Hydrogen transport in nickel-base alloys
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INTRODUCTION
NICKEL-base alloys have been used extensively in light water reactor environments due to their good resistance to general corrosion and their mechanical strength. The material preferred for pressurized water reactor steam generator tubing has historically been alloy 600, but because of its greater resistance to cracking, future steam generators will use alloy 690. For applications requiring high strength and good aqueous corrosion resistance, agehardenable Ni-Cr-Fe alloys, such as X-750 and 718, are often used. Specific applications include fuel assembly hold-down springs, boiling water reactor jet pump beams, and high-strength bolts for core and other structural components. Despite their excellent general corrosion resistance, these materials have been shown to be susceptible to environmentally assisted fracture for certain combinations of heat treatment and environment, m2j Cracking is almost always intergranular. As such, grain boundary chemistry and precipitate morphology are likely to play critical roles. IL2j Electrochemical studies have revealed differences in electrochemical activity associated with A. TURNBULL, Scientist, is with the National Physical Laboratory, Teddington, Middlesex, TW110LW, United Kingdom. R.G. BALLINGER, Associate Professor, I.S. HWANG, Research Scientist, and M.M. MORRA, Research Assistant, are with the Massachusetts Institute of Technology, Cambridge, MA 02139. M. PSAILADOMBROWSKI, Scientist, is with B&W McDermott, Alliance, OH 44601. R.M. GATES, Research Staff, is with Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted June 3, 1991.
METALLURGICAL TRANSACTIONS A
the various microstructures, with significant differences in electrochemical behavior of individual phases noted.131 Hydrogen-assisted cracking has been proposed as the failure mechanism for some environmental conditions, t31 However, the exact mechanism by which hydrogen degrades performance is unknown. This is especially true for precipitation-hardened nickel-base alloys. For example, the role of hydrogen in the accelerated cracking of alloy X-750, subject to cyclic loading in aqueous environments, has been demonstrated, t21 However, the observed crack growth rates exceed any estimate of hydrogen diffusion lengths by order of magnitude. At the same time, grain boundary y' has been shown to corrode actively and, through galvanic coupling to the matrix, may enhance the cathodic generation of hydrogen atoms in the immediate crack tip region. For the specific case of alloy X-750: (1) cracking is intergranular and is accelerated in two temperature regimes, one near 100 ~ and one above 250 ~ (2) the most susceptible microstructure is that where grain boundary y' predominates; (3) ductility is reduced to near zero in hydrogen-charging straining electrode tests independent of heat treatment; and (4) the regime of low-temperature susceptibility coincides with a transition in electrochemical activity of the y' phase.13~ It is thus clear that the interaction of hydrogen atoms with the preci
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