Effects of solution temperature and Mn Content on the passivity of active-passive metals

Abstract

Effects of solution temperature and Mn content on the passivity of pure metals and alloys were investigated by electrochemical impedance measurement and micro-electrochemical technique. To reveal why the corrosion resistance of Ni is degraded with increase in solution temperature, the effects of temperature on the kinetic nature of passive film on Ni were examined. Under the steady state condition, the current density of Ni is associated with cation vacancy flux $\It{I_{ss}}=$F\GammaJ_{V\rdquo_{Ni}}$ in the passive film. With increasing temperature, the cation vacancy generation rate at the film/solution (f/s) interface increases significantly, thereby increasing the cation vacancy flux at the f/s interface ($J^{f/s}_{N\rdquo_i}$). Because diffusivity of cation vacancy is significantly enhanced by solution temperature, the cation vacancy generated at the f/s interface moves to the metal/film (m/f) interfaces more rapidly. While the annihilation rate of cation vacancy decreases by solution temperature, the cation vacancy concentration at the m/f interface ($\C^{m/f}_{N\rdquo_i}$) increases abruptly, thereby increasing the cation vacancy flux at the m/f interface ($\J^{m/f}_{N\rdquo_i}$). The increase in cation vacancy flux in the passive film resulted in the increase of $\It{I_{SS}}$. According to the Point Defect Model (PDM), film breakdown is closely associated with cation vacancy condensation at the m/f interface. The cation vacancy concentration at the m/f interface increased drastically with solution temperature. Therefore, it is suggested that the possibility of formation of “vacancy condensate” increases with temperature and accordingly, the susceptibility to film breakdown increases with solution temperature. The effects of temperature on the film growth kinetics of Fe were examined by analyzing the potentiostatic current flowing during the growth of passive film based on the PDM. Standard rate constants for the oxide lattice formation ($\It{k^{0}_{...