Stress-Corrosion Cracking of Sensitized Type 304 Stainless Steel in Thiosulfate Solutions

R. C. NEWMAN, K. SIERADZKI, and H. S. ISAACS

The stress corrosion cracking of a sensitized Type 304 stainless steel has been studied at room temperature using controlled potentials and two concentrations of sodium thiosulfate. In both constant extension rate and constant load tests, the crack velocities attain extremely high values, up to 8 /xm s -l. Scratching electrode experiments conducted at various pH values on simulated grain boundary material show that both the crack initiation frequency and crack velocity are closely related to the repassivation rate of the grain boundary material as expected on a dissolution-controlled mechanism; however, the maximum crack velocity at any potential is consistently about two orders of magnitude higher than that predicted from the electrochemical data. Frequent grain boundary separation ahead of the crack tip is thought to occur, but retarded repassivation of the grain boundary material is a necessary feature of the cracking. Effects of strain-generated martensite are discussed.

I. INTRODUCTION tions as dilute as 6 • 10 -7 molar caused SCC in constant extension rate (CER) tests. The suggestion was made that IT has been known for some years that several types of this ion was probably responsible for some instances of low environment, often highly dilute, can cause intergranular temperature SCC in borated water in pressurized water reac- stress corrosion cracking (IGSCC) in sensitized stainless tor systems, ~2 particularly as thiosulfate is sometimes used in steels at ambient temperatures. The most damaging species the emergency spray water to react with iodine. Controlled identified to date are fluoride ions I and various metastable potential CER tests in a 6 • 10 -4 M Na2S203 solution con- sulfur compounds. Neutral chloride solutions also cause taining boric acid showed an increase in propagation rate of some cracking at room temperature, particularly if a thick initiated cracks as the potential was raised to +500 mV oxide is grown prior to testing. 2'3 (SCE) [+740 mV (NHE)] and crack arrest when the poten- The dangerous effects of sulfur compounds were first tial was lowered to -500 mV (SCE) [-260 mV (NHE)]. recognized by Dravnieks and Samans, 4 who detected poly- More recently Dhawale et a/13'14 showed that, in 0.01M thionic acids in the condensate from catalytic reformers Na2S:O3 solutions containing boric acid, no cracking oc- where SCC of sensitized steels had occurred. Laboratory curred in CER tests at or above +300 mV (NHE). It was tests using Wackenroder's polythionic acid mixture, 5 pre- proposed that maximum SCC susceptibility was associated pared by bubbling hydrogen sulfide and sulfur dioxide with elemental sulfur formation; sulfur was visible as a through water, showed that severe SCC of sensitized steels yellowish material in and around the cracks. Known effects occurred at concentrations of HzS406 as low as 5 • 10 -a of adsorbed sulfur in accelerating dissolution and hindering molar. This cracking occurs only when the steel is sensitized passivation were cited, and thermodynamic calculations as defined by the acid copper sulfate test. 6 More recent showed that the potential range of rapid cracking corre- studies using near-neutral or mildly acidified tetrathionate sponded approximately to an (Fe 2+ + S) stability field on a solutions 7 showed cracking at all concentrations of $4062- potential-pH diagram. Zucchi et al 7 also noted that sulfur above 3 • 10 -5 molar. Apart from pH effects there is proba- formed in and around cracks; this may show that the tetra- bly no essential difference between SCC in this environment thionate ion is a major cathodic reactant, 9 although sulfur and in the Wackenroder solution. The potential dependence can also form as a result of disproportionation reactions of polythionic acid cracking has been investigated by following localized acidification of the solution in the crack, Matsushima, 8 who found cracking over the range -140 to particularly when thiosulfate is the bulk environment. There +440 mV (NHE) with a maximum around +200 mV. The is also some evidence that transition metal ions such as Cr3+ latter potential is close to that attained at open circuit after catalyze the disproportionation of a variety of unstable sul- a few minutes immersion in the aerated solution; it has been fur oxyanions in already acidified solutions, j5 If sulfide or suggested 9 that the tetrathionate ion rather than oxygen is sulfite ions are present, they react rapidly with tetrathionate responsible for maintaining this potential, and that any to give ($2032- + S) and ($2032- + 53062-), respectively. ~6 acidic environment at the same pH and potential would also The present work has two principal aims: to measure the cause equivalent SCC. Subsequent data 7'~~ have demon- potential dependence of crack initiation frequency and strated, however, that the metastable sulfur species have a propagation rate in a sensitized steel in dilute and concen- much more specific effect. trated thiosulfate solutions, and to deduce features of the Recent studies in this laboratory H have shown that thio- cracking mechanism by studies of simulated grain boundary sulfate, thiocyanate, tetrathionate, and sulfide ions are all material using a scratching electrode technique. highly aggressive SCC agents for sensitized steels at room temperature. In particular, aerated sodium thiosulfate solu- II. EXPERIMENTAL PROCEDURE

