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How to determine the frequency of chemical solution recharge?

We have used various types of chemical solution (Anodize, Chemical Conversion coat and other precleaning solution,..) for surface treatment of Aerospace program. Chemical solution would get aged and contaminated, as we use them. Sometimes, unsatisfactory quality from the surface treated part is obtained, even though the solution is properly under control limit. Therefore, the solution life is limited and it should be replaced periodically, I suppose. Then, How do I determine the frequency of the solution recharge ?

Dong-cheol Park
- Busan, Busan, South Korea
2003

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Years of experience maybe?   :-) This is indeed a difficult question, Mr. Park, since whole books have been written about the effects of various contaminants on various solutions. However, a general principle that will serve as a beginning, is that some processes are unidirectional chemical reactions and some are equilibrium electrochemical reactions. In the former (chemical conversion coating for example), the conversion solution is reacting with the treated metal and exhausting itself and must eventually be replaced. In the latter (many electroplating processes), the solution can essentially be expected to last forever (although contaminants must be removed). Anodizing is closer to being an equilibrium electrochemical reaction where the solution can last forever, precleaning and acid dipping are closer to being one directional chemical reactions where the process solution will become exhausted with use.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003