Strain-aging-assisted localized corrosion of a mill-scaled X-65 pipeline steel

Abstract

This investigation was designed to study whether strain aging could assist localized corrosion of pipeline steels under realistic conditions, which is an unsolved problem in the evaluation of strain-aging-induced stress corrosion cracking of pipeline steels in the field. It was found that the corrosion severity was significantly aggravated after strain aging because of the increase in yield strength and the transformation of oxide scales on the steel surface. These variations caused by strain aging lead to lesser exfoliation and longer life of the oxide scales under cyclic loading in air and during subsequent corrosion exposure, which enhances the galvanic corrosion between oxide scales and the steel substrate. The composition of oxide scales, mechanical properties of steels, and magnitude of the stress cycles, which could all impact localized corrosion, were studied. The findings of this investigation suggest that the severe localized corrosion leading to crack initiation is a vital factor for the higher SCC susceptibility of pipeline steels associated with strain aging, in addition to the enhanced hydrogen embrittlement as usually believed.