Reduction of corrosion current of CoCr alloys by post-PIII oxidation

Nitriding of biomedical CoCr alloys at elevated temperatures leads to a marked increase in the hardness and wear resistance. However, a compromised corrosion resistance is encountered even at relatively low temperatures, not correlated with the appearance of CrN in the XRD spectra. A duplex treatment of initial nitrogen and subsequent oxygen PIII at 350 °C was already shown to partially reduce the corrosion current. Here, it is investigated whether restoring the oxide passivation layer by PIII is necessary, a simple annealing is sufficient or an intermediate processing is more suitable. GDOES, XRD, XPS and corrosion investigations in bovine serum are performed to obtain information on the underlying mechanisms of the improved corrosion resistance. It is shown that post-oxidation by PIII leads to a more favourable corrosion protection with lower corrosion current and more positive corrosion potential compared to thermal oxidation or plasma oxidation. As the XPS and GDOES data show no differences, the cause of this behaviour should be correlated with the microstructure of the oxide.