Structure and corrosion behaviour of electrodeposited Co-Mo/TiO₂ nano-composite coatings

Highlights: • Microstructure of Co-Mo/TiO₂ nano-composite coatings. • Corrosion behaviour of Co-Mo/TiO₂ nano-composite coatings in Ringer's solution at 25 °C. • Properties of the Co-Mo/TiO₂ nano-composite coatings after long-term ageing in the Ringer's solution. - Abstract: The structure and the corrosion behaviour in the Ringer's solution of Co-Mo/TiO₂ nano-composite coatings have been investigated. They consist of aggregates of TiO₂ nanoparticles uniformly distributed in a Co-Mo alloy matrix (crystallite size of about 2 nm). Both nodular (thickness less than 20 μm) and globular structures (thickness greater than 20 μm) have been observed using field-emission scanning electron microscopy. Under potentiostatic control (in Ringer's solution), oxidation of the coating first occurs followed by (with increasing applied potential) both oxidation and selective dissolution of Co. At the OCP value, Co is oxidized in the form of Co²⁺-based compounds (CoO, Co(OH)₂ or α-CoMoO₄) in the coating. This process only occurs in the outermost part of the coating. Therefore, the bulk properties of the coating are not affected after long-term ageing in the Ringer's solution at OCP.