Effect of initial Ni plating on the structure and hardness of electrodeposited Ni-W alloys with and without annealing

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.