EFFECT OF STRESS.ON CORROSION
L
18
I N D U S T R I A L A N D ENGl
ING CHEMISTRY
J. F. BATES designers,
Stress corrosion and corrosion fatzsue are potential hazards that must be avoided operators, and maintenance staffs in all process industries. The current status
of
corrosion,
fatigue, and embrittlement is presented with a summary of preventive measures and testing methods he combined influence of stress and corrosion may
Tcause early failure in common engineering materials
such as metallic alloys, glass, and plastics. When the wide range of materials and environments encountered in industry is considered, the incidence of such early failure is relatively low. Historical examples are the season cracking of brass (Figure l), caustic embrittlement of structural steels, and corrosion fatigue of propeller shafts on marine vessels. The present paper is an educational treatment of metallic stress-corrosion and corrosion-fatigue phenomena sometimes seen by design, operations, and maintenance engineers in the process industries. Examples are drawn from the alloy systems commonly encountered in these industries, although no attempt is made to describe detailed solutions for specific problems. Several major reference sources are cited for the convenience of the reader. Further detailed information can often be provided by the equipment manufacturers and by the alloy producers. T h e voluminous recent scientific literature devoted to this area is cited briefly to indicate the steadily increasing knowledge of this complex subject. Effect of Stress on Geneml Corrosion
The rate of uniform or general corrosion may be affected by the presence of stress. Thermodynamically, a metal under tensile stress should be more susceptible to corrosion, and this effect has sometimes been noted by weight-loss studies as well as by slight active shifts of corrosion potentials. Shvartz and Kristal (43) conclude that this effect is seen only when metals are corroding in acid solutions. When the corrodent is dissolved oxygen, as in neutral or basic solutions, diffusion of oxygen to the cathodic areas is the rate-controlling
process, and metallurgical or mechanical factors have little effect. Cyclic Stress and Corrosion Fatigue
All other effects of stress on corrosion to be discussed herein involve cracking by one mechanism or another. When cyclic stresses are applied to a corroding structure, the part may crack in a manner apparently similar to, but substantially different from, mechanical fatigue failure. Figure 2 shows a fatigue curve made in air; its most important characteristic is that for steel or titanium there is a stress limit below which failure will not occur, regardless of number of stress cycles. This limit, the endurance or fatigue limit, is, for steel or titanium, approximately half the tensile strength. Consequently, within limits, designers can raise the endurance limit simply by using higher strength alloys. Alloy systems other than steel or titanium do not have such well defined endurance limits. Figure 2 also shows a typical S-N curve for fatigue in a corrosive environment; note that at all stress levels the fatigue life is less than in air and that there is no endurance limit. T h e position of this second curve is typically affected by the corrosion rate but not by the tensile strength of the metal. Hence, the life of a structure undergoing corrosion fatigue cannot be prolonged by increasing the strength. Generally, only those preventive measures that reduce the rate of corrosion will be effective. Examples of air fatigue and corrosion fatigue are shown in Figure 3. T h e cracks in both are, typically, transgranular. Air fatigue is frequently characterized by a single crack; corrosion fatigue often shows a multitude of crack sites with possible side branching from the main crack.
I 0'
10'
1ff
104
10'0
NUMlER OF CYCLES
Figure 1. Stress-conasion cracking in a b r m drain pips
Frgue 2. S-N
~ T U far C
alloys subjected to cyclical strcss
" O L 5 8 NO. 2
FEBRUARY 1966
19
Effect of Static Stress on Corrosion
The Corrosion Handbook (55)defines as stress-corrosion cracking the combined action of stress and corrosion which leads to cracking or embrittlement of metal. I n practice, this definition is usually narrowed to include only- those cases of cracking where the crack progresses a t least partly by electrochemical dissolution at the tip of the crack. Under the broader definition, however, come both hydrogen einbrittlement and einbrittlement by liquid metals; all three will be discussed here. I t is characteristic of these three forms of stress corrosion that only tensile stresses are dainaging. Further, combinations of alloy and environiiient that result in cracking are usually quite specific--there are no environments that will crack all alloys and there are no alloys that will crack in all environments. Nevertheless the problem concerns the design engineer and the corrosion engineer because a structure may fail much below the design stress in an environment where general corrosion is very slight. The problem holds further interest for the corrosion scientist because the mechanism has not yet been found for this form of failure. Before discussing the possible mechanism of stress-corrosion cracking, let us consider briefly mechanical fracture. There are two general classifications of mechanical fracture-ductile and brittle. Figure 4 ( a ) shows ductile fracture from an impact test on a high purity iron ( 3 ) . Energy is absorbed by the metal adjacent to the crack, resulting in deformation of the metal grains in this region. Figure 4 ( b ) shows a low-temperature-impact brittle fracture of the same iron. I n brittle fracture the energy is used in crack propagation and there is little deformation of the grains adjacent to the crack. Because the metal grains adjacent to a stress-corrosion crack do not show deformation, Figure 4(c), this cracking is often classified as brittle fracture. Such a classification can be quite misleading, however, because it obliges one to account for a micro- or submicrobrittleness in what may be otherwise a very ductile material. Stresscorrosion cracking is a separate category of metal failure, rather than a form of brittle (mechanical) failure, because its origin is not purely mechanical. Theories of Stress-Corrosion Cracking
Many theories of stress-corrosion cracking have been published and numerous variations of the basic theories have been proposed. I t is not simple to categorize the several divergent vieivs, but most of them fall into two broad groups. One group proposes that stress-corrosion cracking is an electrochemical process, progressing normally to the prevailing tensile stress.
J . F. Bates is Senior Research Chemist at the Applied Research Laboratory, Cnited States Steel Co., Monroeuille, P a . T h e paper w a s originally presented to the Division of Industrial and Engineering Chemistry at the Atlantic City -1’ational Meeting of the A C S in September 1965. T h e author acknowledges the assistance of Mr. A . W . Loginow andpermission of the Editors of ‘‘Corrosion’) to use a previous paper appearing in 7964, a s ihe basis f o r this expanded and updated review.
