Intergranular Corrosion

19.2.3 Intergranular Corrosion

Improper heat treatment of ferritic or austenitic stainless steels causes the grain boundaries, which separate the individual crystals, to become especially suscep- tible to corrosion. Corrosion of this kind leads to catastrophic reduction in mechanical strength. The specifi c temperatures and times that induce susceptibil- ity to intergranular corrosion are called sensitizing heat treatments. They are very much different for the ferritic and the austenitic stainless steels. In this respect, the transition in sensitizing temperatures for steels containing 18% Cr occurs at about 2.5 – 3% Ni [15] ; stainless steels containing less than this amount of nickel are sensitized in the temperature range typical of nickel - free ferritic steels, whereas those containing more nickel respond to the temperature range typical of the austenitic stainless steels.

19.2.3.1 Austenitic Stainless Steels. For austenitic alloys, the sensitizing temperature range is approximately 425 – 875 ° C (800 – 1610 ° F). The degree of

* After transformation, the alloys, otherwise resistant, become susceptible to hydrogen cracking; also, ferritic and martensitic stainless steels on cold working tend to become more susceptible to hydrogen cracking.

344 ALLOYING FOR CORROSION RESISTANCE; STAINLESS STEELS

damage to commercial alloys caused by heating in this range depends on time, with a few minutes at the higher temperature range of 750 ° C (1380 ° F) being equivalent to several hours at a lower (or still higher) temperature range (Fig.

19.2 ) [16, 17] . Slow cooling through the sensitizing temperature range or pro- longed welding operations induce susceptibility, but rapid cooling avoids damage. Hence, austenitic stainless steels must be quenched from high temperatures. Spot welding, in which the metal is rapidly heated by a momentary electric current followed by a naturally rapid cooling, does not cause sensitization. Arc welding, on the other hand, can cause damage, with the effect being greater the longer the heating time, especially when heavy - gage material is involved. Sensitizing temperatures are reached some millimeters away from the weld metal itself, with the latter being at the melting point or above. Hence, on exposure to a corrosive environment, failure of an austenitic stainless steel weld — called weld decay —

occurs in zones slightly away from the weld rather than at the weld itself (see Fig. 19.3 ). The extent of sensitization for a given temperature and time depends very much on carbon content. An 18 – 8 stainless steel containing 0.1% C or more may

be severely sensitized after heating for 5 min at 600 ° C, whereas a similar alloy containing 0.06% C is affected less, and for 0.03% C the alloy heat treated simi- larly may suffer no appreciable damage on exposure to a moderately corrosive environment. The higher the nickel content of the alloy, the shorter the time for sensitization to occur at a given temperature, whereas alloying additions of molybdenum increase the time [16] .

The physical properties of stainless steels after sensitization do not change greatly. Because precipitation of carbides accompanies sensitization, the alloys become slightly stronger and slightly less ductile. Damage occurs only on expo- sure to a corrosive environment, with the alloy corroding along grain boundaries at a rate depending on severity of the environment and the extent of sensitiza- tion. In seawater, a sensitized stainless - steel sheet may fail within weeks or

Figure 19.2. Effect of time and temperature on sensitization of 18.2% Cr, 11.0% Ni, 0.05% C, 0.05% N stainless steel [16] . (Reprinted with permission of ASM International ® . All rights reserved. www.asminternational.org .)

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Intergranular corrosion Weld

Figure 19.3. Example of weld decay (2 × ). After sensitization, specimen was exposed to 25% HNO 3 .

months; but in a boiling solution containing CuSO 4 · 5H 2 O (13 g/liter) and H 2 SO 4 (47 mL concentrated acid/liter), which is used as an accelerated test medium, failure occurs within hours.

19.2.3.2 Theory and Remedies. Since intergranular corrosion of austenitic stainless steels is associated with carbon content, a low - carbon alloy ( < 0.02% C) is relatively immune to this type of corrosion [18] . Nitrogen, normally present in commercial alloys to the extent of a few hundredths percent, is less effective than carbon in causing damage (Fig. 19.4 ) [19] . At high temperatures [e.g., 1050 ° C (1920 ° F)], carbon is almost completely dispersed throughout the alloy, but within (or somewhat above) the sensitizing temperature range it rapidly diffuses to the grain boundaries where it combines preferentially with chromium to form chro-

mium carbides (e.g., M 23 C 6 , in which M represents the presence of some small amount of iron along with chromium). This reaction depletes the adjoining alloy of chromium to the extent that the grain - boundary material may contain less than the 12% Cr necessary for passivity. The affected volume of alloy normally extends some small distance into the grains on either side of the boundary itself, causing apparent grain - boundary broadening of the etched surface. The chro- mium - depleted alloy sets up passive – active cells of appreciable potential differ- ence, with the grains constituting large cathodic areas relative to small anodic areas at the grain boundaries. Electrochemical action results in rapid attack along the grain boundaries and deep penetration of the corrosive medium into the interior of the metal.