R.C. NEWMAN, K. SIERADZKI, and H.S. ISAACS are all A. Specimen Preparation and SCC Testing Metallurgists, Corrosion Science Group, BrookhavenNational Laboratory, Upton, NY 11973. The steel used in the investigation was obtained as Manuscript submitted October 28, 1981. 0. 125 inch (3.2 mm) sheet and contained Cr 18.68 wt pct, ISSN 0360-2133/82/1111-2015500.75/0 METALLURGICALTRANSACTIONS A 1982 AMERICAN SOCIETY FOR METALS AND VOLUME 13A, NOVEMBER 1982--2015 THE METALLURGICAL SOCIETY OF AIME Ni 8.55, Mn 1.70, Si 0.7, C 0.07, P 0.026, and S 0.005. with 500 turns and a current of 2.5 amps. The distribution Smooth specimens for CER testing were cut to give a l inch of the magnetic a' martensite phase was revealed by aggre- (25.4 mm) gauge length and a rectangular cross-section gation of the iron particles. In addition, carbon extraction 3.1 x 1.5 mm. Single edge notched specimens were also replicas were made of crack tip regions on similar polished made, of 3.2 mm thickness and 12.7 mm ligament length. surfaces after SCC testing and reheating to 823 K for All specimens were degreased, annealed in silica tubes con- 24 hours to precipitate characteristic carbides on any mar- taining argon at 1373 K for three hours, sensitized in argon tensite present. Deep etching with bromine was used to at 873 K for 24 hours, and cooled to room temperature by expose sheets of grain boundary carbides prior to replica- immersing the intact silica tubes in water. The yield strength tion. The replicas were examined in a JEOL 100C trans- was 300 MN m -2, the elongation to fracture 70 pet, and the mission electron microscope. grain size about 90 /xm. The notched specimens were fatigue precracked in air (AK = 15 MN m 3n, R -- 0.1). C. Scratching Electrode Experiments No further surface treatment preceded SCC testing. The Potentiostatic scratching tests were carried out, mainly in electrolytes for SCC testing were prepared and used at room 0.5M NatS203 solutions of various pH, with a few tests for temperature (296 - 2 K), and contained 100 ppm comparison in 0.5M Na2SO4. The repassivation of both (6.3 x 10 -4 M) or 0.5 M Na2S203 in deionized water of resistivity > 10 Mohm cm. The conductivity of the dilute matrix and simulated grain boundary material was exam- ined. Sheet specimens of annealed Type 304 steel and an solution was 1.7 x 10 -4 ohm -1 cm -~. A single test was annealed iron-9Cr-10Ni alloy (actual composition Ni carried out in a solution containing 6.3 x 10-4M Na2S203 10.1 pct, Cr 9.22 pct) were mounted face up in an open cell and 6.3 • 10-3M Na2SO4, to study the effect of adding a containing counter and reference electrodes. 9 pet Cr was relatively inert anion; this test was then repeated with further chosen as a compromise between the predicted equilibrium additions of Na2SO4 until inhibition of cracking was ob- value of -7.5 pct given by Tedmon et al for this heat served. CER tests were performed at nominal strain rates treatment and carbon content 17 and the higher values between 10 -6 and 2 x 10 -3 s -1, with the specimen elec- obtained analytically with -20 nm spatial resolution. TM trically insulated from the grips and immersed in 3 1 Manual scratching with a diamond-tipped tool gave a bare of aerated, stationary electrolyte. The potential was con- surface area -18 x 0.2 mm, with a contact time -50 ms. trolled from the instant of immersion by a potentiostat Application of a high frequency alternating current to an (PAR Model 173) using a 3 cm 2 platinum counter electrode edge-on foil specimen with the same dimensions as the and a saturated calomel reference electrode (SCE) with a scratch showed that the ohmic solution resistance to current Luggin probe tip 3 mm from one specimen face. All po- flow was --5 ohms in the 0.5M Na2S203 solution. Thus, all tentials quoted in this paper are relative to this electrode. ohmic potential drops were <25 mV. Repetitive scratching Load, extension, and cell current were monitored contin- at a fixed potential (controlled by a Stonehart Model BC uously in all tests. After each CER test the number of 1200 potentiostat) showed a scatter in the scratch area of identifiably distinct cracks was determined using a • about ---15 pet. Current transients were recorded, stored, microscope. The notched specimens were tested in a servo- and plotted using a Nicolet digital storage oscilloscope hydraulic machine using a small cell to contain the electro- (Model 206). The significant portions of the transients lay at lyte and electrodes. The load was ramped at 0.37 N s -~ large currents compared with the oxygen reduction rate in until SCC initiated, then held constant. Crack initiation and growth were monitored using a DC electrical potential the aerated solution; thus, deoxygenation was unnecessary. This was confirmed by performing a few tests in a deoxy- technique with a crack length resolution of about 40/xm; genated solution. The pH of the 0.5M NazSzO3 solution the cell current measured potentiostatically was also a (initially 9.1) was adjusted by small additions of sulfuric useful monitor. Initiation was defined by a simultaneous sustained increase in both cell current and electrical po- acid; for the more acidic pH values, where sulfur was pre- cipitated, the solution was allowed to stand for 10 minutes tential gauge signal. The results were converted to crack velocity as a function of apparent stress intensity. Fractured to attain a relatively steady pH before making the measure- ments. At the most acidic pH (3.0) the amount of sulfur specimens were studied using a scanning electron micro- precipitated during the experiment was about 5 pet of the scope (SEM) or by optical examination of polished sections. total present as 52032- in the original solution. Particular attention was paid to possible evidence for dis- continuous crack propagation. An ancillary CER test was performed on a notched specimen in one atmosphere of hydrogen sulfide containing less than 10 -6 atmospheres of III. RESULTS impurities, to compare the fracture mode with that observed in SCC. A. Constant Extension Rate Tests 1. 6 • lO-4M Sodium Thiosulfate Solution B. Detection of Strain-Generated Martensite A striking feature of these tests was the occurrence of Two techniques were used to investigate the formation of rapid SCC at very high nominal strain rates. A test at strain generated martensite ahead of the cracks. First, frac- +100 mV (SCE) and a strain rate 2 x 10 -3 s -1 resulted tured CER specimens were mounted in epoxy resin and in failure of the specimen at a nominal strain of 37 pct polished to a 0.25/xm finish. A suspension of colloidal iron (compared with 70 pct in air) and at failure many stress- ("Ferrofluid," Ferrofluidics Corp.) was placed in contact corrosion cracks had propagated 1 mm or more. It was with the surface using a glass cover slip, and optical exami- apparent that the average crack velocities involved here nation carried out with the specimen inside a 3 cm radius coil were unusually high, up to 8/zm s -1. Tests run in this