Another group feels that a stress-corrosion crack advances too rapidly to be caused by corrosion only, and too s l o ~ l yto be caused b>- mechanical failure alone. Thus, an alternating mixed mechanism is postulated. Dix (8) proposed the electrochemical theory in 1940, stating that there must exist in the alloy a susceptibility to selcctive corrosion along more or less continuous paths, for instance grain boundaries. In 1944, Mears,
(6)
Figure 3. ( a ) Aii-futigue crack in structural carbon steel. ( 6 ) Coriosiongatigue crack in structural carbon steel eulosed to sjnthetzc sea u,ater. ‘Yote multiple cracks emmating from pits. X 700; unetched
AUTHOR
20
INDUSTRIAL A N D ENGINEERING CHEMISTRY
(a)
(6)
(c)
Figure 4. ( a ) Cross-section of a ductile fracture in iron. Etched in nitnl. X60. ( 6 ) Cross-section of a brittle fracture in iron. Etched zn nital. XGO. (c) Crosr-section of a stress-coriosion crack an structural carbon stpel. Etched in nital. X 750 (3)
Brown, and Dix (23) showed that an aluminum alloy containing 4’% copper could be treated so that the grain boundaries were anodic to the base metal, thus providing an active path for corrosion. Another well known active-path system is in austenitic stainless steel that has been sensitized by heating in the range of 800’ to 1500’ F. Chromium carbides precipitated in the grain boundaries by this heat treatment are believed to rob the adjacent areas of chromium. Such a “sensitized” steel may crack by intergranular stress corrosion in a marine atmosphere, Figure 5 ( a ) . There are, however, instances of intergranular stresscorrosion cracking where it is not readily apparent that the grain boundaries are chemically different from the bulk metal. Many impurities are known to segregate in the grain boundaries, however, and it is not difficult to conceive that one or many of these are responsible for the intergranular cracking. Transgranular cracking of single-phase alloys, such as austenitic stainless steel, Figure 5(b), provides a much more interesting test of the electrochemical theory. Here it is not apparent what would constitute an active path. The following possibilities have their proponents : -static crystal imperfections-Robertson and Tetelman (36) -moving crystal imperfections (yielding metal) at the tip of the crack-Hoar (16, 17) -microsegregation of solute atoms to moving crystal and imperfections at the tip of the crack-Swann Pickering (52) -new corrosion-sensitive phases nucleated by metal deformation at the tip of the crack-Edeleanu (10) -new corrosion-sensitive phases formed with corrosion products (hydrogen) at the tip of the crackVaughan (57) -film rupture by yielding metal at the tip of the crack-Logan (20) The following support the existence of a mechanical step : -Pickering
(0)
and Swann postulate that during stress-
corrosion cracking a tunneling corrosion produces in the metal a weakened, spongy structure which fails in tension by ductile fracture (33). Using oxide replica techniques, Nielsen (26) has shown such tunnels at the tip of an advancing crack in an austenitic stainless steel, Figure 6 -Forty (12) proposes that the corrosion reaction triggers a brittle crack that can advance for short distances through ductile metal -Uhlig (56) postulates that certain ions selectively adsorbed at metallurgical imperfections cause easy rupture of the metal bonds There has been much good work done on stresscorrosion cracking that is outside the scope of the mechanistic controversy. Particularly noteworthy is the work of Graf (14) on stress-corrosion cracking of homogeneous copper-gold and silver-gold alloys in a variety of solutions. His plots for these alloys in aqua regia, Figure 7, are typical of his findings for a variety of alloy systems. Graf claims that stress-corrosion cracking susceptibility is caused by alloy components that are more noble than the principal metallic component of the alloy. He further shows that the alloy ceases to be susceptible at something less than 50 atomic yo of the noble constituent. The work of Copson (7), Figure 8, tends to confirm Graf’s ideas; austenitic alloys with greater than 45% nickel do not crack in boiling 42y0 magnesium chloride solution, the standard test medium for these alloys. Copson presents evidence to show that nickel is noble in conditions believed to be representatve of those at t h e tip of a stress-corrosion crack. Graf’s rule does not encompass the entire picture, however. Whereas Graf’s data form a smooth curve, the variations in the cracking time in Figure 8 exceed two orders of magnitude for most levels of nickel. The recent advent of electron-transmittance micrography has given metal physicists a powerful new tool to examine the interior structure of metals with a resolution of less than 100 atom diameters. Thus, it is now possible to relate metal substructure with transgranular stress-
(b)
Figure 5. (a) Intergranular cracking i n an austenitic stainless steel. ( 6 ) Transgranular cracking in an austenitic stainless steel. Etched electroiytically in 70% oxalic acid. X250 (3)
Figure 6. Electron micrograph of corrosion tunnels at leading edge of a stress-corrosion crack in an austenitic stainless steel. From iVielsen (26) VOL. 5 8
NO. 2 F E B R U A R Y 1 9 6 6 21
corrosion cracking. The initial work of Swann and Nutting in this area (57) has been continued and expanded (2, 50) until there are many studies being made by means of these new techniques. An austenitic stainless steel with a tangled arrangement of dislocations is likely to be more resistant than one with a linear array of dislocations. Examples of each are shown in Figure 9. Because of the specificity of a cracking environment for an alloy, it is unlikely that any single mechanism can account for all known cases of stress-corrosion cracking. It is probable that several, and possible that all, of the previously mentioned mechanisms may operate under some conditions. Surveyed as a whole, the proposed mechanisms have these two characteristics: Most do not imply a pre-existing crack path but, rather, that yielding metal at the tip of the crack generates the path of the crack. All recognize electrochemical corrosion as a step in the process. The universality of the second characteristic suggests immediately one method used to halt stress-corrosion cracking, viz., cathodic protection. Cathodic protection will prevent a crack from initiating- and will halt a crack already started. This principle is so universally applicable that its success or failure can be used to distinguish stress-corrosion cracking from other modes of cracking. Of particular interest are the instances in which cathodic protection accelerates cracking. These are called hydrogen embrittlement or hydrogen stress cracking.
