Solution-Side Transport Processes in the Electropolishing of Copper in Phosphoric Acid

The unsteady‐state anodic dissolution of copper in 5–10M phosphoric acid was carried out in the absence of convection, under galvanostatic and potentiostatic conditions. At a given bulk concentration of , the product of current density, , and the square root of the time of onset of sharp potential rise (or abrupt decline of current) is nearly constant in the active dissolution range. decreases with increasing acid concentration. The transition times found are in substantial agreement with those reported by Elmore and Edwards. By using the most reliable diffusivity data available for phosphoric acid and for anodically formed copper phosphate, it is shown that the rate‐limiting step in the active dissolution regime is the transport of copper phosphate from the electrode surface into the bulk solution. The potential jump in galvanostatic dissolution, or the peak current phenomena reported by numerous authors for steady‐state potentiostatic dissolution can be best interpreted by assuming rapid increase of coverage of the anode surface with solid reaction product after the critical solubility limit of copper phosphate is reached at the surface. The concentration of phosphoric acid at the surface up to and including the peak current density where the critical solubility of copper phosphate is exceeded, remains finite. The magnitude of the peak current density can be predicted by considering the transport of copper phosphate away from the surface to be the limiting transport process.