TOPOLOGY OF IRON SURFACES IN THE EARLY STAGES OF ELECTROCHEMICAL CORROSION

The formation and growth of microcrystalline corrosion centers on polished iron are studied in the initial stages by angular-resolved scattering of coherent laser light and by scanning electron microscopy. The corrosion process is controlled by cyclovoltammetry in 1M aqueous NaOH. The primary indication of corrosion is observed by scattering at the cathodic end of the first oxidation-reduction cycle. The signal forms a speckle pattern with strong spatial intensity fluctuations arising from interferences between corrosion crystallites. The intensity of the pattern increases in each cycle proportional to the square of the mass of the corrosion crystallites, whereas the spatial shape of the pattern is preserved. It is concluded that the corrosion product is formed by precipitation of ferrate(II) as well-separated crystallites of iron(II) hydroxide with diameters in the order of 40-400 nm. Once formed, the crystallites remain fixed in their positions during all consecutive voltammetric cycles. The crystallites are reversibly oxidized and reduced in a solid-state topochemical reaction by proton ex- and incorporation, Fe(OH)2 reversible FeOOH + H+ + e-. The crystallites grow in each cycle by deposition of newly formed ferrate(II).