Evidence for a Logarithmic Oxidation Process for Stainless Steel in Aqueous Systems

Upon immersion of a passive stainless steel electrode into an oxidizing ferrous‐ferric aqueous solution, the mixed potential of the electrode gradually approaches the reversible redox potential of the solution. During this period, an oxidation process occurs, the rate of which decreases with time. The process is most likely oxidation of metal since the potential range over which it occurs is more active than the estimated reversible oxygen potential for the system, and no other oxidation reaction is likely. The kinetics of oxidation of stainless steel are similar to kinetics reported for anodic oxidation of metals which form thin amorphous oxides. Therefore, it is believed that evidence has been obtained for growth of a similar type of oxide on stainless steel. At constant potential, the oxidation process can be described closely by a logarithmic oxidation equation. Also, the kinetics are consistent with equations which describe ion current through thin oxides as a function of the field across the oxide.