The low frequency corrosion fatigue performance of some surgical implant alloys: with reference to stress concentration, environmental factors and the behaviour of mild steel

A review of the literature on corrosion fatigue and fatigue testing is followed in this work by an analysis of the variables involved and of the various mechanisms which have been advanced. Particular reference is made to the corrosion fatigue of surgical implant alloys. An experimental programme was devised to be particularly relevant to the environmental variations possible in implant/body systems.
A multi-specimen, reverse-bend, strain facility was designed and produced for this research. Small smooth specimens were tested at a 'walking pace' of 1.7 Hz. A multi-channel,
mini-potentiostat was also designed and produced for electrochemical studies during corrosion fatigue. Scanning and transmission electron microscopy was used to examine fracture surfaces.
The corrosion fatigue performance of some alloys (stainless
steel type 316, titanium 130 and 318) currently used for
surgical implants has been evaluated. A comparative study of
the performance of mild steel, a material which has in the past been used for implants, was also made. A series of experiments was conducted to simulate normal conditions with a 0.17M saline solution buffered at pH 6.5. Some extreme conditions were also investigated at pH 1.5 and pH 11.5. In order to evaluate the comparative influence of saline, additional experiments were conducted in air and in distilled water for each material.A considerable number of metal/environment combinations was thereby investigated, involving corrosion fatigue in both the passive-state and the active state. Since it is common place for some implants to be drilled for fitting, a further series of experiments was conducted using specimens containing a drilled hole. The influence of such stress concentrating features was thus established for each material.
The behaviour of mild steel in all the environments tested,
and that of S316 in pH 1.5 saline was described using a single model for interaction between stress concentration and corrosion during fatigue. A different model was found to represent the behaviour of all the other metal/environment combinations tested. The two models were seen to represent, respectively, activestate corrosion fatigue and fatigue under passive state conditions. The drilling of holes in mild steel and T130 showed a small stress concentration effect. No such effect was produced with holes in stainless steel. By contrast, however, the drilling of a hole in T318 resulted in a dramatic reduction in the fatigue limit: a reduction of the order of 50% was obtained in both distilled water and saline solutions. Unlike other materials S316 produced a wide scatter band of results in saline pH 6.5. This was attributed to a naturally fluctuating corrosion potential indicative of an irregular film breakdown and repair. This was attributed to a naturally fluctuating corrosion potential indicative of an irregular film breakdown and repair.
The results are generally discussed and the evidence
produced shows that the use of S316 for surgical implants cannot be recommended because of poor resistance to corrosion fatigue at low pH. T130 was superior in this respect. However, T318 showed the best resistance to corrosion fatigue even allowing for the drastic reduction in performance with a drilled hole.

Test rigs for the experimental work were built and used at three sites: Aldermaston A.W.R.E., Middlesex Polytechnic, and City of London Polytechnic.