Anodic oxidation of ferrous ion on passive iron

Abstract Anodic polarization curves of passive iron electrodes were measured in deaerated boric-acid-sodium-borate solutions (pH 6-9) to which ferrous ions were added in various concentrations. A limiting current was observed in the passive region; it increased linearly with [Fe 2+ ]. For constant [Fe 2+ ] the limiting current decreased almost linearly with decrease of pH and at pH 5·8 reached a small value corresponding to the “passivity maintaining current” which would be observed in a solution without ferrous ion addition. The net electrode reaction corresponds to the oxidation of ferrous ion in which hydrous ferric oxide is formed and is eventually converted by dehydration into “passive oxide” having an electronconducting character. The rate of reaction appears to be controlled by the diffusion of hydrolysed ferrous ion through the hydrous oxide layer, which is at the outermost part of the film. The existence of this hydrous layer was confirmed by a cathodic reduction experiment combined with a radiochemical technique with tritiated water. The decrease of the limiting current with [H + ] is explained by assuming a change of the concentration of hydrolysed ferrous ion in the solution and the change of the charged state of hydrous oxide.