Development of Tools to Estimate Actual Corrosion Growth Rates of Gas Pipelines

A model is developed to predict the chemistry, corrosion potential and rate of pipeline steels in a crevice formed when a coating disbonds from a pipe surface. The gap of the coating disbonded region is assumed to vary with distance from the mouth. The effect of this gap variation on the chemistry and corrosion rate in the coating disbonded region is investigated in this study through modeling. The preliminary model results suggest that overall, the variation of the disbondment gap with distance has an insignificant effect on the pH, corrosion potential and rate in the coating disbonded region. Unlike some conventional crevice corrosion often associated with a large cathodeto-anode area ratio, the area ratio here is rather relatively small and the pH commonly falls in the neutral or alkaline range. Within this pH range, even if the pH varies within a few units across the crevice length, the variation of the crevice corrosion rate is not significant. This paper reports on and discusses the fundamental principles used for the model, some key model results, and the practical implications of the results.