ABSTRACT

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.

INTRODUCTION

The present paper is oriented towards the understanding of the mechanism operating during the stress corrosion cracking (SCC) of some h.c.p, metals in iodine-containing environments. This subject is of high technological interest in the nuclear industry because zirconium alloys are used in nuclear power reactors as fuel rod cladding and also as structural material in the reactor core. The fuel rod cladding is reported to be susceptible to SCC induced by the iodine liberated during the nuclear fission of uranium. The present subject has been investigated for the past 40 years but there is no general agreement about its mechanism and it is still matter of dispute. In recent years, the surface-mobility SCC mechanism, developed by Galvele 1'2, was satisfactorily applied to f.c.c, alloys in numerous media, being a promising mechanism to explain the SCC phenomenon in cases like the present one. The mechanism predicts the crack propagation rate as a function of the characteristics of the material and environment.

In a previous work 3-s, the SCC susceptibility of zirconium (UNS R60702) and one of its alloys, Zircaloy-4 (Zry-4) (UNS R60804), was studied in 10 g/L iodine dissolved in various alcohols: methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol and 1-octanol. SCC was observed in all the systems studied. In the initial stages the cracks had an intergranular crack path. In later stages the cracks propagated transgranularly with cleavage and fluting, and the wires finally ruptured by mechanical overload where the crack surfaces showed the typical ductile fracture dimples. The overall crack propagation rate was found to vary depending on the size of the solvent molecule. As the solvent molecular weight increased, the crack propagation rate decreased. Since the corrosion rate of Zr single crystals was similar in all the solutions tested, it was concluded that the decrease in the crack propagation rate was due to a steric effect, that hindered the access of the corrosive species to the tip of the crack. However, it was not possible to differentiate the time associated with the intergranular attack from that associated with the SCC process (the transgranular mod

This content is only available via PDF.
You can access this article if you purchase or spend a download.