Hydrogen-facilitated corrosion and stress corrosion cracking of austenitic stainless steel of type 310
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I.
INTRODUCTION
MANY investigators have proven that the mechanism o f stress corrosion cracking (SCC) f o r austenitic stainless steel (ASS) in hot chloride solution is due to anodic dissolution instead o f hydrogen-induced cracking ( H I C ) . H-7] For example, cathodic polarization increased hydrogen content in a specimen but decreased the susceptibility to SCC. [2j The threshold SCC intensity factor, K~scc, o f ASS in boiling 42 pet MgC12 solution was much l o w e r than that o f HIC, KIH, during severely dynamic charging both at r o o m temperature and in melted salt o f 160 °C.[3-6] This means that when the applied stress intensity factor KI is l a r g e r than K~scc but b e l o w KIH, the HIC cannot occur even if during heavily dynamic charging, but the SCC can occur. Macroscopic compressive stress could induce SCC for types 304 and 321 steels in 42 pct MgC12 solution but not HIC because o f lack o f hydrogen enrichment in the absence o f hydrostatic stress a t the c r a c k tip. t7] The SCC initiated at the site o f the maximum normal stress in mode II and mode III notched specimens, but HIC initiated at the site o f the maximum shear stress, t5,6] The fracture surface o f SCC in 42 pet MgC12 solution consisted o f cleavage and occasionally intergranular but that o f HIC was dimple f o r high stress o r quasi-cleavage for low stress, t4,5'6~ However, several investigations indicated that hydrogen could enter into the specimen and enrich at the c r a c k o r notch tip during SCC o f ASS in hot chloride solution. [8'9,1°J The hydrogen entering into the specimen could promote the anodic dissolution based on the studies about the effects o f hydrogen on polarization curve and weight lOSS. [n-14] The effect o f hydrogen precharged on SCC and a synergistic effect o f hydrogen and stress o n corrosion rate were investigated in the present work. II.
EXPERIMENTAL P R O C E D U R E S
The chemical composition o f Type 310 ASS used in this study is as follows (in weight percent): 23.6Cr, QIAO L i l l E , Associate Professor, CHU WUYANG, Professor, MIAO HUIJUN, Lecturer, XIAO JIMEI, Professor, and GUO PEIXIN, Lecturer, are with the Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, People's Republic of China. Manuscript submitted October 2 2 , 1991. METALLURGICAL TRANSACTIONS A
20.43Ni, 2.04Mn, 1.43Si, 0.06C, and Fe balance. The single-edge notched tensile and smooth tensile specimens were prepared from cold-rolled foils with a thickness o f 0.5 mm. The specimens were solution-treated under vacuum at 1050 °C for 30 minutes, followed by air cooling. Hydrogen was introduced by cathodic charging at 280 °C in a melted salt o f 43 pet NaHSO4 + 47 pct KHSO4 + 10 pet Na2SO4. Cathodic charging was carried out with a current density o f 40 m A / c m2 for 25 hours to get a homogeneous hydrogen concentration in whole specimen. Plasma emission spectroscopy (ICP) was used to determine the ion concentration o f Fe, Cr, and Ni dissolved in the 0.5 m o l / L H2SO4 + 0.1 m o l / L HC1 solu
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