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

IT has been known for some years that several types of environment, often highly dilute, can cause intergranular stress corrosion cracking (IGSCC) in sensitized stainless steels at ambient temperatures. The most damaging species identified to date are fluoride ions I and various metastable sulfur compounds. Neutral chloride solutions also cause some cracking at room temperature, particularly if a thick oxide is grown prior to testing. 2'3

The dangerous effects of sulfur compounds were first recognized by Dravnieks and Samans, 4 who detected poly- thionic acids in the condensate from catalytic reformers where SCC of sensitized steels had occurred. Laboratory tests using Wackenroder's polythionic acid mixture, 5 pre- pared by bubbling hydrogen sulfide and sulfur dioxide through water, showed that severe SCC of sensitized steels occurred at concentrations of HzS406 as low as 5 • 10 -a molar. This cracking occurs only when the steel is sensitized as defined by the acid copper sulfate test. 6 More recent studies using near-neutral or mildly acidified tetrathionate solutions 7 showed cracking at all concentrations of $4062- above 3 • 10 -5 molar. Apart from pH effects there is proba- bly no essential difference between SCC in this environment and in the Wackenroder solution. The potential dependence of polythionic acid cracking has been investigated by Matsushima, 8 who found cracking over the range - 1 4 0 to +440 mV (NHE) with a maximum around +200 mV. The latter potential is close to that attained at open circuit after a few minutes immersion in the aerated solution; it has been suggested 9 that the tetrathionate ion rather than oxygen is responsible for maintaining this potential, and that any acidic environment at the same pH and potential would also cause equivalent SCC. Subsequent data 7'~~ have demon- strated, however, that the metastable sulfur species have a much more specific effect.

Recent studies in this laboratory H have shown that thio- sulfate, thiocyanate, tetrathionate, and sulfide ions are all highly aggressive SCC agents for sensitized steels at room temperature. In particular, aerated sodium thiosulfate solu-

tions as dilute as 6 • 10 -7 molar caused SCC in constant extension rate (CER) tests. The suggestion was made that this ion was probably responsible for some instances of low temperature SCC in borated water in pressurized water reac- tor systems, ~2 particularly as thiosulfate is sometimes used in the emergency spray water to react with iodine. Controlled potential CER tests in a 6 • 10 -4 M Na2S203 solution con- taining boric acid showed an increase in propagation rate of initiated cracks as the potential was raised to +500 mV (SCE) [+740 mV (NHE)] and crack arrest when the poten- tial was lowered to - 5 0 0 mV (SCE) [ -260 mV (NHE)]. More recently Dhawale et a/13'14 showed that, in 0.01M Na2S:O3 solutions containing boric acid, no cracking oc- curred in CER tests at or above +300 mV (NHE). It was proposed that maximum SCC susceptibility was associated with elemental sulfur formation; sulfur was visible as a yellowish material in and around the cracks. Known effects of adsorbed sulfur in accelerating dissolution and hindering passivation were cited, and thermodynamic calculations showed that the potential range of rapid cracking corre- sponded approximately to an (Fe 2+ + S) stability field on a potential-pH diagram. Zucchi et al 7 also noted that sulfur formed in and around cracks; this may show that the tetra- thionate ion is a major cathodic reactant, 9 although sulfur can also form as a result of disproportionation reactions following localized acidification of the solution in the crack, particularly when thiosulfate is the bulk environment. There is also some evidence that transition metal ions such as Cr 3+ catalyze the disproportionation of a variety of unstable sul- fur oxyanions in already acidified solutions, j5 If sulfide or sulfite ions are present, they react rapidly with tetrathionate to give ($2032- + S) and ($2032- + 53062-), respectively. ~6

The present work has two principal aims: to measure the potential dependence of crack initiation frequency and propagation rate in a sensitized steel in dilute and concen- trated thiosulfate solutions, and to deduce features of the cracking mechanism by studies of simulated grain boundary material using a scratching electrode technique.

