An environment-friendly calcium phosphate conversion coating on AZ91D alloy and its corrosion resistance

Abstract

An environment-friendly phosphate conversion coating free of chromate, fluorides, and nitrite was developed on AZ91D alloy through a facile chemical deposition method. The surface morphology and phase composition of the coating were characterized by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The corrosion resistance of the coating was evaluated by potentiodynamic polarization test in 3.5 wt% NaCl solution. Results indicate that the calcium phosphate coating was composed of a large number of leaf-like and flower-like particles. The chemical composition of the conversion coating was CaHPO4·2H2O crystals. The coating can remarkably reduce the corrosion current density of the substrate. Introduction of triethanolamine (TEA) additive into the phosphating bath leads to denser coating and high anticorrosive property of the phosphate coating.