The evaluation of polarization parameters as quality predictors for zinc electroplated from acidic sulphate solutions

Abstract

Successful commercial electrowinning and refining of metals typically requires the optimization of the electrolyte composition and operating parameters to produce high quality compact electroplated metal. The prediction of the structure and morphology of the plated metal from the operating parameters remains a challenge. The aim of this work was to develop a reliable galvanodynamic technique to measure nucleation and plating potentials that was specifically applicable to the plating of zinc from sulphate electrolytes, with which plant operation may be controlled and optimized. The work was focussed on determining whether there would exist a definite unique or range of values for both the nucleation and plating potentials for which it would be possible to obtain high quality zinc electrodeposits repeatably. Such a tool could be modelled and be particularly useful in plant situations to suggest remedies for process upsets and development. In the current study a repeatable and reliable galvanodynamic polarization technique using two scan rates was developed to perform the polarization measurements. The values were used to predict the nature and morphological characteristics of the metal plated from the respective acidic aqueous zinc sulphate electrolytes The developed galvanodynamic technique was found to be an effective and reliable technique to measure nucleation and plating potentials systematically and repeatably. The results indicated that nucleation and plating potentials could be obtained with reasonable repeatability of ±3mV. These potentials provided useful data and information on the electrowinning processes and the effect of changes in parameters and conditions of zinc deposition on the morphology. The repeatability and relevance of the potential measurements were strongly dependent on strict adherence to consistency in electrode preparation procedures, electrolyte preparation and control of experimental conditions. . The relationship between initial nucleation of zinc on an inert aluminium substrate and further growth provided a useful framework for relating the physical changes in the structure of electrodeposits. The difference between the nucleation and plating potentials was observed to be an indicator of the type, quality and characteristics of the deposits formed, and could be used to identify a unique region which is considered to be the region in which the most desirable quality deposit will be obtained. However the idea suggested by Adcock et al. (2002), that nucleation is favoured over growth if the plating potential is more negative than the nucleation potential ΔE, and should therefore result in unoriented dispersion (UD) type deposits turned out to be only partly so with such deposits rather obtained for ΔE values around zero and with field oriented texture (FT) and basis reproduction (BR) type deposits obtained at more positive as well as negative values. The effect of temperature on the plating characteristics was not significant. This may have been attributed to the influence of hydrogen ion reduction on the potential whereby a substantial current flow and a shift of potentials to positive values is observed, but not necessarily due to zinc plating. Higher temperatures showed a considerable reduction in current efficiency which is a consequence of high rates of hydrogen reduction. This indicated that the approach to characterise zinc plating by polarization characteristics would probably not be valid for conditions where significant rates of hydrogen ion reduction occur. The study indicated that grain refiners influence the nucleation process while levellers change the growth process. The difference between the plating and nucleation potentials ΔE was found to be an indicator for identifying and predicting grain refinement but not as much as in the case with levelling. This limits the usefulness of the two dimensional representation of the polarization parameters, such as suggested by Adcock, for the evaluation of additives for zinc electrowinning from sulphate electrolytes.