If the alloy is rapidly cooled through the sensitizing zone, carbon does not have time either to reach the grain boundaries or to react with chromium if some carbon is already concentrated at the grain boundaries. On the other hand, if the

346 ALLOYING FOR CORROSION RESISTANCE; STAINLESS STEELS

Figure 19.4. Intergranular corrosion measured by change in electrical resistance of 18 – 8 stainless steels containing nitrogen or carbon immersed in 10% CuSO 4 + 10% H 2 SO 4 . All speci- mens sensitized at stated temperatures for 217 h [19] . ( Reproduced with permission. Copyright 1945, The Electrochemical Society .)

alloy remains within the sensitizing temperature zone an especially long time (usually several thousand hours), chromium again diffuses into the depleted zones, reestablishing passivity of the grain boundaries and eliminating suscepti- bility to preferred attack. Although nitrogen forms chromium nitrides, it is less effective than carbon in causing damage, perhaps in part because nitrides pre- cipitate more generally throughout the grains or they form islands along grain boundaries, interrupting a continuous path along which the corrosive agent can proceed [19] . Carbides, on the other hand, form continuous paths of chromium -

depleted alloy. Other experimental data can be cited that supports the chromium - depletion mechanism of intergranular corrosion of austenitic stainless steels. For example, carbides isolated from grain boundaries of sensitized stainless steels have shown an expected high chromium content. Along the same lines, corrosion products of grain - boundary alloy obtained by choosing corrosive conditions that avoid attack of carbides show a lower chromium content than corresponds to the alloy. In this respect, Schafmeister [20] , using cold concentrated sulfurous acid acting on sensitized 18% Cr, 8.8% Ni, 0.22% C stainless steel for 10 days, found only 8.7% Cr in the alloy that had corroded from grain - boundary regions. Accompa- nying analyses for Ni and Fe of 8.4% and 83%, respectively, showed no appre- ciable depletion in nickel and an increase in iron content.

Microprobe scans of sensitized stainless steels have indicated chromium depletion and nickel enrichment at grain boundaries [21] . Radioactive C 14 intro- duced into an austenitic 18% Cr, 12.8% Ni, 0.12% C stainless steel has demon-

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strated enrichment of carbon at grain boundaries observable immediately after quenching from 1000 ° C to 1350 ° C. This indicates that time for sensitization may depend, in part, on the reaction rate of chromium with carbon already at grain boundaries and not alone on the time required for additional carbon to diffuse to grain boundaries.

There are at least three effective means for avoiding susceptibility to inter- granular corrosion:

1. Heat Treatment at 1050 −1100 ° C (1920−2000 ° F) Followed by Quenching. At high temperature, precipitated carbides are dissolved, and rapid cooling pre- vents their re - formation. This treatment is recommended, for example, after welding operations. It is not always a possible treatment, however, because of the size of the welded structure or because of a tendency of the alloy to warp at high temperatures.

2. Reduction of Carbon Content. Carbon content can be reduced in the commercial production of stainless steels, but at extra cost. Alloys of low carbon content (e.g., < 0.03% C) are designated by letter L — for example, Types 304L and 316L. These alloys can be welded or otherwise heated in the sensitizing temperature range with much less resultant susceptibility to intergranular corro- sion. However, they are not immune.

3. Addition of Titanium or Niobium (Columbium). By alloying austenitic alloys with a small amount of an element (e.g., Ti or Nb) having higher affi nity for carbon than does chromium, carbon is restrained from diffusing to the grain boundaries, and any which is already at the boundary reacts with titanium or columbium instead of with chromium. Alloys of this kind are called stabilized grades (e.g., Types 321, 347, 348). Except as noted below, they can be welded or otherwise heated within the sensitizing zone without marked susceptibility to intergranular corrosion. Optimum resistance to intergranular corrosion at 675 ° C (1250 ° F) is obtained by a prior carbide stabilizing heat treatment for several hours at about 900 ° C (1650 ° F) [17, 22] . This heat treatment converts available carbon to stable carbides of titanium or niobium in a temperature range of lower carbon solubility compared to that of the usual higher quench temperatures.

