The Influence of Surface Conditions on the Electropolishing of Metals

There has been a great deal of work published regarding the theoretical aspects of diffusion phenomena in the liquid layer, formed at the metal/solution interface during electropolishing, but a full explanation of the relationship between this layer and any solid surface film, and the possible practical implications of such studies is yet to be successfully achieved. It was therefore desired to obtain an improved understanding of the electrochemical and physical surface conditions which support the process of electropolishing. Aluminium and stainless steel are probably the materials which are used most widely in industrial electropolishing practice and have therefore been selected as subjects for examination in the present work. Potentiostatic polarisation curves have been obtained for a wide variety of electropolishing solutions. Their interpretation has provided evidence regarding the changes occurring at the metal surface when controlled at various potentials. A factor which can be shown to be of particular significance is the degree of polarisation of the oxygen evolution reaction, which is influenced by the concentration of the electrolyte and by dissolved metal. This feature of the polarisation curves has been tentatively correlated with the incidence of pitting defects due to gas evolution. Capacitance measurements on stainless steel have been successful in demonstrating the existence of a solid film on the specimen surface after electropolishing, Cyclic voltametry, used during electropolishing has shown that the film is formed as a result of the polishing process. Valency changes occurring within the solid films have also been investigated using this method. Transmission electron microscopy has been employed to examine the morphology of the solid surface films. For aluminium the _type of film formed depends mainly on the properties of the electrolyte in imparting high ionic conductivity to the film. Since the film is so thick the role of the liquid diffusion layer appears to be restricted. In the case of stainless steel the film material seems basically similar in morphology to the oxide films formed in air and in passivating conditions. Similarities and contrasts between the anodic polishing of steel and the bright electrodeposition of chromium have been assessed in terms of random processes occurring through surface films. The behaviour of hexavalent chromium is important in each case. Electropolishing characteristics are found to be determined by the degree of modification of cationic conduction, resulting from the valency changes occurring in the chromium-bearing solid film. In most cases established theory is confirmed in that effective polishing requires a critical balance between the solid film and the liquid layer. The relative importance of these factors can be exemplified when specimen behaviour with or without a solid film present can be compared in the same electrolyte. A theory is proposed to explain the origin of gas pits, which to date have been ascribed to the presence of adherent gas bubbles. This has enabled general principles to be founded regarding the possible methods of preventing or curing gas pitting in practice