Determination of Mo(VI) species and composition in Ni-Mo alloy elating baths by Raman spectra factor analysis

The equilibrium constants of the formation of isopolymolybdates HrMo7O24(6-x)- (x = 0-2), heteropolymolybdate NiMo6O24H64- and Mo(VI)-citrate complexes H-r(MoO4)(2)Cit((7-r)-) and H(r)MoO(4)Cit((5-r)-) (Cit(3-) = C6H5O73-) were determined from factor analysis of Raman spectra and visible absorption spectra to calculate the chemical species present in acidic Ni-Mo alloy plating baths. On the addition of citrate to a Ni(II)-Mo(VI) bath, (i) the polymolybdates were decomposed by the formation of H-r(MoO4)(2)Cit((7-r)-) complex, and then (ii) H(r)MoO(4)Cit((5-r)-) and NiCit(-) complexes formed. This complex formation was necessary for the alloy deposition, but an excess addition of citrate, which results in the formation of NiCit(2)(4-) complex, lowered the current efficiency. The deposition behavior of Ni-Mo alloy from Ni(II)-Mo(VI)-citrate baths was compared with that of pure Ni from Ni(II)-citrate baths, in which concentrations of Ni(II)-species are identical to those in the corresponding Ni(II)-Mo(VI)-citrate baths. The alloy deposition behavior is strongly governed by the ease of Ni deposition, indicating that the electrodeposited nickel induces the deposition of molybdenum. (C) 2000 The Electrochemical Society. S0013-4651(99)08-032-5. All rights reserved.