Likewise, periodic rainfall may be beneficial causing a rinsing action onthe surface compared to a climate where the surface is continually wet. Thus,time of wetness and wet/dry cycles can also be quite important, especially iffrequent periods of wet and dry can limit the formation and development of aprotective oxide layer. Moreover, the existence of insoluble corrosion productscan act to entrain water during short dry cycles, keeping the metal surface suffi-ciently wet to continue corrosion.

Two other external environments that can cause significant deteriorationof ferrous alloys are soils and concrete. The majority of buried structures in the

Ferrous Alloys 27

world are made of cast irons and steels. Soils have a great variability in theirtendency to cause corrosion of ferrous alloys. Some of the more important factorsthat contribute to the aggressiveness of soils are resistivity, pH, moisture, oxygen,bacterial activity, and temperature of the ferrous alloy (i.e., hot pipelines) in con-tact with the soil. Decreasing resistivity and pH, increasing moisture content,oxygen content, and bacterial activity all enhance corrosion of ferrous alloys insoils. The elevated temperature of the steel surface not only can increase thecorrosion rate but also can lead to other forms of more serious attack such asEC. As shown in Fig. 9, the external SCC of pipelines has become a particularproblem for those pipelines that operate above ambient temperature and the trendin the future is to operate pipelines at even higher temperatures. Often, coatingsand cathodic protection can be used to limit these problems; however, they canalso exacerbate them if not properly maintained. One of the major reasons olderpipelines have become susceptible to SCC is that the coatings have degradedover time and the cathodic protection systems cannot effectively limit corrosion.

Corrosion of steel reinforcing bar in concrete has gained attention due tothe widespread problem of the crumbling infrastructure (bridges, highways,buildings, etc.) in many countries. Typically, the steel rebar corrodes as a resultof the pH of the cement paste in contact with the steel and the diffusion of chlo-rides and oxygen into and through the concrete. As corrosion products form onthe steel, they represent a larger volume than the original iron in place, therebyspalling and cracking the concrete. It has been found that temperature and relativehumidity are important factors in rebar corrosion as well as chlorides and oxygen.Thus, tropical climates that are hotter and more humid than temperate climateswould be expected to have a greater problem with rebar corrosion than coolerclimates. However, even northern climates have had problems for other reasons;for example, deicing salts used on bridges and roadways to eliminate snow andice can cause severe rebar corrosion. Several of the methods currently used tofight rebar corrosion are organic coated steel, galvanized steel, stainless-steelrebar, and cathodic protection.

As can be appreciated from the foregoing comments, coatings on ferrousalloys are an important means of extending the applicability and service life ofthese materials. As this is a book on engineered materials, it is beneficial to atleast mention the general types of coatings used on ferrous alloys. Coatings canbe grouped into four general categories: organic, inorganic, conversion, and me-tallic. Table 4 lists some of the typical coatings under each of these categories.Under the category for metallic coatings, the specific metal is not listed becausemany metals can be applied; rather, the process is provided because it will deter-mine which metal can be applied. The selection and use of a particular coatingis a function of many factors and care must be taken in selecting the right coatingfor a ferrous alloy.

It must always be borne in mind that a coating is part of a system that

28 Craig

Table 4 General Categories of Coatings

Organic MetallicCoal tars GalvanizingEpoxy PlatingPhenolics Ion implantationAlkyds CladdingVinyls Flame sprayUrethanes Chemical vapor depositionAcrylics Physical vapor deposition

Conversion InorganicAnodizing SilicatesPhosphating CeramicsChromate GlassMolybdate

Fig. 18 Coating degradation and corrosion of HY80 in artificial seawater.

Ferrous Alloys 29

includes the steel substrate; therefore, if the coating is damaged, corrosion oftenoccurs by different mechanisms than if only the steel were involved. For example,galvanizing (Zn) acts as a sacrificial anode to the underlying steel if the coatingis damaged. Thus, the steel substrate is protected against corrosion. However, amore noble coating such as Ni on steel can act as a large cathode, thereby acceler-ating corrosion at a damaged location (referred to as a holiday). Even organiccoatings can display accelerated corrosion at holidays in the coating depending onthe particular environment to which they are exposed (Fig. 18) (19,20). Generally,coatings show a slow decrease over time in the coating (film) resistance, indicat-ing the gradual permeation of water and other ionic species through the coating.Thus, coating life in any environment is finite and organic coatings are not trulybarriers, as is so often mistakenly suggested. However, the application of thecorrect coating can often double or triple the useful life of a ferrous alloy incertain environments and is, therefore, an important factor in the selection offerrous alloys.

