Effect of Pickling and Activation Treatments on the Performance of Electroless Plating of Ni-B Coating on AZ31 Mg Alloy

Abstract

Pickling is particularly critical before Ni-B electroless plating to create a uniform and catalytically active surface.  In the present work, pickling treatment of AZ31 Mg alloy was performed in phosphoric acid followed by immersion in 40% HF.  The effect of concentration of both H₃PO₄ and HF as well as the effect of time were investigated.   The pickled samples were then electroless plated with a Ni-B composite.  The performance of the Ni-B composite coating was investigated under the SEM.Results show that the highest quality coating was obtained upon pickling AZ31 Mg alloy samples in 50% phosphoric acid for 1 min, followed by activation in 40% HF for 15 min. Higher concentration of H₃PO₄ results in a higher concentration of PO₃⁻⁴, leading to a higher tendency to produce insoluble films (mainly Mg₃(PO₄)₂ and AlPO₄) on the substrate surface slowing down the rate of Mg alloy oxidization and dissolution.  Activation of pickled AZ31 Mg samples in HF results in the formation of corrosion resistant dense layer of magnesium fluoride. SEM investigation predicted the electroless deposition of an adherent compact layer of Ni–B composite coating on the H₃PO₄/HF pickled AZ31 samples.

Key Words: AZ31 Mg alloy, pickling, activation, HF, electroless plating

DOI: 10.7176/CMR/11-5-04

Publication date:May 31^st 2019