Niobium addition enhancing the corrosion resistance of nanocrystalline Ti5Si3 coating in H2SO4 solution

In this paper, novel Nb-containing Ti5Si3 (i.e., Ti56.2Nb6.3Si37.5 and Ti50.0Nb12.5Si37.5) nanocrystalline coatings were deposited onto Ti–6Al–4V substrates by a double glow discharge plasma technique. The effects of Nb alloying on the electrochemical behavior of the Ti5Si3 nanocrystalline coatings were systematically investigated in a naturally aerated 5 wt.% H2SO4 solution, for which various electrochemical techniques, including potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), potentiostatic polarization and Mott–Schottky analysis, were employed. Moreover, to evaluate the corrosion performance of the as-deposited coatings over an extended period, their corrosion resistance was analyzed after 7 days’ immersion in a 5 wt.% H2SO4 solution by EIS measurements and observations of corroded surface morphologies. The results showed that the Ti62.5−xNbxSi37.5 (x = 0, 6.3, 12.5) nanocrystalline coatings exhibit superior corrosion resistance compared with Ti–6Al–4V, and their corrosion resistance is enhanced with increasing Nb content, suggesting that Nb alloying is an effective strategy for improving the corrosion protection ability of the Ti5Si3 nanocrystalline coating. The roles of Nb additions in enhancing the corrosion resistance of the Ti5Si3 nanocrystalline coatings can be summarized as: (a) reducing the residual tensile stresses of the as-deposited coatings and (b) tailoring the composition, compactness and electronic structure of the passive films formed. These findings are expected to broaden the application of Ti5Si3 as a highly corrosion-resistant coating for engineering components operating under aggressive conditions.