II. EXPERIMENTAL PROCEDURE

R.C. NEWMAN, K. SIERADZKI, and H.S. ISAACS are all A. Specimen Preparation and SCC Testing Metallurgists, Corrosion Science Group, Brookhaven National 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

METALLURGICAL TRANSACTIONS A �9 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. Smooth specimens for CER testing were cut to give a l inch (25.4 mm) gauge length and a rectangular cross-section 3.1 x 1.5 mm. Single edge notched specimens were also made, of 3.2 mm thickness and 12.7 mm ligament length. All specimens were degreased, annealed in silica tubes con- taining argon at 1373 K for three hours, sensitized in argon at 873 K for 24 hours, and cooled to room temperature by immersing the intact silica tubes in water. The yield strength was 300 MN m -2, the elongation to fracture 70 pet, and the grain size about 90 /xm. The notched specimens were fatigue precracked in air (AK = 15 MN m 3n, R -- 0.1). No further surface treatment preceded SCC testing. The electrolytes for SCC testing were prepared and used at room tempera tu re (296 - 2 K), and con ta ined 100 ppm (6.3 x 10 -4 M) or 0.5 M Na2S203 in deionized water of resistivity > 10 Mohm cm. The conductivity of the dilute solution was 1.7 x 10 -4 ohm -1 cm -~. A single test was carried out in a solution containing 6.3 x 10-4M Na2S203 and 6.3 • 10-3M Na2SO4, to study the effect of adding a relatively inert anion; this test was then repeated with further additions of Na2SO4 until inhibition of cracking was ob- served. CER tests were performed at nominal strain rates between 10 -6 and 2 x 10 -3 s -1, with the specimen elec- trically insulated from the grips and immersed in 3 1 of aerated, stationary electrolyte. The potential was con- trolled from the instant of immersion by a potentiostat (PAR Model 173) using a 3 cm 2 platinum counter electrode and a saturated calomel reference electrode (SCE) with a Luggin probe tip 3 mm from one specimen face. All po- tentials quoted in this paper are relative to this electrode. Load, extension, and cell current were monitored contin- uously in all tests. After each CER test the number of identifiably distinct cracks was determined using a • microscope. The notched specimens were tested in a servo- hydraulic machine using a small cell to contain the electro- lyte and electrodes. The load was ramped at 0.37 N s -~ until SCC initiated, then held constant. Crack initiation and growth were monitored using a DC electrical potential technique with a crack length resolution of about 40/xm; the cell current measured potentiostatically was also a useful monitor. Initiation was defined by a simultaneous sustained increase in both cell current and electrical po- tential gauge signal. The results were converted to crack velocity as a function of apparent stress intensity. Fractured specimens were studied using a scanning electron micro- scope (SEM) or by optical examination of polished sections. 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 impurities, to compare the fracture mode with that observed in SCC.

B. Detection of Strain-Generated Martensite

Two techniques were used to investigate the formation of strain generated martensite ahead of the cracks. First, frac- tured CER specimens were mounted in epoxy resin and polished to a 0.25/xm finish. A suspension of colloidal iron ("Ferrofluid," Ferrofluidics Corp.) was placed in contact with the surface using a glass cover slip, and optical exami- nation carried out with the specimen inside a 3 cm radius coil

with 500 turns and a current of 2.5 amps. The distribution of the magnetic a ' martensite phase was revealed by aggre- gation of the iron particles. In addition, carbon extraction replicas were made of crack tip regions on similar polished surfaces after SCC testing and reheating to 823 K for 24 hours to precipitate characteristic carbides on any mar- tensite present. Deep etching with bromine was used to expose sheets of grain boundary carbides prior to replica- tion. The replicas were examined in a JEOL 100C trans- mission electron microscope.

