Corrosion protection of composite coating combining ceramic layer, copper layer and benzotriazole layer on magnesium alloy

A novel composite coating was fabricated on AZ91 magnesium alloy by applying a composite surface treatment which combined the methods of plasma electrolytic oxidation (PEO) pre-treatment, electroless copper and benzotriazole (BTA) passivation. The cross-section microstructures and chemical compositions of coating were examined using scanning electron microscopy (SEM) equipped with energy dispersive analysis of X-rays (EDX). Potentiodynamic polarization curves and salt spray tests were employed to evaluate corrosion protection of the coating to substrate in 5% NaCl solution. It is indicated that electroless copper produces a rough interface between the electroless copper layer and the ceramic layer. The corrosion potential shifts to the positive direction significantly and the current density decreases by more than one order of magnitude. There is no visible galvanic corrosion pits on the surface of the composite coating combination of PEO and electroless copper after 168 h neutral salt spray testing. The color of copper after BTA immersion could be held more than 60 d.