Ion Selective Properties of Rust Membranes and Protective Effect of Stable Rust Layer Formed on Weathering Steel

Abstract

In order to elucidate the mechanism for the protective ability of final stable rust layer formed on a weathering steel by atmospheric corrosion, the ion selective property of rust layer was studied by the use of synthetic iron rust membranes. Anion selective property was increased in this order: Fe₃O₄<α-Fe₂O₃<α-FeOOH<γ-FeOOH<β-FeOOH. Fe₃O₄ has little effect on the ion selective property. Anion selective property decreased with increasing Cr content in α-(Fe_1-x, Cr_x) OOH (Cr-substituted goethite), cation selective property came out when the Cr content exceeds 3.8 mass%. As the effect of adsorbed oxyanions, all of PO₄^3-, MoO₄^2- and SO₄^2- reduced the anion selective property or changed to cation selective property. Among these anions, PO₄^3- has significant effect on the ion selective property of rust membrane and suppresses the permeation of Cl^- through rust layer. In acidic solution, the ion selective property of α-(Fe_1-x, Cr_x) OOH changed from cation to anion selective one, though little effect for α-FeOOH. The anion selective property of α-(Fe_1-x, Cr_x)OOH was reduced as the concentration of KCl solution increased under the constant ratio of KCl solutions (C_I/C_II). Furthermore, it was thought that protective rust layer formed on a weathering steel acts as a kind of bipolar membrane which was composed of outer γ-FeOOH layer of anion selective property and inner α-(Fe_1-x, Cr_x) OOH layer of cation selective property.