is not the result of a corrosion reaction at the tip of the crack, but is a mechanical failure resulting from the joint action of tensile stresses and embrittlement induced by hydrogen penetration of the metal. From size considerations, it is deduced that the hydrogen penetrates the metal as atomic hydrogen, H, rather than as molecular hydrogen, H2. One theory is that the hydrogen atoms segregate to dislocations and vacancies, where they combine into hydrogen molecules, exerting very high pressures. These high pressures may cause submicroscopic voids and add to the stresses already present in the metal. I t is not a simple matter to distinguish between stresscorrosion cracking and hydrogen embrittlement just from the microscopic appearance of the crack. An example of each in a modified 12% chromium martensitic stainless steel, USS 12 MoV stainless steel, is shown in Figure 10. A test procedure that can be used to distinguish between stress corrosion and hydrogen embrittlement has been used by Brown (6) and also by Bhatt and Phelps (4). In the latter work, specimens cf the 12 MoV stainless previously mentioned were mad
a
Hydrogen Embrinlement
Hydrogen embrittlement of the high yield strength alloy steels, caused by plating operations, has long been known. Of more recent knowledge is hydrogen embrittlement caused by the corrosion processor by cathodic protection. I n such instances the crack in the alloy
0 Ag
20
Au
CU.
Figure 7. Lifclime of copper-gold and silver-gold olloys in q u a regia in which gold is atfacked. From Graf (74) 22
I
IC0 ATOMIC PER CENT GOLD
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
I
-0 NICKEL IERCENT
Figure 8. Slress-cmosion mocking of iron-chromium-&ktl ulirrs in boiling 4 v 0 magmsim chloride. From Copson (7)
either cathodic or anodic in a n oxygenated 3% sodium chloride solution. Without polarization, specimens cracked in about 100 minutes (Figure 11). Anodic polarization greatly reduced the time to cracking, indicating that the specimens had failed by stress-corrosion cracking. Slight cathodic polarization increased the specimen life to about 8000 minutes indicating that cathodic protection was possible in a narrow range of current densities. Cathodic polarization at higher current densities markedly decreased the time to cracking, indicating hydrogen stress cracking. The recognized susceptibility of high yield strength martensitic steels to cracking in hydrogen sulfide is considered an example of hydrogen embrittlement. In addition to high yield strength steels, the zirconium alloys that are used for nuclear applications are also susceptible to hydrogen embrittlement. Although these laboratory experiences should caution the corrosion engineer regarding the use of cathodic protection for higher strength alloy steels, the present author is not aware that cathodic protection has caused hydrogen stress cracking in service.
metal may penetrate an alloy to produce a crack very similar in appearance to a stress-corrosion crack. For this reason, cracking caused by tensile stresses and exposure to liquid metals is included in this review. T h e recent monograph by Rostocker, McCaughey, and Markus (38) summarizes the literature to date and reports on the very extensive work by these authors
Cracking by liquid Meals
The reaction of a n alloy with a molten metal is in some respects akin to corrosion and in others unlike corrosion. In the presence of tensile stresses, however, a molten
"
mlmm!H
_I 6 5 4 3 2 1 0 1 2 8 4 - 5 t -CATHODIC ANODIC _.)
CURRENT DENSIN lma./sq. in.) Figure 11. Effect of applied current on lime to failure. USS 72 MoV stmnlcrr in mygcnoted 3% NaC6 solution at p H 6.5. From Bhalt and Phelps (4)
Figure 9. Direct-transmillance rlectron photomicrographs of dislocation arrangemcnb in austenitic stainless steels that ( a ) fauor and (b) do not famr rcrihncc to transgranular strewcorrosion macking. X 7 6 , w O . From Swam (50)
Figure 7 0 . Appearance of cracks in USS 7 2 MoV sloinless exposed to 3% NoCI ~olulion: (a) cathodic current, (6) no applied arrmt, and (c) onodic arrent. Etched clectrolyticoliy in 70% ammonium persu&ale, opproXimalc~7 0 0 X ( 2 9 )
2
0 TEMPERING I E M M N I R E 1°F)
Figure 72. Effect of tempering temperature on stress-cowosion cracking andyicld strength of USS 72 MoV stainless steel (3) VOL 58
NO.
2
FEBRUARY 1966
23
such systems. Table I, excerpted from their work, summarizes the cracking effect of 13 different liquid metals on aluminum alloys, magnesium alloys, steels, and titanium allol-s. As with stress-corrosion cracking, the interaction is specific. None of the alloys will crack in all environments and none of the environments will crack all the alloys. This table should be used only as a guide, however, because quite plausible conclusions drawn from it may, in fact, be incorrect. For example, the table indicates that neither molten lead nor molten tin will crack steel, but a lead-tin solder is known to cause stress cracking of high yield strength alloy steels. Effect of Stress and Mechanical Properties
With most alloy sb-stems. increasing the stress will increase the frequency of cracking and increase the number of environments that caused cracking. Thus, any measures to raise the mechanical properties of a given alloy type will increase the stress it can bear and increase the susceptibility to cracking. The principal methods used to increase strength are changing the chemical composition, heat-treating, and strain- or work-hardening. Figure 1 2 illustrates the effect of tempering temperature on yield strength and consequently on hardness for 1 2 MoV stainless steel ( 3 ) . Sheet specimens were stressed to 50 and 75770 of their yield strength and exposed to marine atmosphere at Kure Beach, N. C., approximately 80 feet from the ocean. The great spread that is typical of stress-corrosion data often precludes the development of quantitative relationships, but the results confirm the general statenlent, above : The temperature for highest yield streiigth coincides with that of poorest stress-corrosion behavior : the temperature for lowest strength coincides with the area of best behavior; for a given strength (temperature) the lower stress level gave the longer cracking time. These generalizations do not hold from one alloy type to another, however, because cold-worked austenitic stainless steels are much less susceptible to cracking than the heat-treated martensitic stainless steels of the same strength. From the above discussion it is apparent that four factors are necessary for stress-corrosion cracking : a cracking environment, a susceptible alloy, a source of stress, time. Sources of Stress1
Stress in an engineering structure can come from many sources and it can be in tension or compression.
