Modeling pipeline crevice corrosion under a disbonded coating with or without cathodic protection under transient and steady state conditions

Abstract

Underground steel pipelines are protected by coatings and cathodic protection (CP). The pipeline corrosion occurs when the coating is disbonded away from a defect or holiday to form a crevice and the corrosion rate varies temporally and spatially in the crevice. In the presence of dissolved oxygen (O 2) in soil ground water, a differential O 2 concentration cell may develop in the crevice because O 2 diffuses more readily into the crevice through the holiday than through the disbonded coating. CP can decrease or eliminate the O 2 concentration cell depending on the potential applied at the holiday. Since the coatings are usually non-conductive, CP is unable to protect the steel surface deep inside the crevice. The transport of dissolved O 2, and that of dissolved carbon dioxide (CO 2) if present, into the crevice through holiday can be key to determining the crevice corrosion rate. In this work, the transient and steady state behavior of the corrosion process is investigated. The effect of the cathodic portion of iron vs. ferrous ion redox reaction on the crevice corrosion rate, which is often neglected traditionally, is further studied. At steady state, the effect of dissolved O 2 on the crevice corrosion rate and the added effect of dissolved CO 2 are mathematically modeled.