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

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