Abstract
The principal aim of the work described in this thesis was to produce a sintered cobalt-chromium-molybdenum alloy, and to study its corrosion resistance in order to assess its value as a surgical implant material. A literature and clinical survey indicated that sintering, as opposed to casting, the alloy produces a required increase in mechanical properties but that the sintered alloy had not been shown to have sufficient corrosion resistance for use in the human body. A potentiostatic method of corrosion testing was chosen for this work after a consideration of the theory of aqueous corrosion and a review of previously used techniques. Preliminary tests were carried out on specimens of Vitallium and these provided information about the corrosion resistance of the cast alloy as well as helping to standardise a testing procedure. Specimens of the sintered alloy were produced by the author using a previously developed method. This material was found to be unsuitable for use in the body by virtue of its low passive breakdown potential in Hanks' balanced salt solution. Electron probe analysis suggested that this inadequate corrosion resistance was the result of an unexpected low chromium content. A sintered alloy with an increased chromium content was produced but this was found to have similarly low corrosion resistance. Further corrosion tests indicated that this was the result of a chromium deficient phase within the grain boundaries. It was concluded that, without further development, the powder metallurgy technique was incapable of producing a cobalt-chromium-molybdenum alloy with sufficient corrosion resistance for surgical implants. The metallurgical examination of several removed cast Vitallium implants is described. The results, together with those of further laboratory corrosion tests, suggest that the cast alloy is susceptible to crevice corrosion in the human body.