Improvement of DC coloring of anodized aluminum with use of certain diphenylmethane derivatives

Deep bronze coloration is obtained during d-c treatment of aluminum samples, covered by a porous oxide layer, in acidic tin(II) sulfate solutions containing the dyes Nile Blue, Methylene Blue, Pyronine B, and Acridine Orange in concentrations of 2.55, 1.39, 1.70, and 0.21 g/L, respectively, The optimum conditions are 13 VDC and 3 min, The dye and metal cations migrate to the cathode, to the insides of the pores, and are deposited at the bottoms of the pores, Reduction of hydrogen cations and dissolved oxygen occurs as well, which increase the local pH in the film, causing dissolution of the oxide to an extent dependent on the voltage and duration of the cathodic treatment. These dyes, present at the bottom of the pores, by influencing these reactions, affect the extent of spalling or fine peeling of the film and increase its tin content and, consequently, the intensity of its color, The oxazine derivative, Nile Blue, added as the sulfate, yielded the best results.