Stress corrosion cracking behavior of high strength pipe steels in an acidic soil environment

This work mainly reviews the progress of my group's research on stress corrosion cracking (SCC) of pipeline steels in the acidic soil area in China. It has been acknowledged that SCC occurs to high strength pipeline steels in high susceptibility. Heat treatment alters the microstructure of the steel, resulting in a change of SCC susceptibility. In particular, the quenched or normalized steel with a bainite microstructure has a high susceptibility to SCC in acidic soils, while the as-received steel with a ferrite matrix have a low SCC susceptibility. Inclusions existing in X70 pipeline steel have different effect on SCC initiation.The inclusions rich in Al are susceptible to initiate cracks, and inclusions containing mainly Si are less susceptible to crack initiating. With different applied potentials, the dominance of SCC process changes. The potential ranges for different SCC mechanism can be determined according to the comparison between the fast and slow scan rate polarization curves. At a relatively less negative potential, the steel SCC is based on anodic dissolution mechanism, when the applied potential is shifted negatively, hydrogen is involved in the cracking process. With further negative shift of applied potential, the SCC of the steel follows completely a hydrogen-based mechanism. A theory model was developed to illustrate the critical role of strain rate in SCC of the steel. According to this model, the generation of surface active spots introduces an additional negative potential locally, affecting the local electrochemical reaction and, consequently, the susceptibility of the steel to SCC.