Some thoughts on the mechanisms of in-reactor corrosion of zirconium alloys

In recent years sufficient new information has accumulated to change current views of which mechanisms of corrosion are operating in water-cooled reactors. The total number of publications is now so enormous that it is impossible for a short review to be completely comprehensive. This review concentrates on those studies that have resulted in changed views of the importance of various mechanisms. There seems to be insufficient evidence to support an hypothesis that increased corrosion rates in reactor result directly from displacement damage to the oxide by fast neutron bombardment. Replacing this hypothesis are the observations that redistribution of Fe from second phase particles (SPPs) into the Zr matrix by fast neutron recoil reduces the corrosion resistance of Zircaloy type alloys both in-reactor and in laboratory tests. There is little support for the idea that an irradiation induced phase change from monoclinic to tetragonal (or cubic) zirconia is important in-reactor, since such transformed zirconias are unstable in ∼300 °C water and revert to the monoclinic phase; as do chemically stabilised zirconias. New alloys with improved corrosion resistance in PWRs are generally low in Fe (and Sn), or have Fe in the form of more radiation resistant SPPs than those in the Zircaloys. Similarly, the hypothesis that nodular corrosion in BWRs was directly related to an effect of irradiation produced radical species in the water is unsupported. However, local dissolution of the oxide film by radiation produced species such as H2O2 may be occurring, and the close mechanistic relationship between nodular corrosion and ‘shadow corrosion’ is very evident. Thus, galvanic potentials between large SPPs (or clusters of SPPs) and the Zr matrix, aided by greatly increased electronic conduction of zirconia in irradiated systems appears to offer an hypothesis that provides a rationale for the observed effects of SPP sizes and numbers. Irradiation induced redistribution of Fe from the SPPs into the Zr matrix eliminates nodular corrosion susceptibility in Zircaloys.