Localized passivity breakdown of iron in chlorate- and perchlorate-containing sulphuric acid solutions: A study based on current oscillations and a point defect model

Current oscillations are used to study the effect of chlorate and perchlorate ions on the iron passivity in sulphuric acid solutions. Quasi steady-state current–potential and potentiostatic current–time curves show the emergence of complex current oscillations, besides the simple periodic ones attributed to general corrosion occurring across the passive–active transition of the Fe|0.75 M H2SO4 electrochemical system. The complex current oscillations arising at the iron passive state are indicative of pitting corrosion. Experimental results support that pitting is due to chlorides produced via the reduction of chlorates and perchlorates by ferrous ions either during the active phase of current oscillations or in the passive phase during the H+-catalyzed dissolution of the oxide. Thus, chloride is the aggressive ion that causes pitting corrosion and not chlorates and perchlorates themselves. Chloride production induced via the reduction of perchlorates is much slower than that induced by chlorates. A point defect model (PDM) is employed to explain the oxide growth and its breakdown induced by general and pitting corrosion.