Anodic Polarisation of Mild Steel in Molten NaNO 3 —KNO 3 Eutectic

Abstract

The galvanostatic oxidation of mild steel in NaNO₃–KNO₃ melts has been investigated at temperatures ranging from 250°-450°c. The anodic current/voltage curves show that with the first small applied current there is a large shift of the steel electrode potential towards more noble values, denoting passivity. With further increase in applied current the potential changes are small, but give rise to one clear step, which has been attributed to the oxidation of nitrite ions present in the melt. Beyond the nitrite oxidation step the E/log i curves are complex in shape and not very reproducible, especially at higher temperatures. This further step has been assigned to Fe₂O₃ formation. Further increases in current produce no significant change of potential, indicating breakdown of passivity. These results are interpreted in relation to the available thermodynamic predictions for the system and compared with previous work on the electrochemical behaviour of iron in molten nitrates.