Effect of Annealing Temperature on the Corrosion Resistance of Electroless Produced Ni-B-W Coatings

Abstract

The present work deals with the formation of Ni-B-W coating on steel by electroless plating process and evaluation of their corrosion resistance after applying heat treatments at different temperatures for 1 h. The Ni-B-W coating was prepared using alkaline borohydride- reduced electroless nickel bath. Scanning electron microscopy of the surface cross-sectional view of the electroless Ni-B-W coating was analyzed and layer characteristics was investigated. Coating structure was investigated using XRD. The study reveals that the Ni-B-W coating is amorphous in their as-plated condition and upon heat treatment at 400 0C for 1 h, Ni-B-W coating crystallize and produce nickel and nickel borides in the coatings. Annealing temperature dependence of the corrosion resistance of the coating was investigated by potentiodynamic polarisation measurements. These results show that the Ni–B-W coating annealed at 650 0C exhibit better corrosion resistance than those of coatings with other annealing temperature. The corrosion resistance increased after the crystallisation of the coating, due to factors like; decrease of porosity and internal stress and the formation of tungsten oxide on the surface acting as a protective layer.