Effect of Surface Preparation on Intergranular Stress Corrosion Cracking of Alloy 600 in Hydrogenated Steam

In the literature it is a common belief that electropolishing mitigates primary water stress corrosion cracking (PWSCC) because it removes superficial cold work. Here, it is shown that electropolished Alloy 600 (UNS N06600) exposed to hydrogenated steam undergoes internal Cr oxidation, whereas mechanical polishing induces external oxidation. This has implications for SCC initiation, which has been tested with different surface preparations (electropolishing and mechanical polishing) using reverse-U-bend (RUB) and C-ring samples. The results show a systematic trend that mechanically polished surfaces are more resistant to stress corrosion cracking (SCC) than electropolished surfaces. The mechanism involved in this increased resistance is thought to he related to short-circuit diffusion of Cr to the surface, which promotes external rather than internal oxidation. The role of compressive stress induced by mechanical polishing is a less-likely explanation of the observed effects. Mechanical polishing does not suppress PWSCC completely because if the external oxide layer breaks, the exposed material may continue to oxidize internally.