Selective Internal Oxidation as a Mechanism for Intergranular Stress Corrosion Cracking of Ni-Cr-Fe Alloys
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INTERGRANULAR stress corrosion cracking (IGSCC) of alloy 600 (Ni-16Cr-9Fe) exposed to pressurized water reactor (PWR) primary water continues to be a major concern for the nuclear power industry. The IGSCC of alloy 600 was first observed in steam generator tubes, but has developed into a generic problem affecting all components made of alloy 600 exposed to PWR primary water such as the control rod drive penetrations and the reactor vessel outlets nozzles.[1] Thermally treated alloy 600 (A600TT) and alloy 690 (Ni-30Cr-9Fe) have been developed and are more resistant to IGSCC in laboratory testing,[2] but the root mechanism of IGSCC is still not understood. Without a firm understanding of the root mechanism, IGSCC in both current and future plants, and in the 690 TT replacement alloy, will be a concern for the continued viability of nuclear power. Over the years, a number of mechanisms for IGSCC have been proposed such as slip oxidation,[3] hydrogenenhanced localized plasticity,[4] corrosion-enhanced plasticity,[5] enhanced surface mobility,[6] and bubble formation.[7] Of these mechanisms, slip oxidation has been used successfully as the basis for the prediction of BRENT M. CAPELL, formerly Graduate Student with the Department of Nuclear Engineering and Radiological Sciences, The University of Michigan, Ann Arbor, MI 48109, USA, is Staff Scientist, Bettis Atomic Power Laboratory, Pittsburgh, PA, USA. GARY S. WAS, Professor, is with the Department of Nuclear Engineering and Radiological Sciences and the Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109, USA. Contact e-mail: [email protected] Manuscript submitted November 14, 2005. Article published online June 13, 2007. 1244—VOLUME 38A, JUNE 2007
cracking in stainless steel BWR components.[3] Slip oxidation is based on periodic rupture of the alloy surface film, due to the buildup of strain, that exposes the underlying bare metal to the environment. Then, either by dissolution or oxidation processes, the bare metal is removed, resulting in crack extension until the surface can repassivate. However, the mechanistic interpretation of the slip-oxidation model does not seem consistent with at least three experimentally observed trends in PWSCC. First, the crack growth rate for alloy 600 in primary water is a strong function of the corrosion potential with a peak in the crack growth rate that is centered at the Ni/NiO equilibrium potential and extends ±80 mVSHE from the equilibrium potential.[8] For the slip-oxidation model, the corrosion potential affects the repassivation rate that controls the amount of time during which crack propagation occurs and, therefore, the crack growth rate. Because the repassivation rate and crack growth rate are directly linked, they should follow the same behavior with the corrosion potential. However, the anodic excess current, which is a measure of the repassivation rate, has been measured during tensile straining experiments, and instead of a peak at the Ni/NiO equilibrium potential, it changes
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