Effects of solution temperature and $Cl^-$ion on the passivity of active-passive metals

Abstract

Effects of solution temperature and $Cl^{-}$ ion concentration on the electronic properties of passive film on pure metals were investigated by the photocurrent measurement and the Mott-Schottky analysis in order to elucidate the relationship between the electronic properties of passive film and corrosion properties of metals. The Mott-Schottky analysis and photocurrent measurement revealed that as solution temperature increased, the concentration of oxygen vacancy in the passive film formed on Fe in pH 8.5 buffer solution increased accompanying with an increase in the concentration of $Fe^{2+}$ ion in the $γ-Fe_2O_3$ passive film due to a charge neutrality reaction in the film. Further, the increase in passive current density of Fe with solution temperature was found due primarily to the increase in the concentration of oxygen vacancy in the passive film, which is well explained by the Point Defect Model. Diffusivity of point defects (DO) in the passive film formed on Fe in deaerated pH 8.5 buffer solution was estimated by the Mott-Schottky analysis based on the Point Defect Model and surface charge approach assuming that donors are oxygen vacancies. From the exponential decay of the concentration of donors with film formation potential, DO was calculated to be $1.69 × 10^{-20} ㎠s^{-1}$. Mott-Schottky analysis revealed that, as the concentration of $Cl^{-}$ in pH 8.5 buffer solution increased, the concentration of metal vacancies $(V_{Ni}^{χ-})$ in the passive film on Ni increased. The addition of $Cl^{-}$ to pH 8.5 buffer solution after passivity has been established in the absence of $Cl^{-}$ also resulted in an increase in the concentration of metal vacancy in the passive film. Finally, by using electrochemical impedance spectroscopy to interrogate the point defect generation and annihilation reactions occurring at the metal/film and film/solution interfaces, the observed increase in cation vacancy concentration in the passive film is presumably due to chlo...