C. Scratching Electrode Experiments

Potentiostatic scratching tests were carried out, mainly in 0.5M NatS203 solutions of various pH, with a few tests for comparison in 0.5M Na2SO4. The repassivation of both matrix and simulated grain boundary material was exam- ined. Sheet specimens of annealed Type 304 steel and an annealed iron-9Cr-10Ni alloy (actual composition Ni 10.1 pct, Cr 9.22 pct) were mounted face up in an open cell containing counter and reference electrodes. 9 pet Cr was chosen as a compromise between the predicted equilibrium value of - 7 . 5 pct given by Tedmon et al for this heat treatment and carbon content 17 and the higher values obtained analytically with - 2 0 nm spatial resolution. TM Manual scratching with a diamond-tipped tool gave a bare surface area - 1 8 x 0.2 mm, with a contact time - 5 0 ms. Application of a high frequency alternating current to an edge-on foil specimen with the same dimensions as the scratch showed that the ohmic solution resistance to current flow was --5 ohms in the 0.5M Na2S203 solution. Thus, all ohmic potential drops were

solution at 10 -~ s -~, at open circuit or at controlled potentials between about - 2 0 0 and +200 mV, produced average crack velocities around 1 kcm s -~ and showed negligible specimen ductility (e.g., 0.2 pct at +200 mV). Open circuit tests in the aerated solution showed a gradual fall in potential as cracking proceeded, beginning at - - 5 0 mV and falling as low as - 3 0 0 mV.

In a previous publication 11 it was shown that crack propa- gation rates in a borated version of this solution (as indicated by the rate of load decay in a constant elongation test) increased monotonically with more anodic potential in the range - 5 0 0 to +500 mV (SCE). This was confirmed in the present work provided that cracking was first initiated at a relatively low potential; however, it was found that cracking would not initiate from the smooth surface at +500 mV. By propagating cracks at + 100 mV and 10 -6 S -1 until the load began to fall, then varying the potential, it was found that cracks arrested by applying - 500 mV for 60 seconds would reinitiate immediately only if a potential lower than about +250 mV was applied. This "critical" potential was some- what higher ( + 3 0 0 mV) if the cracks were allowed to propagate almost through the specimen before beginning the potential variation. Crack initiation probability at the smooth surface at potentials of +300 to +400 mV was, however, finite for the 10 -6 S -1 tests: either one or two cracks were obtained reproducibly at + 300 mV and in one 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 the specimen outside the gauge length was imperfectly applied, cracking occurred readily in the crevice at +400 or +500 mV. At 10 -4 S -1 no cracking was observed above +200 mV and, moreover, propagating cracks could be a r res ted by a p p l y i n g a po ten t i a l more pos i t i ve than +400 mV. The corresponding potential for 10 -6 S -I was 750 • 50 mV.

Figures 1 and 2 summarize the results described above. Figure 1 shows the variation of number of cracks initiated, normalized to the total strain at fracture. This arbitrary pro-

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P O T E N T I R L (mV S C E ) Fig. 1 - -Crack initiation during CER tests in 6 • 10-4M Na2S203 at 2 strain rates: Q . . . 10 -6 s-l; + . .. 10 -4 s -l .

cedure was justif ied by the observation that the crack lengths at failure were distributed rather evenly as estimated from optical sections. Whether the normalization is used or not, the m a x i m u m crack ini t ia t ion f requency is at around - 1 0 0 mV for two strain rates. Table I contains the original data used to construct this figure. Figure 2 shows mean

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Table I. Data from Constant Extension Rate Tests in Which SCC Occurred

Potential (mV, SCE)