2016--VOLUME 13A, NOVEMBER 1982 METALLURGICALTRANSACTIONS A solution at 10 -~ s -~, at open circuit or at controlled potentials cedure was justified by the observation that the crack lengths between about -200 and +200 mV, produced average at failure were distributed rather evenly as estimated from crack velocities around 1 kcm s -~ and showed negligible optical sections. Whether the normalization is used or not, specimen ductility (e.g., 0.2 pct at +200 mV). Open circuit the maximum crack initiation frequency is at around tests in the aerated solution showed a gradual fall in potential -100 mV for two strain rates. Table I contains the original as cracking proceeded, beginning at --50 mV and falling data used to construct this figure. Figure 2 shows mean as low as -300 mV. In a previous publication 11 it was shown that crack propa- , , gation rates in a borated version of this solution (as indicated o -5 \ N by the rate of load decay in a constant elongation test) E increased monotonically with more anodic potential in the 4- + range -500 to +500 mV (SCE). This was confirmed in the >: • present work provided that cracking was first initiated at a I-- Q relatively low potential; however, it was found that cracking H 4- e -6 would not initiate from the smooth surface at +500 mV. By U 0 0 § 0 propagating cracks at + 100 mV and 10 -6 S -1 until the load _1 began to fall, then varying the potential, it was found that W > cracks arrested by applying - 500 mV for 60 seconds would reinitiate immediately only if a potential lower than about V +250 mV was applied. This "critical" potential was some- O: -7 what higher (+300 mV) if the cracks were allowed to n, propagate almost through the specimen before beginning the potential variation. Crack initiation probability at the Z smooth surface at potentials of +300 to +400 mV was, O: however, finite for the 10 -6 S -1 tests: either one or two hi cracks were obtained reproducibly at + 300 mV and in one I I I out of two tests at +400 mV a single crack was obtained; the other test showed full ductility. If the lacquer used to stop off o -200 0 200 the specimen outside the gauge length was imperfectly O J applied, cracking occurred readily in the crevice at +400 or POTENTIRL (mV SCE) +500 mV. At 10 -4 S-1 no cracking was observed above Fig. 2--Mean crack velocities for CER tests in 6 x 10-4M Na2SzO3, +200 mV and, moreover, propagating cracks could be measured from yield and taking the longest crack. Strain rates: C)... 10-6 S-I; X . . . 10-5; -k . . . 10-4; * . . . 8.2 • 10-4; . . . 2 • 10 -3. arrested by applying a potential more positive than +400 mV. The corresponding potential for 10 -6 S-I was 750 • 50 mV. Table I. Data from Constant Extension Figures 1 and 2 summarize the results described above. Rate Tests in Which SCC Occurred Figure 1 shows the variation of number of cracks initiated, Nominal True Plastic Number of normalized to the total strain at fracture. This arbitrary pro- Potential Strain Rate Strain to Cracks (mV, SCE) (s-l) Fracture eI (Pct) Initiated, N z H (1) 6 • 10-4M Na2SzO3

iv -300 10 6 3.7 18 I- o 80 10 -4 23.1 418 -200 10 6 0.65 29 u 10 -4 15.2 500 I- - 100 t0 -6 0.39 32 tn 10 -4 13.2 702 IV" 60 0 10 4 8.9 240 --.I + n + 100 10 -6 0.25 9 o 10 5 1.44 21 40 o 10 4 8.30 50 rw 8 • 10 -4 22.5 149 + 0 n 2 • 10 -3 3t.5 276 + +200 10 -6 0.20 8 ~fJ 10 -4 8.9 70 20 + +300 10 -6 0.39 2

rw + (2) 0.5M Na2SzO3 U 0 + o -400 10 -4 25.1 3 -300 10 -4 12.9 390 -400 -200 0 200 400 -200 10 -a 6.6 312 -100 10 4 4.5 338 POTENTIRL (mV SCE) 0 10 -4 5.4 441 Fig. 1--Crack initiation during CER tests in 6 • 10-4M Na2S203 at +100 10 -4 7.3 111 2 strain rates: Q... 10 -6 s-l; + . .. 10 -4 s -l.

METALLURGICAL TRANSACTIONS A VOLUME 13A, NOVEMBER 1982--2017 crack velocities for the CER tests, established by taking the (+1190 mV, 2 • 10 -3 s-~; see Figure 2). The effect of the longest crack and assuming that crack initiation occurred at preexisting fatigue crack on initiation was evidently similar yield. This assumption was justified by visual observation of to that of a crevice in the CER testing, as a crack was crack initiation and by examination of the current flowing, initiated readily at +500 mV. as described below. In the solution containing 6 x 10-3M Na2SO4 as well as C. Scratching Electrode Tests 6 x 10-4M Na2S203, SCC occurred at a strain rate 10 -6 S-l Most of these tests were carried out on the iron-9Cr- 10Ni and potential -100 mV, but the mean crack velocity was alloy in 0.5M Na2S:O3 of various pH. For the original pH of greatly reduced (0.04 p~m s-~). By adding extra sodium 9.1, it was found that a highly characteristic delayed re- sulfate it was found that crack propagation could be in- passivation was obtained within a potential range of about hibited altogether (within the resolution of the technique) by increase of the sulfate concentration to 1.2 x 10-2M. For a maximum crack depth of -1 mm, the time to crack arrest Z I ! I I I following the addition of the extra Na2SO4 was 120 --- 20 H IT" seconds. It was noted, however, that the sulfate-containing OF. mixtures tended to cause a general grain boundary etching O which was never observed in plain thiosulfate solutions. BO O U 2. 0.5M Sodium Thiosulfate Solution H A set of CER tests was run in this solution at 10 -4 s -~, for n- GO comparison with the scratching electrode tests and to isolate _J possible effects of electrolyte resistance. Figure 3 shows the 0_ mean crack velocities and crack initiation frequencies ob- tained. A single test was also run at 10 -6 S-l to examine 40 crack initiation by the method described above. In this solu- 0~ W tion there was a close correspondence between the crack 0_ initiation frequency, mean crack velocity, and crack arrest 20 potential, in contrast to the more dilute environment; the CO Y 0 apparent critical potential above which cracking did not rO occur was about + 100 mV, independent of whether crack- OT. ing was already occurring. The lower critical potential was IZ - I i ~ i U around -500 mV as in the dilute solution. -400-200 0 200 400

3. Current Flowing during SCC POTENTIAL (mV SCE) (a) It was reported earlier" and confirmed by Dhawale et a113'14 that the currents flowing during potentiostatically ol -5 I I I \ controlled SCC of sensitized steels in thiosulfate solutions E are closely associated with dissolution within the cracks. In 0 the present work, the wider range of strain rates employed >: O permitted a closer examination of this phenomenon. At a I-- G strain rate of 10 -6 s -~ the current rose continuously from just 1=4 U after yield in all tests where cracking was observed; when O cracking did not occur, no noticeable current rise occurred. _i W At the high strain rates (->10 -4 s -a) the current flowing to > -G G e cracking specimens tended to level off after a few pct strain v (Figure 4). By making electrical connections to both ends of U a specimen, the decay of the current after fracture was IZ followed; this typically showed a sharp (less than 1 second) U fall of between 10 and 30 pct followed by a slow decay to 0 Z the original passive (mixed) value. Figure 4 shows a repre- (Z sentative example. W E B. Constant Load Tests -7 I I I

Two tests were carried out in the 6 x 10-aM solution, O ._1 -400 -200 and resulted in typically shaped curves of the apparent stress intensity factor K o against crack velocity (Figure 5). It is POTENTIRL (mY SCE) emphasized that the plane strain limit, for this material and (b) geometry, occurs at about 10 MN m -3/2. At the higher po- Fig. 3--(a) Crack initiation during CER tests in 0.5M Na2S20~ at a strain tential of +500 mV the stage II crack velocity was similar rate of 10 -4 s-L (b) Mean crack velocities for CER tests in 0.5M Na2S203, to the highest mean crack velocity obtained in the CER tests measured from yield and taking the longest crack. Strain rate = 10_4 s-'.

