Mechanistic insights into the corrosion of heat-treated wollastonite-zirconia coatings reinforced with carbon nanotubes

This research work investigates the corrosion behavior of as-deposited and heat-treated wollastonite-zirconia (WaZr) composite coatings, including those enhanced with carbon nanotubes (WaZr-CNT), and compares these against established hydroxyapatite (HA) coatings. The investigated coatings were deposited on titanium substrates using plasma spraying method. Microstructural investigations reveal that the addition of CNTs results in a denser and more uniformly bonded WaZr-CNT coating. Moreover, the coatings became denser after heat treatment due to effective melting of splats. Electrochemical impedance spectroscopy (EIS) indicates that HA coatings maintain superior performance under EIS tests. Nevertheless, potentiodynamic polarization tests show that the WaZr coatings achieve a substantial decrease, approximately 80%, in corrosion current density and about 70% in corrosion potential relative to HA coatings. The reinforcement with CNTs, however, leads to an increase in corrosion current density and a shift towards more negative corrosion potentials. Heat treatment of these coatings resulted in inferior performance despite better microstructure. To elucidate the underlying corrosion mechanisms, SEM, XPS, and XRD analyses were employed. A novel aspect of this study is the first-time application of electrochemical corrosion testing on CNT-reinforced WaZr coatings with and without heat treatment, revealing effect of CNT addition and heat treatment on corrosion resistance.