Structure and electrochemical behaviour in the Ringer's solution at 25 °C of electrodeposited Co-Mo nanocrystalline coating

•Microstructure of coatings consisting of hard oxide particles uniformly dispersed in a CoMo nanocrystalline alloy matrix.•Corrosion behaviour of these coatings in Ringer's solution at 25 °C.•Properties of the native passive film formed on these coatings in air.

The structure and the passive properties of coatings consisting of hard oxide particles (diameter less than 1 μm) uniformly dispersed in a CoMo alloy matrix (crystallite size of about 2 nm) have been investigated. The native passive film formed on this coating is composed of Co2+, Mo6+ and Moδ+ (2<δ < 4) species. Different regions are found in the anodic branch of polarisation curves (Ringer's solution at 25 °C, pH = 7.2). A narrow passive range is first observed (presence of a native passive film). From −600 mV vs. Ag/AgCl, the current density increases continuously. This is first associated to slight oxidation of the CoMo coating, followed by strong oxidation and selective dissolution of Co (−490/−190 mV vs. Ag/AgCl). Oxidation of metallic elements is the main reaction proceeding at the surface coating. Under certain conditions, internal stresses developed drastically in the coating, leading to cracks and peeling off. From −190 mV vs. Ag/AgCl, corrosion of the coating starts.