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1000 lg for iron-9 Cr-10 Ni scratched in U 100 0.51vl Na2S203 (pH 9.1). 500 Od I E 1000 2000 O 60 I I I I I TIME (s) U Fig. 4-- Variation of load and current during a CER test: - 100 mV (SCE), E 10-4 s-L (a) load; (b) current. 50 0 0 I I I I I I n, 40 N bJ L >--5 m 30 0 I" X X X " I-4 (J 0 ao J . " > g ~ 10 v O. . -100 mV U X... +500 mV O ~ X X X X 00 E E-7 (SCE) IlK I I I I U X e I -400-200 0 200 400 U POTENTIRL (mV SCE) O Fig. 7--Variation of repassivation rate parameter with potential for 1 I I I I I I scratching tests in O,5M Na2S2Oa, pH 9.1, (~)... iron-9 Cr-10 Ni; X... annealed "l'ype 304 steel. 20 40 80 80 100 120 corresponds to 20 nm penetration by dissolution of iron as K (MN m-3/2) Fe 2+. The quantity -5 mC cm -2 measured for the matrix F~. 5--V~at~n ~ crack vel~i~ wi~ app~nt s~ss in~nsity for alloy at the same potential is of the order expected for precrac~d s~cimens ~ 2 ~nti~s in 6 x 10-4M NazS203. reestablishment of a 2 nm passive film J9 without signifi- cant dissolution. 100 mV around -200 mV (SCE), whereas at higher or The sodium thiosulfate solution at pH 9.1 is not a good lower potentials the charge passed during repassivation was simulation of a crack tip environment in this system, as small. Figure 6 shows current transients at three potentials. acidification by hydrolysis undoubtedly occurs. Accord- A convenient parameter for characterizing the transients was ingly, the same procedure described above was applied with the charge density q~ passed after 1 second; at -200 mV the successively greater degrees of acidification by H2SO4 or relatively slow current decay in the range -0.1 to 1 second Cr2(804) 3. It was noticeably difficult to maintain a pH below means that an electrochemical analysis of crack propagation 3, probably owing to the development of a sulfite/sulfurous is rather insensitive to the frequency of bare surface genera- acid buffering action following the decomposition of the tion. The scratch area used was the mean projected area. thiosulfate ion. Thus, the lowest pH examined was 3.0. Figure 7 shows the variation of q~ with potential for the Figure 8 shows q~ values for a variety ofpH; the results were "matrix" and "grain boundary" alloys, showing that the independent of the means used to lower the pH. For pH 7.0 attack would be most selective on bared grain boundaries and 4.4, the results are very close to those for pH 9.1; around -200 mV. Assuming zero effective roughness for however, a further decrease to pH 3.6 (when sulfur precipi- the scratch, the value 50 mC cm -2 obtained at -200 mV tation became noticeable) resulted in a large increase in the

METALLURGICAL TRANSAEHONS A VOLUME 13A, NOVEMBER 1982--2019 Od 1 D. Microscopy IE o 250 As reported earlier," the fracture surfaces obtained in I I I I I thiosulfate environments were intergranular (Figure 9(a)). 0 Some attention was paid, in view of the very high crack E X velocities, to evidence for discontinuous crack propagation. - 200 X During high magnification examination, however, by no O) means a majority of the grain facets showed any noticeable micro-dimpling other than the micron-scale roughness asso- Od X ciated with the carbides, so that if mechanical failure is w 150 occurring it must be a brittle process closely following the b_ grain boundaries. n- The carbon extraction replicas of a reheated specimen >- 100 I X showed needle-like carbide precipitates just ahead of the crack tips in the spaces between the original coarse grain boundary carbide particles (Figure 9(b)). These were not O3 X Z present at distances more than about one grain diameter w50 ahead of the crack tip. The radius of the zone containing Q these needles appeared to be about 50/xm for a specimen X + OI bJ strained in 0.5M Na2S203 at 10 -4 S 1 and --100 mV (mean X i o e velocity of crack examined ~3 p,m s-~). Carbides of this rr 09 90 n- type characteristically precipitate on martensite in reheated 7- -400-200 0 200 400 austenitic stainless steels. 2~ U POTENT3RI (mV SCE) Specimens in which crack propagation had occurred at a Fig. 8--Variation of repassivation rate parameter with potential for strain rate of 10 -6 S -I (total strain to fracture <0.5 pct) iron-9 Cr-10 Ni alloy in 0.5M Na2S~O3 adjusted to various pH with sul- showed very localized and sporadic grain boundary mar- furic acid. Q)... pH 9.0; +... 7.0; *... 4.4; O... 3.6; X... 3.0. tensite formation in the Ferrofluid test, with little matrix martensite visible. Where martensite was visible, the dis- tance uniformly affected along grain boundaries ahead of the peak charge density and a shift in peak position to about crack was less than 20/zm (Figures 9(c-e)). However, a -150 mV. At pH 3.0, the peak was at -100 mV and re- specimen strained at 10 -4 S-I and -300 mV (strain to frac- tarded repassivation then became evident at 0 and at ture --12 pct) showed extensive martensite both at grain + 100 mV. The steady-state current densities remained low boundaries (particularly within - 1 grain diameter of a crack at all potentials (less than 20/zA cm-2), consistent with the tip) and within the grains. Figures 9(f-g) illustrate both the observation of no noticeable intergranular corrosion on a extensive transformation and the very large density of sensitized steel specimen immersed in the pH 3 solution for cracks in this specimen. 24 hours. Both iron and an iron-5Cr-10Ni alloy were, in As noted in the previous publication, u the sensitized contrast, highly active, generating a loose black film which material used in this study shows a tendency for limited after washing and drying, dissolved in sulfuric acid with intergranular separation when tested to fracture in air (Fig- evolution of hydrogen sulfide. This material was also pro- ure 9(h)). This separation generally follows planes roughly duced during anodic polarization of these materials at perpendicular to that of the main ductile failure, and is also -200 mV. When several cm 2 of the iron-9Cr-10Ni alloy visible in the micrographs of Dhawale et al. 13,14 Figure 9(j) were abraded under the pH 9.1 solution, left for 24 hours, shows the fracture obtained in hydrogen sulfide, where the washed and dried, swabbing with sulfuric acid again gener- specimen showed very low ductility and a mean crack ve- ated a noticeable HzS odor indicative of sulfide corrosion locity of ~1 /zm s-l; this shows that hydrogen sulfide pro- products. Microscopic examination showed a faint tarnish- duced at the crack tip could, in principle, account for the ing of the surface. SCC behavior. According to Briant 21 a high susceptibility to To ensure that the effect of the thiosulfate ion on bare hydrogen embrittlement is indicative of extensive grain surface dissolution kinetics was a specific one rather than boundary martensite formation. being merely related to potential and pH, several scratching tests were carried out on the iron-9Cr-10Ni alloy in a 0.5M Na2SO4 solution, pH 5.8. At no potential in the range IV. DISCUSSION -500 mV to +500 mV did the value of q~ exceed 15 mC cm -2, and no transients of the type shown in Figure 6(b) A. Effect of Thiosulfate Concentration on the Potential were observed. This establishes that thiosulfate has a Dependence of SCC specific aggressive effect. It was noticeable that in the sul- fate solution, where the anion is inert to electroreduction, The results obtained in the more dilute electrolyte suggest the bare surface "corrosion potential," where no detectable a marked difference between the potential dependences of current transient occurred, was several hundred mV lower crack initiation and propagation. However, in the 0.5M than in any of the thiosulfate solutions -- e.g., in pH 9.1 solution these correspond closely. Figure 10 summarizes the thiosulfate it was --600 mV. This results from the ca- various cracking/potential relationships. These observations thodic reactivity of the thiosulfate ion. The transients at have a number of implications. Since there is probably no -200 mV are far enough anodic to this potential for the significant alteration in the immediate crack tip environment cathodic component of the charge density to be small. for changes in the bulk thiosulfate concentration, one can

