Chromate and HF free pretreatment for MAO/electroless nickel coating on AZ31B magnesium alloy

Abstract

Micro arc oxidation (MAO) coating was developed as an interlayer for the electroless nickel (EN) top coat to improve the corrosion resistance of Mg alloy. Prior to the electroless nickel deposition, oxide layer was activated by using NaBH4 solution as a replacement for conventional chromate and HF activation processes. EN coatings were prepared using the alkaline carbonate bath. As-prepared coatings were characterized for the surface morphology and composition using Field Emission Scanning Electron Microscopy (FESEM) attached with energy dispersive analysis of X-ray (EDX). Potentiodynamic polarization studies were carried out in non-deaerated 3.5% NaCl solution to find out the corrosion resistance of the coatings. The MAO coating showed porous morphology with micro cracks whereas, MAO/Ni-P coating exhibited dense nodular structure. The cross-Ni-P and MAO/Ni-P/Ni-W-P duplex al images showed good adhesion between MAO and Ni-P layers. This clearly indicates that the present activation process results in dense with uniform pores of MAO coating along with excellent bonding at the interface for Ni-P coat. The MAO/EN coating combination showed two orders improvements in corrosion resistance as compared with the substrate.