Electrochemical behavior of plasma electrolytically oxidized niobium in simulated physiological environment

Niobium is a beta-stabilizing element in the new generation of low modulus Ti alloys. Like titanium, it is characterized by good chemical stability in physiological conditions, however, it suffers from lack of bioactivity as well. In the last few decades plasma electrolytic oxidation (PEO) was used to form bioactive interfaces on Ti and its alloys, which promoted bone growth. However, little attention was devoted to the PEO treatment of pure niobium. This paper describes the process of formation of the oxide coatings on Nb from Ca(H2PO2)(2) solutions with addition of either Ca(HCOO)(2) or Mg(CH3COO)(2) salts via galvanostatic DC PEO up to 200, 300, 400 or 500 V. The coatings were all good quality (essentially crack-free), had a tri-layer structure and their thickness ranged from 2.45 to 120 mu m, depending on the limiting anodization voltage. The corrosion resistance of the modified surfaces was tested using electrochemical impedance spectroscopy (EIS) and DC polarization techniques, such as linear polarization resistance (LPR) and potentiodynamic polarization (PDP) in Ringer's solution at 37 degrees C. A new equivalent electrical circuit (EEC) was proposed to model the PEO coatings on niobium and possibly other metals, such as titanium in dilute chloride environments. The coatings formed at lower voltages (200 V) in the more concentrated PEO solutions were more resistant, while the higher voltage (300-400 V) during the processing in the less concentrated solution had a positive effect on the corrosion resistance of the modified niobium surfaces.