Dense and Sparse Stress Corrosion Crack Initiation in an X65 Pipeline Steel With Mill Scale

Near-neutral pH stress corrosion cracking (SCC) is a significant threat to the operational safety and reliability of gas and oil pipelines. The SCC cracks are typically formed in colonies with different crack density populations on the external surface of the pipe. The density of SCC cracks affect how pipeline integrity and remaining lifetime are assessed. Although sparse and dense crack colonies are commonly observed on pipelines, it has not been well established how these crack colonies with different crack populations were developed in the field. This research was made in an attempt to replicate near-neutral pH SCC cracks with different crack densities in the laboratory with realistic loading conditions commonly found during field operation. An X65 pipeline steel with different surface preparations was used. The results showed that the dense near-neutral pH SCC cracks were successfully reproduced on the primer-coated samples, whereas sparse cracks were reproduced on the mill-scaled and polished samples. The densely spaced cracks could transform into sparsely spaced cracks when most of the primer layer and mill scale had been removed during the long period of corrosion under cyclic loading and further corrosion occurred thinning the crack density. The results of crack initiation obtained from this investigation have also been found to be quite consistent with crack initiation scenarios found during field operation.