Effects of Corrosion and Cracking of Sulfide Inclusions on Pit Initiation in Stainless Steel

Microelectrochemical measurements were carried out to ascertain the effects of applied stress on pit initiation behavior at (Mn,Cr)S inclusions, which contained around 15 at% chromium. During anodic polarization in 1.5 mol kg −1 MgCl 2, the MnS selectively dissolved and the composition changed from (Mn,Cr)S to CrS covered with a surface oxide film. Stable pitting did not always occur when no stress was applied, even when the MnS dissolved and meta-stable pits formed. Under applied stress, stable pitting was consistently initiated in the potential range of the MnS dissolution, and micro-cracks were generated perpendicular to the tensile direction. The cracks were thought to be stress corrosion cracking induced by the formation and rupture of the oxide film on the inclusion under applied stress. The inclusion was perforated by the crack, and the steel matrix under the inclusion was then exposed to the solutions, and stable pitting was initiated.