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Aluminum chemical polish question

 

In our Type II anodize line we utilize a chemical polish solution, composed of approx. 95:1 phosphoric acid (85%) to nitric acid (42baumé). We occasionally have problems controlling this bath. The amount of water is critical. During normal operation, we add 50:50 nitric acid/water a couple of times a day, and the work is mirror-bright. Besides a hydrometer, does anyone know a method to analyze this solution for phosphoric, nitric, water, and aluminum? At one time we were using a proprietary additive consisting of acetic and nitric acid, and the vendor would analyze the solution. He (she) claimed it was a very complex procedure but didn't go into detail. Since then I determined that nitric acid alone gave more satisfactory results, but obviously, lost my contact with the vendor lab.

George Brackett III
- Utica, New York

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This process depends upon aluminium conc, water conc, nitric conc, temperature, phosphoric/sulfuric acid conc. There are a number of parameter combinations that will give a bright finish. I recommend that you monitor nitric acid & aluminium by analysis and maintain the density within a narrow band. An experienced operator can estimate the nitric acid content within 1% by the level of bubbling. I recommend that you find the analysis in The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick, R. Pinner, P.G. Sheasby or request a Technical Data Sheet from your supplier.

John Tuohy
- Ireland