The role of surface oxidation on primary water stress corrosion cracking of Ni-base alloys

Ni-based Alloy 600 (Ni - 16 wt.% Cr - 8 wt.% Fe), which has been used extensively for structural components in nuclear power plants, is well known to be highly susceptible to primary water stress corrosion cracking (PWSCC) in the primary side environments of a primary water reactor (PWR). Although the exact failure mechanism involved is still not well understood, there is currently rising evidence to support intergranular oxidation, in which oxygen diffusion into the grain boundaries can increase the degree of susceptibility to PWSCC. As a result, intergranular oxidation is responsible for grain boundary embrittlement, and therefore has a major contribution to IGSCC There is a noticeable difference in the intergranular oxidation behavior between Alloy 600 and Alloy 690. Alloy 690 is known to be more resistant to PWSCC than Alloy 600 due to its higher Cr content, which promotes the formation of protective films on the surface when exposed to primary water. Many studies on the surface oxidation of Alloy 600 have been performed. However, there is only limited literature on the surface oxidation of Alloy 690. The aim of the present study is to investigate the resistances of Alloy 600 and Alloy 690 to PWSCC, and to reveal the reasons for their different resistance to PWSCC in terms of surface and intergranular oxidation behavior. Alloy 600 cracked easily in normal testing conditions simulating PWR primary water, but Alloy 690 didn't. The surface grain boundaries of Alloy 600 were oxidized due to the oxygen penetration into the specimen. However, intergranular oxidation was not found in Alloy 690. The surface oxidation layers of Alloy 690 were much thicker than those of Alloy 600, and faceted nodules composed of purely metallic Ni were found outside the oxidation layer of Alloy 690. The difference in cracking susceptibilities between Alloy 600 and 690 can be attributed to the different surface and intergranular oxidation of the alloys.