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

Ni-W alloys were electrodeposited onto steel sheets with an initial Ni plating. The electrodeposition was conducted in unagitated sulfate solution containing citric acid at pH 5 and 60°C under coulostatic (9.0 × 105 C•m^-2) and galvanostatic (100-5 000 A•m^-2) conditions. The effect of the initial Ni plating on the structure and hardness of the deposited Ni-W alloys was investigated before and after annealing. At all current densities and in the presence and absence of the Ni plating, Ni4W and NiW were precipitated in the annealed deposits. Without initial Ni plating, the Ni in the deposits diffused into the steel substrate during annealing, increasing the W content near the steel substrate. Consequently, many large precipitates were found in that vicinity. With initial Ni plating, the Ni in the plating diffused into the Ni-W deposits during annealing, and the W content of the deposits decreased around the plated Ni. The precipitates in the Ni-W deposits with initial Ni plating were finer than those without initial Ni plating. Before annealing, the W content in the deposits was lower in the initially Ni-plated sheet than in the unplated sheet. During annealing, Ni diffused from the plated Ni to the Ni-W deposits, apparently repressing the formation of large Ni4W and NiW precipitates. The fine, uniformly deposited Ni4W and NiW precipitates increased the hardness of the deposited Ni-W alloys after annealing.