A review of pretreatment techniques for electroless nickel-phosphorus plating on magnesium alloys with enhanced corrosion resistance and mechanical properties

Magnesium alloys, known for their high hardness-to-density ratio, have significant potential in aerospace, automotive, and other industries but face challenges like high corrosion susceptibility and low hardness. Electroless nickel-phosphorus (Ni-P) plating can address these limitations but requires suitable pretreatment to manage high electrochemical activity, surface heterogeneity, and a loose oxide/hydroxide layer. This article reviews various pretreatment methods for electroless plating of the magnesium alloys, discussing their application procedures and the properties of the final electroless coatings. It emphasizes the significance of pretreatment in overcoming the challenges of magnesium alloys, ensuring uniform, adhesive, and corrosionresistant Ni-P coatings. Extensive research has focused on various pretreatment methods, including zincating, non-electrical conversion coatings (chromating, phosphating, rare elements, titanate, zirconium, and hydroxide), electrical conversion coatings (anodizing and PEO), etc. The pretreatments aim to enhance the properties of the final electroless coatings, primarily targeting anti-corrosion performance. Electroless Ni-P coatings on pretreated magnesium alloys typically exhibit a nodular, cauliflower-like morphology with phosphorus content ranging from about 3.78-9.6 wt%. The corrosion current of the applied electroless coatings depends on pretreatment, thickness, porosity, and the corrosive medium. Most studies have used 3.5 wt% NaCl as corrosive media, revealing corrosion current densities from 0.0102 to 22.47 mu A cm-2. The strategic importance of magnesium alloys and the advancements in pretreatment methods highlight the ongoing efforts to enhance their practical applications while addressing environmental concerns.