The Atmospheric Corrosion of Iron — A Discussion of the Physico‐Chemical Fundamentals of this Omnipresent Corrosion Process Invited Review

Abstract In this review the physico‐chemical fundamentals of the atmospheric corrosion of iron are discussed, as they result from a number of electrochemical and spectroscopic techniques, which have been applied in order to analyze the corrosion mechanism of the atmospheric corrosion during wet‐dry‐wet transitions. — After wetting a dry corroded metal surface the cathodic partial reaction of the corrosion process is given by the reduction of Fe 3+ ‐ to Fe 2+ ‐states within the lattice of γ‐FeOOH. If the corrosion potential is rather negative, then also magnetite is formed. As a consequence of the formation of Fe 2+ states the electronic conductivity of oxide crystals close to the metal/oxide interface is increased. This results in an acceleration of the kinetics of the oxygen reduction. It could be shown, that oxygen is reduced predominantly at the oxide/electrolyte and not at the metal/electrolyte interface and therefore the rate of the oxygen reduction is determined by the electronic properties of the oxide crystals, the kinetics of the electron‐transfer reaction being very fast at oxides, which are highly doped with Fe 2+ states. If the metal surface is covered with a thick electrolyte layer , then the charge transfer reaction is so fast within the reduced oxide scale, that the overall rate of the oxygen reduction is given by the diffusion of oxygen through the water filled pore system of the scale and by the surface area of the highly doped oxide scale, which allows an electron transfer to occur at the oxide/electrolyte interface. The size of this effective surface area is determined by the amount of the reduction of the oxide scale immediately after wetting the dry metal surface. During drying , oxygen diffuses rapidely into the pore system of the oxide scale by gas diffusion. Then the corrosion rate is increased dramatically due to the fast oxygen reduction at the electron‐conducting large surface area oxide cathode. Due to the fast corrosion a dense oxide layer is formed on top of the metal surface and the surface is passivated. The corrosion stage of drying is terminated by a reoxidation of the oxide scale by oxygen, which causes the rate of the oxygen reduction to decrease as the electronic conductivity of the oxide scale is reduced.