Characterization and corrosion properties of electrodeposited Ni-W alloys

Ni-W alloys electrodeposited from citrate solution consist of three different phases, which are all present in high W-content alloys: a solid solution of W in a Ni matrix; an intermetallic compound Ni4W, as well as another solid solution of Win Ni with a W content higher than 20 mol.%. XPS analysis revealed that the alloys were covered with a surface layer of complex structure containing the pure metals Ni and W, Ni(OH)2 andWO3, on the very surface as well as some partially reduced oxideWO2.72 (most probably a solid solution of WO2.72 in Ni) and tungsten carbide in the layer underneath. It is highly likely that some of the oxide species in the layer act as intermediates in the cathodic deposition process. Identifying these species should be the clue to a more detailed understanding of the mechanism of induced deposition of W than has been achieved so far. Corrosion of Ni-W alloys in sulfuric acid solution at OCP, occurs by the preferential dissolution of nickel from the surface layer. The longer the corrosion process lasts, the more the surface behaves like pure W. The lowest initial corrosion rates were recorded with alloys rich in W, but after aging at OCP the lowest corrosion c.d. was found for the Ni-W alloy with the most homogenous phase structure, consisting of the solid solution only. nema