The passive behavior of biomedical alloys in simulated physiological solutions

Passive alloys are commonly considered to exhibit a current that is essentially independent of potential over the passive range in neutral chloride solutions. However, the current-potential dependence of Ti and its alloys, CoCr alloys, and 316L stainless steel in buffered, simulated physiological solutions containing phosphate differs from that usually reported for the alloys in chloride-only solutions. An analysis of potentiodynamic polarization data from previous studies showed that these alloys typically exhibit an exponential dependence of current on potential-as reflected by Tafel-type behavior-over the initial part of the passive range in buffered solutions. This behavior is consistent with that predicted by the Generalized Growth Model for film growth and dissolution in the case of thin films where cation migration through the film is the rate-controlling step and where the anodic current is associated predominantly with dissolution. Film growth is thought to be impeded in the buffered solutions, allowing the rate of migration and hence dissolution to remain relatively high over a wider potential range. Analysis of the apparent Tafel slopes indicated that the Ti and CoCr alloys exhibit a similar corrosion mechanism, with a small difference in the slopes for the two groups of alloys being attributable to the difference in the type of oxide. Stainless steel 316L exhibits a distinctly higher apparent Tafel slope, which is likely associated in large part with the duplex nature of the oxide.