Stress corrosion cracking assessment of API X65 steel non‐conventionally heat treated

Abstract

Stress corrosion cracking (SCC) susceptibility of API X65 steel non‐conventionally heat‐treated was evaluated through slow strain rate tests (SSRTs) using the synthetic soil solution NS4 at 20, 40 and 60 °C. Polarization curves (PCs) and corrosion potential (Ecorr) measurements were also performed. The non‐conventional heat treatment (HT) was made by heating plates at 1050 °C for 30 min followed by water quenching. The tensile strength of the heat‐treated plates increased to values similar to API X80 steel. The SCC index obtained by ratios of mechanical properties indicated that X65 steel without HT is not susceptible to SCC whereas with HT is slightly susceptible to SCC. The SCC mechanism in the soil solution in the heat‐treated plates was influenced by hydrogen diffusion as confirmed by the internal cracks and the brittle fracture observed in the specimens. The electrochemical study suggests that the corrosion process of X65 steel with and without HT occurs via a mix mechanism where the charge transfer resistance is influenced by a mass transfer process. Crevice corrosion features were found in the surface of the steel. It is likely that this type of corrosion could have sensitized the X65 steel and induced cracks when the steel was under tensile stress.