Anodic Treatment of Aluminum in the Chromic Acid Bath

Abstract

CHROMICacid has extensive uses in the anodic treatment of aluminum for all U. S. Naval aircraft. Uncertain and inefficient operation has resulted in many instances from lack of data regarding the factors influencing the process. In this report complete fundamental data are presented for the first time showing the relationship between corrosion resistance, film weight and thickness, current density, concentration and pH of the chromic acid bath, temperature, time of treatment, electrical and practical efficiencies, chromic acid economy, alumina dissolved, cathode area, throwing power, etc., in the treatment of aluminum and some of its alloys. From these data may be calculated the most economical procedures which will give satisfactory films under various operating conditions. * Since this work was started, revised specifications {10) have reduced the salt spray exposure from 30 days to 250 hours.

Regeneration of Chromic Acid Solutions by Cation Exchange

“…In anodizing, where a moderate amount of alumina in solution is not harmful, it may be more economical to operate the resin to exhaustion in each cycle. A convenient check on the exhaustion of the column may be obtained by pH determinations on the influent and effluent, as pH is directly affected by alumina content in anodic liquors (5,10); in other liquors, the replacement of metal ions by hydrogen ions will also reduce the pH. When the pH of the effluent and influent are the same, the resin capacity is exhausted.…”