Silicone

Q. What would be the appropriate solvent to preclean aluminum surfaces to assure they are free of silicone prior to painting? D.S.

 

A. Silicone is a major problem with any organic coating process. It will cause areas of the coating to dewet and create very poor paint adhesion. For this reason, it is important that any silicone be removed from a part prior to coating it with paint.
Not knowing the form the silicone is in (i.e., residual silicone from a mold release agent, silicone oil, etc.) and the relative amount, I would suggest starting with a cleaning and degreasing step since the silicone could be suspended in a oil. This could be in the form of a solvent or aqueous cleaning process, whatever you may be currently using. If not currently utilizing a cleaning process, you would have to examine the relative ease of implementation of either. Some factors for consideration would be effectiveness, floorspace, utility cost and capital investment. This may be an effective first step as many times silicone is used in some form of oil.

To insure there are no traces of silicone remaining on your part, it would be advisable to follow up the degreasing step with an alkaline etchant. This would insure that any traces of the silicone that were not removed in the cleaning process would be taken care of before painting. This would work by literally etching the aluminum surface from under the residual silicone. This would not be advisable as a first step, since the etchant would not uniformly penetrate under the silicone. This would result in an uneven etch surface.

Following the alkaline etch step would be a nitric acid desmut of the aluminum surface. This would be necessitated by the residual alloying elements left behind on the surface after the aluminum has been etched from the matrix of the metal. The desmut step typically consists of an immersion in approximately 50% nitric acid. This is effective as dissolving the alloying elements while leaving the aluminum intact.

With all the additional process steps required to insure all traces of silicone are removed from you aluminum surface prior to coating, I would suggest that you examine the manufacturing process in more detail to identify where the source of the silicone is and then what you could do to eliminate it from the surface of your parts. The amount of additional process steps and labor would make it worth the time to eliminate it if at all possible.