Effect of Initial Ni Plating on the Structure and Hardness of Electrodeposited Ni-W Alloys with and without Annealing

摘要

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℃ under coulostatic (9.0 × 10~5 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, Ni_4W 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 Ni_4W and NiW precipitates. The fine, uniformly deposited Ni_4W and NiW precipitates increased the hardness of the deposited Ni-W alloys after annealing.