Stress Corrosion Cracking and Corrosion Behavior of Ti and Ti-Al Alloys in Methanol Iodine Solutions

Studies have been made of the corrosion characteristics and susceptibility to stress-corrosion cracking of pure titanium and Ti-Al binary alloys in methanol-iodine solutions. The results indicate that two distinct stress-corrosion mechanisms may be operative, depending on the composition of the solid. In pure titanium and a Ti-2.09 w/o Al alloy failures were found to be intergranular, while in the higher alloys (>5 w/o Al) only the initial part of the crack was intergranular, the part adjacent to the final ductile fracture being largely transgranular. In all the materials examined, the intergranular stress-corrosion process could be inhibited by cathodic polarization or the introduction of water in the methanol-iodine solution. On the basis of these and other observations, it is suggested that this process represents stress-accelerated intergranular corrosion involving dissolution of metal at a crack tip. The transgranular crack propagation stage observed in the higher alloys has the characteristics of mechanical failure, and its occurrence appears to be related to a reduced propensity for cross-slip.