The Nature of the Film Formed on Copper during Electropolishing

The thin film shown by Hoar and Farthing to exist on the surface of copper during electropolishing in orthophosphoric acid has been found, by electron diffraction methods, to be composed of a phosphate of copper. This evidence would appear to confirm the basic assumption of Elmore's theory of electropolishing of a limit to the solubility of copper in the boundary layer of electrolyte. Since the film is soluble in phosphoric acid in the absence of an applied potential, this limit is attributed to a decrease in hydrogen ion concentration at the anode surface during polishing.