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 rang es present during atmospheric corrosion. On pre-corroded iron, oxidati on 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 re latively 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 dry ing in a wet-dry atmospheric cycle. On uncorroded iron, free Fe2+ spec ies were found during cathodic sweeps after anodic polarisation. These were absent on pre-corroded iron electrodes confirming their consumpt ion during the production of a solid oxide. On uncorroded iron, no pas sivation was found up to 0 mV(SCE) which suggests either that pre-exis ting solid oxides are required to initiate passivation either by takin g part in the reaction or by acting as nucleation sites for growth of new oxide during passivation. No evidence was found for redox cycles i nvolving Fe3O4 and FeO(OH). The results suggest that Fe(OH)2 may be ox idised to Fe3O4 or Fe2O3. These data largely confirm Strattman's modif ication to the Evans' scheme for cyclic oxidation during wetting and d rying during atmospheric corrosion of iron. The importance of reductio n of FeO(OH) supporting anodic dissolution during wet periods is limit ed by the volume of rust available for reaction at the iron/rust inter face. A primary limitation of these redox processes is likely to be th e electrical conductivity of the rust or the electrolyte.