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This paper describes an electrochemical technique for growing adhesive copper oxide films on copper with attractive colors ranging from gold-brown to pearl with intermediate colors from red violet to gold green. The technique consists of anodically dissolving copper at transpassive potentials in hot sodium hydroxide, and then depositing brilliant color films of Cu 2 O onto the surface of copper after the anodic potential has been turned off. The color of the copper oxide film depends on the temperature, the anodic potential, the time t 1 of polarization, and the time t 2 , which is the time of immersion after potential has been turned off. (At potentials lower than the transpassive region, that is, in the passive region, the oxide film is a dark gray or dark green matte with a blue powder hydroxide on the surface and it is not attractive.) The brilliant colored films were characterized using glancing angle x-ray diffraction, and the film was found to be primarily Cu 2 O. Cyclic voltammetry, chronopotentiometry, scanning electron microscopy, and x-ray photoelectron spectroscopy were also used to characterize these films. These techniques show that the different colors were due to differences in porosity and thickness rather than changes in composition.

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.Anodization of metals is an old and relatively simple method to functionalize material surfaces, by electrochemically growing a metal oxide film on the surface. For valve metals such as Al, Ti, Ta, or Zr, anodization has been extensively explored and, by a controlled variation of the electrochemical parameters used, the thickness, morphology ͑e.g., compact or porous layers͒, or crystal structure of the anodic films can be tailored to achieve the desired functionality. As for the valve metals, high voltages can be applied without substantial oxygen evolution reaction ͑OER͒ and thick anodic oxide layers can be grown before dielectric breakdown occurs. However, in the case of iron or steel, strong oxygen evolution at high anodic potentials takes place and, hence, the potential regime of oxide growth is limited.As has been reported previously by Burleigh, in spite of oxygen evolution ͑and, hence, lower current efficiency͒, thick anodic oxide layers can be grown on steel in hot, concentrated caustic solutions in the transpassive region. 1 The present article presents a continuation of the previous work, with the aim to understand the growth mechanisms, to analyze and optimize different properties of the anodic oxide layer, and to explore the use of the anodic layer for corrosion protection of steel. This is a relatively new field of research even though there have been many diverse studies on the electrochemical behavior of steel in caustic solutions. For example, previous studies include the dissolution and passivation of iron and steel ͑e.g., Ref.2-6͒, the production of ferrate ͑VI͒ compounds ͑e.g., Ref. 7 and 8͒, and the iron-nickel battery ͑e.g., Ref. 9 and 10͒. Only a few studies have described the use of hot caustic solutions to grow protective iron oxides on steel. 1,11-14 Burleigh has shown that when steel was polarized in the transpassive region, the applied electric current promoted the growth of a magnetite ͑Fe 3 O 4 ͒ film in addition to oxygen evolution and iron dissolution. 1 This magnetite film has many different optical appearances, including black, dichroic, and a light brown color, depending on the temper...