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

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 approximate to 0.048 V/decade for the E-ss vs. pH relation in the whole pi-I 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 (HMoO4- and MoO42-). 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 (approximate to 28.3 kJ mol(-1)) than in neutral (approximate to 52.5 kJ mol(-1)) or acidic solutions (approximate to 62.4 kJ mol(-1)) which reflects the relative instability of the passive film in basic solutions. (C) 1998 Elsevier Science Ltd. All rights reserved.