Near-neutral pH corrosion and stress corrosion crack initiation of a mill-scaled pipeline steel under the combined effect of oxygen and paint primer

Highlights: • We studied near-neutral pH SCC crack initiation under testing conditions realistic to the field. • Pre-cyclic loading and hydrostatic testing lead to local exfoliation of primer layer and/or oxides. • FeCO₃ precipitation indicates a high concentration of Fe²⁺ retains underneath the primer layer. • Primer layer and oxygen concurrently increase corrosion severity but reduce the crack initiation. • Pit coalescence reduces its aspect ratio and thus the susceptibility to pit-to-crack transition. This investigation was initiated to study near-neutral pH stress corrosion crack initiation of a mill-scaled X65 pipeline steel under cyclic loading and the combined effect of oxygen and paint primer. It was found that the combined action of oxygen and primer layer under cyclic loading increases the corrosion severity but reduces the aspect ratio of the localized corrosion areas because of the vast coalescence of the localized corrosion areas, leading to less tendency towards pit-to-crack transition as compared to the primer-coated specimen in the absence of oxygen. Various factors that affect the corrosion and crack initiation have been studied.