Nominal True Plastic Number of Strain Rate Strain to Cracks

(s-l) Fracture e I (Pct) Initiated, N

(1) 6 • 10-4M Na2SzO3

-300 10 6 3.7 18 10 -4 23.1 418

-200 10 6 0.65 29 10 -4 15.2 500

- 100 t0 -6 0.39 32 10 -4 13.2 702

0 10 4 8.9 240 + 100 10 -6 0.25 9

10 5 1.44 21 10 4 8.30 50

8 • 10 -4 22.5 149 2 • 10 -3 3t.5 276

+200 10 -6 0.20 8 10 -4 8.9 70

+ 3 0 0 10 -6 0.39 2

(2) 0.5M Na2SzO3

-400 10 -4 25.1 3 -300 10 -4 12.9 390 -200 10 -a 6.6 312 -100 10 4 4.5 338

0 10 -4 5.4 441 +100 10 -4 7.3 111

METALLURGICAL TRANSACTIONS A VOLUME 13A, NOVEMBER 1982--2017

crack velocities for the CER tests, established by taking the longest crack and assuming that crack initiation occurred at yield. This assumption was justified by visual observation of crack initiation and by examination of the current flowing, as described below.

In the solution containing 6 x 10-3M Na2SO4 as well as 6 x 10-4M Na2S203, SCC occurred at a strain rate 10 -6 S - l and potential - 1 0 0 mV, but the mean crack velocity was greatly reduced (0.04 p~m s-~). By adding extra sodium sulfate it was found that crack propagation could be in- 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 following the addition of the extra Na2SO4 was 120 --- 20 seconds. It was noted, however, that the sulfate-containing mixtures tended to cause a general grain boundary etching which was never observed in plain thiosulfate solutions.

2. 0.5M Sodium Thiosulfate Solution

A set of CER tests was run in this solution at 10 -4 s -~, for comparison with the scratching electrode tests and to isolate possible effects of electrolyte resistance. Figure 3 shows the mean crack velocities and crack initiation frequencies ob- tained. A single test was also run at 10 -6 S - l to examine crack initiation by the method described above. In this solu- tion there was a close correspondence between the crack initiation frequency, mean crack velocity, and crack arrest potential, in contrast to the more dilute environment; the apparent critical potential above which cracking did not occur was about + 100 mV, independent of whether crack- ing was already occurring. The lower critical potential was around - 5 0 0 mV as in the dilute solution.

3. Current Flowing during SCC

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

B. Constant Load Tests

Two tests were carried out in the 6 x 10-aM solution, and resulted in typically shaped curves of the apparent stress intensity factor K o against crack velocity (Figure 5). It is emphasized that the plane strain limit, for this material and geometry, occurs at about 10 MN m -3/2. At the higher po- tential of +500 mV the stage II crack velocity was similar to the highest mean crack velocity obtained in the CER tests

(+1190 mV, 2 • 10 -3 s-~; see Figure 2). The effect of the preexisting fatigue crack on initiation was evidently similar to that of a crevice in the CER testing, as a crack was initiated readily at +500 mV.

C. Scratching Electrode Tests

Most of these tests were carried out on the iron-9Cr- 10Ni alloy in 0.5M Na2S:O3 of various pH. For the original pH of 9.1, it was found that a highly characteristic delayed re- passivation was obtained within a potential range of about

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2018--VOLUME 13A, NOVEMBER 1982 METALLURGICAL TRANSACTIONS A

4

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100 mV around - 2 0 0 mV (SCE), whereas at higher or lower potentials the charge passed during repassivation was small. Figure 6 shows current transients at three potentials. A convenient parameter for characterizing the transients was the charge density q~ passed after 1 second; at - 2 0 0 mV the relatively slow current decay in the range - 0 . 1 to 1 second means that an electrochemical analysis of crack propagation is rather insensitive to the frequency of bare surface genera- tion. The scratch area used was the mean projected area. Figure 7 shows the variation of q~ with potential for the "matrix" and "grain boundary" alloys, showing that the attack would be most selective on bared grain boundaries around - 2 0 0 mV. Assuming zero effective roughness for the scratch, the value 50 mC cm -2 obtained at - 2 0 0 mV

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corresponds to 20 nm penetration by dissolution of iron as Fe 2+. The quantity - 5 mC cm -2 measured for the matrix alloy at the same potential is of the order expected for reestablishment of a 2 nm passive film J9 without signifi- cant dissolution.