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(c) if) Fig. 9--Microscopy and ffactography of sensitized Type 304 steel: (a) Constant load test, 6 x 10-4M Na2S203, + 500 mV (SCE) (from Stage I region, v ~ l p.m s-~). Crack propagation bottom to top. (b) Transmission electron micrograph of carbide precipitation in a grain boundary just ahead of a stress-corrosion crack tip (0.5M Na2S203, -100 mV (SCE), 10-4 s-~), following reheating to 550 ~ for 24 h. Shows characteristic ribbon morphology of carbides precipitated on martensite. Carbon extraction replica following deep bromine etch. (c) to (e) Crack tip region from CER specimen (6 • 10-4M Na2S203, l0 -6 s -~) showing crack morphology (c), oxalic acid etch of carbide-rich grain boundaries (d), and "Ferrofluid" etch of martensite (e). (]), (g): SCC in 0.5M Na2S203, -300 mV, 10-" s -~ , showing crack distribution (t) and "Ferrofluid" etch (g). METALLURGICAL TRANSACTIONS A VOLUME 13A, NOVEMBER 1982--2021 (g)

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Fig. 9--(cont.) (f), (g): SCC in 0.5M Na2S203, -300 mV, 10 -4 S -l, showing crack distribution (f) and "Ferrofluid" etch (g). (h) Onset of over- load fracture from a CER test in 6 x 10-4M Na2SzO3, showing typical grain boundary separation on ductile overload fracture surface. (j) Fracture sur- face after CER test in 1 atmosphere H2S. Crack propagation bottom to top.

(h)

deduce that there are large ohmic potential differences in the MAXIMUM HEAN CRACK VELOCITY more dilute solution resulting from current flow out of the (10 -6 s -1 STP,MN PATE) propagating cracks. The similarity in the cathodic protection potentials for the two solutions is consistent with this, as the MAXIMUMCRACKINITIATION ohmic drop approaches zero for low currents. The current FREQUENCY (i0 -& S "l) measured during SCC which causes this ohmic drop does CATHOOIC PROTECTION not correspond to the rate of metal removal at the crack tips, POTENTIAL (I0 "6 s "I , but probably originates chiefly from dissolution of exposed PROP~AT[NG CPJ~CK) chromium-depleted material on the crack walls: the evi- ...... [] 7777-~ dence for this is the slow current decay after specimen (INITIATION, 10 -6 s -1) fracture shown in Figure 4. Dissolution of the susceptible material on the crack walls will, of course, occur indepen- ...... (PROPAC4~TING CRACK, 10 -6 5 "I) [] 7/P dently of whether crack advance occurs by highly localized i l ~ i i i 1 i i i I l i i J dissolution, hydrogen embrittlement, or mechanical rup- -0,5 0 0,5 ture. Refinements in the simulation of the crack tip environ- POTENTIAL (VOt.TS SCE) ment, based on analysis of this slow current decay, are Fig. 10-- Summary of data from CER tests. ~: 6 x 10-4M Na25203 reported in another paper. 23 [~'%~X~: 0.5M Na2S203

