Study on water adsorption-desorption on metal surfaces and the early stage of atmospheric corrosion in steels

Atmospheric corrosion of steel is comparatively different from the corrosion in bulky water or seawater in the sense of smaller corrosion loss. In general, steel exposed to the air forms thin rust layers under wet and dry alternating atmospheric conditions, reaching a stabilized lower corrosion rate with time. This paper was aimed at studying the early stage of rust formation on steel for clarifying the mechanism of high corrosion resistance of weathering steel. First, water adsorption-desorption phenomena were measured with an ellipsometer and Kelvin microprobe for pure Fe, Ni, Cr, and weathering steel. Second, corrosion potential of weathering steel was measured with a Kelvin microprobe as a function of the thickness of water layers on the metal surface. All specimens of as a wet polished surface condition increase water adsorption with relative humidity (RH), whereas the specimens pre-oxidized at 523K hardly adsorb water irrespective of RH. Work function phi calculated from Kelvin potential responds well to the amount of water adsorbed on metal surfaces or trapped within oxides. Specimens as polished show decrease in phi with the amount of adsorbed water, on the other hand, specimens with pre-oxides on their surfaces indicate a little decrease in phi for pure Fe, Ni, and almost no change for pure Cr. The corrosion potential for the weathering steel under the 200 and above 200 mu m thickness of 1M sodium sulfate solution was around -600mV (SHE), meaning that this steel corroded at an active dissolution potential under the control of oxygen diffusion through the Solution layer. When the thickness of the solution layer was reduced down to between 0.2 and 10 mu m, the corrosion potential was shifted upward to -100mV, meaning that the steel was in a passive potential region where ferric oxyhydroxide could be formed.