The sodium thiosulfate solution at pH 9.1 is not a good simulation of a crack tip environment in this system, as acidification by hydrolysis undoubtedly occurs. Accord- ingly, the same procedure described above was applied with successively greater degrees of acidification by H2SO4 or Cr2(804) 3. It was noticeably difficult to maintain a pH below 3, probably owing to the development of a sulfite/sulfurous acid buffering action following the decomposition of the thiosulfate ion. Thus, the lowest pH examined was 3.0. Figure 8 shows q~ values for a variety ofpH; the results were independent of the means used to lower the pH. For pH 7.0 and 4.4, the results are very close to those for pH 9.1; however, a further decrease to pH 3.6 (when sulfur precipi- tation became noticeable) resulted in a large increase in the

METALLURGICAL TRANSAEHONS A VOLUME 13A, NOVEMBER 1982--2019

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peak charge density and a shift in peak position to about - 1 5 0 mV. At pH 3.0, the peak was at - 1 0 0 mV and re- tarded repassivation then became evident at 0 and at + 100 mV. The steady-state current densities remained low at all potentials (less than 20/zA cm-2), consistent with the observation of no noticeable intergranular corrosion on a sensitized steel specimen immersed in the pH 3 solution for 24 hours. Both iron and an iron-5Cr-10Ni alloy were, in contrast, highly active, generating a loose black film which after washing and drying, dissolved in sulfuric acid with evolution of hydrogen sulfide. This material was also pro- duced during anodic polarization of these materials at - 2 0 0 mV. When several cm 2 of the iron-9Cr-10Ni alloy were abraded under the pH 9.1 solution, left for 24 hours, washed and dried, swabbing with sulfuric acid again gener- ated a noticeable HzS odor indicative of sulfide corrosion products. Microscopic examination showed a faint tarnish- ing of the surface.

To ensure that the effect of the thiosulfate ion on bare surface dissolution kinetics was a specific one rather than 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 - 5 0 0 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) were observed. This establishes that thiosulfate has a specific aggressive effect. It was noticeable that in the sul- fate solution, where the anion is inert to electroreduction, the bare surface "corrosion potential," where no detectable current transient occurred, was several hundred mV lower than in any of the thiosulfate solutions - - e.g., in pH 9.1 thiosulfate it was - - 6 0 0 mV. This results from the ca- thodic reactivity of the thiosulfate ion. The transients at

- 2 0 0 mV are far enough anodic to this potential for the cathodic component of the charge density to be small.

2020--VOLUME 13A, NOVEMBER 1982

D. Microscopy

As reported earlier," the fracture surfaces obtained in thiosulfate environments were intergranular (Figure 9(a)). Some attention was paid, in view of the very high crack velocities, to evidence for discontinuous crack propagation. During high magnification examination, however, by no means a majority of the grain facets showed any noticeable micro-dimpling other than the micron-scale roughness asso- ciated with the carbides, so that if mechanical failure is occurring it must be a brittle process closely following the grain boundaries.

The carbon extraction replicas of a reheated specimen 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 present at distances more than about one grain diameter ahead of the crack tip. The radius of the zone containing these needles appeared to be about 50/xm for a specimen strained in 0.5M Na2S203 at 10 -4 S 1 and --100 mV (mean velocity of crack examined ~3 p,m s-~). Carbides of this type characteristically precipitate on martensite in reheated austenitic stainless steels. 2~

Specimens in which crack propagation had occurred at a strain rate of 10 -6 S - I (total strain to fracture

(d)

(a)

(b) - , 4 V *

(e)

(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)

(/)

