Improvement of Adhesion Strength of Electroless Nickel Coating on AZ91D by Anodized Oxide Interlayer

Abstract

Nickel-oxide duplex coatings were successfully deposited on magnesium alloy (AZ91D) by anodizing and electroless nickel-phosphorus plating processes. The anodizing interlayer was used to increase the adhesion strength of Ni-P layer. The electroless Ni-P coating enhances the corrosion resistance of the anodic oxide layer. Specimen of AZ91D magnesium alloy was polished firstly. The anodizing process was preceded in alkaline anodizing solution at 24 °C, and the bath voltage maintained at 70 volts for 2 hours. Successive electroless Ni-P plating was used to achieve the sandwich structure. The surface morphologies of the coatings were observed by field-emission scanning electron microscopy (FE-SEM). The adhesion strength was measured by pull-off tester. The electrochemical behavior of coatings with corrosion resistance in 3.5 wt.% NaCl solution was evaluated by potential polarization curve. The experimental results showed that the adhesion strength of directly deposited coating and duplex coating were 115.4 kgf/cm2 and 142.2 kgf/cm2, respectively. The adhesion strength of coatings on AZ91D magnesium alloy was improved by the synergistic effect between anodized magnesium oxide and Ni-P layer in duplex coating.