Electroless Ni–P–B coatings on magnesium alloy AZ91D: influence of nano Al2O3 on corrosion, wear, and hardness behaviour

Abstract

This study examines the effect of nano Al2O3 additions on the corrosion, wear, and hardness properties of an electroless Ni–P–B protective coating on AZ91D magnesium alloy. In this work, nano alumina particles with a diameter of 40–50 nm and a weight percentage of 1, 2, and 3% were used in an alkaline Ni–P–B electroless bath. The surface morphology of the nano-composite coating was studied by scanning electron microscopy (SEM) and the crystalline structure of the coating was examined by x-ray diffractometer (XRD). The inclusion of alumina nanoparticles, an electroless deposit produces a homogeneous coating on the AZ91D magnesium alloy surfaces. The experimental results show that incorporating 2 wt% alumina nanoparticles into an electroless Ni–P–B coating increases microhardness (986.4-VHN200), specific wear rate (1.2 × 10–10 Kg N−1m−1) and has the lowest average friction coefficient (0.08) when compared to other coated and uncoated samples. Electrochemical polarisation and electrochemical impedance spectroscopic (EIS) tests in 3.5 wt% NaCl solution were used to investigate the corrosion resistance of various nano Al2O3 additive Ni–P–B coatings on Mg alloy surfaces. Potentiodynamic polarisation studies show that 2 wt% of nano Al2O3 incorporation in Ni–P–B coating on magnesium substrates has good corrosion resistance, with the coating resulting in the lowest corrosion rate (icorr = 1.47 × 10–8 A cm−2) and the most positive corrosion potential (Ecorr = 0.55 V).