Fig. 9--(cont.) (f), (g): SCC in 0.5M Na2S203, - 3 0 0 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 more dilute solution resulting from current flow out of the propagating cracks. The similarity in the cathodic protection potentials for the two solutions is consistent with this, as the ohmic drop approaches zero for low currents. The current measured during SCC which causes this ohmic drop does not correspond to the rate of metal removal at the crack tips, but probably originates chiefly from dissolution of exposed chromium-depleted material on the crack walls: the evi- dence for this is the slow current decay after specimen fracture shown in Figure 4. Dissolution of the susceptible material on the crack walls will, of course, occur indepen- dently of whether crack advance occurs by highly localized dissolution, hydrogen embrittlement, or mechanical rup- ture. Refinements in the simulation of the crack tip environ- ment, based on analysis of this slow current decay, are reported in another paper. 23

MAXIMUM HEAN CRACK VELOCITY

(10 -6 s -1 STP, MN PATE)

MAXIMUMCRACKINITIATION

FREQUENCY (i0 -& S "l)

CATHOOIC PROTECTION

POTENTIAL (I0 "6 s "I ,

PROP~AT[NG CPJ~CK)

...................... [] 7777-~ (INITIATION, 10 -6 s -1)

...................... [] 7/P (PROPAC4~TING CRACK, 10 -6 5 "I)

i l ~ i i i 1 i i i I l i i J -0,5 0 0,5

POTENTIAL (VOt.TS SCE)

Fig. 1 0 - - Summary of data from CER tests. ~ : 6 x 10-4M Na25203 [~'%~X~: 0.5M Na2S203

2022--VOLUME 13A, NOVEMBER 1982 METALLURGICAL TRANSACTIONS A

The relative importance of the crack contents and the bulk solution in contributing to the ohmic potential drop cannot be assessed accurately from the present data; however, it is easy to see that both will have significant effects, particu- larly in the case of the 6 x 10-4M solution (conductivity 1.7 • 10 -4 ohm -~ cm-~). Thus, the potential difference AE associated with the flow of (typically measured) 20/zA per cm length of crack tip out of an external opening of (typically) 20/xm width can be estimated from the formula given by Pearson et al:22

1 R ~- ~ ln(4b/a)

2 7r trb

where R is the resistance to flow of current to a rectangular strip electrode, b its half-length, a its half-width, and cr the conductivi ty. For o- = 1.7 • l0 -4 ohm -~ cm -~, b -- 0 .5 cm and a = 10 -3 c m , th i s y i e l d s R 14,000 ohms and hence AE ~ 280 mV. If the same model crack is 1 mm deep and wedge-shaped, the resistance of the crack contents gives an ohmic drop of

and the upper cracking potential of - + 340 mV (NHE), a dissolution mechanism of SCC must be considered likely. The apparent equilibrium restrictions on hydrogen for- mation may conceivably be circumvented if a resistive sur- face layer is present near the crack tip which absorbs most of the applied potential while allowing some interaction between the metal surface and water; however, this is un- likely at extremely high crack velocities where the crack tip is essentially bare. The only other significant source of hydrogen sulfide will be the small amount produced chemi- cally as a result of acidification of the thiosulfate solution. This will be rapidly oxidized at the crack tip, although if hydrogen entry into the metal is very rapid following adsorption, some absorption and embrittlement could con- ceivably occur. These considerations focus our primary at- tention on the effects of sulfur or sulfide ions in promoting dissolution of iron and nickel, which are well documented (e.g., References 28, 29). As indicated earlier, the effect of the thiosulfate is shown in the scratching tests to be a spe- cific one and not merely characteristic of a particular pH and potential. Kowaka and Kudo ~~ showed that a surface nickel sulfide was formed as a result of exposure of a simulated grain boundary alloy to polythionic acid (pH 1, potential +150 mV NHE); this is consistent with the composite potential - p H diagram for Ni-S-H20 shown in Figure 12, which shows a NiS stability field. Under the same condi- tions, iron (Figure 11) and chromium 2s are highly soluble at equilibrium.

