An oxide-semiconductance model of nodular corrosion and its application to zirconium alloy development

The mechanisms of nodule initiation and growth on susceptible zirconium alloys both in- and ex-reactor remain unclear. Hypotheses citing locally inadequate concentrations of alloying elements generally can explain more experimental observations than, for example, those relying solely on electrochemical activity at second phase particles, or residual stress, or texture. Since the alloying additions that protect zirconium from rapid oxidation in high-temperature, high-pressure steam all have normal oxidation states other than IV, it seems likely that the incorporation of aliovalent oxides into the monoclinic zirconia lattice plays an important role in providing that protection. Corrosion experiments indicated that effective elements are those forming aliovalent cations that can substitute for Zr(IV) and alter the defect structure without disrupting the oxide matrix. Laboratory alloy optimization for a combination of immunity to nodular corrosion in steam and minimum uniform corrosion in water requires in-reactor confirmation. (orig.)