ROLE OF OXIDE-ION CONCENTRATION ON THE OXIDATION BEHAVIOR OF ZIRCONIUM IN MOLTEN NANO3-KNO3 EUTECTIC

The oxidation behaviour of zirconium in molten (Na, K)NO3 eutectic in the presence of Na2O2, KNO2, K2CrO4 or K2CrO7 was investigated under open circuit and polarization conditions. The corrosion potential was found to increase linearly with the logarithm of time until a steady state value was reached, which is a function of oxide ion concentration in the melt. In the presence of KNO2, K2CrO4 or low concentrations of Na2O2, the rate constant of oxide growth is independent of [O2-] while it increases significantly as temperature rises. In contrast, it decreases in the presence of K2Cr2O7 with rising temperature. At [Na2O2] > 0.05 molal, the rate constant of oxide growth and the steady corrosion potential increase sharply with increasing [Na2O2]. The shape of the polarization curves depends mainly on the anodizing current density and the additive type. Oxide film grows efficiently at low current densities in the presence of K2Cr2O7 or Na2O2 while the oxide growth is suppressed in the presence of KNO2 or K2CrO4. Regardless of the additive type, the field strength and the rate of oxide formation decreases as temperature rises in agreement with the solid state mechanism of oxide growth under high electric fields. At high current densities, a potential maximum appears and ii is explained in terms of change in the conductivity profile of the growing oxide. The dependences of the maximum potential and the time required to reach the maximum on current density, temperature additive type and concentration were analysed.