In conclusion, even though ferrous alloys (ferritic and martensitic) are themost widely used engineered materials, they are also the least corrosion resis-tantreadily degrading in most environments. Because of this behavior, theymost often require additional means of controlling corrosion (i.e., coatings, ca-thodic protection, inhibitors) to provide a satisfactory service life.

REFERENCES

1. RL Martin. Application of Electrochemical Polarization to Corrosion Problems. St.Louis, MO: Petrolite Corp., 1977.

2. ASM Metals Handbook, Vol. 13, Corrosion. ASM International, Materials Park,OH, 1989.

3. ND Stolica. Pitting corrosion on FeCr and FeCrNi Alloys. Corrosion Sci 9:455460, 1969.

4. D Wheeler. Treating and monitoring 450,000 b/d injection water. Petrol Eng Int1975; November: Vol. 38, 6872.

5. BD Craig. Selection guidelines for corrosion resistant alloys in the oil and gas industry.Technical Publication No. 10073, The Nickel Development Institute, Toronto, 1995.

6. BD Craig, RH Jones. Environmentally induced cracking. In: ASM Metals Hand-book, Vol. 13, Corrosion, ASM International, Materials Park, OH, 1989, pp. 145171.

7. FP Ford. Quantitative prediction of environmentally assisted cracking. Corrosion51:375395, 1996.

8. EH Phelps. Stress corrosion behavior of high yield strength steels. Proc. SeventhWorld Petroleum Congress. Amsterdam: Elsevier, 1967.

9. RN Parkins, R User. The effect of nitrate solutions in producing stress corrosion

30 Craig

cracking in mild steel. First International Congress on Metallic Corrosion, London,1961, p. 289.

10. Corrosion Data SurveyMetal Selection, 6th ed. Houston, TX: NACE Interna-tional, 1985, p. 176.

11. RN Parkins. Predictive approaches to stress corrosion cracking failure. CorrosionSci 20:147166, 1980.

12. E Sato, M Hashimoto, T Murata. Corrosion of Steels in a Wet H2S and CO2 Environ-ment. Second Asian Pacific Corrosion Control Conf., Kuala Lumpur, 1981.

13. CS Carter. Fracture toughness and stress corrosion characteristics of a high strengthmaraging steel. Met Trans 2:16211625, 1971.

14. Corrosion Basics. LS Van Delinder, ed. Houston, TX: NACE International, 1984,p. 35.

15. BD Craig. Fundamental Aspects of Corrosion Films in Corrosion Science. NewYork: Plenum Press, 1991.

16. NG Galindez Ruiz. The effect of crude oil on corrosion of alloys in H2S/CO2 envi-ronments. Masters thesis, Colorado School of Mines, 1993.

17. JL Crolet. Acid corrosion in wells (CO2, H2S): Metallurgical aspects. J Petrol Tech-nol 35:15521558, August 1983.

18. MG Fontana. Corrosion Engineering, 3rd ed., New York: McGraw-Hill, 1986, p. 94.19. BD Craig, DL Olson. Corrosion at a holiday in an organic coated-metal substrate

system. Corrosion 32:316321, 1976.20. JR Scully. Evaluation or organic coating deterioration and substrate corrosion in

seawater using electrochemical impedence measurements. Corrosion/86, NACE,Houston, TX, 1986.

2Austenitic Stainless Steels

Russell H. Jones, Stephen M. Bruemmer, and Mike J. DanielsonPacific Northwest National Laboratory, Richland, Washington

Bruce CraigMetCorr, Denver, Colorado

I. INTRODUCTION

Austenitic stainless steels derive their stainless properties from the presenceof a very effective passive film. This film forms spontaneously within aqueousenvironments and the stability of this film in various environments determinesthe corrosion resistance of stainless steels. Alloy composition is also verysignificant in determining the stainless character of this family of alloys. Chro-mium concentration is the one element that directly affects the passive film stabil-ity, and Ni, Mn, Mo, C, and N also play a role. Passive behavior begins at about10% Cr, with increasing film stability occurring with increasing Cr concentration.Repassivation can occur in aqueous environments and it is the rate at which abreak in the passive film re-forms that also contributes to the stainless charac-ter of these materials. Under selected conditions, the passive film is not stableand this can lead to general or localized corrosion phenomena. Austenitic stain-less steels are often prepassivated in an acid bath, but the removal of surfacecontaminants that would hinder passivation in aqueous solutions is the primarypurpose of this prepassivation treatment. Given a clean surface in an alloy with

31

32 Jones et al.

sufficient Cr, austenitic stainless steels will passivate upon immersion in an aque-ous environment. Also, there will always be an air-formed oxide even withoutthis prepassivating treatment.

