Properties of the Nanoporous Anodic Oxide Electrochemically Grown on Steel in Hot 50% NaOH

Abstract

The anodic oxide grown on steel in 50% NaOH at different temperatures and applied potentials has many remarkable properties. Optically the appearance of the oxide can vary from black to dichroic, to a light brown color. Field-effect scanning electron microscopy and X-ray diffraction reveal the oxide to be a nanoporous oxide composed of a network of 100 nm diam magnetite channels. Electrochemically the oxide is a very good conductor and, when polarized cathodic or anodic, can absorb charge by switching between Fe+2 and Fe+3. Although the film provides excellent corrosion resistance in pure water, it provides only temporary resistance to corrosion in oxygenated saltwater. Different methods were used to seal the porous oxide, and it was found possible to reduce the corrosion rate by 2 orders of magnitude in oxygenated 0.1% NaCl solution by penetrating the oxide with a commercial inhibiting oil spray. © 2008 The Electrochemical Society.