A study of stress corrosion in alloys of orthopaedic interest

A number of failed surgical implants have been examined and a brittle type of fracture shown to be a major cause of failure. It was not possible to establish unequivocallythe mechanism of fracture from these examinations but the evidence suggested the importance of mechano-chemical effects. Static stress corrosion tests on both smooth and notched specimens of currently used and potentially useful implant materials were performed. The materials tested being iron, cobalt and titanium based alloys. Tests were run for long periods (up to 10,000 hrs) and elevated temperature used as an accelerant. These tests showed that stress corrosion was not a major cause of failure in present implant devices. The problem of predicting the likelihood of SCO failure in very long term implantation eg up to 50 years for a hip joint has been investigated using a dynamic strain test. In this test specimens were strained at a constant rate in 0. 17M NaCl solution; the potential of the specimen was controlled and the corrosion current flowing during the test monitored. Stress corrosion susceptibility was revealed by a reduction in mechanical properties in the stress corrosion environment compared with those in air. Data obtained on 316S16 stainless steel have been interpreted in terms of a slip-step dissolution model. Extrapolation of susceptibility to long exposure times has been made on a semiquantitative basis. The theory predicts that 316S16 should be free from SCC in implant conditions for the periods of use envisaged. Slight susceptibility has been observed in cobalt based alloys which was dependent on the pre-existence of a stress concentrator eg a notch root. A model for cracking based on grain boundary attack has been developed for Co-Cr-Mo and metallurgical treatments to minimise cracking suggested. The effect of strain rate, environment, alloy structure and alloy hydrogen content on susceptibility of titanium alloys to SCC was investigated and a mechanism based on slip step dissolution and titanium halide hydrolysis postulated. The development of a critical crack tip deformation rate has been shown to be the most important mechanical parameter. Alloy structure, environment composition and alloy hydrogen content are also significant, the effect of hydrogen being via its influence on the stability of the oxide film rather than any bulk metallurgical effect. Using the model, heat treatment conditions which may induce susceptibility have been predicted for the commercially important alloy Ti-6Al-4V. The deleterious effect of these conditions has been shown experimentally and recommendations made for their avoidance during implant manufacture.