Role of Prior Cyclic Loading in the Initiation of Stress-Corrosion Cracks on Pipeline Steels Exposed to Near-Neutral pH Environment

Abstract

X-52 and X-80 pipeline steels have been used to study the effect of cyclic loading prior to corrosion exposure upon the initiation of stress-corrosion cracks in a near-neutral pH environment. To simulate realistic loading conditions, a cyclic loading scheme was constructed based on pressure fluctuation data collected on operating pipeline systems. It has been observed that cyclic loading either prior to corrosion exposure under static stress or during corrosion exposure enhances pitting corrosion and micro-cracking on the steel surface. These pits are seen to be aligned along scratches on the steel surface and to have coalesced to form micro-cracks at some locations. Some micro-cracks appeared not to have developed from pits, but were instead observed in a direction approximately 45 degree to stress axis. They are believed to have developed at locations where localized deformation such as persistent slip bands has occurred due to cyclic loading. In contrast to cyclic loading, no pits or micro-cracks were seen on the specimen that was subjected to static loading only. The pits and micro-cracks were only observed when the specimens were immersed under a cathodic potential of −790 mV SCE. At the open circuit potential only uniform corrosion occurred on the specimens whether they were cyclically or statically stressed.