Tribocorrosion of Pulsed Plasma-Nitrided CoCrMo Implant Alloy

In the present study, a forged CoCrMo (ISO 5832-12) has been subjected to pulsed plasma treatment in a N-2/H-2 atmosphere at low temperatures (below 500 A degrees C). This treatment resulted in the formation of a layer composed by dispersed chromium nitride particles in a N-enriched metal matrix. The materials were tested for corrosion and tribocorrosion performance in 0.9 wt% NaCl at room temperature under controlled electrochemical conditions. After the treatment, the alloy loses its passive nature. The electrode potential was found to critically affect the corrosion and the tribocorrosion rates. In the nitrided alloy, a significant increase of corrosion rate was found at high potentials, while tribocorrosion was determined mainly by mechanical wear and not affected by potential. On the other hand, the untreated CoCrMo alloy exhibited stable corrosion over a wide range of potentials. Its tribocorrosion rate was similar to the nitrided alloy samples at low potentials, but it increased dramatically at high potentials where passivity triggered severe wear-accelerated corrosion and promoted mechanical wear. The present study shows that the electrochemical conditions determine material deterioration and should therefore be considered when selecting materials for tribocorrosion applications such as biomedical implants.