2015 Volume 101 Issue 11 Pages 590-597
Electrodeposition of Ni-W alloys was conducted from an unagitated sulfate solution containing citric acid at pH 5 and 60 °C under coulostatic (9.0×105) and galvanostatic (100-5000 A·m–2) conditions onto steel sheets with an initial Ni plating, and the effect of initial Ni plating on the structure and hardness of deposited Ni-W alloys was investigated before and after annealing. The precipitates of Ni4W and NiW were observed in deposits obtained at all the current densities after annealing, irrespective of having undergone initial Ni plating or not. Without initial Ni plating, a lot of large precipitates occurred in the vicinity of steel substrate, while the fine precipitates occurred with initial Ni plating. Without initial Ni plating, the W content in deposits increased in the vicinity of steel substrate due to diffusion of Ni in deposits into steel substrate during annealing. With initial Ni plating, the W content in deposits in the vicinity of Ni plating decreased due to diffusion of Ni in Ni plating into deposits during annealing. Before annealing, W content in deposits was lower with initial Ni plating than that without Ni plating. With initial Ni plating, W content in deposits was lower and Ni diffused from Ni plating to deposits during annealing, as a result, the formation of large precipitates of Ni4W and NiW seems to be suppressed. The hardness of deposited Ni-W alloys after annealing increased when the precipitates of Ni4W and NiW became uniform and fine.
