Establishing Environment Friendly Surface Treatment for AZ91 Magnesium Alloy for Subsequent Electroless Nickel Plating

Abstract

Asystematic study was made to develop the non-chromate and non-fluoride surface treatment for the AZ91 alloy to enable an adherent and corrosion resistant electroless nickel (EN) coating. The performance of the surface treatments is substantiated by the EN coating, in a single and two (duplex) layers on the prior surface pretreated AZ91 alloy, in the subsequent bath. The surface treatment in a solution containing cerium nitrate and subsequent single layer EN coating exhibited good adhesion and an excellent corrosion resistance in 0.5% NaCl solution. The duplex EN layer improved both corrosion as well as adhesion resistance. A remarkable long term (100h exposure) corrosion resistance, assessed using electrochemical impedance spectroscopy (EIS), was shown by the duplex EN coating with an outer layer produced in a bath of pH similar to 5. In contrast, the duplex coating with an outer EN layer obtained in a bath of pH similar to 10.5 failed within 4h of exposure. The porosity index of the coating in the order of 10(-6) appears critical for achieving an excellent corrosion resistance. The adhesion resistance, as indicated by the critical load for coating delamination, was successively increased from 5.2 to 10.39N using different pretreatment processes. The adhesion resistance was influenced by the agglomerate particle size and coating thickness. (C) 2018 The Electrochemical Society.