Corrosion protection of AM60B magnesium alloy by application of electroless nickel coating via a new chrome-free pretreatment

Abstract

Cerium-vanadium (Ce-V) conversion coating was proposed as a new pretreatment for application of electroless Ni-P coating on AM60B magnesium alloy to replace the traditional chromium oxide pretreatment. Morphology and chemical composition of the conversion coating were investigated. The subsequent Ni-P coating deposited on the conversion coating was also characterized by morphology, chemical composition, microstructure, corrosion protection performance and micro-hardness. A uniform, compact and pore-free electroless coating with a moderate concentration of phosphorus (7.78 wt%) was obtained. The electroless coating showed a nobler opencircuit potential than that of the bare alloy during the first 13 h of immersion in 3.5wt% NaCl solution. Also, the sample plated with the Ce-V conversion coating showed lower corrosion current density than the sample plated with traditional chromium oxide pretreatment. The micro-hardness of the bare alloy was significantly increased after electroless coating. The electroless coating is pore-free and there is suitable adhesion between the coating and alloy substrate.