Electrodeposition of Molybdenum Alloys

A phenomenon of induced codeposition, similar to that discussed above for W, is observed when Mo is codeposited with iron-group metals. Similarly to tungsten, molybdenum cannot be deposited alone from aqueous solutions. Electrodeposition of Mo alloys exhibits similar dependencies on experimental variables as that of W. It should be noted that, although the two systems are very similar, some differences are found in the literature, as described bellow. 

An early comprehensive review on electrodeposition of Mo alloys with iron-group metals was presented by Brenner. The development of different baths, as well as the effect of operating conditions on the Mo content of the alloy is described in detail in that work. Electrodeposited alloys of Mo were claimed to be of limited practical value, because of their poor physical characteristics compared to the corresponding alloys of W. 

Newer studies have demonstrated that the properties of Ni-Mo alloys can be improved by application of pulsed electrodeposi-tion. Compared to DC electrodeposition, a higher Mo content in the alloy was obtained, which was accompanied by reduced residual stresses, improved mechanical properties and improved corrosion resistance. 

In addition to the effects of convection discussed above, it was found that increasing the concentration of the M0O4 ion, the concentration of Ci (up to a limit of 0.75 M) or the temperature resulted in an increase in the Mo-content in the deposit over a wide range of applied current densities. The concentration of ammonia was also found to have a significant effect—as it was increased 

On the basis of the observations described above, it was postulated that the precursor for the deposition of the Mo-Ni alloy was an adsorbed intermediate mixed-metal complex of the form [Ni-Cit(Mo02)]. This intermediate can be reduced, thus allowing Mo deposition. The Ni + ion, complexed either with Cit or with NH3 (as discussed earlier for W codeposition), can be reduced in parallel with Mo, following the simple equations 

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