In welding operations, the weld rod usually contains niobium rather than titanium; the latter tends to oxidize at elevated temperatures with the danger of its residual concentration becoming too low to stabilize the weld alloy against corrosion. Niobium, on the other hand, is lost by oxidation to a lesser extent.

If, during welding the stabilized grades of stainless steel, the region of the base metal adjacent to the weld is heated to temperatures at which the carbides of titanium and niobium dissolve [above about 1230 ° C (2250 ° F)] and the cooling rate is suffi ciently rapid to prevent the formation of the stabilizing carbides, then these steels can be sensitized if they are subsequently heated in the temperature range in which chromium carbides precipitate. In such a situation, a narrow region immediately adjacent to the weld is susceptible to intergranular corrosion,

348 ALLOYING FOR CORROSION RESISTANCE; STAINLESS STEELS

which, if it occurs, is called knife - line attack . This type of intergranular corrosion can be prevented by selecting the welding process parameters to avoid tempera- tures at which carbides of titanium and niobium dissolve and by using a carbide -

stabilizing heat treatment as described above.

19.2.3.3 Intergranular Corrosion of Nonsensitized Alloys. Sigma phase is a hard, brittle, intermetallic compound, rich in chromium and molybdenum, with a tetragonal structure that can precipitate at grain boundaries in Type 316 stainless steel when heated in the range 540 – 1000 ° C (1000 – 1800 ° F). Sigma phase impairs corrosion resistance in highly oxidizing nitric acid environments. The mechanism of corrosion may be based on the high molybdenum content of sigma phase, which results in preferential direct attack on the sigma phase particles in preference to the molybdenum - depleted zones surrounding them [23] .

Sigma phase may also form in Types 310, 430, and 446 stainless steels, but in these alloys the phase forms at a rate so low that it is important only when the alloys are used in the temperature range in which sigma phase is formed [23] .

In strongly oxidizing media (e.g., boiling 5N HNO 3 + Cr 6+ ), austenitic stain- less steels, including stabilized grades, quenched from 1050 ° C, undergo slow intergranular attack [24] . Stress is not necessary. The attack occurs only in the

transpassive region; hence, oxidizing ions, such as Cr 6+ (0.1 – 0.5 N K 2 Cr 2 O 7 ), Mn 7+ , or Ce 4+ , are necessary additions to the boiling nitric acid. The rate of attack increases with the amount of alloyed nickel [25] , being more than 10 times higher for a 78% Ni, 17% Cr, bal. Fe alloy compared to a similar 10% Ni alloy (70 - h test). This trend is opposite to the benefi cial effect of nickel on the resistance of stainless steels to stress - corrosion cracking.

Grain - boundary attack of 19% Cr, 9% Ni stainless steel is most rapid after quenching from 10 ° C to 1200 ° C; it is less pronounced on quenching from either 900 ° C or 1400 ° C [26] . High - purity alloys are immune. Alloyed carbon, nitrogen, oxygen, or manganese added in small concentrations have no effect, whereas silicon and phosphorus ( > 100 ppm) are damaging. Silicon causes increased inter- granular attack in the intermediate range of 0.1 − 2% for the 14% Cr, 14% Ni stainless steel; in larger or smaller amounts the alloy is not susceptible [27] . The necessity of strongly oxidizing conditions and presence of phosphorus for inter- granular attack was confi rmed for a quenched low carbon 20% Cr, 20% Ni, 0.1%

P stainless steel [28] . When polarized anodically in 1 N H 2 SO 4 or in 27% HNO 3 at 40 ° C, intergranular attack occurred only at very noble potentials in the trans- passive region. With only 0.002% P in the alloy, such attack was not observed at any potential.

