Transient Species Participating in the SCC of Zirconium Alloys

A study of the chemistry of the intergranular dissolution of zirconium has been made in a range of iodine/organic solutions. This process is the second of the 3 stages through which the stress corrosion cracking (SCC) of the Zircaloys proceeds. The first being cracking or breakdown of the surface oxide film and the last a relatively fast transgranular propagation process. From a study of the effects of various additives on the time to failure of smooth split-ring specimens, together with fractography of the resulting cracks, it is concluded that competition reactions between the various components of the solution and transient chemical species in solution control the extent of the intergranular dissolution step. A comparison with published chemical reaction rates suggests a hypothesis whereby this dissolution process proceeds via solvated electron and low valence zirconium intermediates. Additives which react rapidly with solvated electrons, or which complex with the lower valence states of zirconium may compete for the intermediate species and thereby slow or stop the SCC. The mechanism by which the SCC is slowed is not always a simple reduction in the rate of intergranular penetration, some additives operate by eliminating the specificity for grain boundaries in the dissolution process, while giving enhanced overall dissolution.