Caustic Stress Corrosion of Fe-Cr-Ni Austenitic Alloys⁽¹⁾

The susceptibility to stress corrosion cracking (SCC) in caustic solutions of austenitic materials for steam generator tubing, both for liquid metal fast breeders and pressurized water reactors (Type 316, and Alloys 600 and 800) has been compared as a function of: (1) The concentration of the sodium hydroxide solution; (2) the stress level (the actual stress levels for different types of specimens have been carefully evaluated and discussed), and (3) the composition and structure of the metal (in particular, the grain boundary precipitation in high nickel alloys has been characterized by different techniques). It has been shown that the first two factors have a strong influence on the classification of the alloys, which can be completely reversed when the stress and the caustic concentration change. The possible cracking of Alloy 600 for very low caustic concentrations (4 g/l) suggested the hypothesis that cracking in pure water could be the same phenomenon as caustic cracking. Thermal treatments (such as 16 hours at 700 C) can significantly improve the behavior of Alloy 600 in caustic solutions.