Evaluation of polarisation parameters as predictor of morphology of nickel electrodeposits produced from sulfate electrolyte

Abstract

Industrial electrowinning of nickel requires specific control and optimisation of the electrolyte composition, the presence of impurities, the addition of additives (to achieve particular deposit properties) and the operating parameters (such as pH and temperature) in order to produce electroplated nickel of high quality and desirable morphological characteristics. Without understanding and strict control of the electrocrystallisation process, nickel can delaminate due to internal strain, frequent pitting of the deposit can occur, current efficiency can decrease significantly or dendritic growth can cause short circuits. Prediction of the effect of the electrolyte composition and operating parameters on the structure and morphology of the plated metal in the early stages of electrodeposition could be paramount to controlling and/or eliminating such problems during the later stages of electrowinning. Such prediction remains an enormous challenge. The use of polarisation measurements to investigate the electrocrystallisation process and predict the outcome of the resulting deposit quality and morphology was used with variable success in early investigations. Some of the main problems with techniques such as cyclic voltammetry and the later-developed continuous monitoring techniques are inaccuracy and unreliability of the results. The aim of the present work was to develop a galvanodynamic polarisation technique to investigate the electrocrystallisation process of nickel metal from sulfate electrolyte in order to examine the effect of electrolyte composition, operating parameters and the presence of impurities or additives. This could then be used to optimise these factors and thereby predict the outcome of the quality and morphological characteristics of the produced nickel deposit. The idea was that a relatively easy and concise method needed to be developed that could be implemented industrially to monitor and detect problems in the early stages of electrowinning in order to take control and rapid corrective action if needed. A two-step galvanodynamic method was developed to measure plating and nucleation potentials accurately and repeatably. It was shown that the relationship between the two potentials could be used as an indication of the effect of electrolyte composition and operating parameters on the composition and morphological characteristics of the produced nickel electrodeposits. This method, together with studies on the buffering capabilities of electrolyte solutions, can be used to investigate the influence of additives and impurities industrially introduced during the process in the electrolytes. Typical variations in commercial electrolytes and nickel electrodeposits were evaluated using this developed technique and results compared with those obtained from synthetic electrolyte. The insights gained from this work can be useful to predict and manipulate the electrodeposition process in order to optimise electrocrystallisation and the production of high quality nickel deposits.