Eco-Friendly Surface Pretreatment for Electroless Ni-W-Sn-P Coating on Micro-Arc Oxidized AZ91D Magnesium Alloy: Evaluation of Corrosion Performance and Coating Properties

Abstract

This investigation presents a sustainable surface modification strategy to enhance the functional performance of AZ91D magnesium alloy. A duplex coating architecture was engineered, by integrating micro-arc oxidation (MAO), followed by an eco-friendly pretreatment and subsequent direct electroless nickel (DEN) plating to improve the surface protection of the alloy. Binary (MAO-Ni-P), ternary (MAO-Ni-W-P, MAO-Ni-Sn-P), and quaternary (MAO-Ni-W-Sn-P) alloy coatings were systematically deposited onto the substrate. The coated surfaces were characterized for morphological features and elemental composition using field emission scanning electron microscopy (FE-SEM) and energy-dispersive x-ray spectroscopy (EDS), respectively. Quantitative assessment of surface texture was undertaken via profilometric roughness measurements, while three-dimensional surface topography was examined with atomic force microscopy (AFM). Electrochemical behavior, including corrosion resistance, was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. The quaternary alloy coating demonstrated a marked improvement in surface smoothness, approximately 5.6-fold compared to the uncoated alloy, and exhibited superior electrochemical corrosion resistance, indicating substantially enhanced protective efficacy relative to bare AZ91D magnesium alloy.