2022--VOLUME 13A, NOVEMBER 1982 METALLURGICAL TRANSACTIONS A The relative importance of the crack contents and the bulk susceptibility with some confidence on the potential axis of solution in contributing to the ohmic potential drop cannot a potential -pH diagram, with due attention to the kinetic be assessed accurately from the present data; however, it is limitations of such a procedure. A lower limit for the pH easy to see that both will have significant effects, particu- may also be estimated, in view of the buffering action en- larly in the case of the 6 x 10-4M solution (conductivity countered in trying to obtain values below 3.0. A small 1.7 • 10 -4 ohm -~ cm-~). Thus, the potential difference AE concentration of Cr3§ ions produces such a pH quite readily, associated with the flow of (typically measured) 20/zA since the first hydrolysis constant 24 is 1.58 • 10-4: this per cm length of crack tip out of an external opening of gives a pH of 3.0 for only 0.006M Cr3+ . Later we show that (typically) 20/xm width can be estimated from the formula this pH is also a very reasonable estimate in view of the given by Pearson et al:22 relationship between the scratching tests and crack veloci- ties; this analysis is elaborated more fully elsewhere. 23 1 R ~- ~ ln(4b/a) Dhawale et all3't4 placed their potential range of maxi- 2 7rtrb mum cracking susceptibility on a composite potential -pH where R is the resistance to flow of current to a rectangular diagram generated by superimposing the Fe-H20 diagram of Pourbaix 25 and the metastable S-H20 diagram of Valensi. 26 strip electrode, b its half-length, a its half-width, and cr the conductivity. For o- = 1.7 • l0 -4 ohm -~ cm -~, The latter corresponds to metastable equilibrium conditions b -- 0.5 cm and a = 10 -3 cm, this yields R when oxidation of $203z-, $40~-, and SO 2- to SO42- is slow. 14,000 ohms and hence AE ~ 280 mV. If the same model Figure 11 shows a more precise form of this diagram, where crack is 1 mm deep and wedge-shaped, the resistance of the a stability field for iron sulfide has been added. Cracking crack contents gives an ohmic drop of <50 mV for a con- ranges for three studies are shown: those of Matsushima s in stant tr of 0.1 ohm -~ cm-]; however, if only 100/zm depth polythionic acid of pH ~ 1, of Dhawale et al in borated near the metal surface has the bulk conductivity of thiosulfate solution (assumed here to have a crack tip pH 3 1.7 • 10 -4 ohm -1 cm -l, the crack tip potential will fall by rather than the unaltered bulk value of 5 assumed by the about 600 mV for a current of 20/xA. Further theoretical authors), and of the present work using 0.5M Na2S203. The progress is difficult; however, it is worth noting that the correlation with the (Fe 2§ + S) stability field is striking, differing anodic protection potentials recorded earlier for a and there can be little doubt that the formation of elemental sulfur is somehow important in promoting cracking. 10 -6 strain rate (-250 and -750 mV) suggest a 500 mV increment in ohmic drop on going from the 0.5M to the At this stage, a sulfur effect on hydrogen entry 27 with 1.7 x 10-4M solution at a crack velocity of -1 tzm s -~. consequent embrittlement cannot be ruled out at potentials around -200 mV (SCE) [+40 mV (NHE)]. However, in The greater ease of anodic protection at 10 -4 S -l suggests that the geometrical effect of extra crack opening is more view of the low equilibrium partial pressures of hydrogen (-10 -17 atm) and hydrogen sulfide (--10 -12 atm) at pH 3 significant than any dilution effect as a result of flow of the bulk solution into the cracks. Some evidence that the crack contents make the major contribution to the ohmic drop may I I I possibly be deduced from the greater effect of dilution on propagation compared with initiation, and from the ten- FE205 + HSO~ dency for the anodic protection potential to increase with average crack length. In addition, one can cite the closer + $40~- 0-4 (though not exact) correspondence of crack initiation in the FE203 + SO~- dilute solution to the scratching tests, compared with crack propagation in the same solution.

B. Effect of Sodium Sulfate Additions Additions of relatively inert anions are expected to inhibit localized corrosion which relies on a high local accumu- -r FE++ + H2S ~ FE203 + S " lation of an aggressive ion, as they will electromigrate pref- erentially. Accumulation of large amounts of sulfate within FES the cracks evidently causes passivation of the crack tip. -0-4 The ratio of [5042-]/[52032-] ~ 20 for inhibition of SCC is high, and shows that very high concentrations of thio- sulfate within the cracks are not a prerequisite for SCC to FE304 + occur; however, they are evidently necessary for very rapid FE + H2S . FE304 + crack propagation. A similar inhibition affect is seen at low -0"8 thiosulfate concentrations on addition of boric acid," which is very weakly ionized. I I I I I I I I-"-4 0 2 4 6 8 10 12 pH C. Thermodynamic Interpretation of Cracking Fig. 11 --Potential -pH diagram for Fe-S-H20 at 298 K, not considering Susceptibility SO~-. All equilibria involving dissolved species are drawn for unit activity; equilibria involving H2S are for 1 atmosphere gaseous H2S. /.t ~ Ohmic potential drops are probably on the order of a few (FeS) = -97.7 kJ mol-~; other values from Pourbaix "Atlas". Severe tens of millivolts, at most, for the tests performed in 0.5M cracking ranges are indicated by arrows: a (ref. 8), b (refs. 13-14), and Na2S203. One may, therefore, place the region of cracking c (present work with 0.5M Na2S203).

METALLURGICAL TRANSACTIONS A VOLUME 13A, NOVEMBER 1982--2023 and the upper cracking potential of -+ 340 mV (NHE), a difficulties which are unusual. The principal problem is that dissolution mechanism of SCC must be considered likely. the highest current densities measured in the scratching tests The apparent equilibrium restrictions on hydrogen for- are quite inadequate to account for the crack velocities ob- mation may conceivably be circumvented if a resistive sur- served. Application of Faraday's second law shows that a face layer is present near the crack tip which absorbs most penetration rate of 1 /zm s -l by dissolution of iron as-Fe2+ of the applied potential while allowing some interaction requires a mean current density of 2.5 A cm-2; therefore, between the metal surface and water; however, this is un- the highest crack velocities in Figures 2, 3, and 5 would likely at extremely high crack velocities where the crack tip require about 20 A cm -2. The highest value of q] in is essentially bare. The only other significant source of Figure 8 is, however, only 220 mC cm -2, and the corre- hydrogen sulfide will be the small amount produced chemi- sponding peak current density at 50 ms about 300 mA cally as a result of acidification of the thiosulfate solution. cm -2. There are two ways of rationalizing this discrepancy: This will be rapidly oxidized at the crack tip, although if 1. Partial repassivation occurs during scratching and the hydrogen entry into the metal is very rapid following true instantaneous peak current density attains 20 A cm -2. adsorption, some absorption and embrittlement could con- 2. Crack propagation is predominantly mechanical in na- ceivably occur. These considerations focus our primary at- ture, possibly incorporating a hydrogen effect. tention on the effects of sulfur or sulfide ions in promoting Although higher current densities are obtained by more dissolution of iron and nickel, which are well documented rapid scratching, the charge density passed at these rates is (e.g., References 28, 29). As indicated earlier, the effect of very small--even at the 50 ms current peaks, such as that the thiosulfate is shown in the scratching tests to be a spe- in Figure 6, it amounts to only a few monolayers of metal cific one and not merely characteristic of a particular pH and oxidized. Such shallow attack is unlikely to be able to sus- potential. Kowaka and Kudo ~~ showed that a surface nickel tain crack propagation: a number of authors have shown that sulfide was formed as a result of exposure of a simulated the systems in which SCC propagation is considered to grain boundary alloy to polythionic acid (pH 1, potential occur by anodic metal removal are those in which many +150 mV NHE); this is consistent with the composite layers of alloy are oxidized or dissolved for each surface potential -pH diagram for Ni-S-H20 shown in Figure 12, baring event (e.g., References 33, 34). The theoretical basis which shows a NiS stability field. Under the same condi- of such a requirement has been discussed by Scully. 35 tions, iron (Figure 11) and chromium 2s are highly soluble The mean crack velocities from CER tests in 0.5M at equilibrium. Na:S203 at 10 -4 s -t strain rate (where mean crack velocities are approximately Stage II in nature) have been used in D. Interpretation of Scratching Electrode Tests Figure 13 for comparison with the penetration rates pre- The purpose of these tests was to provide some answers dicted from scratching tests on the iron-9Cr-10Ni alloy at to the following questions: pH 3.0. The agreement in curve shape is remarkable with 1. How is crack initiation related to repassivation of matrix the exception of the discrepancy of a factor of -50 between crack velocity and predicted penetration rate. We discuss and grain boundary material in 0.5M NazS203? elsewhere 23 the details of the dependence of these results on 2. Does the potential dependence of crack propagation cor- relate with repassivation in an acidified thiosulfate solution, and if so, at what pH? "-5 I I I ! ! 3. Can the maximum crack velocity at any potential be I= g g related to a bare surface dissolution rate? II g While these considerations are rather conventional in this r (a) type of investigation, 3~ the present system offers some g Z -6 g I-I 1,2 N1203" + HSO~I' ~ NxO~ + HSO~~ ' , '~'~ " ~, Z g 0 H l- -7 X X O: X n,' X (b) I-" X l.iJ Z -g X X O.C Nz +++ $20~-/ ~/~ NIO+ SO~- l.d O. N~O +$20 ~- ~-0.4 N~S X 0 -I- 9 I I I I I -0.8 -4BB-2BB g 20R 4BB -1,2 ~ ~ ~ ~ ~ ~ i 0 2 4 6 8 10 12 14 POTENTIflL (mV SCE) pH Fig. 13--Comparison of penetration rates: (a) CER tests in 0.5M Na25203 Fig. 12--Potential-pH diagram for Ni-S-H20 at 298 K. Conditions as in at 10 -4 s-a; (b) scratching of iron-9 Cr-10 Ni alloy in 0.5M Na2S~O3 acidi- Fig. 11, using ~~ (NiS) = -108.9 kJ mol -l. Ni3S2 not considered. fied to pH 3.0.