Austenitic stainless steels undergo all the common forms of corrosion, in-cluding (1) general, (2) galvanic, (3) pitting, (4) crevice, (5) intergranular, and(6) stress corrosion. General corrosion occurs when stainless steel is immersedin an environment in which the passive film is not stable, such as hot sulfuricacid, boiling MgCl2, or another very aggressive environment. Pitting corrosionoccurs, as in other metals, because of a local discontinuity in the passive filmsuch as an inclusion or local chemistry change. Halogen ions are the most preva-lent cause of pitting in stainless steels, with chloride being the most commonhalogen to initiate pitting. Pit growth depends on a variety of factors, with thelocalized corrosion environment in the pit being the most significant factor. Sur-face condition can also contribute to pitting, with the presence of deposits beinga significant factor. Prepassivating treatments are used to clean the surface ofdeposits. Intergranular corrosion and intergranular stress-corrosion cracking arerelated phenomena which occur when the grain-boundary microchemistry is al-tered by thermal treatment such as welding or heat treatment. The process thatalters the grain-boundary corrosion resistance is called sensitization and occurswhen chromium carbides precipitate at the grain boundaries.

II. CORROSION BEHAVIORA. Alloy ClassificationStainless steels are classified by the phases that are present. Table 1 shows theclassifications and compositions of the more commonly available alloys, but thistable is not exhaustive. The five classifications are austenitic, ferritic, duplex (con-taining both austenite and ferrite), martensitic, and precipitation hardening (PH).Alloying affects the predominant phase that is present, and the phase has a pro-found effect on the mechanical and corrosion characteristics. Often, the mechani-cal properties are the driving force for choosing the alloy for the application, andthe corrosion properties must then be optimized within that alloy classification.A brief description of each classification is given in the following subsections.

1. Austenitic Stainless SteelsAustenitic stainless steels have a face-centered-cubic (fcc) crystal structure. Themost commonly used stainless steels, the 300 series, belong to this group. Theaustenite is a high-temperature phase that is stabilized by the addition of nickel,manganese, or nitrogen, but they also contain significant amounts of chromium,which gives them good overall corrosion properties. These alloys are fairly low

Austenitic Stainless Steels 33

in strength, but they exhibit a high fracture toughness and are used over a widerange of temperatures. As a class, they have good resistance to hydrogen embrit-tlement. In recent years, a new class has emerged called superaustenitics. Theyare very high in Mo (47%) and nickel, which results in the highest resistance(within the austenitics) to any localized attack processes in chloride-containingmedia.

2. Ferritic Stainless SteelsFerritic stainless steels have the body-centered-cubic (bcc) crystal structure, usingchromium as the major alloying element. They can have higher strengths thanthe austenitic steels at ambient temperatures, but they suffer from a lower fracturetoughness, particularly at lower temperatures. In an attempt to improve the frac-ture toughness and corrosion behavior, a new class of superferritics was devel-oped. These materials are low in carbon and higher in Cr and Mo than the olderferritics. Successful use of ferritics requires careful attention to detail in control-ling the heat treatment.

3. Duplex Stainless SteelsDuplex stainless steels have lower amounts of nickel than the austenitic grades,with the result that some of the austenite transforms to ferrite. Generally, thealloying element and heat treatments are controlled to form equal amounts offerrite and austenite. The principal advantages over the fully austenitic gradesare higher strength, improved resistance to stress-corrosion cracking (SCC), anda high immunity to sensitization. In order to improve the localized corrosionbehavior, a superduplex series of alloys has emerged. These are alloyed to containlarger amounts of Cr, Mo, and N.

4. Martensitic Stainless SteelsMartensitic stainless steels contain significant amounts of chromium (1018%)and carbon but are low in Ni. Although austenitic at high temperatures, they canbe transformed into the martensite structure by rapid cooling. These alloys arevery strong, but they suffer a loss of fracture toughness and have generally infe-rior localized corrosion resistance. Hydrogen embrittlement has been identifiedas a cause of fracture. Recently, supermartensitics have been formulated that arehigher in Mo to improve localized attack behavior.

5. Precipitation-Hardening Stainless SteelPrecipitation-hardened stainless steels superficially resemble the 300 series aus-tenitic steels in nickel and chromium composition, but, in addition, they containsmall amounts of copper, aluminum, or titanium that can be precipitated with

34Jones

etal.

