Electrochemical state conversion model for occurrence of pitting corrosion on a cathodically polarized carbon steel in a near-neutral pH solution

Pitting corrosion occurs frequently on oil/gas pipelines which are protected by both coating and cathodic protection (CP). Since the steel is in an active state, the classic theories attributing pitting to the passive film breakdown do not apply. This work is anticipated to advance the understanding of the fundamentals of pitting corrosion of carbon steel pipelines occurring in an active system. In this work, a square wave polarization method was used to simulate the CP fluctuation and its effect on pipeline steel pitting in a near-neutral pH solution. Moreover, an electrochemical state conversion model was developed to illustrate the pit initiation and growth on the cathodically polarized steel. According to ESCM, when a potential fluctuation is generated on the steel electrode, the local double-charge layer structure is disturbed. The defect area undergoes a temporary anodic potential field, resulting in the local anodic dissolution to nucleate pits. Thus, local anodic dissolution (pitting) of steel would occur under an unstable cathodic polarization. Furthermore, it is found that the potential shifting range plays an important role in pitting initiation and growth. Pits nucleate in high-density under square wave polarization if the polarization potential range is relative narrow and the upper potential is not so negative, i.e., close to OCP. When under a wide potential range and a relatively negative upper potential, pits tend to grow into big size with a low density. Therefore, although the pipeline is protected by CP, the CP fluctuation could occur during service, which would introduce the polarization fluctuation on the steel, resulting in pitting corrosion.