Corrosion and related electrochemical studies on ziroconium (in nitrate melts, nitrats fluoride melts and aqueous nitrate solutions)

Abstract

The thesis embodies the results of the investigations on the corrosion of zirconium in nitrate melts. The predicted thermodynamic behaviour of zirconium in 2-component NaNO?-KNO? eutectic melts based on E-pO diagrams is compared with the experimental results. The thermodynamic predictions seem to be largely substantiated. A few electrolysis experiments were carried out from NaNO?-KNO?-K?ZrF? baths. Zirconium deposits could be obtained which were liberally contaminated with ZrO?. The current efficiency was about 40%. The kinetics of corrosion of zirconium in nitrate melts is followed by the weight-change method; the corrosion results are explained in terms of oxidative corrosion (ZrO?), metal dissolution (sub compound formation) and zirconate formation. The influence of temperature and the initial concentration of potassium fluorozirconate have also been studied. The polarisation studies of zirconium in the nitrate eutectic with and without the addition of potassium fluorozirconate have been carried out under galvanostatic conditions; the anodic and cathodic polarisation behaviour in pure nitrate melts is compared with their aqueous analogues. The metal exhibits active to passive transition in both aqueous and fused nitrate systems. The susceptibility of zirconium to stress corrosion cracking has been studied; the susceptibility to stress corrosion cracking increases with pH up to pH = 6, which falls off markedly at pH > 6. This behaviour of zirconium in aqueous nitrate solutions is explained on the basis of the passivity of the metal and from the point of view of the E-pH diagram.