Comparative study of microstructure and corrosion resistance of electroless Ni-W-P coatings treated by laser and furnace-annealing

A comparative study of amorphous electroless Ni-W-P coatings on mild steel substrate treated by a high power diode laser and furnace annealing was presented. Effects of different laser operating parameters and furnace-annealing conditions on microstructures, in terms of crystallisation, pores formation and grain growth, were investigated using SEM/EDX and XRD. Corrosion behaviours of these coatings before and after various treatments were evaluated with anodic polarisation in 0.5 mol/L H2SO4 solution. The results show that the furnace-annealing produces either a mixture of nanocrystallined Ni and amorphous phases or precipitated Ni3P phase distributed in nanocrystallined Ni-based matrix, depending on annealing temperatures, whilst the laser treatment under the operating conditions only produces nanocrystallined Ni-based matrix with Ni3P precipitates. Corrosion performance of the coatings treated by both the laser and the furnace-annealing is dependent on the annealing temperature and laser operating conditions. Corrosion mechanisms of various treated-coatings were discussed in the consideration of phase constitutes and proportion, grain sizes of both Ni and Ni3P phases, pores formation and residual stresses.