Q. What technique is used to avoid staining on aluminum parts when chemical film per Mil-C-5541 is applied? It seems that the bottom few drops dry on the very edge and leave a stain. J.C.
A. Here are some points to think about in this regard when doing the chem film process:
- Although the bath concentration may vary a small amount depending on its condition, make sure it is not too concentrated. I recommend making up a new chem film bath at 1/2 oz/gal. An older, well-used bath may require 1 oz/gal to perform well.
- Bath temperature of 70-75°F (21-24°C) is ideal.
- Rinse thoroughly in a double-counter flow-rinse tank system, followed by a clean deionized (DI) water rinse, then a final warm rinse (150°F, 65°C maximum). That’s four rinses altogether.
The bottom line is, if the bath is too concentrated and/or the parts are not rinsed well enough, the staining is more likely to occur.