TABLE I.
I n stress-corrosion cracking, hydrogen embrittlement, and embrittlement by liquid metals, however, we are concerned only with tensile stresses ; compressive stresses do not cause cracking. T h e tensile stress of concern is the vector sum of all stresses present, and this must be present on the metal surface that is in contact with the environment. The origin of the stresses can be varied; the stresses may be operational (design) stresses, stresses from thermal cycling, residual stresses from heat treatment and from fabrication of the working structure, or stresses generated by corrosion products (32). Residual stresses are frequently the most dangerous ones because they are often the highest, their magnitude and direction are frequently impossible to predict, and it is not usually practical to measure them. Residual stresses irom fabrication may be caused by bolting, riveting, shrink or press fitting, shearing, punching, or cold forming. Depending on the degree of restraint, such residual stresses may be very high, often exceeding the initial yield strength of the metal. Cold deformation will usually increase the yield strength and lower the ductility by work hardening. This increase in yield strength enables the alloy to support higher stresses and may result in a higher frequency of stresscorrosion cracking. Residual stress may also be caused by welding and by heat-treating operations designed to confer on the alloy desired mechanical properties. Both operations can cause warping and breaking even in the absence of a corrosive environment, giving evidence of the severity of the stresses involved. In heat-treatable alloys, welding produces a heat-affected zone that rnay be less resistant to cracking than the base metal because of metallurgical changes such as formation of imteinpered martensite in steels. I n austenitic stainless steels, welding may sensitize the heat-affected zones unless the steel is formulated to prevent grain-boundary precipitation of chromium carbides. I n certain environments, corrosion along the precipitates at the grain boundaries rnay be accelerated by stress and lead to cracking. When design or operational stresses are added to already high residual stresses, an engineering structure will often plastically deform slightly at a few critical overloaded places to adjust to the load. With an alloy of even minimal ductility in an unaggressive environment this yielding does no harm-such accommodation without damage is one of the most important charac-
C R A C K I N G OF ALLOYS B Y L I Q U I D M E T A L 9
I
Liquid M e t a l s
Engineering Allow
Aluminum alloys Magnesium alloys Steels Titanium alloys a
24
~
~
Ec Nd
hE
E N N N
_
_
_
E N N
..
N
N
~
D a t a taken from Rostocker, McCaughey, and Markus (38).
N N I N N Hg-37, zinc amalgam. c E = embrittlement.
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
*V = nonembrittlement.
teristics of metals. This serves to point up the critically important fact that the actual stress will depend more on the mechanical properties of the alloy than on the design stresses, because high-strength alloys can support higher residual stresses than can low-strength alloys. Consequently the susceptibility to stress-corrosion cracking will depend strongly on the mechanical properties of the alloy. General Preventive Measures
Residual stresses may be removed by thermal stress relieving, but this is often not practical-the fabricated structure may be too large or it may warp if its shape is complicated. I t is often not practical to stress-relieve austenitic stainless steels with normal carbon contents because they may be sensitized to intergranular corrosion by heating or slow cooling in the range 800' to 1500' F. If the environment will cause this type of corrosion, the structure would have to be heated at a higher temperature and then cooled rapidly through the sensitizing range. Such a procedure may be ineffective because stresses may be reintroduced by the rapid cooling. This difficulty may be overcome by using the steels designed to prevent the precipitation of intergranular carbides; AIS1 Types 321 and 347 stainless steels can safely remain in the sensitizing range long enough to permit thermal stress relief. Residual surface-tensile stresses may be reduced or eliminated by the use of shot peening to introduce compressive stresses. This method can generally be expected to be more effective on materials that are easily work-hardened, such as austenitic stainless steels and the softer aluminum alloys, than on the hard highstrength alloys. One method that may be used to reduce the intensity of local residual stresses is called mechanical stress relief. A pressure vessel, for example, will be expanded in a hydrostatic test to a slight permanent elongation of the metal. I n the process of yielding, highly localized residual stresses will be reduced to about the level of those in the rest of the structure. Although this method is a recognized one for reducing peak stresses, the author is not aware of its use to prevent stress-corrosion cracking. Another method sometimes used to reduce surfacetensile stresses is to cause a surface reaction that will result i n a voluine increase of the metallic phase, thus putting the surface in compression. An example of this is the nitriding of some alloy steels in ammonia at high temperatures. A new method of protection recently developed by DLIPont has very interesting possibilities ( 9 ) . An alloy itnmersed in molten calcium or zinc containing dissolved alloying elements quickly forms a diffusion layer on the surface in equilibrium with the liquid metal. As an example, austenitic stainless steels containing 8 to 12y0 nickel are markedly susceptible to stress corrosion in hot chloride solutions ; stainless steels containing over 20% nickel, and those with very low nickel are much less susceptible. I n the D U Pont studies, a steel with 18y0 chromium and 8% nickel had its stress-corrosion per-
formance greatly improved by placing it either in a bath that removed nickel from the surface, or in one that added nickel to the surface. For some environments it is possible to specify a maximum strength or hardness level for alloys to be used. An example of this approach is the avoidance of high yield strength steels in environments in which sulfide stress-corrosion cracking is possible (24). Similarly, high-strength aluminum alloys are avoided where chloride contamination is possible. When the stresses cannot be conveniently eliminated, steps can be taken to halt the corrosion. This can be done by cathodic protection, with the reservation noted that hydrogen embrittlement of zirconium alloys or very high yield strength alloy steels may cause a problem. Also, if the environment contains hydrogen sulfide, it is probable that cathodic protection would cause or accelerate such cracking and should, of course, not be used. Brasses have been coated with zinc to prevent stresscorrosion cracking. Protection of high-strength aluminum alloys by cladding with a more anodic aluminum alloy is an example of sacrificial