The stress corrosion cracking (SCC) susceptibility of h.c.p, metals was studied. The alloys used were: zirconium (UNS R60702), Zircaloy-4 (UNS R60804) and titanium (UNS R50400). The studies were performed in a 10 g/L iodine-methanol solution. SCC was observed in all the systems studied, which was always preceded by intergranular attack. It was found that the rate-controlling step of the intergranular propagation was the diffusion of iodine to the tip of the crack. It is not clear if the mechanism of the intergranular attack is due to some sort of chemical attack of the disrupted atomic structure along the grain boundaries, or to the anodic dissolution of an unidentified species segregated along the grain boundaries. After the intergranular penetration reached a certain value, a transition to transgranular cracking mode took place. The length of intergranular attack as a function of time was measured using interrupted strain rate tests. From these data and the overall (Intergranular + Transgranular) crack propagation rate, the rate for transgranular cracking was evaluated. It was concluded that the transgranular part of the cracking was the real SCC process. When analyzed this part of the process, it was found that the crack velocities measured agreed with the predictions of the surface mobility-SCC mechanism. In the systems studied in the present work, the species responsible for inducing SCC are believed to be titanium tetraiodide (TiI4) and zirconium tetraiodide (ZrI4) for titanium and zirconium alloys respectively.
Keywords" titanium, zirconium, Zircaloy-4, stress-corrosion-dracking, intergranular attack, iodine