In this work the changing structure of nickelphosphorus deposits on aluminium and its alloys at the early stage of electroless nickel phosphorus deposition using hypophosphite ion as reducing agent has been studied. Prior to electroless nickel deposition, zincating is used for pre-treatment of aluminium substrates. The surface morphology and structure of the electroless Ni-P layers were characterized by scanning electron microscopy and X-ray diffraction analysis. Results show that Ni-P deposition is closely related to the dissolution of the zincating layer, followed by progressive nickel nucleation. The nuclei serve as a catalytic surface for further Ni-P deposition which increases with deposition time. The growth and coalescence of the nuclei on the aluminium substrate results in crystalline layers of Ni-P.

The growing demand for chromium coated aluminium components especially for the automotive industry is due to their favourable physical properties (density, strength to weight ratio etc.). However, their frequent use under harsh environmental conditions renders them corrosion sensitive and consequently they need to be protected. An approach that has been applied in industry is to directly electroplate nickel onto aluminium substrate prior to a top metallic finish; however, in components with complex geometry, certain areas could become exposed to corrosion attack due to poor surface coverage during plating. In this study, a modified electroless nickel undercoat was applied to pre-treated aluminium alloys prior to duplex nickel and chromium plating with a view to enhance corrosion resistance, improve coating adherence and durability, and overall, to achieve substrate protection. Hexavalent and trivalent chromium were applied to pre-treated Al 1050 and Al 6061 following electroless nickel deposition, and plating performance was assessed by surface and corrosion techniques, while durability was measured by scratch, adhesion and hardness tests. Overall, while chromium plating with an electroless nickel undercoat did not improve corrosion resistance or hardness of the materials, it provided an additional protective layer for the substrate with a potential for longer term durability.