For the purpose of obtaining integral color films in a mixture of malefic acid-sulfuric acid, studies have been made on electrolyte composition, electrolyte temperature, current density, electric wave form and also conditions for the initial two-step current control method. Under the anodizing conditions of 200 to 250g/l malefic acid and 3 to 6g/l sulfuric acid concentration, 20 to 40°C electrolyte temperature and 1 to 3A/dm2 current density, unifom integral color anodic films of dark amber to light gold were obtained on commercially pure aluminum and Al-Mg, Al-Mg-Si and Al-Zn alloys. In addition, it was found that the initial two-step current control method, in which anodizing is carried out at a low current density for the initial stage of anodizing and at a specified current density thereafter, was very effective to prevent the formation of pittings on the film surface. With respect to the applied electric wave form, it was found that low ripple waves such as direct current or three-phase full waves were effective to obtain colored films of uniform thickness. The electron microscopic observation of the anodic films revealed that the addition of sulfuric acid caused the cell to be small in size and more like the regular hexagonal structure, with an indication that the pore diameter also became small. The anions incorporated in the colored film were studied by using chemical analysis, IMA, IR and ESR, and it was revealed that coloring of the anodic film depended greatly on the amount of SO42- from sulfuric acid and corresponded to the amount of organic compounds, incorporated into the film from maleic acid as well. As the result, it was assumed that coloring has a close correlation to the behavior of organic anion radicals. Further, it was shown that when the colored films, were heated, the decoloration of the films, began at about 500°C or over, but by means of DTA and TGMS, it was found that the organic compounds in films were decomposed into CO2 at about 875°C, while SO42- decomposed into SO2 at about 950°C. This decoloring process corresponds to the variation of ESR spectra of the anodic oxide films.
This paper describes the mechanism of built-in color of anodic oxide films formed in various electrolytes in terms of the crystal structure of the alumina films. The film color seems to be dependent on the co-ordination number of oxide ion to Al3+ ion. The co-ordination number was determined either by chemical shift of the fluorescent X-ray spectoscopy or by radial distribution method. The co-ordination number of γ-Al2O3 is 5.3, whereas that of the colorless anodic oxide films is 4.7, those of sulfosalicylic acid film and oxalic acid film being 5.8 and 5.9 respectively. The film formed in mixed electrolyte of oxalic-sulfuric acid bath showed the co-ordination number of 5.3. In conclusion, the oxide film having the coordination number of less than 5.3 is colorless, and one having the number of more than 5.3 shows built-in color.
The origin of golden color of Al2O3 films formed in organic acid electrolytes has been investigated by using ESR, IR and pyrolytic gas-chromatography. In the course of the pyrolysis process in air or argon atmosphere, the film color changed and the deepness of the color was proportional to the spin concentration of the Al2O3 films, whereas the amount of the effluent CO2 gas during pyrolysis was not always proportional to the color. The location of spin free electron was determined by the ESR spectra of stable isotopes, 2H, 17O, and 13C introduced to the constituents of the anodizing electrolyte. Spin free electron was found to be situated near C atom rather than either H or O atom. From the ESR spectra of pyrolysis products of Al(OH)3 and organic acid mixture, it seems that the golden color is chiefly due to some intermediates formed by carbonizing process of the organic acids incorporated in the Al2O3 matrix, although the substances are not identified yet.
Al (99.85%) and Al-Cu (Cu: 3.5-4.5%) alloy anodized at various voltages in a 5% ammonium borate solution, were reanodized in 1M sulfuric acid and 1/2M oxalic acid solutions, respectively. For the secondary anodizing, V-T curves and surface states were observed. Irregular black spots formed on the anodized surface of Al and Al-Cu alloy specimens in sulfuric acid. In oxalic acid anodizing, films were brown at initial voltage (100V or lower), violet at 150V, blue at 200V, but brown for Al-Cu alloy. The film formation on Al took place initially at the corners of each specimen and eventually over the entire surface. The peak voltage of the V-T curves for the secondary electrolysis in oxalic acid was higher than the initial voltage in the ammonium borate solution. The peak voltage is an indication of breakdown of the barrier layer. In anodizing of Al-Cu alloy in oxalic acid, the final voltage in the V-T curves was constricted, owing to the dissolution of copper, to the low voltage as in the case of anodizing the alloy in the ammonium borate solution.
Pure aluminum (2S-Al) and alloy (61S-Al) sheets, anodized in sulfuric acid or in phosphoric acid, were electrolyzed with alternating current (50Hz, 5-25V) in a solution of NiSO4·6H2O: 25g/l, (NH4)2SO4: 15g/l and HBO3: 3g/l. Anodizing in sulfuric acid gave a better coloring than anodizing in phosphoric acid, although the oxide film obtained in phosphoric acid is more suitable as a substrate for electroplating than that obtained in sulfuric acid. It was also shown that, for coloring, pure aluminum was more suitable than the alloy. The optimum voltage for the AC electrolysis was 10V and, at higher voltages, nickel deposition was observed on the colored coatings. The voltage and current wave forms during the AC electrolysis recorded by using an oscilloscope were discussed. It has been shown that the anodized aluminum electrode of the optimum condition for coloring could be represented by an equivalent parallel circuit of a capacitor and a diode.
Morphology of integrally colored anodic films in some electrolytes such as sulfosalicylic acid+ sulfuric acid, oxalic acid +sulfuric acid, and sulfuric acid has been studied by electron microscope. The electron micrographs of these film sections showed the branched pores developing not exactly perpendicular to the surface and the scallop shaped pore colonies as fiberous bundles. Impurities in metal substrates caused severe disorder of cellular structure of the film around them, which means termination and/or branching of the pore. The initiation process of the film formation at constant voltage is assumed as follows; (1) barrier type film is formed, and (2) branching pore colonies appear in some places subsequent to final dense pore development on the barrier film. Apparently there exists a correlation between intensity of color and disorder of cellular structure of the films.