Passivity and Pitting Corrosion of Carbon Steel in Chloride Containing Borate Buffer Solution

Abstract

Passivity breakdown of high manganese steel in deaerated borate buffer solution has been studied in the following thesis. The steel used here is a special high strength C-Mn steel used extensively in pressure vessels. This article describes an attempt to predict and interpret the pitting corrosion of the steel in terms of the point defect model (PDM) in deaerated borate buffer solution at different pH (6, 8, 9, 10) at different chloride ion concentration ([Cl¯] = 0.01, 0.1, 0.6, 1M). The objectives of the work is to study the state of passivity and the related characteristics, determine the breakdown potential (Vc) as functions of pH, chloride concentration and potential sweep rate (?), analyze the relationship between Vc and [Cl¯], pH and scan rate (v) in terms of PDM to extract parameter values for passivity breakdown and assess the ability of PDM to account for passivity breakdown. The passive film was found to contain n-type defect with film thickness depending directly on the passivation potential. The near normal distribution in breakdown potential is in satisfactory agreement with the analytical prediction of the breakdown potential distribution obtained from PDM. The linear dependence of breakdown potential on the square root of potential scan rate and polarizability dependence of the barrier layer/solution interface upon potential and pH are 0.83 and ­ 0.01 respectively, as predicted by the PDM, yields an estimate of the critical areal concentration of condensed vacancies at the metal/film interface (? = 4.9×1014 cm-2) that leads to passivity breakdown. The value of the critical areal concentration of condensed vacancies falls in well agreement with the value obtained from structural argument. These provide convincing evidence for the validity of PDM for modelling passivity breakdown on high manganese carbon steel.