Table 1 Stainless Steel Compositions (wt%)Type UNS C Mn Si Cr Ni P S Other

Austenitics201 S20100 0.15 5.57.5 1 16.018.0 3.55.5 0.06 0.03 0.25 N202 S20200 0.15 7.510.0 1 17.019.0 4.06.0 0.06 0.03 0.25 N301 S30100 0.15 2 1 16.018.0 6.08.0 0.045 0.03 302 S30200 0.15 2 1 17.019.0 8.010.0 0.045 0.03 304 S30400 0.08 2 1 18.020.0 8.010.5 0.045 0.03 304L S30403 0.03 2 1 18.020.0 8.012.0 0.045 0.03 304LN S30453 0.03 2 1 18.020.0 8.012.0 0.045 0.03 0.100.16 N316 S31600 0.08 2 1 16.018.0 10.014.0 0.045 0.03 2.03.0 Mo316LN S31653 0.03 2 1 16.018.0 10.014.0 0.045 0.03 2.03.0 Mo;316L S31603 0.03 2 1 16.018.0 10.014.0 0.045 0.03 2.03.0 Mo321 S32100 0.08 2 1 17.019.0 9.012.0 0.045 0.03 5 3 % C min Ti347 S34700 0.08 2 1 17.019.0 9.013.0 0.045 0.03 10 3 % C min Nb348 S34800 0.08 2 1 17.019.0 9.013.0 0.045 0.03 0.2 Co; 10 3 % C min Nb;

0.10 Ta384 S38400 0.08 2 1 15.017.0 17.019.0 0.045 0.03

Superaustenitics254 SMO S31254 0.02 1 0.8 19.5020.50 17.5018.50 0.03 0.01 6.006.50 Mo; 0.501.00

Cu; 0.1800.220 NAL-6X N08366 0.035 2 1 20.022.0 23.525.5 0.03 0.03 6.07.0 MoAL-6XN N08367 0.03 2 1 20.022.0 23.5025.50 0.04 0.03 6.007.00 Mo; 0.180.25 N20Cb-3 N08020 0.07 2 1 19.021.0 32.038.0 0.045 0.035 2.03.0 Mo; 3.04.0 Cu; 8

3 % C min to 1.00 maxNb

904L N08904 0.02 2 1 19.023.0 23.028.0 0.045 0.035 4.05.0 Mo; 1.02.0 CuFerritics

405 S40500 0.08 1 1 11.514.5 0.04 0.03 0.100.30 Al409 S40900 0.08 1 1 10.511.75 0.5 0.045 0.045 6 3 % C min0.75 max Ti429 S42900 0.12 1 1 14.016.0 0.04 0.03 430 S43000 0.12 1 1 16.018.0 0.04 0.03 436 S43600 0.12 1 1 16.018.0 0.04 0.03439 S43035 0.07 1 1 17.019.0 0.5 0.04 0.03 0.15 Al; 12 3 % C min

1.10 Ti442 S44200 0.2 1 1 18.023.0 0.04 0.03 446 S44600 0.2 1.5 1 23.027.0 0.04 0.03 0.25 N

Austenitic

StainlessSteels

35Superferritics

Sea-Cure S44660 0.025 1 1 25.027.0 1.53.5 0.04 0.03 2.53.5 Mo; 0.2 1 4 (% C(SC-1) 1 % N) min to 0.8 max

(Ti 1 Nb); 0.035 NAL 29-4C S44735 0.03 1 1 28.030.0 1 0.04 0.03 3.604.20 Mo; 0.201.00 Ti

1 Nb and 6 (% C 1 %N) min Ti 1 Nb; 0.045 N

AL 29-4-2 S44800 0.01 0.3 0.2 28.030.0 2.02.5 0.025 0.02 3.54.2 Mo; 0.15 Cu; 0.02N; 0.025 max (% C 1% N)

Duplex329 S32900 0.2 1 0.75 23.028.0 2.505.00 0.04 0.03 1.002.00 MoUranus 50 S32404 0.04 2 1 20.522.5 5.58.5 0.03 0.01 2.03.0 Mo; 1.02.0 Cu;

0.20 NFerralium S32550 0.04 1.5 1 24.027.0 4.506.50 0.04 0.03 2.004.00 Mo; 1.502.50255 Cu; 0.100.25 N

Martensitic403 S40300 0.15 1 0.5 11.513.0 0.04 0.03 410 S41000 0.15 1 1 11.513.5 0.04 0.03 414 S41400 0.15 1 1 11.513.5 1.252.50 0.04 0.03 416 S41600 0.15 1.25 1 12.014.0 0.06 0.15 0.6 Mo(b)

min420 S42000 0.15 min 1 1 12.014.0 0.04 0.03 422 S42200 0.200.25 1 0.75 11.513.5 0.51.0 0.04 0.03 0.751.25 Mo; 0.751.25 W;

