MASS-TRANSPORT EFFECTS DURING ELECTROPOLISHING OF IRON IN PHOSPHORIC SULFURIC ACIDS

Anodic dissolution of iron in a mixture of phosphoric and sulfuric acids has been investigated using a rotating disk electrode. Potentiostatic and potentiodynamic anodic polarization studies were performed to determine experimental conditions that lead to electropolishing. Results indicate that the onset of electropolishing corresponds to a mass transport controlled limiting current plateau. An appreciable change in the water content of the electrolyte had little influence on the measured limiting current. On the other hand, addition of small amounts of Fe3+ ions in the electrolyte tremendously reduced the limiting current. These data strongly suggest that electropolishing is due to formation of a salt layer at the anode that involves rate limiting transport of dissolved Fe3+ ions from the anode surface into the bulk electrolyte.