Stress corrosion cracking behavior of X70 pipe steel in an acidic soil environment

Stress corrosion cracking (SCC) behavior of X70 pipe steel was investigated in an extracted acidic soil solution by slow strain rate test (SSRT), potentiodynamic polarization curve measurements and surface analysis technique. The SCC process and mechanism of X70 steel in the acidic soil solution is mixed-controlled by both anodic dissolution and the hydrogen involvement. With the different applied potentials, the dominance of SCC process changes. At a relatively less negative potential, the steel SCC is based primarily on the anodic dissolution mechanism. When the applied potential is shifted negatively, hydrogen is involved in the cracking process, resulting in a transgranular cracking mode. With the further negative shift of applied potential, the SCC of the steel follows completely a hydrogen-based mechanism, with a river-bed shaped brittle feature of the fracture surface. Heat treatment alters the microstructure of the steel, resulting in a change of SCC susceptibility. In particular, the quenched steel with a bainite microstructure has a high susceptibility to SCC in the acidic soil, while the as-received steel with a ferrite matrix have a low SCC susceptibility.