THE STRESS-CORROSION CRACKING AND HYDROGEN EMBRITTLEMENT OF TITANIUM IN METHANOL HYDROCHLORIC-ACID SOLUTIONS

Pre-exposure tests on unstressed specimens of IMI 125 (low-oxygen, com mercially pure titanium) in MeOH/HCI solutions revealed that intergran ular anodic dissolution was occurring. In addition it was also observe d that for pre-exposure times of 25 h or more, absorbed hydrogen was r esponsible for an intergranular strain-rate dependent embrittlement wh en specimens were tested in air, in the strain rate range 5.56 x 10(-4 )-5.56 x 10(-6) s-1 immediately upon removal from the test environment . If an ageing period of 20 h at 150-degrees-C was inserted in between removing the specimens from the environment and testing, then the emb rittlement effect could be eliminated. Constant crosshead speed stress corrosion tests were also performed in MeOH/HCI in the range of cross head speeds 166.67 mum s-1 to 1.67 nm s-1. The mode of cracking was ob served to be predominantly intergranular, with small regions of transg ranular cleavage evident. Crack velocities of in excess of 1.2 mm h-1 were observed for the intergranular SCC. Additions of selenium, a cath odic poison, to the test environment produced a significant increase i n the observed crack velocity, whilst additions of platinum were obser ved significantly to reduce crack propagation rates and, in some insta nces, cause complete crack arrest. Significantly less intergranular fr acture was evident in specimens tested in platinum-containing solution s compared with those tested in platinum-free solutions. The above obs ervations supported the argument that hydrogen embrittlement is the me chanism responsible for the intergranular SCC of titanium in MeOH/HCI solutions.