A mechanism for hydrogen-induced intergranular stress corrosion cracking in alloy 600
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I.
INTRODUCTION
CORIOU et al. were the first to call attention to the fact that solution-annealed nickel-base alloys, e.g., Alloy 600, will exhibit IGSCC in deaerated water at elevated temperatures, tll At the time, this was novel, but since then, IGSCC has been recognized as an important failure mechanism in the primary system of pressurized water nuclear power plants, most frequently being observed in steam generator tubing, t2'31 At elevated temperatures, Alloy 600 can show two distinctly different types of IGSCC. In oxygenated water, the alloy is prone to IGSCC in the sensitized condition but resistant to IGSCC in the solution-annealed condition. t41 Sensitization gives chromium-depleted grain boundaries, and a mechanism which involves the easy breakdown of the protective film at the chromium-depleted boundary is generally accepted. [Sj Similar behavior is shown by sensitized 304 and 316 stainless steels in oxygenated water. In deoxygenated water, or steam, the reverse is found. The solution-annealed (mill-annealed) Alloy 600 is prone to IGSCC, and sensitization makes it more resistant to IGSCC. Several studies have shown that IGSCC in deaerated water shows the following characteristics: (1) It is substantially faster in samples in which carbides did not precipitate along grain boundaries after annealing than in samples in which they did. That is, IGSCC is faster in grain boundary regions supersaturated with carbon. t2,3,6~ A corollary of this is that IGSCC is found in mill-annealed low-C heats where no carbide forms on air cooling but may not be found in high-C heats when enough C is put in solution by the anneal to form grain boundary carbides on cooling, m (2) It is enhanced by cathodic potentials but hampered CHIH HAO SHEN, Research Engineer, is with National Steel Corporation, Trenton, MI 48183. PAUL G. SHEWMON, Professor, is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted August 2, 1989. METALLURGICAL TRANSACTIONS A
by anodic potentials, tS] This is just the opposite of the behavior in aerated water in the same temperature range. (3) It is enhanced by saturating water with about 1 atm of hydrogen, which aids in the deaeration. However, if the hydrogen overpressure is raised to I0 or 100 atm, IGSCC slows or stops, t3,91 (4) It increases rapidly with temperature, the activation energy being variously reported between 74tt~ and 138 kJ/ mol. 131 (5) It is accelerated by residual stress and plastic strain. [9,11] Several workers have tried to explain why sensitization reduces the rate while solution annealing accelerates the development of IGSCC in hot, deaerated water. Van Rooyen suggested that in solution-annealed material, some grain boundary segregation occurred, and that carbide precipitation somehow cleaned the boundary, t2J Carbon segregation has been postulated to play a role, 1121 and several studies using Auger spectroscopy have reported grain boundary segregation of phosphorus after 700 ~ anneals (e.g., see Guttmann et
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