Abstract
Rust electrodes were prepared by fixing a piece of rust plate, which was formed on pure iron during four months exposure in urban environment, on an acrylic plate with a conductive adhesive. The galvanostatic catholic polarization of them was studied in 0.1M Na2SO4 solution at 20°C. The rest potential gradually recovered after polarization current was switched off and approached the value which depended on the amount of charge passed through the rust electrode. The change in the amount of crystalline substances in the rust plate was observed by X-ray diffraction. As the reduction proceeded, the amount of lepidocrocite (γ-FeOOH) decreased, crystalline magnetite (Fe3O4) increased and goethite (α-FeOOH) did not change. The bulk of electro-chemically active substances, which we call “intermediate substances, ” could not be identified by X-ray diffraction but their behavior was estimated from that of crystalline substances. After sufficient reduction, the intermediate substances were converted to crystalline magnetite which could not be oxidized any more electrochemically. The rest potential of the rust electrodes is controlled by the equilibrium potential of the intermediate substances which is defined by solid state Fe(III)/Fe(II) redox system. From our findings, “an overlapping dual electrode” model is proposed for rusted steel in aqueous solution, namely an equilibrium electrode of solid state system overlaps on the corroding electrode controlled by the reduction of oxygen.
