Effect of Heat Treatment on Corrosion Resistance of Electroless Duplex Ni–P/Ni–B Coatings

Abstract: As-plated and heat-treated Ni–P, Ni–B, and Ni–P/Ni–B coatings (Ni–P as an internal layer) on steel by electroless plating and their morphology, microstructure, and corrosion performance were evaluated in this study. Scanning electron microscopy analysis demonstrated that all coatings are uniform and adhesion between the substrate and coating was good. Ni–P and Ni–B coatings were amorphous-like structures in their as-plated condition, and by applying heat treatment nickel fully crystallized, nickel borides and nickel phosphides were formed. Immersion tests in 10% HCl and 5% H₂SO₄ solutions and potentiodynamic polarisation measurements in 3.5% NaCl aqueous solution were applied to investigate the corrosion resistance of the coatings. The results demonstrated that all coatings exhibit better corrosion performance than the substrate steel. Applying heat treatment did not change the corrosion resistance of Ni–P coating, conversely, heat treatment had a dominant positive effect on the corrosion performance of Ni–B and a minor effect on Ni–P/Ni–B duplex coatings.