49062

Chemical for easy removal of MIL-C-5541, CL3 ?  

June 17, 2008

We are a producer of military airborne radar systems. Some of our dip brazed, 6061 Al housings are finished per Mil-C-5541 [link is to spec at TechStreet], CL3 all over, then externally finished with an epoxy paint system. Internally, there are certain areas of the chassis that require very good electrical bonding with circuit boards, that the chemical conversion coating is preventing. We know that we can abrade the coating off in these areas, but this is very labor intensive and can change surface finish and flatness, so I was curious if anyone knows of a chemical that is commercially available that will etch away the coating easily, without attacking the base aluminum.

Ken
Sr. Mechanical Engineer

Kenneth Miller
radar systems engineer - Huntington, New York

It is cheaper and easier to mask the small area that you need contact with a proper tape that has been burnished with a wood or plastic paddle (stick). This can be removed after the process and you will have bare aluminum. There are several chemicals that will remove the coating, but you do not want that chemical running around on the inside of your part.
PS, this is a time consuming process also, so try the masking. The tape choice is very important, so try several to get the best.

James Watts
- Navarre, Florida

I won't say that James' response is incorrect, however, hand application of tape on internal surfaces is sometimes difficult if not altogether impossible. Furthermore, tapes are reknowned for leaving adhesive residue that would degrade the ability to "electrically" bond to the otherwise bare aluminum surface. One technique that my firm has been particularly successful with is designing mechanical fixtures that prevent chromating on interior, masked off features and chromates only the exterior portions. This is obviously more costly upfront but once the investment is made it's (1) faster, (2) cleaner in that there is no residue on masked surfaces, and (3) less labor per piece required (i.e. cheaper).

Milt Stevenson, Jr. Anoplate Corp Syracuse, NY, USA

You might try 1:3 aqua ammonia, or a 10% solution of sodium carbonate in water, along with gentle abrasion. I don't think these would attack the Al basis metal, at least not very quickly...try it on a scrap part first.

Dave Wichern - The Bronx, New York

You might try application of one of our paintlike peelable maskants. Apply by brush to the surface not requiring processing, then finish, then peel off.

Jim Baldwin Mask Coat LLC West Monroe, Lousiana