A MATHEMATICAL MODEL FOR ELECTRODEPOSITION OF Fe-Ni-SiOZ COMPOSITES

A mathematical model was developed which evaluates the effect of the applied deposition parameters on the composition of deposited Fe-Ni-Si02 composites. This model assumes that the discharge rates of Fe(OH)+ and Ni(OH)+ are proportional to their respective surface coverage fraction. The inclusion of Si02 with the deposited alloys is considered. Introduction Fe-Ni deposition is classified as anomaleous. Under most operating conditions, the less noble Fe has a much higher deposition rate than Ni. According to Dhams and Dhams and Croll (1'2) anomaleous codeposition appears closely related to the local pH rise at the surface. They assumed that an increase in surface pH causes the adsorption of ferrous hydroxide on the cathode which suppresses the discharge of Ni. Romankiw [3] revised Dahms and Croll's mechanism suggesting that a trace amount of Fe+3 in the solution causes precipitation of Fe(OH)3, and that such a film accounts for the selective discharge. Nicol and Phillip (4) who also reviewed other theories of anomaleous codeposition, claimed that the anomaleous deposition of Fe on Fe-Ni accounted for the elevated Fe content of the deposit. However, this model has not been supported by experimental evidence. Andricacos et a]. [5,6] used a rotating ring-disc electrode and a rotating-disc electrode to study Fe-Ni deposition. They found that Fe deposition is under mass transfer limitation, while Ni deposition is kinetically controlled. [7,8] and Hessami and Tobias [9] proposed that the charge transfer of Fe(OH)+ and Ni(OH)+ species is responsible for anomaleous codeposition of Fe-Ni alloys. According to these authors higher Fe(OH)+ concentration, compared to Ni(OH)+, causes the higher iron plating rate. The importance of metal hydroxide ions in the iron-nickel plating system was suggested by Bockris et al. [ 101 and Matulis and Slizys [ 111 on the single metal species deposition. They suggested that the reduction of adsorbed Fe(OH)+ is the rate determining step for the iron reduction. Matlosz et al. [ 121 proposed a model in which the competitive surface adsorption of Ni(1) and Fe(1) intermediates determine their respective deposition rates and alloy deposition. We used galvanostatic pulse and pulse reverse and potentiostatic pulse techniques to study the plating of Fe-Ni alloys in the presence of organic additives [ 13,141. Electrodeposition of the alloy was explained by the eoncentration depletion of reactants and the surface coverage of the additives on the electrode The objective of this work was to develop a mathematical model which will evaluate the effect of the applied deposition parameters on the composition of deposited Fe-Ni-Si02 composites. Talbot et al.