Process Classification


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Q. A facility uses a dilute solution of hydrofluoric and phosphoric acids to pretreat aluminum parts prior to painting. Should this process be classified as conversion coating, etching, cleaning orsomething else? Would the acids produce a phosphate coating, or is it necessary to have a cathodic metal in the solution to conversion coat?
Thanks for your help. D.F.


A. In my opinion (and that is key here), you are only acid etching the aluminum. The hydrofluoric acid will dissolve and attack the aluminum while the phosphoric acid will act as a brightener. Although I have not studied this on aluminum, phosphoric acid cleaning of steel will produce a very light “phosphate” coating (far lighter than that recommended for a traditional iron phophating process), although I am not aware of this ever being classified as a conversion coating or metal finishing process.

I assume you are asking the question in order to determine how to classify a manufacturing facility environmental permit application. I believe part of the classification goes toward intent. For instance, if the intent is to produce a phosphate coating (without a formal phosphating process), then this may still be considered a conversion coating. On the other hand, if it is an artifact of the cleaning process, it may not be considered as such. Subsequent treatment may also dictate classification (i.e., nothing or painting?). As mentioned above, I would consider the information above opinion and consult further with an environmental specialist in the geographic area where the permit is needed.

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