We demonstrate the ability of a polyaniline film, into which highly dispersed platinum microparticles were introduced at a fairly low level (ca. 25μg cm −2 ), to protect stainless steel (containing 15.86% Cr) against corrosion in strong acid solution (2M H 2 SO 4 ). We show that the electrocatalytic reduction of oxygen (from air) on the platinized polyaniline film can be coupled to the self-passivation of steel, i.e. to the anodic formation of a passive layer composed of the oxides or oxyhydroxides of iron/chromium. The reduction of O 2 at dispersed platinum microcenters, is apparently fast enough to replenish the polyaniline charge consumed by the oxidation of stainless steel, and to poise the steel’s potential at about 0.4V vs SCE. Consequently, the corrosion potential of stainless steel is stabilized within the passive range, and the rate of its dissolution is minimized. Also a low overpotential for the reduction of protons seems to facilitate the active-to-passive transition of stainless steel.