Corrosion and stress corrosion cracking of austenitic alloys in supercritical water

The purpose of this study is to determine the corrosion and stress corrosion cracking (SCC) behavior of austenitic alloys for potential use as structural materials in the supercritical water reactor concept. SCC of 304L and 316L stainless steels was investigated using constant extension rate tensile experiments in either deaerated or non-deaerated supercritical water at 500 or 550degC. Corrosion experiments on heated tubes were conducted on 316L and Inconel 625 in high purity, non-deaerated water between 300 and 500degC. Results reveal that 304L is susceptible to intergranular SCC in 550degC non-deaerated water and, to a lesser extent, in 500degC deaerated water. The 316L sample failed by ductile rupture after 35% strain in 500degC deaerated water. Oxides formed on the SCC samples varied in thickness from ∼5 microns in non-deaerated water to 1.5-2 microns in deaerated water. The oxides were predominantly iron oxides with a significant amount of chromium oxides. The probable form of the oxides are FeO and Fe(OH)3 and Cr2O3, Cr(OH)3, or CrOOH. In corrosion tests, the weight gain of 316L increases by a factor of three between 300degC and 500degC, with an oxide thickness of 5 microns after 8 days at 500degC. The oxide development in 625 is less than that in 316L. (author)