D. Interpretation of Scratching Electrode Tests

The purpose of these tests was to provide some answers to the following questions:

1. How is crack initiation related to repassivation of matrix and grain boundary material in 0.5M NazS203? 2. Does the potential dependence of crack propagation cor- relate with repassivation in an acidified thiosulfate solution, and if so, at what pH? 3. Can the maximum crack velocity at any potential be related to a bare surface dissolution rate?

While these considerations are rather conventional in this type of investigation, 3~ the present system offers some

1,2 N1203 + HSO~ ~ NxO~ + HSO~ " I' ' ~ ' ~ " ~, ~ ' ,

O.C Nz +++ $20~-/ ~ / ~ NIO+ SO~- N~O +$20 ~-

~-0.4 N~S

-0.8

-1,2 ~ ~ ~ ~ ~ ~ i 0 2 4 6 8 10 12 14

pH

Fig. 12--Potential-pH diagram for Ni-S-H20 at 298 K. Conditions as in Fig. 11, using ~~ (NiS) = -108 .9 kJ mol -l . Ni3S2 not considered.

difficulties which are unusual. The principal problem is that the highest current densities measured in the scratching tests are quite inadequate to account for the crack velocities ob- served. Application of Faraday's second law shows that a penetration rate of 1 /zm s -l by dissolution of iron as-Fe 2+ requires a mean current density of 2.5 A cm-2; therefore, the highest crack velocities in Figures 2, 3, and 5 would require about 20 A cm -2. The highest value of q] in Figure 8 is, however, only 220 mC cm -2, and the corre- sponding peak current density at 50 ms about 300 mA cm -2. There are two ways of rationalizing this discrepancy:

1. Partial repassivation occurs during scratching and the true instantaneous peak current density attains 20 A cm -2. 2. Crack propagation is predominantly mechanical in na- ture, possibly incorporating a hydrogen effect.

Although higher current densities are obtained by more rapid scratching, the charge density passed at these rates is very smal l - -even at the 50 ms current peaks, such as that in Figure 6, it amounts to only a few monolayers of metal oxidized. Such shallow attack is unlikely to be able to sus- tain crack propagation: a number of authors have shown that the systems in which SCC propagation is considered to occur by anodic metal removal are those in which many layers of alloy are oxidized or dissolved for each surface baring event (e.g., References 33, 34). The theoretical basis of such a requirement has been discussed by Scully. 35

The mean crack velocities from CER tests in 0.5M Na:S203 at 10 -4 s -t strain rate (where mean crack velocities are approximately Stage II in nature) have been used in Figure 13 for comparison with the penetration rates pre- dicted from scratching tests on the iron-9Cr-10Ni alloy at pH 3.0. The agreement in curve shape is remarkable with the exception of the discrepancy of a factor of - 5 0 between crack velocity and predicted penetration rate. We discuss elsewhere 23 the details of the dependence of these results on

" - 5 I=

I I

r

Z I - I

Z 0 H l - O: n,' I-" l.iJ Z l.d O.

- 6

- 7

- g

0 - I - 9

I I I

g g g

( a ) g

g

X X X

X

X

g

X

! !

( b )

X

X

I I I I I

- 4 B B - 2 B B g 20R 4BB

P O T E N T I f l L (mV S C E ) Fig. 13--Comparison of penetration rates: (a) CER tests in 0.5M Na25203 at 10 -4 s-a; (b) scratching of iron-9 Cr-10 Ni alloy in 0.5M Na2S~O3 acidi- fied to pH 3.0.

2024-- VOLUME 13A, NOVEMBER 1982 METALLURGICAL TRANSACTIONS A

pH and alloy chromium concentration. It is worth repeating at this point that the pH at which this agreement is obtained is also that which tends to be obtained for quite a wide range of acid additions to the 0.5M NazSzO3, as a result of the buffering action described earlier. A critical test of the esti- mate of the pH is the correct prediction of the anodic protec- tion potential against cracking, which would be predicted as about - 5 0 mV rather than + 100 mV if the pH was even as high as 3.6.

