Structure and Codeposition Behavior of Ni-Mo Alloys from Ammoniacal Tartrate Baths.

Abstract

The structure of Ni-Mo alloys electrodeposited from ammoniacal tartrate baths was discussed in terms of their codeposition behavior. The results obtained are as follows:(1) With increasing Mo content in the alloys, the partial current efficiency of Mo first increased and then decreased after a maximum was achieved. This showed the existence of two characteristic alloy compositions, i. e., a Mo content at a maximum partial current efficiency of Mo and one at the upper limit of Mo deposition.(2) Alloys of lower Mo content were found to consist of α-Ni solid solution by X-ray diffraction analysis and the diffraction peaks became broader with increasing Mo content.(3) From SEM observation of the surface appearance of the deposits, the most compact and smooth deposits were found in alloy which gave the maximum partial current efficiency of Mo. With a further increase in Mo content, blackish deposits were observed in the alloy.(4) TEM study also revealed that the main phase of the electrodeposited Ni-Mo alloys was α-Ni. When the Mo content exceeded the value of the maximum partial current efficiency of Mo, the X-ray diffraction patterns of the deposit showed broad and diffuse peaks similar to those from an amorphous phase. However, the alloys were found from TEM observation to consist of α-Ni crystallites smaller than 30nm in diameter and fine particles of MoO2 crystals. This finding supports the multi-step reduction mechanism based on intermediate Mo (IV) compound in the codeposition of Mo with Ni.