Role of chloride ion in passivity breakdown on iron and nickel

Abstract

The effects of Cl- on the electronic properties of the n-type passive film on Fe and the p-type passive film on Ni in pH 8.5 buffer solution were investigated using photocurrent spectroscopy and Mott-Schottky analysis in order to elucidate the mechanism of passivity breakdown on metals. Metastable pitting events were observed in the potentiostatic current transients (chronoamperograms) for Fe measured at 0.7 V-SCE when 0.1 M of NaCl was added to the solution, demonstrating that Cl- induces passivity breakdown. However, the Mott-Schottky analysis revealed chloride ion, present at a concentration of 0.10 M, did not change the concentration of oxygen vacancies in the passive film on Fe, indicating that the oxygen vacancy is not responsible for the passivity breakdown. It was observed by Mott-Schottky analysis that as the concentration of Cl- in the solution increased, the concentration of cation vacancy in the passive film on Ni also increased, which is in concert with the view that metal vacancies are responsible for the breakdown of passivity on nickel. Finally, by using electrochemical impedance spectroscopy to interrogate the point defect generation and annihilation reactions that occur at the metal/film and film/solution interfaces, we show that the observed increase in cation vacancy concentration in the passive film on Ni is due to chloride-catalyzed ejection of cations from the film/solution interface. These findings are inconsistent with chloride-catalyzed film dissolution and chloride penetration mechanisms for passivity breakdown, but they are entirely consistent with cation vacancy generation at the barrier layer/solution interface and subsequent cation vacancy condensation at the metal/barrier layer interface, as postulated in the point defect model. (c) 2005 The Electrochemical Society.