Stress Corrosion Cracking Mechanisms of Fe-based Alloys in Hydrogenated Hot Water

Abstract

It is generally accepted that carbon steel resists Stress Corrosion Cracking (SCC) in hot water provided the oxygen content is low enough1; however, isolated cracking events have occurred2, apparently in fully reducing conditions, and these may or may not be true SCC. There are also occasional reports of SCC of austenitic stainless steel in reducing conditions3 where cold work may play a critical role. SCC though originally seen in sensitized material at oxidizing potentials persists to low potentials and in cold-worked but unsensitized material. We suspect from the literature that alloying with Ni introduces a susceptibility to SCC in reducing hot water as well as in caustic solution4. Our hypothesis is that Ni causes SCC, and Cr retards it; so stainless steel has just enough Cr to protect against SCC under most conditions. We are examining this using model Fe-based materials and environments designed to separate the effects of different parameters. Early results suggest that Fe-Ni alloys and austenitic SS both undergo de-alloying and SCC in reducing caustic solutions. It is hypothesised that if these alloys are indeed susceptible to dealloying (and hence to SCC) in this environment then we can reasonably project that their behaviour in reducing, hot pure water may be similar. This is pertinent to understanding recent failures in high temperature, high pressure aqueous systems. As a first step we have investigated the surface reactions that occur in caustic solutions and in hot water.