2024-- VOLUME 13A, NOVEMBER 1982 METALLURGICAL TRANSACTIONS A pH and alloy chromium concentration. It is worth repeating Cowan and Gordon9 reported that polythionic acid crack- at this point that the pH at which this agreement is obtained ing did not produce acoustic emission, but the crack veloci- is also that which tends to be obtained for quite a wide range ties were low in their U-bend tests and therefore acoustic of acid additions to the 0.5M NazSzO3, as a result of the emission would not be expected. Lee and Vermilyea37 sug- buffering action described earlier. A critical test of the esti- gested that intergranular SCC in Inconel 600, where mar- mate of the pH is the correct prediction of the anodic protec- tensite does not form, might involve localized fracture of tion potential against cracking, which would be predicted as grain boundaries--this idea has not been tested. Recent about -50 mV rather than + 100 mV if the pH was even as advances in studies of transgranular SCC have shown that high as 3.6. localized mechanical cleavage of macroscopically ductile Crack initiation frequency and propagation rate have material often occurs. 38 about the same potential dependence in the 0.5M solution. This would not be predicted if initiation was considered to F. Role of the Thiosulfate Ion and Relationship to occur at pH 9.1 and propagation at pH 3.0: thus, initiation Polythionic Acid Cracking may probably be considered to occur as a result of many film rupture events at the same site. Even one film rupture event The extraordinary potency of the thiosulfate ion, even will lower the interfacial pH of a near-neutral solution con- compared with its relatives such as the tetrathionate ion, is siderably at such low repassivation rates as obtained at probably the result of two important effects: -200 mV, owing to hydrolysis by Fe z+ ions: this is illus- 1. Of all the metastable sulfur anions, $202- is the most trated neatly by the almost identical values of q~ for initial readily converted to elemental sulfur, which seems to play pH values of 9.1, 7.0, and 4.4 (Figure 8). A similar effect an important part in the cracking. operates in chloride pitting of stainless steels, 36 where the 2. Ferrous thiosulfate is highly soluble and can exist as a pitting potential is approximately constant for 2-< pH -<8. supersaturated solution during localized corrosion. 39 Thus, sulfur is generated at the advancing crack tip surface, E. Mechanism of Cracking retarding repassivation, while a high concentration gradient To be consistent with the electrochemical measurements, of Fe 2+ ions can be maintained without precipitation. the mechanism of SCC in this system must incorporate a The polythionic acid test for SCC induced by sensitization mechanical step in cracking. If one accepts the analysis has been standardized. 4~ The present results indicate that a given earlier, hydrogen embrittlement must be considered dilute thiosulfate solution may have advantages as a testing unlikely. However, the formation of strain-generated mar- medium owing to its low cost and ease of preparation. tensite at grain boundaries and the occurrence of limited Comparison of the two environments is in progress-- intergranular separation in tests carried out in air both indi- preliminary results show that sensitized Alloy 600 is also cate that some intrinsic grain boundary brittleness is present. susceptible to thiosulfate SCC at concentrations as low This will be revealed most clearly when a sharp inter- as 10 -5 molar, and that in stainless steels the dependence granular crack is already available; thus, one can develop a of both polythionic acid and thiosulfate SCC on sensiti- mechanism containing the following steps: zation temperature is related to repassivation of grain boundary material. 1. Crack initiation occurs by repeated film rupture at grain boundaries, with rapid dissolution to about 150 nm depth per film rupture event for 9 pct Cr, pH 3, and - 100 mV SCE. V. CONCLUSIONS 2. At some stress intensity value, grain boundary mar- tensite films start to form to a distance ahead of the crack 1. SCC of sensitized Type 304 steel in thiosulfate solutions determined by the local strain. occurs only when repassivation of simulated grain 3. Narrow penetration by dissolution sharpens the crack, boundary material is retarded in a simulated crack tip facilitating localized grain boundary separation through the environment. Crack advance by dissolution is about transformed region. 150 nm per film rupture event at the potential of maxi- 4. Crack runs along the grain boundary through the trans- mum susceptibility. formed region. 2. The observed maximum crack velocity in this material at 5. Crack blunts on emerging from the transformed region any potential is 50 to 100 times that calculated on a and is resharpened by dissolution. dissolution model. The highest crack velocities observed correspond to a 3. Grain boundary cracking through a zone containing ratio of 50 to 100 between the areas of the fracture surface strain-generated martensite most probably constitutes the generated by fracture and by dissolution. Thus, one can majority of crack propagation in the Stage II regime. This make three predictions which are currently being tested: is possibly but not necessarily associated with hydrogen. 4. Addition of sufficient concentration of sulfate ions to a 1. There maybe a lower plateau crack velocity (--0.1 /~m s-~) dilute thiosulfate solution inhibits SCC; the molar ratio at low stress intensities, corresponding to pure dissolu- required is -20 for [S:O~-] = 6 • 10-4M. tion control. 2. Acoustic emission should occur at all crack velocities >0.1 /xm s -~. 3. The crack velocities which contain an element of me- ACKNOWLEDGMENTS chanical fracture should be sensitive to alloy compositions The assistance of Kenneth Sutter, Donald Becker, and or sensitization treatments which increase the intrinsic grain Ronald Graeser in the experimental work is gratefully ac- boundary strength without altering the chromium deple- knowledged. Professor Andr6 Vinckier carried out the trans- tion profile. mission electron microscopy of grain boundary carbides

