The nature and role of passive films in controlling the corrosion resistance of MoSi2-based nanocomposite coatings

Abstract

Novel MoSi₂-based nanocomposite coatings were synthesized in situ on Ti–6Al–4V substrates by sputter-deposition of a MoSi₂ layer, with differing Al concentrations, followed by plasma nitriding. The electrochemical behaviors and electronic properties of the passive films developed on the MoSi₂-based nanocomposite coatings in 3.5 wt% NaCl solution were characterized using various electrochemical analytical techniques, including potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), potential of zero charge and Mott–Schottky analysis. The results revealed that both plasma nitriding and alloying with Al have a positive effect on the corrosion resistance of the resultant coatings by altering the microstructure of the coatings as well as the chemical composition and semiconducting properties of the passive films. Both plasma nitriding and Al alloying can suppress transpassive dissolution of molybdenum by increasing the ratio of Mo(IV) to Mo(VI), and, thus, reduce the donor density of the passive films. In addition, it is observed that both treatments increase the differences between corrosion potential and potential of zero charge and thereby enhance the capability to repulse chloride ions. These findings are expected to provide valuable guidance for the design of corrosion-resistant coating materials.