0.150.3 V440A S44002 0.600.75 1 1 16.018.0 0.04 0.03 0.75 Mo440B S44003 0.750.95 1 1 16.018.0 0.04 0.03 0.75 Mo440C S44004 0.951.20 1 1 16.018.0 0.04 0.03 0.75 MoLapelloy S42300 0.270.32 0.951.35 0.5 11.012.0 0.5 0.025 0.025 2.53.0 Mo; 0.20.3 V

Precipitation Hardening138 Mo S13800 0.05 0.2 0.1 12.2513.25 7.58.5 0.01 0.008 2.02.5 Mo; 0.901.35 Al;

0.01 N15-5 PH S15500 0.07 1 1 14.015.5 3.55.5 0.04 0.03 2.54.5 Cu; 0.150.45 Nb17-4 PH S17400 0.07 1 1 15.517.5 3.05.0 0.04 0.03 3.05.0 Cu; 0.150.45 Nb17-7 PH S17700 0.09 1 1 16.018.0 6.57.75 0.04 0.04 0.751.5 AlAM-350 S35000 0.070.11 0.51.25 0.5 16.017.0 4.05.0 0.04 0.03 2.53.25 Mo; 0.070.13 N

(Type 633)

36 Jones et al.

the appropriate heat treatment. These alloys can develop extremely high strengthand can also share some of the good corrosion properties of the austenitics, buthydrogen embrittlement has been identified as a cause of fracture. This grade ofalloy can be martensitic or austenitic.

These five classes of alloys can also be obtained as cast alloys. Microstruc-ture is a key variable in controlling the strength, localized corrosion behavior,and SCC behavior of all these classes. It needs to be pointed out that the existenceof cast microstructures increase the complexity of choosing the ideal materialand making certain the microstructure is under control.

B. Composition Effects on CorrosionStainless steels are vulnerable to all forms of corrosive attack. Their successfuluse requires (1) knowledge of the alloying constituents that impact resistance,(2) detailed information on the chemical composition and temperature of the testenvironment, and (3) knowledge of the failure processes to which the particularalloy or alloy class is subject. Each of the major alloying elements will be brieflydescribed to give the reader a general knowledge of how to fit the alloy composi-tion to the environment.

1. ChromiumChromium is the single most important element contributing to the stainlessbehavior of these iron-based alloys, and in general, the higher the chromium level(once it gets above 10%), the better the performance. Additions of chromiumgreatly improve the behavior over that of iron in neutral and acidic pH rangesbut, curiously, has little effect in high-pH environments. The chromium appearsin the oxide film and acts to inhibit the transport of corrosion products across it,leading to the formation of a passive film. In particular, the chromium acts todecrease the general corrosion rate and improve crevice corrosion resistance.

2. MolybdenumMolybdenum is perhaps the second most important alloying element from a cor-rosion standpoint. Small additions have a profound effect by enhancing the pas-sive character of the passive film, particularly in chlorides and reduced sulfurenvironments. In particular, the pitting and crevice corrosion behavior are im-proved relative to similar alloys without the molybdenum. Every class of stain-less-steel alloy has members that are very high in Mo, and these are called su-per austenitics, martensitics, and so forth. The alloys with high Mo levels havethe disadvantage in that certain undesirable phases (sigma, chi, laves) can formunless care is taken in heat treatment and welding.

Austenitic Stainless Steels 37

3. NickelNickel is the most important austenite stabilizer, but it has a complex effect onthe corrosion behavior. It acts to decrease the general corrosion rate in reducingacidic environments. In sufficiently high levels, it acts to improve the SCC behav-ior in chloride and caustic solutions, but at intermediate levels, it can decrease theSCC resistance. The austenite structure results in a high resistance to hydrogenembrittlement.

4. ManganeseAt the low levels used in most stainless steels, manganese can be considered asubstitute for nickel. It controls the solubility of sulfur by precipitating the sulfuras MnS inclusions. Modern steel-making practice is to keep the sulfur as lowas possible because localized corrosion events such as pitting initiate at MnSinclusions.

5. CarbonCarbon is used at low levels as a strengthener in stainless steels, but it also canrender the stainless steels vulnerable to localized attack or SCC from sensitiza-tion. In sensitization, the carbon reacts with chromium to precipitate a Cr23C6carbide causing a very local decrease of the chromium concentration in the metal,making this depleted region vulnerable to localized attack or SCC. In the austenit-ics, this has led to the L grades, which are low in carbon (less than approxi-mately 0.03 at.%) and fairly immune to this problem.

