Stress Corrosion Cracking of Austenitic Alloys in Supercritical Water

Abstract

The purpose of this study was to determine the stress corrosion cracking behavior of austenitic alloys in pure supercritical water (SCW) and to gain a better understanding of the stress corrosion cracking (SCC) mechanism. Austenitic stainless steel alloys Type 304 (UNS S30400) and Type 316L (UNS S31603) and nickel-based alloys 625 (UNS N06625) and 690 (UNS N06690) were tested in constant extension rate tensile (CERT) mode in deaerated supercritical water over the temperature range of 400°C to 550°C. All alloys displayed some degree of SCC over the temperature range. Significant intergranular stress corrosion cracking (IGSCC) occurred in Alloy 625 and Type 304 stainless steel as measured by fracture surface analysis, gauge surface crack length and density, and crack depths of cross-sectional samples. While both transgranular (TG) and intergranular (IG) cracks were observed in Alloy 690 and Type 316L stainless steel, the degree of IG cracking was less severe. TG cracking was caused predominantly by a tarnish rupture mechanism. Crack growth rates were approximated from crack depth and time under load and the activation energies for crack growth ranged between 84 kJ/mol and 105 kJ/mol. These values are consistent with oxygen diffusion along short circuit paths. The observed cracking in SCW may be due to either a slip-oxidation type of mechanism or a selective internal oxidation mechanism.