cathodic protection that serves to prevent stress-corrosion cracking. The incidence of cracking may be reduced by the use of proper inhibition. I n sour-crude-oil service, it is fortunate that the inhibitors commonly used to decrease general corrosion are also effective in reducing sulfide stress cracking. Another example is the use of nitrates as inhibitors in boilers where sodium carbonate or sodium hydroxide is used for alkalinity control. I t has been known for some time that fibrous inorganic thermal insulation can cause stress-corrosion cracking of hot austenitic stainless steel piping ; chlorides are leached from the insulation by rain or spillage and are concentrated on the hot stainless steel surface. I t has recently been found that a soluble inhibitor may be included in the formulation of the insulation; the inhibitor is leached from the insulation and concentrated on the hot steel at the same time as the chloride, preventing attack. At times it is possible to modify the environment to prevent stress-corrosion cracking. I t has been shown that water containing 1 p.p.m. of chlorides, when splashed on a heat transfer surface, can concentrate sufficiently to cause cracking of an austenitic stainless steel (75). Thus for water-cooled nuclear reactors constructed of stainless steels, the chloride content of the water is kept very low. Sometimes the alloy is isolated from the environment by use of nonconductive coatings. The effectiveness of this method is, however, no better than the continuity of the coating used. Coatings with a high content of metallic zinc, which offers cathodic protection, can be quite beneficial in enhancing the life of the high yield strength alloy and stainless steels (30). Design changes can prevent stress-corrosion cracking. A notable example is the use of welded rather than riveted steel boilers. I n the latter, caustic embrittlement was caused by the concentration of solids at small leaks in riveted joints. Such leaks do not occur in welded boilers. VOL. 5 8
NO. 2
FEBRUARY 1966
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When the above protective measures are not practical, a way to prevent stress-corrosion cracking may be the selection of an alloy that does not crack under the specific conditions. For instance, austenitic stainless steels can safely handle hot, concentrated nitrate solutions that crack structural carbon steels. I n some applications that cause stress-corrosion cracking of austenitic stainless steels, a ferritic Type 430 stainless steel has been a successful substitute ( 7 ) . Cra c k-In d uc i ng Env ironrnent s
Process equipment may fail either from the inside or from the outside. Process streams may contain unexpected contaminants, or known contaminants may concentrate unexpectedly in crevices or at heat-transfer surfaces. The equipment may be contaminated on the inside by accident during fabrication, erection, startup, operation, shutdown, downtime, or during cleaning. The equipment may crack from the outside because of contamination by the natural environment, by accidental spillage, by leachings from thermal insulation, by windborne contaminants, or by other and unusual sources. Several lists of corrodents that may cause stress-corrosion cracking have been published. These lists are based on typical experience within the process industries where the true cracking agent may be an unknown and minor contaminant in the main stream. Thus the actual aggressive agents are probably fewer in number than those given. Tables I1 and I V to IX, inclusive, are compilations of those previously published. There has been no attempt made to trace each report to its original source to determine the actual corrodent. Except for a few well documented corrodents for each alloy, the corrosive agents listed in these tables should be used as indications of product streams in which something
has been known to cause stress corrosion or hydrogen embrittlement. Austenitic Stainless Steels
A compilation of corrodents reported to have caused stress-corrosion cracking of austenitic stainless steels is given in Table 11. Hot chloride and hydroxide solutions are the principal agents that cause transgranular stress-corrosion cracking of austenitic stainless steels. Of the alloys frequently used in the process industries, AIS1 Types 304, 304L, 316, 316L, and 310 stainless steels, the first four are about equally susceptible to cracking; Type 310 stainless, with its higher nickel content is somewhat less susceptible to cracking. Higher nickel steels such as Incoloy 800 (33y0 Ni) have been used if cracking is severe. I t is anticipated that current research efforts will produce new and improved alloys for this service. Intergranular cracking of stainless steels, sensitized in the weld zone, has been recognized recently in the polythionic acids encountered in the petroleum industry. This may be minimized by use of stainless steels such as Types 304L, 316L, 321, or 347 that do not sensitize during welding. The High Alloys Committee of the IVelding Research Council has recently surveyed the metals fabricating industry to learn the practices employed for thermal stress relief of austenitic stainless steels; their findings are presented in Table 111. TABLE, 111. STRESS-RELIEF T R E A T M E N T FOR A U S T E N I T I C STAINLESS STEELSa
I Environment, 1 Material Condition , or Objective of Treatment ,
1
Recommended Thermal Treatmentb ,302
304L 316L 317L
TABLE I I. CORROSIVE AGENTS REPORTED T O CAUSE C R A C K I N G I N A U S T E N I T I C STAINLESS STEEL (300 SERIES) Corrosive Agent
1 Reference
Corrosiue Agent
Reference
Steam condensate 5 p.p.m. N H 3 Sulfate liquor
+
I
Inorganic chlorides (25, 41) (hot) I Organic chlorides (hot) (25) Ethyl benzene H2SiF6
HC1, " 0 3 , HF pickling solutions Organic acids and chlorides KOH, NaOH NaA102 NalS04
26
Sulfite liquor
+
HzS03 metal chlorides Brackish and sea water (coldworked steel) H2S (cold-worked steel) HzS04 and CuSOa Methamine methylation stream High temperature steam NaaCOs NaCl
+
Polythionic acids Hap04
I N D U S T R I A L A N D E N G I N E E R I N G C H E M STRY
Remove peak E stresses No stress corrosion Intergranular corx, Cd rosion Stress relief After severe
Dimensional stability
318 .?2 1 347
304 308 309s 370s 316 317
305
B, .I
CC
B, A, C
CC
B, A, C, D, E
c, E
E
E
None required
.I, C, Bd
C
A, C
C
B, A, C
Ce
A, C, B
C
F
F
a Sangdahl (40). Treatments are listed above i n order of preference: A = anneal at 7850' to 2050' F., slow cool. B = stress r t l i e u at 7650' F., slow cool. C = anneal at 7 8 5 0 O t o 2050°F., quench or rapid cool. D = stress relieoe at 900" to 1200" F., slow cool. E = stress relieve at 800"to 900" F. slow cool. F = stress relieve at 400 to 900' F , slow cool. U s u a l holding time is 4 hour5 per inch of section thickness. T o allow use of the optimum stress-reliej treatment? stabilized or extra low carbon grades of stainless are recommended. U s e where fabrication procedures have sensitized the stainless steel. e A or B is satisj'actory $ f o l i o z e d b? C a t completion of forming.