6. NitrogenNitrogen is used at very low levels and acts as a strengthener and austenite stabi-lizer. Nitrogen greatly improves the pitting and crevice corrosion performanceof the austenitics and superaustenitics, particularly in concert with Mo. It helpsprevent (slows down) the undesirable chi (and probably sigma, laves) phase fromforming during welding or heat-treating operations with the high-Mo alloys. Itsbehavior is mixed in other classes of stainless steels.

C. Corrosion BehaviorChoosing the right material is a multistage process. Once the mechanical proper-ties and certain other properties (e.g., wear resistance, weldability, machinability,availability in the needed form, etc.) of the metal are defined for the application,the next level requires a careful definition of the chemical (e.g., pH, chloridelevel, sulfide level, temperature, velocity, oxygen, hydrogen gas) and exposureenvironment (single or multiple phases present, occasional dryout, differential

38 Jones et al.

aeration, etc.). Once this multistage process is completed, the general class ofalloy can be defined. Because cost is always an important parameter for thematerial choice, the tendency will be to use the lowest alloyed material becausenickel and chromium are expensive. As a general rule for iron-based stainlesssteels, the higher the alloy is in nickel and chromium (and molybdenum), themore corrosion resistant the material will be. The task of the materials scien-tist is to find the least expensive material that will perform adequately. Often,the exact answer will not be found in the literature; consequently, testing of var-ious representative alloys will be needed. Many test methods are described inRef. 1.

From their inception, stainless steels were developed to have low generalcorrosion rates. This simplification is still fairly true as long as one avoids certainenvironments such as reduced sulfur environments or highly acid environ-mentsonly highly alloyed stainless steels will perform in these environments.The surprises with stainless steels are usually due to localized attack such aspitting, crevice corrosion, and SCC. Because the chemical environment has infi-nite variability, the corrosion behavior for each class of stainless steel will beexamined using certain defined environments which will act as benchmarks ormodels. Examples will be potable water (low chloride), seawater (high chloride),and so forth. The book by Sedriks (2) is a particularly good reference.

1. Atmospheric EnvironmentsAtmospheric environments are those exposed to the natural elements in the airat ambient temperatures. City and marine environments present the most difficultenvironments because they are contaminated with sulfur compounds and chlo-rides. The austenitics, particularly those containing molybdenum, give the bestoutdoor performance from the standpoint of pitting and crevice corrosion, but itshould be noted that they will not be pit-free. Rather, the pit density and depthwill usually remain low enough that the cosmetic use of the material will not beimpacted. Localized corrosion rates for all classes of stainless steels are highestwhere the chlorides and sulfides are highest. The ferritics have also been usedin this environment but not as successfully as the austenitics. Both the austeni-tics and ferritics have good immunity to SCC. The higher-strength alloys suchas the martensitics and precipitation-hardened alloys are prone to pitting andSCC. This environment can get extremely aggressive if there is a source of heatsuch that condensation and dryout takes place, resulting in very concentratedsolutions. One extreme of this environment might be found under insulation, and,here, SCC and pitting can occur with all the standard stainless classes, even theaustenitics and ferritics. The high-Mo (super types) alloys provide the greatestresistance to pitting and SCC when the atmospheric environment is the mostaggressive.

Austenitic Stainless Steels 39

2. Deionized (Pure) Water (Ambient Temperature)Pure water with low concentrations of dissolved salts (although it may containdissolved carbon dioxide from the air) is the most ideal environment for stainlesssteels. They should all perform well and be free of any localized corrosion prob-lem and SCC.

3. Deionized (Pure) Water (High Temperature)The nuclear power industry successfully uses austenitic stainless steels underelevated temperature conditions. Initially, 304 stainless (high C) was used, whichwhen welded gave rise to sensitized grain boundaries which were very prone toSCC. This was a very major problem which has been solved by using low-carbonaustenitics. Some ferritics and precipitation-hardened alloys have been success-fully used in other components such as turbine blades. The key to successful useis attention to heat treatment and maintaining the purity of the chemical environ-ment. The other classes of stainless steel have not been used extensively in thisenvironment. There is an extensive literature on this subject and the reader isdirected to reviews by Hanninen (3) and Cragnolino (4).

