The Phenomenon of Electropolishing

In some electrolytes (e.g., phosphoric acid) an effect opposite to amplification of surface roughness is found upon anodic dissolution, i.e., the surface becomes increasingly smooth, until a mirrorlike finish is achieved. This phenomenon has been widely used for practical purposes of electropolishing, since the work of Jacquet in 1936.  

This indicates the existence of a photosensitive semiconducting film and its possible role in obtaining the electropolishing effect. 

Nevertheless, there seems to be sufficient evidence that electropolishing is an essentially transport-controlled process. Thus, the transition time, r, is observed under galvanostatic conditions, with the Sand product, being constant for a given electrolyte and reciprocal with the square root of its viscosity. Yet the Faradaic efficiency is found to be virtually 100%, and hence there can be no doubt that there is no other process leading to electropolishing except anodic dissolution of the metal. 

A number of reasons have been put forward for the smoothing effect of the anodic dissolution current. Thus, Jacquet ascribed it to the primary current distribution, i.e., higher concentrations of current lines arrive at the more elevated parts of the surface than on the bottoms of recesses. However, although it should exist in principle, this effect cannot be significant under normal conditions of conductivity of the metal and the electrolyte. Moreover, if this model was valid, no dependence on stirring should be observed. 

The Kelvin-type effect has also been forwarded as a reason, since the surface free energy of metal atoms should be larger at peaks of elevations than at flat portions of the surface/ This should be true also of any defects or imperfections caused, e.g., by mechanical treatment of the surface/  

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The phenomenon of decreasing the surface coarseness of a metal upon anodic dissolution under certain conditions is defined as electropolishing. In cases when polishing occurs, the current-voltage curve was found to exhibit a plateau characteristic for diffusion cmitrol of the dissolution process. Some facts point to the complex nature of the phenomenon of electropolishing. 

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