Microstructure Dependence of Stress Corrosion Cracking Initiation in X-65 Pipeline Steel Exposed to a Near-Neutral pH Soil Environment

A study was carried out to understand mechanisms of stress corrosion crack initiation in an X-65 pipeline steel exposed to a near-neutral pH soil environment under a mechanical loading condition typical of a pipeline operating in the field. Microcracks initiated on the polished surface of the X-65 pipeline steel after long-term exposure at open-circuit potential in a near-neutral pH synthetic soil solution. It was found that these microcracks were initiated mostly from pits at metallurgical discontinuities such as grain boundaries, pearlitic colonies, and banded phases in the steel. Strong preferential dissolution was observed along planes of the banded structures in the steel. Selective corrosion at these metallurgical discontinuities is attributed to the anodic nature of those areas relative to the neighboring steel surface. Consistent with previous observations, no increased susceptibility to crack initiation was found at physical discontinuities mechanically introduced into the surface of steel exposed to synthetic soil solution at open-circuit potential.