4. Fresh (Potable) WaterFresh water can contain a few hundred ppm chloride, and when used in a heatexchanger, it can reach temperatures near 100 C. The 300 series austenitics havebeen used successfully in this environment, but as the temperature and chloridelevel increase, the amount of alloying (particularly Mo) must increase to preventpitting and SCC. Sensitization is an issue and the low carbon grades must beused. For years, there was a widely accepted belief that SCC did not occur below60 C, but long-term experience has now demonstrated that there is no thresholdtemperature or chloride level for the 300 series austenitics (5). Consequently,there will be a large variability in the observations of success with the 300 series.The superaustenitics with high levels of molybdenum should perform very wellagainst pitting and SCC degradation. The standard ferritics are low in molybde-num and will be prone to pitting, but they can be resistant to SCC. The behaviorof the duplex stainless steels is complex and must be considered carefully. Theyare more resistant to SCC relative to the 300 series of austenitics but may besimilarly prone to pitting. Both the martensitic and precipitation-hardening gradesare prone to pitting and SCC. The SCC process for these higher-strength steelsis considered to actually be hydrogen embrittlement.

5. SeawaterSeawater is the most challenging environment for stainless steels, and the prob-lems are compounded if reduced sulfur species are present due to the action of

40 Jones et al.

bacteria or decaying vegetation. In general, the standard alloys within each gradewill be prone to pitting and SCC with much variability in the reported results. Formaximum safety and reliability, only the high-Mo alloys should be considered forthis environment. A review paper by Streicher (6) on 30- and 60-day ambient-temperature crevice tests clearly shows only the superaustenitics, superferritics,high-Mo duplex, and nickel-based alloys are suitable. All localized corrosionand SCC problems worsen at elevated temperatures. Here, the famous boilingmagnesium chloride test can give insight into a materials behavior. The 300series alloys have completely unsuitable SCC behavior in this test (unless theyare cathodically protected). The nickel level has to be above 20% before theaustenitics show significant improvement in SCC behavior in the boiling magne-sium chloride test. Molybdenum is also beneficial in raising the threshold stressintensity for SCC. Austenitics (high Ni), ferritics, and duplex alloys all can showresistance to SCC, but it is clear that they must be high in Mo. For the mostdifficult applications, titanium or nickel-based alloys will need to be used.

6. Acidic EnvironmentsChromium is the most important element that imparts resistance to acidic envi-ronments; consequently, the highest resistance is associated with the highest Crlevels within each grade of stainless steel. In general, the 300 series austeniticstainless steels can be used in nitric acid over a wide concentration range (065%), even up to the boiling point. However, stainless steels are completely un-suitable for the HCl environment, and mixed acids containing HCl and otherhalides are also very problematic. The alloy 20Cb-3, containing copper, was espe-cially developed for use in sulfuric acid. Clearly, it is important to define theacid compositions and temperatures and then to utilize the literature for alloyrecommendations. General corrosion and localized attack due to microstructuralproblems are the major causes of failure. Much less is known about the SCCbehavior. Organic acids are less corrosive than the mineral acids.

7. Basic EnvironmentsIn general, all stainless steels are quite resistant to general corrosion in concen-trated caustic solutions, even up to boiling temperatures. However, there is asevere SCC problem, and the alloys can act in a very brittle manner. Alloys withthe highest nickel content are the most resistant to general attack and SCC. Thethreshold for SCC is a function of the temperature, caustic concentration, andnickel content. The austenitics 304 and 316 are resistant to SCC up to about 60 Cin 60% concentrated caustic. At higher temperatures, nickel-based alloys mustbe used. Microstructural effects are very important because sensitization makesthe materials more susceptible to SCC. Austenitic structures are more resistantthan ferritic structures.

Austenitic Stainless Steels 41

III. INTERGRANULAR STRESS-CORROSION CRACKING

Grain-boundary composition has been inferred to control intergranular (IG) frac-ture in a wide range of materials systems. Although many authors have attemptedto link grain-boundary composition and environmental cracking susceptibility,few have made direct measurements. In most cases, bulk composition and/or heattreatment is varied and it is assumed that interfacial segregation is systematicallychanged. Indirect measurements are often made (e.g., IG corrosion tests) indicat-ing the grain-boundary composition of an isolated element. Within selected well-understood cases, such approaches can give reproducible results. However, quan-titative measurements of grain-boundary composition are essential to enable anyreasonable assessment of variables controlling cracking susceptibility. With thecommonplace use of high-resolution techniques such as analytical transmissionelectron microscopy (ATEM) and scanning Auger microscopy (SAM), quantita-tive relationships have been established between interfacial composition andcracking susceptibility for many metallic alloy systems (7). Perhaps the alloysystem that has been most closely examined has been austenitic stainless steeldue to its widespread use as a corrosion-resistant structural alloy in nuclear powersystems. The vast majority of failures have been in high-carbon, 300-series stain-less steels thermally sensitized during fabrication. Extensive basic and appliedresearch activities were initiated about 25 years ago to develop a mechanisticunderstanding of the IGSCC process and, more importantly, to identify remedialactions and corrective measures to cracking problems in boiling-water reactor(BWR) power plants. For the most part, those research activities were highlysuccessful. IGSCC of sensitized stainless steel is probably the best understoodand effectively modeled environmental cracking process (8). However, recentobservations of IG cracking in cold-worked or in irradiated stainless steels, havebeen difficult to explain.

