Microstructure and corrosion performance of laser-annealed electroless Ni–W–P coatings

This paper presents an investigation of amorphous electroless Ni–W–P coatings on mild steel substrate treated by a high-power diode laser. Effects of the laser operating parameters on microstructures, in terms of porosity formation, crystallisation, phase transformation, grain growth, microstrain and residual stress, were investigated using SEM/EDX and quantitative XRD technique. Corrosion behaviors of the coatings before and after laser treatments were evaluated by anodic polarisation in 0.5 M H2SO4 solution and immersion test in 10 wt.% HCl solution. The results showed that the laser treatment produced nanocrystallined Ni phase and Ni3P precipitation with remaining amorphous structure. Corrosion performance of the coatings was significantly improved by the laser treatment. Further, corrosion mechanisms of the as-plated and laser-treated coatings were discussed in the consideration of porosity, integrated intensity ratios of individual phases, grain sizes of both Ni and Ni3P phases, microstrain and residual stress.