Environmental Cracking Susceptibility of Low-Alloy Steels Under a High H₂S Pressure and High-Temperature Sour Environment Available to Purchase

Environmental cracking (EC) susceptibility of low-alloy steels with a specified minimum yield strength of 655 MPa (95 ksi) and 758 MPa (110 ksi) manufactured by quenching and tempering heat treatments was investigated in high H₂S partial pressures (more than 1.0 MPa) using four-point bend tests in autoclaves. The H₂S partial pressures and testing temperatures varied from 1.0 MPa to 10 MPa and 24°C to 150°C, respectively. Materials of grades 95 ksi and 110 ksi containing high Cr and Mo showed no macrocracking under all tested conditions. Localized corrosion occurred at several locations after exposure for 1 month under high H₂S pressure and high-temperature conditions. It was concluded that the localized corrosion did not form macrocracking even after long-term (3 months) immersion tests. On the other hand, 110 ksi grade material containing low Cr and Mo suffered from sulfide stress cracking at low temperatures (below 66°C) and at an H₂S pressure of 1.0 MPa. The material also showed EC at an H₂S pressure of 10 MPa and temperature from 107°C to 150°C. The difference of EC susceptibility among the materials is discussed based on corrosion reactions, hydrogen absorption, and morphologies of the corrosion products on the steel surface.