Oxidation behavior of electroless Ni-P, Ni-B and Ni-W-B coatings deposited on steel substrates

High temperature oxidation behaviors of electroless Ni-P, Ni-B and Ni-W-B coatings that are deposited on mild steel substrates were investigated in a temperature range of 600-800 degrees C. Oxide films formed on the surfaces of the coated samples were characterized by X-ray diffraction analyses. Oxidation behaviors of the coatings were evaluated by monitoring the oxide character and iron diffusion through the structure by glow discharge optical emission spectroscopy (GDOES). The results indicated that oxidation properties of the coated steel depend not only on the coating itself, but also on the diffusion and interdiffusion processes, which take place between the substrate and the deposit. For Ni-P deposited steel, iron started to take part in the oxide structure during treatment at 700 degrees C, resulting in a loss of protective character of the oxide. Depletion of boron within the coating and formation of nickel borates were observed for B containing deposits. Diffusion of iron to the surface also took place at 700 degrees C for Ni-B coatings, but not as drastic as for Ni-P. Oxidation behavior of Ni-W-B coatings was similar to Ni-B coatings in general. However, the presence of W decreased the diffusion of iron to the surface and delayed the formation of iron oxides. At 800 degrees C, all three coatings started to degrade due to higher oxidation rate and iron diffusion to the surface. The results of this study showed that it is possible to increase the oxidation resistance of electroless nickel coatings by alloying them with W. (C) 2015 Elsevier B.V. All rights reserved.