A mechanistic study of initial atmospheric corrosion kinetics using electrical resistance sensors

This paper describes a novel experimental approach to the study of atmospheric corrosion of iron and zinc, utilising electrical resistance sensors that are sensitive to corrosion losses of the order of one atomic monolayer. Using such devices, a mechanistic study of the initial stages in the atmospheric corrosion of iron and zinc was performed in a rectangular flow cell using controlled relative humidity (RH), temperature and gas flow rate. Additionally, the effects of SO2 contamination in the gas phase and prior NaCl contamination of the metal surface were studied. It was found that the initial corrosion kinetics of iron and zinc are, not unexpectedly, dominated by the development of surface corrosion product films, but that the growth kinetics vary with metal, humidity, etc. Specifically, in the presence of gas-phase SO2, activation energies and kinetic and chemical rate orders were consistent with control of the atmospheric corrosion process by solution-phase oxidation of sulphite–sulphate ion. For iron, this implies that the well-known sulphate-nest theory is inoperative at least during the early stages of atmospheric corrosion. In contrast, for chloride–contaminated zinc, the data were consistent with a rate-controlled diffusion of a species, probably water vapour or oxygen, through a thickening corrosion product film. Finally, the kinetic and chemical rate orders for corrosion of chloride–contaminated iron precluded a diffusion-controlled mechanism, but were consistent with a rate-controlling process involving some regeneration of chloride: e.g. as in metal–ion hydrolysis in a pit or similar localised corrosion events.