Pitting Resistance of New and Conventional Orthopedic Implant Materials—Effect of Metallurgical Condition Available to Purchase

The pitting resistances of four high strength, high ductility, stainless steels (TRIP steels) were compared with those of the two alloys most widely used in implants today: Type 316L stainless steel and a cast Co-Cr-Mo alloy (Vitallium). Each of the alloys in several metallurgical conditions was tested in a physiological saline solution. Results from long term immersion tests were compared with the results obtained using two short term electrochemical techniques. The currently popular electrochemical technique, the cyclic polarization method, ranked materials differently than the newer pit propagation rate (PPR) method, but neither method gave results that were inconsistent with the results from the long term tests. Vitallium has a high resistance to localized corrosion in both the as cast and annealed conditions. Cold work has a detrimental effect on the pitting resistance of Type 316L stainless steel, but it has a generally beneficial effect on the TRIP steels. The results indicate that several of the TRIP steels have substantially better pitting resistance than Type 316L stainless steel. If these high strength, high ductility TRIP steels are as resistant to other forms of corrosion (such as fretting corrosion and corrosion fatigue), they must be considered prime candidates for future implants.