METALLURGICAL TRANSACTIONS A VOLUME 13A, NOVEMBER1982--2025 during a period as Visiting Scientist; a full account will 18. C.S. Pande, M. Suenaga, B. Vyas, H. S. Isaacs, and D.F. Harling: be published elsewhere. The work was supported by the ScriptaMet., 1977, vol. 11, p. 681. Department of Energy, Division of Basic Energy Sciences, 19. J.E. Castle and C. R. Clayton: Corros. Sci., 1977, vol. 17, p. 7. 20. A. Vinckier: Brookhaven National Laboratory, Upton, NY, unpub- under Contract No. DE-AC02-76CH00016. lished research, 1981. 21. C.L. Briant: "Hydrogen Effects in Metals," I.M. Bernstein and REFERENCES A.W. Thompson, eds., TMS-AIME, 1981 p. 527. 22. H.J. Pearson, G.T. Burstein, and R.C. Newman: J. Electrochem. 1. C.T. Ward, D.L. Mathis, and R.W. Staehle: Corrosion, 1969, Soc., 1981, vol. 128, p. 2297. vol. 25, p. 394. 23. R.C. Newman, K. Sieradzki, and H. S. Isaacs: Presented at confer- 2. J.C. Danko and S. W. Tagart, Jr.: unpublished research, presented at ence on "Electrochemical Techniques in Corrosion Testing and Re- Investigative Laboratory Reports on Stainless Steel Intergranular SCC search," Manchester, 1982; proceedings to be published in Corrosion in PWR Boric Acid Piping Systems, EPRI, Palo Alto, CA, 1980. Science. 3. B. Vyas and H.S. Isaacs: Brookhaven National Laboratory, Upton, 24. J.W. Oldfield and W. H. Sutton: Br. Corros. J., 1978, vol. 13, p. 13. NY, unpublished research, 1980. 25. M. Pourbaix: "Atlas of Electrochemical Equilibria in Aqueous Solu- 4. A. Dravnieks and C.H. Samans: Proc. American Petroleum Inst., tions," Pergamon Press/Cebelcor, 1966, p. 307. 1957, vol. 37, p. 100. 26. G. Valensi: CEBELCOR RT 207, 1973, p. 121. 5. H. Wackenr&ter: Arch. Pharm., 1846, vol. 97, p. 272. 27. W. Palczewska: Bulletin de l'Academie Polonaise des Sciences 6. C.H. Samans: Corrosion, 1969, vol. 20, p. 256t. (Sciences Chimiques), 1964, vol. 12(3), p. 183. 7. F. Zucchi, A. Frignani, M. Zucchini, and G. Trabanelli: La Metal- 28. D.D. Macdonald, B. Roberts, and J.B. Hyne: Corros. Sci., 1978, lurgia Italiana, 1979, vol. II, p. 49. vol. 18, p. 411. 8. I. Matsushima: Boshoku Gijutsu, 1973, vol. 22, p. 141. 29. J. Oudar and P. Marcus: Appl. Surf. Sci., 1979, vol. 3, p. 48. 9. R.L. Cowan and G.M. Gordon: Proc. Conf. "Stress-Corrosion 30. R.N. Parkins: in Corrosion, L.L. Shreir, ed., 2nd edition, Newnes- Cracking and Hydrogen Embrittlement of Iron Base Alloys," Firminy, Butterworth, London, 1976, p. 824. 1973, NACE, Houston, TX, 1977, p. 1023. 31. E P. Ford: Metal Sci., 1978, vol. 12, p. 326. 10. M. Kowaka and T. Kudo: Nippon Kinzoku Gakkaishi, 1979, vol. 43, 32. R.C. Newman and G.T. Burstein: Corros. Sci., 1981, vol. 21, p. 595. p. 119. 11. H.S. Isaacs, B. Vyas, and M. W. Kendig: Corrosion, 1982, vol. 38, 33. J.F. Newman: Corros. Sci., 1981, vol. 21, p. 487. p. 130. 34. F.P. Ford and M. Silverman: Corrosion, 1980, vol. 36, p. 558. 12. U.S. Nuclear Regulatory Commission, Report NUREG 0691, Sep- 35. J.C. Scully: Metal Sci., 1978, vol. 12, p. 290. tember 1980, ch. 2. 36. H.P. Leckie and H. H. Uhlig: J. Electrochem. Soc., 1966, vol. 113, 13. S. Dhawale, G. Cragnolino, and D. D. Macdonald: EPRI Project RP p. 1262. 1166-1, Progress Report, July-December 1980. 37. D. Lee and D. A. Vermilyea: Metall. Trans., 1971, vol. 2, p. 2565. 14. S, Dhawale: M. S. Thesis, The Ohio State University, 1981. 15. B. Meyer, M. Rigdon, T. Burner, K. Koshlap, M. Ospina, and 38. D.A. Beggs, M. T. Hahn, and E. N. Pugh: Proc. "A. R. Troiano Hon. K. Ward: Proc. ACS National Meeting, Atlanta, GA, 1981; Lawrence Symposium on Hydrogen Embrittlement and SCC," Cleveland, OH, Berkeley Lab. Report LBL-11969, 1981. 1980, in press. 16. O.H. Tuovinen: Talanta, 1978, vol. 25, p. 408. 39. H.S. Isaacs and R.C. Newman: "Corrosion and Corrosion Protec- 17. C.S. Tedmon, Jr., D. A. Vermilyea, andJ. H. Rosolowski: J. Electro- tion," Electrochemical Society, Princeton, NJ, 1981, p. 120. chem. Soc., 1971, vol. 118, p. 192. 40. ASTM Standard Recommended Practice G35-73, 1980.

2026--VOLUME 13A, NOVEMBER 1982 METALLURGICAL TRANSACTIONS A