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Good explanation of what happens in an aqueous alkaline aluminum cleaning system

We are trying to convert our washing system from TriChlor to Aqueous on aluminum tubes. We are successful at getting off the machining oils with the alkaline soap and water levels that have been prescribed by the soap supplier. However, my customer does not understand why the aluminum tubing gets a yellowish-brown oxide layer on it caused by the aqueous washing process, And I have not been able to explain it to them in such a way that they can understand it. Can someone please give me a thorough explanation on what happens to the aluminum in the alkaline bath and why the color change happens - something that you do not have to be a metallurgist to understand.

Margery Mabus
quality engineer - Battle Creek, Michigan
2007

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Probably, you are using a high-pH cleaner suitable for steel.
Aluminum is chemically attacked (etched away) in alkaline solutions. This leaves a surface enriched in certain alloying elements, some of which form colored hydroxide smut. Most notably, copper. Also, silicon is not attacked and gives a dark color to over-etched alloys.

For short-term cleaning at about 120-160 °F, the maximum tolerable pH is about 9.2. Inhibited (silicate-containing) cleaners can be used if the silicate doesn't interfere with subsequent processing. Chemical suppliers below offer etching, mildly-etching and non-etching aluminum cleaners. Specify a non-etching, aluminum cleaner.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2007