Materials Chemistry and Physics, Vol.183, 121-130, 2016
Structure and electrochemical behaviour in the Ringer's solution at 25 degrees C of electrodeposited Co-Mo nanocrystalline coating
The structure and the passive properties of coatings consisting of hard oxide particles (diameter less than 1 mu m) uniformly dispersed in a Co-Mo 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 Mo delta+ (2< delta < 4) species. Different regions are found in the anodic branch of polarisation curves (Ringer's solution at 25 degrees 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 Co-Mo 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. (C) 2016 Elsevier B.V. All rights reserved.