Electrocrystallisation kinetics

As Girginov et al have pointed out [15], very little attention has been paid to the electrocrystallisation kinetics of refractory metals and factors influencing the process, which are fundamental for the monitoring of the process. Nevertheless, we have recently reported chronoamperometric investigations on niobium electrocrystallisation [20] and concluded that the nucleation is instantaneous and further growth of the nuclei is controlled by diffusion. This situation favours an unequal current distibution when high current densities are applied to the cathode and so it is conducive to the inequality of the thickness of the plate. 

Vetter. K. J.. Electrochemical Kinetics, English edn.. Academic Press. New York (1967) Bockris. J. O M. and Razurnney. G. A.. Fundamental Aspects oj Electrocrystallisation, Plenum Press (1967)... 

A literature survey shows that the corresponding data are rather contradictory, as far as the solubilities of CrCk and CrCls are concerned, and as far as the Cr(II) / Cr deposition step is concerned. In fact, the kinetics of this step has been reported to be fast, or slow, and there is some agreement to think that this step is complicated by adsorption or electrocrystallisation phenomena. In this talk, we will tackle the questions of the chromium chlorides solubility, of the Cr(III) or Cr(II) adsorption on various substrates and the kinetics of the final electron exchange. 

Although, in practical terms, the last of these steps is of most interest, it is necessary to look more closely at the earlier stages of the electrocrystallisation process in order to gain a fundamental understanding of the thermodynamics and kinetics of electrocrystallisation. Unfortunately, the complexity of commercially important electrodeposition processes has restricted attempts at satisfactory... 

The kinetics of heterogeneous nucleation differs from the treatment given above, since nuclei may form at preferred or active sites on a surface. The value of AGn therefore depends on the nature of the substrate, and, in the case of electrocrystallisation, on the electrode potential. If we define the number of active sites under particular experimental conditions as TVq, the rate of appearance of stable growth centres is expected to follow first order kinetics, with the number density of centres given by... 

Bewick, A., Fleischmann, M. and Thirsk, H.R. (1962) Kinetics of the electrocrystallisation of thin films of calomel. Transactions of the Faraday Society, 58, 2200. 

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