Austenitic stainless steels provide an example alloy system to demonstratethe influence of grain-boundary composition on IGSCC. Emphasis is placed onidentifying equilibrium and nonequilibrium segregants that may promote suscep-tibility or improve resistance to cracking, respectively. In each case, current un-derstanding of grain-boundary composition development in stainless steels is re-viewed and assessed relative to IG fracture in corrosive environments.

A. Grain-Boundary Composition and IGSCCThe general conditions necessary to promote IGSCC are a susceptible materialmicrostructuremicrochemistry, a sufficiently corrosive environment, and thepresence of tensile stresses. Many of the important aspects controlling environ-mental crack advance are illustrated in Fig. 1. In nearly all cases of IG cracking,grain-boundary composition plays a dominant role. Interfacial composition can

42 Jones et al.

Fig. 1 Schematic illustrating intergranular stress corrosion processes.

be significantly changed from the matrix by equilibrium (nonequilibrium) pro-cesses resulting in segregation (depletion) of alloying (impurity) elements andprecipitation of second phases. These compositional changes in the grain-bound-ary region can influence IG crack advance through effects on electrochemicalbehavior (e.g., dissolution, repassivation, and hydrogen recombination) as wellas effects on interfacial mechanical behavior (e.g., deformation and cohesivestrength).

B. Precipitation and Grain-BoundaryComposition Changes

The dominant material variable controlling IGSCC susceptibility in austeniticstainless steels results from the precipitation of Cr-rich M23C6 carbides at high-energy interfaces. This promotes the development of a Cr-depleted region adja-cent to carbide precipitates. This depletion is controlled by the thermodynamicsof carbide formation and differences between the diffusivities of Cr and C.ATEMEDS has enabled Cr-depletion profiles to be routinely measured demon-strating that interfacial Cr concentrations decrease (from ,18% to ,10%) as theheat-treatment temperature is decreased due to changes in C and Cr activities. Thewidth of the depleted zone increases with time after IG carbides are nucleated.

The extent of grain-boundary Cr depletion has been directly linked to theIG corrosion and SCC susceptibility of austenitic stainless steels (912). Thethreshold concentration to promote IG degradation can be quite different for cor-

Austenitic Stainless Steels 43

Fig. 2 Grain-boundary Cr concentration width on IGSCC in BWR water environment.

rosion and SCC, as illustrated in Fig. 2. Classical IG corrosion is detected in astandard sensitization test when the grain-boundary Cr concentration drops below,13.5 wt%. On the other hand, IGSCC in high-temperature aerated-water envi-ronments can be initiated during slow-strain-rate (SSR) tests when local Cr levelsdrop below ,17 wt%. Additional tests varying the width of the Cr-depletionzone (and keeping boundary Cr concentration approximately constant) reveal thatonly a very narrow (,4 nm) width is necessary to promote cracking (Fig. 3).

Fig. 3 Grain-boundary Cr depletion on IGSCC in BWR water environment.

44 Jones et al.

IGSCC susceptibility is not sensitive to increases in depletion width beyond thatnecessary to establish a continuous path for crack advance. On the other hand,standard sensitization tests will show much more aggressive IG corrosion withincreasing depletion widths (11). The correlations presented in Fig. 2 point outthe critical importance of Cr depletion, and Cr minimums in particular, on IGdegradation of austenitic stainless steels. Specific relationships between IGSCCand grain-boundary composition will always depend on many other critical fac-tors, including mechanical loading characteristics and environmental conditions,as well as secondary material variables. For example, the threshold grain-bound-ary Cr concentration has been shown to depend on the strain rate during SSRtests (10,11)

C. Equilibrium Impurity SegregationImpurity elements present at low levels in austenitic stainless steels can reachhigh levels at grain boundaries due to equilibrium segregation. The most preva-lent segregant is P, which can reach grain-boundary P contents .10 at% in com-mercial stainless steels after intermediate temperature heat treatments (500750 C). Thus, materials in the sensitized condition will most likely have con-siderable P segregation along with M23C6 carbides and Cr depletion defining thelocal microchemistry. Segregation of other impurity elements to stainless-steelgrain boundaries has been observed, but this often requires high bulk contents orspecial thermal treatments. Sulfur segregates rapidly to boundaries if preexistingsulfides are dissolved by a high-temperature (.1200 C) exposure