Enhancement of electric conductivity of the rust layer by adsorption of water

The rust layers on the weathering steels exposed for 17 or 18 y at seven different sites in Japan were studied by electrochemical impedance under wet condition and measurement of amount of water vapor adsorbed. The transient of amount of adsorbed water vapor into the rust layer during humidity jump from 0% to 80% RH indicated that saturation of adsorbed water vapor took place within 1 h. From the semi-infinite model of diffusion, apparent diffusion coefficients of H2O in the rust were calculated. It was found the apparent diffusion coefficient was higher when the amount of air-borne salt of exposure sites was higher. The impedance diagram of the rusted steels was analyzed with an equivalent circuit consisting of a series connection between an ohmic resistance and a parallel circuit of charge transfer resistance-redox capacitance. The double layer capacitance on the gold electrodes pressed onto the rust layer was further added on the circuit. The impedance showed that the rust layer behaved as a dielectric layer under dry condition, while the conductance was greatly enhanced with wet condition. The charge transfer resistance evaluated from the simulation under wet condition was largely dependent on the amount of air-borne salt in environment. It was concluded that the rust layer formed in the site with relatively large amount of air-borne salt revealed high conductivity under wet condition. For the rust formation followed by the Evans model during wet–dry cycles, the higher conductivity induced the more facilitative reduction of the rust layer under wet condition and the larger growth of the rust layer in a wet–dry cycle.