Crack initiation frequency and propagation rate have about the same potential dependence in the 0.5M solution. This would not be predicted if initiation was considered to occur at pH 9.1 and propagation at pH 3.0: thus, initiation may probably be considered to occur as a result of many film rupture events at the same site. Even one film rupture event will lower the interfacial pH of a near-neutral solution con- siderably at such low repassivation rates as obtained at - 200 mV, owing to hydrolysis by Fe z+ ions: this is illus- trated neatly by the almost identical values of q~ for initial pH values of 9.1, 7.0, and 4.4 (Figure 8). A similar effect operates in chloride pitting of stainless steels, 36 where the pitting potential is approximately constant for 2 - < pH -0.1 /xm s -~. 3. The crack velocities which contain an element of me- chanical fracture should be sensitive to alloy compositions or sensitization treatments which increase the intrinsic grain boundary strength without altering the chromium deple- tion profile.

METALLURGICAL TRANSACTIONS A

Cowan and Gordon 9 reported that polythionic acid crack- ing did not produce acoustic emission, but the crack veloci- ties were low in their U-bend tests and therefore acoustic emission would not be expected. Lee and Vermilyea 37 sug- gested that intergranular SCC in Inconel 600, where mar- tensite does not form, might involve localized fracture of grain boundaries--this idea has not been tested. Recent advances in studies of transgranular SCC have shown that localized mechanical cleavage of macroscopically ductile material often occurs. 38

F. Role of the Thiosulfate Ion and Relationship to Polythionic Acid Cracking

The extraordinary potency of the thiosulfate ion, even compared with its relatives such as the tetrathionate ion, is probably the result of two important effects:

1. Of all the metastable sulfur anions, $202- is the most readily converted to elemental sulfur, which seems to play an important part in the cracking. 2. Ferrous thiosulfate is highly soluble and can exist as a supersaturated solution during localized corrosion. 39

Thus, sulfur is generated at the advancing crack tip surface, retarding repassivation, while a high concentration gradient of Fe 2+ ions can be maintained without precipitation.

The polythionic acid test for SCC induced by sensitization has been standardized. 4~ The present results indicate that a dilute thiosulfate solution may have advantages as a testing medium owing to its low cost and ease of preparation. Comparison of the two environments is in p rogress - - preliminary results show that sensitized Alloy 600 is also susceptible to thiosulfate SCC at concentrations as low as 10 -5 molar, and that in stainless steels the dependence of both polythionic acid and thiosulfate SCC on sensiti- zation temperature is related to repassivation of grain boundary material.

V. CONCLUSIONS

1. SCC of sensitized Type 304 steel in thiosulfate solutions occurs only when repassivation of simulated grain boundary material is retarded in a simulated crack tip environment. Crack advance by dissolution is about 150 nm per film rupture event at the potential of maxi- mum susceptibility.

2. The observed maximum crack velocity in this material at any potential is 50 to 100 times that calculated on a dissolution model.

3. Grain boundary cracking through a zone containing strain-generated martensite most probably constitutes the majority of crack propagation in the Stage II regime. This is possibly but not necessarily associated with hydrogen.

4. Addition of sufficient concentration of sulfate ions to a dilute thiosulfate solution inhibits SCC; the molar ratio required is - 2 0 for [S:O~-] = 6 • 10-4M.

ACKNOWLEDGMENTS

The assistance of Kenneth Sutter, Donald Becker, and Ronald Graeser in the experimental work is gratefully ac- knowledged. Professor Andr6 Vinckier carried out the trans- mission electron microscopy of grain boundary carbides

VOLUME 13A, NOVEMBER 1982--2025

during a period as Visiting Scientist; a full account will be published elsewhere. The work was supported by the Department of Energy, Division of Basic Energy Sciences, under Contract No. DE-AC02-76CH00016.

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40. ASTM Standard Recommended Practice G35-73, 1980.

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