Structural Carbon and Alloy Steels
Nitrates, hydrogen sulfide, hydrogen cyanide, hot caustics (NaOH, KOH), and agricultural-grade anhydrous ammonia are ~7ellknown corrodents that cause either stress-corrosion cracking or hydrogen embrittlement of these alloys. Other corrodents that have been reported are given in Table I V . Uhlig (53) outlines a series of heat treatments that reduce incidence of cracking in nitrates. I t has also been noted (28) that steels containing 0.01 to O.25y0carbon are more susceptible than those with lesser or greater amounts. Cyanide cracking may be reduced by removal of cyanide from the process stream, by substitution of more resistant alloys, or by thermal stress relief. TABLE IV. CORROSIVE AGENTS REPORTED T O CAUSE C R A C K I N G I N STRUCTURAL CARBON AND ALLOY STEELS
Corrosive Agent
Corrosive Agent
~
cracking can be essentially prevented by thermal stress relief of the pressure vessels along with the use of 0.2Oj, water as an inhibitor (37). Martensitic Stainless Steels
The high yield strength martensitic or semiaustenitic precipitation-hardenable stainless steels may be especially susceptible to cracking in marine environments and in the presence of hydrogen sulfide. Anodic coatings of zinc or aluminum can appreciably prolong service life in marine environments. These steels should not be used in the presence of hydrogen sulfide. Table V lists the corrodents which are reported to have caused failure of such steels. TABLE V. CORROSIVE AGENTS REPORTED T O CAUSE C R A C K I N G OF M A R T E N S I T I C STAINLESS STEELS
:! : R
NaOH, K O H HCN HCN SnC12 4AsC13 CHC13
+
+
HzSOa
NaF Phosphate solutions NHiCl MgClz WNOd2
KMn04 NaAlOn
Acrylonitrile HCN, HaP04, HAC CaC12
HzS NHiCNS Concd. " 0 8 HsS04 -t " 0 3 NH3 (anhyd.) NaF
Caustic embrittlement of boilers has been largely eliminated by the substitution of welded steel construction for riveted boilers. The welded construction eliminates the large number of crevices in which the sodium hydroxide used for alkalinity control may concentrate. Caustic embrittlement has been further reduced by the use of phosphate buffers and nitrate inhibitors. Carbon steel equipment used to handle caustic solutions should be given a thermal stress relief, although nickel alloys are generally used for such service if the temperature and composition are within the critical range for steel
Aluminum Alloys
The stress-corrosion behavior of aluminum alloys is indicated in Table V I . The principal corrodent of concern is chloride ion. Stress corrosion is seldom encountered in the process industries because the highstrength strain-hardening aluminum magnesium alloys and the precipitation-hardening alloys are seldom used. The lower-strength alloys are not subject to stress-corrosion cracking. Occasional trouble is encountered, however, when aluminum nuts, bolts, and screw stock (which are normally made of high-strength alloys) are used. TABLE VI. CORROSIVE AGENTS REPORTED TO CAUSE C R A C K I N G OF A L U M I N U M ALLOYS
(25). The usual approach in the selection of steels to be used in hydrogen sulfide service is to keep the strength low enough that cracking is not a problem; a maximum hardness of Rockwell C22 is recommended by Committee T-1B of the hlational Association of Corrosion Engineers (24). For severe environments austenitic stainless steels (free from cold work) or Inconel may be specified. Stress corrosion of cold-formed steel pressure vessels used for the transporting of and in the application of agricultural-grade anhydrous ammonia has recently come to light. I t has been found that cracking is caused by an air contamination of the ammonia. T h e
Reference
"03
+
MgClz Monoethanol amine Nix03 "03 f MnC12
I
Corrosive Agent Marine atmosphere HzS NaOH NH3 solution
Alloy Type Air NaCl H202 solution, NaCl solution, air Sea water
AI-Zn
+
NaC1, NaCl
+ H2Oz solutions 1
A1-Mg A1-Mg Al-Cu-Mg Al-Mg-Zn Al-Zn-Mg-Mn Al-Zn-Mg-Cu-Mn Al-Zn-Cu
Copper-Base Alloys
The copper-base alloys are usually susceptible to cracking in ammonia, amines, or nitrogen-containing materials that can react to release ammonia or amines. VOL. 5 8
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27
These are also subject to cracking in mercury or mercury compounds by liquid metal embrittlement. Uhlig (54) suggests minimizing stress corrosion by stressrelief heat treatment, removal of ammonia and/or oxygen, cathodic protection, and use of hydrogen sulfide as an inhibitor. A list of aggressive corrodents is given in Table V I I .
