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Sulfuric/boric mixed acid anodizing has been proposed as a replacement for chromic acid anodizing for certain aircraft components. The present paper investigates the effects of additions of boric acid to sulfuric acid on the current density-time response, morphology and growth rate of porous anodic films formed under conditions recommended for industrial processing. Further, the chemical dissolution rate of the film material was determined from the rate of thinning of the barrier regions of the porous films during immersion in the electrolytes. Scrutiny of the anodic films revealed no significant differences in the film morphology, including steady pore diameter, pore population density, barrier layer thickness, and growth and dissolution rates between films formed in the sulfuric acid and in the mixed acids. Further, the pitting behaviors of the filmed substrates in 3.5 wt% sodium chloride solution were similar for films produced in the two types of electrolyte.
Zhang et al. (Tue,) studied this question.