Correlation between the microstructure and corrosion performance of the HIPIMS nitrided bio-grade CoCrMo alloy

Corrosion performance of CoCrMo alloy (F75) plasma nitrided with High-Power Impulse Magnetron Sputtering (HIPIMS) technique was thoroughly investigated. Open Circuit Potential (OCP) measurements and potentiodynamic polarisation tests exhibited a strong correlation between the transmuting microstructure (as a result of varying nitriding voltage from −700 V to −1100 V) and its corrosion performance. A significant improvement in the ECorr values was noticed (around −590 mV for untreated as compared to −158.17 mV for −1000 V) when analysed against 3.5% wt. NaCl solution. Similarly, results against Hank's solution also exhibited a significant increase in ECorr values (around −776 mV for untreated as compared to −259 mV for −1000 mV). Irrespective of the nitriding voltage, HIPIMS nitriding led to a significant improvement in the corrosion resistance of the alloy. For nitriding voltages −700 V and −900 V, a diffusion based S phase layer played a significant role in imparting corrosion resistance. On the contrary, precipitation of chromium-based nitrides (CrN and Cr2N), observed in samples nitrided at relatively higher voltages of −1000 V and −1100 V, resulted in its relative deterioration. A preferential dissolution of the grains and its grain boundaries, along with a sluggish dissolution of the grains and metal carbides appeared to be the dominant corrosion mechanism for the nitrided alloys. Specimens nitrided at −700 V and −900 V displayed the best corrosion resistance, which was deemed to be derived from the right combination of a thicker S phase layer and the compound layer consisting of M2–3N and M4N phases.