Titanium Alloys
Titanium alloys are relatively free of problems in stress-corrosion cracking. They will crack, however, in hot red-fuming nitric acid, and in the presence of minutest amounts of sodium chloride at temperatures exceeding 500’ F. For these environments they are not suitable. Other environments in which cracking has been reported are listed in Table I X .
TABLE V I I. CORROSIVE AGENTS REPORTED T O CAUSE CRACKING I N COPPER-BASE ALLOYS
Steam NH3 -4mines Air Water NHdNOs BaClz Butane SO2
+
HCle HFa HgdN03)z Mercury “01 vapor Organic chlorides KOH NaOH a
1 Reference
Alloy Type
Corrosive Agent
A1 bronze, Si bronze Brasses, bronzes Brasses, bronzes P bronze, Cu-As M n bronze Brass Brass Brass Brass Brass Brass Brass Brass Brass Brass Brass
Nitrogen compounds also present.
Nickel-Base Alloys
Nickel and nickel-base alloys are subject to cracking in hydrogen fluoride, hydrofluosilicic acid, and high-temperature sodium hydroxide. For nickel itself, it is recommended that a low carbon grade be used for molten alkali service (78). Thermal stress relief is effective for reducing cracking in most nickel alloys. The highnickel alloys are particularly good for service in which stainless steels may be cracked by hot chlorides. The list of reported aggressive environments is given in Table VIII. TABLE V I I I . CORROSIVE AGENTS REPORTED T O CAUSE C R A C K I N G O F NICKEL-BASE ALLOYS
Corrosive Agent H2SiFe
HgdN03)z Mercury NaOH, KOH HF HTa Steam Organic chlorides HTa water Steam SO2 Concd. NazS HTa Sulfur HCN (Impure)
+
a
28
I
Alloy, Reference Monel (25) Kickel (47) Monel (25) Monel (25) Nickel ( 2 5 ) Inconel (47) Monel ( 4 7 ) Nickel (47) Monel(47) Monel (47) Inconel (47) Inconel ( 4 7 ) Inconel (47) Kickel (47) Nickel (41)
H T , high temperature. I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
TABLE I X . CORROSIVE AGENTS REPORTED T O CAUSE C R A C K I N G OF T I T A N I U M ALLOYS Corrosihe Agent
Red-fuming “ 0 3 High temperature C1HC1 Methanol C1Chlorinated hydrocarbons
+
NaCl
Reference (27) ~
I ~
I
(27) (79)
(42) (42) (5)
Testing Methods
There are two general purposes for conducting stresscorrosion tests : the development of alloys, protection systems, or inhibitors; and the evaluation of existing materials. For development of new materials or techniques it is desirable that a specimen be used in which the stresses are quantitatively knolvn and reproducible, and that the time to failure be measured accurately. Such tests are usually conducted in the laboratory under carefully controlled and somewhat artificial conditions. For example, the specimens are often stressed below the yield point because only then can the stresses be known accuratel).. I n this way, small improvements can be noted and used to advantage. Such test procedures and specimens are not usually of immediate interest to the process industries. Testing under conditions as severe as any likely to be encountered in service, and as realistic as possible, is of greater interest. If a fabricated structure will contain welds, shrink fits, crevices, or heat-transfer surfaces the specimens should be designed to duplicate as many of these conditions as possible. The specimens should be tested in the process stream, preferably in a pilot plant, before final material selection is made. The specimens should be such that pieces do not come loose and enter the process stream if a specimen fractures. Because the stresses in the equipment will usually exceed the yield strength some place, the specimens should be coldworked ; this latter requirement usually rules out the quantitative specimen, and the difficulty of inspecting operating equipment usually precludes an accurate knowledge of the time of failure. Thus, for material selection, “pass or fail” type tests are usually adequate. Examples of specimens that have been used by U. S. Steel for material selection are shoivn in Figures 13 and 14. The cup-and-circular-weld specimen that may include stamped identification, punched and drilled holes, and sheared edges is especially realistic for sheet
material. The tuning-fork specimen can include almost as many variables for a plate material. Many other specimens have been used that are not shown here. Representative ones are listed in the references (22, 39, 49).
on the surface; surface alloying; cathodic protection when hydrogen embrittlement is not a problem; modification of environment; use of inhibitors, especially in thermal insulation; use of protective coatings; design changes; selection of a resistant alloy for the intended use.
