Initiation of Environment Induced Cracking in Pipeline Steel: Microstructural Correlations

Abstract

Initiation of environment-induced cracks in pipeline steels involves interactions among microstructural features, loading conditions and environmental variables. Cyclic, or dynamic, loading has an important role in crack initiation, a competitive process in which cracks form first at the most favorable sites.</jats:p> <jats:p>Under simultaneous cyclic loading and exposure of a Grade 448 (X-65) pipeline steel to aqueous solution of near-neutral pH, NS-4 solution saturated either with CO2 or 5% CO2/ balance N2, cracks that initiated early in the process were associated with pits. A correlation between pits and non-metallic inclusions has been observed. Other locations favorable for localized corrosion attack, such as along the steel surface at the edge of a coating, were also found to be sites for crack initiation. The dense population of cracks that appeared at a later stage of exposure most likely developed from slip-dissolution along certain crystallographic planes. Cracks that formed at an early stage did not always remain as the largest cracks, as crack coalescence, dormancy and/or re-activation, as well as initiation of new cracks, all occurred simultaneously on different parts of the exposure surface.