Methyl iodide as a promoter of the SCC of zirconium alloys in iodine vapour

The presence of organic-iodine contaminants, or specifically methyl iodide, in the iodine environment during stress corrosion tests leads to an increased frequency of crack nucleation, and possibly a more rapid crack propagation process. The unusual fractographic features associated with these impurities suggest that they enhance the reduction in the surface energy of zirconium, cause an instability in the propagating crack front, and possibly enhance the rate of reaction to form zirconium iodides. These impurities are the most potent catalysts of iodine-induced cracking so far identified, although methyl iodide alone cannot initiate cracking.