Studies in the electro deposition of manganese

Abstract

The investigations described in the thesis have been directed towards an understanding of the mechanism of electrodeposition of manganese from manganese sulphate–ammonium sulphate solutions and towards an elucidation of the mode of action of the addition agent, selenious acid. The first chapter consists of a survey of the literature on the electrodeposition of manganese and of a statement of the scope of the investigations. The second chapter is devoted to a description of the experimental techniques which have been used, viz., steady?state polarization curves, and galvanostatic and open?circuit decay transients recorded oscilloscopically. The corrosion behaviour of manganese has been studied by weight loss measurements. Chapter II also contains a description of the procedures employed for the pretreatment of electrodes, and for the pre?electrolysis and de?oxygenation of solutions. The presentation of the experimental results in Chapter III is divided into three main parts. The first part deals with the results of steady?state polarization measurements obtained with variations in pH, and in the concentration of selenious acid and of manganous ions. Current efficiency determinations have been used to resolve the total current?potential curves into partial curves which have revealed distinct regions corresponding to selenium deposition, hydrogen evolution and manganese deposition. Stirring the solution during the recording of the polarization curve has been shown to yield a well?defined limiting current for selenium deposition; in contrast, the polarization curves for hydrogen evolution reaction and manganese deposition are virtually unaffected. The galvanostatic potential?time curves reported in the second part of the chapter are characterized by the existence of a clear?cut transition time, followed by a slow build?up of potential which finally attains a plateau. These regions can be identified with the cathodic reactions of selenium deposition, hydrogen evolution and manganese deposition. The third part of the chapter consists of the results on the open?circuit potential?decay transients which are characterized by a potential arrest which has been attributed to manganese corrosion. A critical discussion of the above experimental results is attempted in Chapter IV. (1) The order of the reaction, the lack of dependence of the exchange current for manganese deposition on the pH of the solution, the pH?independence of the corrosion current, the cathodic and anodic Tafel slopes and the stoichiometric number have been argued to be consistent with a consecutive electron transfer type mechanism (Mn²? + e? ? Mn?; Mn? + e? ? Mn) with the first step as the rate?determining step. (2) The IR?drop discovered to be associated with the hydrogen evolution reaction region of the i?E curves has been ascribed to the presence of a film on the stainless steel cathode. (3) On the basis of the Tafel lines for hydrogen evolution reaction with and without selenious acid, it has been shown that the addition of selenious acid leads to a decrease in the exchange current for the hydrogen evolution reaction on stainless steel. (4) The presence of the addition agent enhances the IR?drop arising from the film on the cathode. These conclusions regarding the effect of selenious acid as an addition agent are consistent with the resulting increase in current efficiency for manganese deposition and the decrease in the corrosion rate of manganese. (5) It has been proved that while selenium deposition occurs under diffusion control, hydrogen evolution reaction is under activation control. (6) For the hydrogen evolution reaction, high Tafel slopes have been observed after accounting for the IR?drop. This high Tafel slope has been attributed to the influence of the metal?semiconducting oxide film junction on the current?potential relation. (7) A new method for current efficiency measurements in the case of non?passivating metallic deposits has been developed in the present work. The method is based on the fact that the time of arrest in the open?circuit potential?decay transients is related through the corrosion current to the weight of metal which had been deposited galvanostatically prior to the decay. It has been demonstrated that there is good agreement between the current efficiencies calculated by the open?circuit decay method and the conventional method based on weight gain measurements. (8) The exchange current for the hydrogen evolution reaction has been calculated from the galvanostatic transient curves assuming that the current is used for double layer charging as well as for a simultaneous activation?controlled, totally irreversible H? discharge reaction. The exchange current values obtained by the various methods have been shown to be in good agreement. (9) The lack of constancy of ? for selenium deposition has been ascribed to simultaneous oxide reduction. The study has thus led to an understanding of the mechanism of electrodeposition of manganese as well as of the mode of action of selenious acid used as an addition agent in the above process. A novel method has been developed for current efficiency determination.