AN ELECTROCHEMICAL STUDY OF THE ATMOSPHERIC CORROSION OF IRON .2. CATHODIC AND ANODIC PROCESSES ON UNCORRODED AND PRE-CORRODED IRON

Redox processes occurring on uncorroded iron, and on iron pre-corroded by atmospheric exposure for 30 days, have been studied by voltammetry utilising a thin-film electrolyte in the likely potential and pH ranges present during atmospheric corrosion. On pre-corroded iron, oxidation of Fe(OH)2 or Fe2+ to FeO(OH), depending on surface pH, occurs as a passivation-type phenomenon and stifles further anodic activity at relatively anodic potentials (> -160 mV(SCE) at pH 5.5 and almost-equal-to 60 mV(SCE) at pH 3.5) such as occur towards the final stages of drying in a wet-dry atmospheric cycle. On uncorroded iron, free Fe2+ species were found during cathodic sweeps after anodic polarisation. These were absent on pre-corroded iron electrodes confirming their consumption during the production of a solid oxide. On uncorroded iron, no passivation was found up to 0 mV(SCE) which suggests either that pre-existing solid oxides are required to initiate passivation either by taking part in the reaction or by acting as nucleation sites for growth of new oxide during passivation. No evidence was found for redox cycles involving Fe3O4 and FeO(OH). The results suggest that Fe(OH)2 may be oxidised to Fe3O4 or Fe2O3. These data largely confirm Strattman's modification to the Evans' scheme for cyclic oxidation during wetting and drying during atmospheric corrosion of iron. The importance of reduction of FeO(OH) supporting anodic dissolution during wet periods is limited by the volume of rust available for reaction at the iron/rust interface. A primary limitation of these redox processes is likely to be the electrical conductivity of the rust or the electrolyte.