Structure and Codeposition Behavior of Ni–W Alloys Electrodeposited from Ammoniacal Citrate Solutions

The microstructure of Ni–W alloys electrodeposited at from ammoniacal citrate solutions was discussed in terms of their codeposition behavior. When the W content in the alloys was increased, the partial current efficiency of W first increased toward a maximum value and then decreased. This showed the existence of two characteristic alloy compositions, i.e., a W content at a maximum partial current efficiency of W and an upper limiting content of W. The deposition behavior of Ni–W alloys was explained by the multistep mechanism of the reduction of the electrochemically reduced W(IV) oxide to W by atomic hydrogen adsorbed using the unpaired 3d electrons of the freshly deposited Ni. X-ray diffraction (XRD) analysis revealed that alloys with a low W content consisted of a Ni solid solution, and the diffraction peaks became broader with an increase in W content. Ni–W alloys with high W contents showed a smooth surface composed of fine grains. Transmission electron microscopy studies revealed that the major phase in Ni–W alloys was the Ni solid solution, and the XRD pattern of the Ni–53.9 mass % W alloy revealed a halo pattern characteristic of an amorphous structure. This alloy consisted of crystal grains smaller than in diameter.