Anodic Oxidation and Local Corrosion of Aluminum in Mono-carboxylic Acids

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Anodic oxidation of 99.99% aluminum sheets were conducted in solutions of several kinds of mono-carboxylic acids by applying d. c. of 1.87Amp/dm2. The concentrations of the solutions for baths were from 0.01mot/l to saturation and their temperature were from 20°C to boiling points.Porous and thick anodic oxide coatings were uniformly formed from formic acid bath at relatively high temperatures and concentrations. On the other hand, at low temperatures and concentrations, pittings occurred on the anode aluminum by local corrosion. It was found that uniform formic acid coatings possessed porous structures similar to those obtained from sulfuric acid bath and they were easily colored with organic dyestuffs.In the baths of other acids such as acetic, propionic, butyric, valeric, and benzoic acids, it was impossible to form uniform coatings and local corrosion of the anode was always observed.Microstructures of the pittings were investigated by microscopic examination and measurement of leakage current by the Hunter's method.
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