Summary
Stress-corrosion cracking and corrosion fatigue are not generally encountered except in very specific applications. The necessary conditions are a susceptible alloy, an aggressive environment, a source of stress, and time. Stress-corrosion cracking and corrosion fatigue may be prevented by: thermal stress relief; mechanical stress relief; shot peening to introduce compressive stresses ; causing volume increase metallurgical reactions
Figure 13. Cz~pand circular weld stress-corrosion sbecimen. Actual sire
Figure 74. Tuning fork sbecimens. Left to right: as-machined; cold-worked; stressed; cold-worked, zuelded, and stressed. Approximately actual size (37)
REFERENCES (1) ASTM Special Technical Publication No. 264, ‘‘Stress Corrosion Cracking of Austenitic Chromium-Nickel Stainless Steels,” pp. 36, 56 (1 960). (2) Barnartt, S.,van Raoyen, D., J. Electrochem Soc. 108, 222 (1961). (3) Bates, J. F., Loginow, A . W., Corrosion 20, 189t (1964). (4) Bhatt, H. J., Phelps, E. H., Ibid., 17, 430t (1961). (5) Brohn, B. F., “A New Stress-Corrosion Cracking Test Procedure for High Strength Alloys,” ASTM, in press. (6) Brown, B. F., NRLReports-Problem KO.: M o l - 0 8 , ProjectNo.: NS611-007, Mav, 1958. ~(7) Copson, H. R., “Physical Metallurgy of Stress Corrosion Fracture,” pp. 456-7 Interscience, New York, 1959. (8) Dix, E. H., Jr., Trans., Inst. of M e l d s Diu., A Z M E 137, 11 (1 950). ( 9 ) Du Pont, Brit. Patent 964,323 ( J u l y 22, 1964). (10) Edeleanu, C., J. Iron Sled Inst. 173, 140 (1953). *Evans, U. R., “An Introduction to Metallic Corrosion,” Edward Arnold, (lZ)td., London, 1948. *Forty, A. J., “Phvsical Metallurgy ofStress Corrosion Fracture,” pp. 256 -7, (‘?kterscience, New Yo;k, 1959. (13) *Glikman, L. A,, “Corrosion-Mechanical Strength of Metals,” Butterworths, London, 1962. (14) Graf, L., “Stress Corrosion Cracking and Embrittlement,” pp. 48-60, Wiley, h‘ew York, 1956. (15) Harwood, J. J., “Stress Corrosion Cracking of Austenitic Chromium-Nickel Stainless Steels,” ASTM Spec. Tech. Pub. No. 264, p. 23, 1960. (16) *Hoar, T. P., Scully, J. C., Proc. 2nd. Intern. Cong. Mefniiic Corrosion, in press. (17) Hoar, T. P., West, J. M., >Vafure,p. 835 (1958). *LaQue F L. Copson, H. R . “Corrosion Resistance of Metals and Alloys,” (‘82)nd ed., p. 47i,AbS Monogr.iph Beries, Reinhold, New York, 1963. (19) Ibid., p. 647. (20) Logan, H. L., J. Res. Nnt. Bur. Sld., C. 48, 99 (1952). (21) *Logan, H. L., “Stress Corrosion of Metds,” IViley, New York, in press. (22) Loginow, A. M’,,“Specimens Used in Stress-Corrosion Tcsting of Alloys,” Mater. Prolecl., in press. (23) *Mears, R. B., Brown, R. H., Dix, E. H., “Symposium on Stress Corrosion Cracking of Metals,” . I S T M - A I M E , pp. 323-44, York, Pa., 1945. (24) NACE CommittecT-1B Report, ‘Wafer. P r o k t . 2, 97 (1963). *Nelson, G. A,, “Corrosion Data Survey,” Shell Development Co., National (22ssociation of Corrosion Engineers, Houston, Texas, 1760. (26) Nielsen, N. A,, Corrosion 20, 104t, (1964). (27) *Parkins, R. N., itleln/irrr&zl Reo. 9, 201-60 (1964). (28) *Parkins, R. N., “Stress Corroqinn Cracking iind Embrittlement,” p. 142, Electrochemical Society Symposium, Wilcy, New York, 1954. (29) Phelps, E. H., Loginow, A. LV., Corrosion 16, 325t, (1960). (30) Ibid., p. 328t. (31) Zbid., 18, 299t-309t (1962). (32) Pickering, H. W., Rcck, F. H., Fontana, M.G., Zbid., p. 230t. (33) Pickering, H . Vv., Swmn, P. R., Zbid., 19, 373t (1963). *Polar, J. P., “A Guide to Corrosion Resistance,” Climax Molybdenum Co., ( 3 3 a t o n Press, New York, 1961. (35) *Proc. First Znlern. Congr. Melailic Corrosion, Butterworths, London, 1962. (36) Robertson, W. D., Tetelman, A. S., “Strengthening Mechmisms in Solids,” p. 218, AShf, Metals Park, Ohio, 1960. (37) *Romanov, V. V., “Stress Corrosion Cracking of Metals,” Israel Program for Scientific Translations, OTS, U. S. Dept. of Comm., W;ishington, D. C., Trans. 60-51084. (38) Rostocker, W., McCaughey, J. M., Markus, H. “Embrittlement by Liquid Metals,” pp. 30-32, Reinhold, New York, 1960. (39) Sager, G . F., Brown, R . H., Mears, R. B., “Symposium on Stress Corrosion Cracking of Metals,” ASTM-AIME, pp. 255-72, York, Pa., 1945. (40) Sangdahl, G. S., Meld Prog. 86, 101-4 (Aug. 1964). (41) Shepard, S. W., Corrosion 17, 19-20 (1961). (42) *Shreir, L. L., “Corrosion,” p. 542, Wiley, New York, 1963. *Shvartz, and Kristal, “Corrosion of Chemical Apparatus,” pp. 24-5, Con(4%tants Bureau, New York, 1959. (44) Zbid., p. 69. (45) Zbid., p. 88. (46) Ibid., p. 139. (47) Ibid., p. 142. (48) Ibid., p. 166. (49) Ibid., pp. 206-26. (50) Swann, P. R., Corrosion 19, lO2t (1963). (51) Swann, P. R., Nutting, J., J . Inst. M e t a l s 90, 133 (1961-62). (52) Swann, P. R., Pickering, H . W., Corrosion 19, 369t (1963). *Uhlig, H . H., “Corrosion and Corrosion Control,’’ p. 116, \Viley, New York, (5:b63. (54) Zbid., p. 293. (55) *Uhlig, H. H., “Corrosion Handbook,” p. 569, Wiley, New York, 1948. (56) Uhl/g, H. H., “Physical Metallurgy of Stress Corrosion Fracture,” pp. 1-15 Interscience, New York, 1959. (57) Vaughan, D. A., Phalen, J., Peterson, C. L., Boyd, 1%’. K., Corrosion 19, 315t (1963).
*References so marked are of a more general nature and are particularly valuable as reference works. VOL. 5 8
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FEBRUARY 1966
29
