The stress corrosion cracking and hydrogen embrittlement of titanium in methanol-hydrochloric acid solutions

Abstract

Pre-exposure tests on unstressed specimens of IMI 125 (low-oxygen, commercially pure titanium) in MeOH/HCl solutions revealed that intergranular anodic dissolution was occurring. In addition it was also observed that for pre-exposure times of 25 h or more, absorbed hydrogen was responsible for an intergranular strain-rate dependent embrittlement when specimens were tested in air, in the strain rate range 5.56 × 10 −4-5.56 × 10 −6 s −1 immediately upon removal from the test environment. If an ageing period of 20 h at 150°C was inserted in between removing the specimens from the environment and testing, then the embrittlement effect could be eliminated. Constant crosshead speed stress corrosion tests were also performed in MeOH/HCl in the range of crosshead speeds 166.67 μm s −1 to 1.67 nm s −1. The mode of cracking was observed to be predominantly intergranular, with small regions of transgranular cleavage evident. Crack velocities of in excess of 1.2 mm h −1 were observed for the intergranular SCC. Additions of selenium, a cathodic poison, to the test environment produced a significant increase in the observed crack velocity, whilst additions of platinum were observed significantly to reduce crack propagation rates and, in some instances, cause complete crack arrest. Significantly less intergranular fracture was evident in specimens tested in platinum-containing solutions compared with those tested in platinum-free solutions. The above observations supported the argument that hydrogen embrittlement is the mechanism responsible for the intergranular SCC of titanium in MeOH/HCl solutions.