Ion selectivity of rust formed on low-alloy steels under a cyclic wet-and-dry condition

Low-alloy steel foils of 20 μm thick were corroded under a cyclic wet-and-dry condition simulating highly corrosive coastal environment. Compositions of Fe-1 mass%Co, Fe-3 mass%Co, Fe-3 mass%Ni and Fe-0.8 mass%Al were selected as testing materials because they were thought to be basic alloys of weathering steels for the use in coastal environments. The steel foils were entirely transformed into rust to form rust membranes by processing more than 20 wet-and-dry cycles. The rust membranes were carefully attached to a dual-compartment cell for the measurement of membrane potential. Membrane potential generated by the difference in ionic diffusion rates of K⁺ and Cl^- ions was measured as a function of concentration difference of KCl between solutions in the compartments separated by the membrane. The membrane potential thus measured was proved to be effective in evaluating the performance of rust formed on steel surface. The rust layer formed on the steels of Fe-3%Ni and Fe-1%W showed anion selectivity, while the rust on Fe-1%Co, Fe-3%Co, and Fe-0.8%Al steels, revealed cation selectivity. The addition of Co and Al to the steel decreases the corrosion rate because penetration of chloride ions into the rust is inhibited by the cation selective permeability of rust. The addition of Ni and W is known to be effective in decreasing corrosion rate of steel, despite their anion selective nature in rust. Alternative mechanisms other than ion selectivity are required for the elucidation of inhibition by Ni and W.