Induced electrodeposition the NiMo system

A special case is observed for the electrodeposition of molybdenum and tungsten from molybdate or tungstate solutions. Deposition of the metallic form is not possible from a solution of the pure molybdate or tungstate ions. Only oxide layers of pure quality are formed. But a deposition is observed in combination with metals of the iron group. These alloys, especially NiW and NiMo, have become of interest because of their mechanical properties like hardness or wear resistance. Therefore, they are discussed as substitutes for hard chromium. 

The authors found in their study of the partial current of Ni and Mo that the partial current of Ni deposition increases with the total current with increasing negative potential but has only a marginal dependence on the rotation rate. The molybdenum content of the deposit is independent of the total current density but increases with the rotation rate (diffusion limitation). 

The partial current densities of both components only slightly depend on the current density after some irregularities at low current densities but are both proportional to the rotation rate. The composition is, however, independent of the rotation rate. 

The authors explained the experimental results by catalytic action of the citrate (Ci) with the following mechanism  

The complex on the electrode surface induces the catalytic reduction of the molybdate to the metallic state releasing the unchanged Ni-complex. The Ni-complex is catalytic active. The mechanism was used as a basis for a simulation of the deposition. 

---