Corrosion and passivation behaviors of molybdenum in aqueous solutions of different pH

Abstract

The corrosion and passivation behaviors of molybdenum in aqueous solutions was investigated. The effect of oxygen on the open-circuit potential behavior of the metal was examined. The steady state electrode potential of molybdenum was found to be a linear function of the solution pH. In naturally aerated solutions, a slope of ≈0.048 V/decade for the E ss vs. pH relation in the whole pH range was obtained. It means that the mechanically polished Mo-electrode behaves in aqueous solutions at constant oxygen concentration like a pH-indicator electrode. In oxygen containing solutions, the steady state potential gets more positive and the value of E ss is controlled by both the oxygen concentration and the solution pH. The metal surface is always covered by a passive film. The electrochemical impedance spectroscopy (EIS) and polarization measurements have shown that the naturally occurring passive film is more stable in acidic solutions. In basic solutions, the stability of the passive film decreases due to the formation of soluble species (HMoO 4 − and MoO 4 2−). The dissolution parameters of the passive film were calculated from both EIS and polarization measurements and the activation energy of the dissolution process in the different solutions was calculated. It turned out that the activation energy of oxide film dissolution is lower in basic solutions (≈28.3 kJ mol −1) than in neutral (≈52.5 kJ mol −1) or acidic solutions (≈62.4 kJ mol −1) which reflects the relative instability of the passive film in basic solutions.