Metal finishing Q&As since 1989

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Will MIL-C5541 alter aluminum extrusion surface finish?

2004

Our extrusions are made from 6063-T5 Aluminum Alloy. Their surface finish have a typical mill finish with "fine hair streaks". We wish to specify clear Iridite MIL-C-5541 as their final surface treatment. My question: will the clear MIL-C5541 remove the "fine hair streaks" on the extrusion and give me a more uniform finish look, or will the extrusion surface need to be buffed to remove the streaks prior to Iridite?

Many thanks,

Denise Carlson
Head of Purchasing Dept. - Santa Fe Springs, California, USA

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First of two simultaneous responses --

Aluminum Chromate per se will not change the surface to which it is applied. If you want the extrusion lines smoothed out, that is done in the alkaline etch stage of the surface preparation cycle. In the case of decorative aluminum extrusions for anodizing, sometimes the extrusion lines are removed in a very corrosive "bright dip" or in a light phosphoric acid dip. Whatever surface finish if left after mechanical and chemical surface preparation, the chromate conversion coat will do no further smoothing.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2004

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Second of two simultaneous responses --

In and of itself, Iridite (or more generically chemical film or chromate conversion coating of aluminum) will not hide or obscure any scratches, blotches, smears, etc. If this is needed, the parts can be alkaline etched chemically or "satin" etched in fluoride bearing acids prior to chromating. These chemical treatments can accomplish the same as the mechanical polish (at a lower cost typically). It should simply called out as "HEAVY ETCH TO REMOVE SURFACE FLAWS FOLLOWED BY CLEAR CHROMATE." Anyone doing Iridite has the chemistry to etch parts prior to chromate. Chemically speaking, good luck!

Milt Stevenson, Jr.
Syracuse, New York
2004