Occurrence and Prevention of Hydrogen Induced Stepwise Cracking and Stress Corrosion Cracking of Low Alloy Pipeline Steels^★

Hydrogen induced damage to carbon and low alloy steels can occur in the absence (HIC), or more frequently, in the presence of external stresses (HSCC). A great variety of pipeline steels was investigated to compare the resistance against HIC and HSCC at different pH values in H₂S containing environments. The response of the materials investigated can be described best by critical potentials E_HIC and E_HSCC for the occurrence of both HIC and HSCC and by threshold stresses $σcr*=σcr/σYS$ for the occurrence of HSCC. At pH = 5.5 to 8, there exists a correlation between the HSCC parameters $σcr*$ and E_HSCC but no correlation was found between E_HIC and E_HSCC. It was confirmed by SCC experiments at pH = 3 that there exists no relation between resistance to HIC and that to HSCC. Lowering the pH to 3 results in lower σ_cr values. At low pH, results of HSCC tests are strongly influenced by films or thin deposits of sulfides formed on the surface of the specimens. The resistance to HIC can be considerably improved by desulfurization of pipeline steels. The beneficial effect of alloying of copper on HIC resistance in H₂S containing environments is due to the formation of surface films which restrict hydrogen adsorption. From the results obtained, methods for preventing or mitigating damage due to HIC are derived and discussed.