A Mathematical Model Developed to Predict the Chemistry and Corrosion Rate in a Coating Disbonded Region of Variable Gap Available to Purchase

A model is developed to predict the chemistry, corrosion potential and rate of pipeline steels in a coating disbonded region. The gap of the disbonded region is assumed to vary with distance from the holiday. The effect of this gap variation on the chemistry and corrosion rate in the coating disbonded region is not well understood and 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 disbonded region. This may be explained as that unlike some conventional crevice corrosion often associated with a large cathode-to-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. In this paper, the fundamental principles used for the model, some key model results and practical implications of the results are reported and discussed.