Investigation of Corrosion and Crack Morphology Behavior under Disbonded Coatings on Pipelines

Abstract

Coating disbondment on pipelines is a common phenomenon that leads to exposure of the pipeline metal to the surrounding soil and ground water solutions. The exposure of pipelines to those corrosive environments is associated with development of corrosion and Stress Corrosion Cracking (SCC) on the pipeline steel. Despite the wide study of the electrochemical conditions underneath disbonded coatings, little information has been provided regarding the correlation between the coating disbondment gap size, the corrosion rate of the steel, and the SCC shallow crack morphology at the disbondment location. This research aims to understand the effect of the pipeline coating conditions on corrosion and near-neutral pH SCC of the underlying pipeline steel. The corrosion behavior and rate of pipeline steel under coating disbondment gaps of varying sizes was investigated through field data and experimental studies. The experimental studies were performed on X-65 pipeline steel coupons in anaerobic conditions, with various CO2 concentrations (2%, 5%, and 20% CO2), without Cathodic Protection (CP). The steel coupons were placed into a vertical coupon holder simulating a coating disbondment on a pipeline. The findings showed that a metal ion concentration cell may develop under narrower coating disbondments. In the analysis of the field data, dig reports provided by a Canadian gas transportation company were analyzed for cases of anaerobic corrosion under tape and asphalt coatings. The results obtained through field data analysis correlated with the experimental findings, suggesting that concentration cells under narrow coating disbondments may be responsible for enhanced corrosion on pipelines in the field. The effect of the coating disbondment gap size on the morphology of shallow cracks was analyzed in X-60 pipeline steel, in 5% CO2 under triangular tensile loading under coating disbondment gaps of various sizes. The results showed that the coating disbondment gap size affects the morphology of the shallow cracks under the disbondment.