Predicting Corrosion and Current Flow within a Disc Crevice on Coated Steels

The objective of this work was to develop a model to describe pipeline corrosion within a disc-shaped crevice in the presence of oxygen (O₂). The model results from modification of the pipeline crevice corrosion model (PCCM) previously developed for pipeline corrosion in a rectangular crevice. The present study compares corrosion rate distribution within the disc to the previous rectangular crevices, and investigates the effect of the geometrical parameters of the disc crevice on the corrosion rate. Compared to a rectangular crevice, for the same holiday potential, the corrosion rate decreases more rapidly from the holiday into the disc crevice. Similar to the rectangular crevice, as the holiday radius increases, O₂ reaches the pipe surface in the crevice more readily and increases the crevice corrosion rate. Increasing the gap between the pipe surface and the coating or increasing the coating thickness decreases O₂ diffusion to the pipe surface and decreases the crevice corrosion rate. External cathodic protection (CP) protects the pipe surface by cathodically reducing O₂ near the holiday. Pipeline corrosion at higher temperatures has a higher corrosion rate but a lower corrosion potential. At a potential believed to be protective, high temperatures may render this protection ineffective.