The role of zincate morphology in electroless Ni-P deposition on AA5052 aluminum alloy

Abstract

The growing need for lightweight aluminum alloys in transportation and aerospace demands advanced surface protection solutions. While electroless Ni-P coating offers excellent corrosion resistance, its performance critically depends on zincating pretreatment - a process whose multi-step optimization remains insufficiently studied. This study investigates the influence of single, double, and triple zincating on the morphology of zincate layers and their subsequent impact on the deposition of electroless Ni-P coatings. AA5052 underwent zincating treatments in 200 g/L NaOH + 10 g/L ZnO solution before electroless Ni-P deposition at 80°C (pH5) in nickel sulfate and sodium hypophosphite. Surface characterization was performed using FE-SEM/EDS, while electrochemical properties were evaluated through EIS in 0.5M H₂SO₄ solution. Key findings demonstrate that double zincating generates an optimal surface morphology, in form of porous microstructure (average pore size of 0.28μm), that enhances Ni-P nucleation. This morphology yields the highest Ni percentage, and corrosion resistance exhibiting the largest arc radius semicircle. Single zincating produces discontinuous deposits (0.33μm granules), while triple treatment creates over-smoothed surfaces compromising adhesion. The study concludes that the porous Zn layer from double zincating optimizes Ni-P coating adhesion and corrosion protection by providing mechanical anchoring sites, whereas excessive pretreatment diminishes nucleation efficacy.