Environment-assisted cracking of Ti in methanol solutions containing CaCl2

Environment-assisted cracking (EAC) of Ti in methanol solutions containing various concentrations of CaCl2 has been investigated by using a slow strain rate testing (SSRT) apparatus with dynamic observation system. In the EAC test, water content in the solutions was strictly controlled and the test solution was fully deaerated. Like the case of HCl addition, it was observed that cracks were initiated from black pits generating during the test and that the cracking mode was intergranular at the initial stage and transgranular at the final stage of the crack propagation. Under open circuit condition, cracking occurred only at 0.02 and 0.2 kmol.m(-3) CaCl2 and the maximum corrosion potentials when cracking occurred, which means the critical potential for EAC, was a negative function of the logarithm of the chloride concentration. The potentiostatic SSRT confirmed the linear relationship between the critical potential and logarithm of chloride concentration in the chloride concentration range from 4 x 10(-4) to 4 x 10(-2) kmol.m(-3). In comparison with the case of HCl addition, CaCl2 addition provided the same slope of the relation but higher critical potential. It was also found that the dissolved oxygen plays a role of increasing corrosion potential as an oxidant, and that the cracking occurs when the corrosion potential exceeds the critical value for EAC. It is concluded that the EAC initiation of this system is controlled by the mechanism of initiation of anodic localized corrosion by chloride ions.