Electrochemical deposition of Ni–W alloys from ammonia–citrate electrolyte

Ammonia–citrate electrolyte is one of the solutions that are used in practice for electroplating Ni–W alloys. In this contribution, a systematic effort was made to clarify some controversies about the mechanism of the deposition reaction. Different complex species are found in the electrolyte, the most important for the alloy deposition process being the protonated tungstate–citrate complex and ammonia–citrate complex of Ni. No complex ions containing both Ni and W together could be detected by the employed (spectrophotometric) method. A critical point was found in the Tafel diagram at which W starts depositing and hydrogen evolution subsides. At potentials more positive than that, the presence of the tungsten oxide ∣ hydroxide layer controls all the reactions at the surface. The critical point coincides with Ni-deposition passing from activation into diffusion/reaction control, which usually leads to rough, or even powdery deposits. This leads to a suggestion that the sudden beginning of W codeposition may be due to a closer approach of the complex tungstate–citrate ion from the solution to freshly deposited Ni particles sticking out and electron tunneling from those into the complex with a resulting reduction of W to the metallic state.