Similar intergranular attack of 15% Cr, 6% Fe, bal. Ni (Inconel 600) in high - temperature water (350 ° C) or steam (600 – 650 ° C) [25] or of stabilized 18 – 8 stain- less steel in potassium hydroxide solutions (pH 11) at 280 ° C [29] has been reported. This matter is of prime interest in view of the common use of Inconel 600 and stainless steels in the construction of nuclear reactors for power produc-

tion. Contaminants in the water [such as traces of dissolved O 2 , NaOH, or dis- solved lead (from heat - transfer tube – tube sheet leakage)], and the presence of

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crevices at which superheating occurs accompanied by concentration of solute accelerate S.C.C. [30] . Laboratory tests indicate that such failures also occur in longer times in relatively pure water [31] (see also Section 23.3.3 ). Increased nickel content favors intergranular corrosion [32] of 18 – 20% Cr stainless steels at 200 – 300 ° C in water containing Cl − and dissolved oxygen, as it does in boiling

HNO 3 – Cr 6+ . Silicon ( > 0.3%) and phosphorus ( > 0.023%) in 14% Cr, 14% Ni stain- less steel exposed to 0.01% FeCl 3 at 340 ° C, analogous to results in HNO 3 + Cr 6+ , cause intergranular corrosion. Unlike the latter medium, the high - temperature 0.01% FeCl 3 solution (21 - day exposure) caused intergranular corrosion [33] of the nonsensitized stainless steel when it contained > 0.05% C. The overall results confi rm that intergranular attack is the result of specifi c impurities in the alloy segregating at grain - boundary regions. The extent of their damaging effect depends on the chemical environment to which the alloy is exposed. Although applied stress has been stated to increase the observed attack in various media, stress is not always necessary; hence, the observed damage in such instances is better described as intergranular corrosion rather than as stress -

corrosion cracking.

The sensitizing range for ferritic stainless steels lies above 925 ° C ( > 1700 ° F), and immunity is restored by heating for a short time (approximately 10 – 60 min) at 650 – 815 ° C (1200 – 1500 ° F). These tempera- tures are the opposite of those applying to austenitic stainless steels. The same

19.2.3.4 Ferritic Stainless Steels.

accelerating media (i.e., boiling CuSO 4 –H 2 SO 4 or 65% HNO 3 ) produce inter- granular corrosion in either class, and the extent and rapidity of damage are similar. In welded sections, however, damage to ferritic steels occurs to metal immediately adjacent to the weld and to the weld metal itself, whereas in aus- tenitic steels the damage by weld decay localizes some small distance away from the weld.

Chromium content of ferritic steels, whether high or low (16 – 28% Cr), has no appreciable infl uence on susceptibility to intergranular corrosion [34] . Similar to the situation for austenitic steels, lowering the carbon content is helpful, but the critical carbon content is very much lower. A type 430 ferritic stainless steel containing only 0.009%C was still susceptible [34] . Only on decarburizing low -

carbon steels containing 16% or 24% Cr in H 2 at 1300 ° C for 100 h was immunity obtained in the CuSO 4 –H 2 SO 4 test solution [35] . Similarly, a low - carbon ( ∼ 0.002%) 25% Cr – Fe alloy was reported to be immune [36] . Addition of tita- nium in the amount of eight times carbon content or more provided immunity

to the CuSO 4 test solution, but not to boiling 65% HNO 3 [34] . Niobium additions, it was stated, behaved similarly, and only heat treatment, as described previously, was effective in conferring immunity to nitric acid. It was reported [37] , however, that columbium additions (8 × C + N content), but not titanium additions, minimize the observed intergranular corrosion of welds

exposed to boiling 65% HNO 3 . This behavior may be explained by the observed marked reactivity of titanium carbides, but not niobium carbides, with HNO 3 along grain boundaries where such carbides are concentrated [38] .

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The Mo – Cr stainless steels of controlled purity, described earlier, although in some instances containing more than 0.01% C, are immune to intergranular corrosion. This is accounted for by their molybdenum content, which slows down diffusion of carbon and nitrogen, and by their titanium or niobium content which, if present, reacts preferentially with carbon and nitrogen.

Intergranular corrosion of ferritic stainless steels is best explained on the basis of the chromium - depletion theory that is generally accepted for the austen- itic stainless steels [39] . Differences in sensitization temperatures and times com- pared to the austenitic steels are explained by the much higher diffusion rates of carbon, nitrogen, and chromium in the ferritic body - centered - cubic lattice com- pared to the austenitic face - centered - cubic lattice. Accordingly, chromium car- bides and nitrides, in solution at higher temperatures, precipitate rapidly (within seconds) below 950 ° C along grain boundaries, depleting the adjoining alloy of chromium to resultant compositions below stainless - steel requirements and which corrode at correspondingly higher rates than the grains. But accompanying high diffusion rates of chromium account for restored immunity to intergranular corrosion on heating the alloy for several minutes in the range 650 – 815 ° C (rather than weeks or months required to restore sensitized austenitic stainless steels), thereby restoring the grain - boundary alloy to normal stainless - steel composition.