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Yellow-Brown stain on Solution heat treated Aluminum

After solution heat treat of Various Aluminum Alloys (mainly 2024 and 7075) There is a yellow brown stain appearing on parts. (This stain seems to be more prevalent on bare alloys) The stain has an odor when the parts come in contact with water. The Salt bath is operated at 920 Deg F and is a mixture of approximately 50/50 sodium nitrate ⇦ this on Amazon [affil link] and potassium nitrate. The stain easily comes off when the material is deoxidized. the stain is not always present , but when it is the smell is extremely unpleasant. Any ideas on what this stain may be?

Jim Morehouse
QA Engineer - Amherst, Nova Scotia, Canada
February 27, 2009

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