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Chromate Conversion per MIL-C-5541 CLASS 1A (MED GOLD)

I am designing circuits and modules for an airborne radar system. We have been using "Alodine" Chromate Conversion per MIL-C-5541 CLASS 1A to finish all of our machined Aluminum chassis and housings. I am very unfamiliar with this finish as far as Electrical Conductivity at RF frequencies (up to 2 GHz in my case) is concerned. I usually finish my Aluminum housings with a Tin type plating since it is a fairly good conductor at RF Frequencies.

I would like to know if anyone has any knowledge or information as to the electrical conductivity of this Chromate Conversion process per MIL-C-5541 CLASS 1A?

Just using logic, it seems to me that since it employs Chromium (not sure about that though) AND since Chromium is in the Ferrite family, this finish would not be desirable at RF Frequencies. I may be totally off in this case, but I know this holds true for the Nickel finishes. ANY help or pointers would be GREATLY appreciated!

Thank you,

Ryan Groulx
- Melbourne, Florida, USA

I can't really help you, Ryan, except to point out that there is an error in the logic. Chromate coatings involves chromate salts (chromium in the +3 or +6 oxidation state), not chromium metal. Property wise, chromate salts are to chromium metal as table salt is to sodium.

Sorry I couldn't help more.

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

Chromate is a conversion coating for aluminum, zinc, cadmium and sometimes other metals and is considered to be non-electrically conductive. There is a Class 3 coating within the Mil-C-5541 which is considered conductive, but the reality is that it's thin enough that mating pressure causes shorts through the coating, thus the "conductivity". I'm not an EMI guy, but I know that chromate coatings are not a favorite of our EMI experts, they usually prefer an electroless nickel finish for EMI shielding.

Bob Denney
avionics Tampa, Florida


Dear Sir,

I cannot tell you of the conductive properties of chem film plating, but from the parts that I have worked on, I can say that if salt or heat is a factor in what you are looking for then you have a good coating--I have personally heated parts that have been previously chem film coated and painted with epoxy primer to the point of 775 degrees with no warpage of the part and no adverse effect to the chem film on the part--as I was to understand that the (chromate)i.e. (chromium) is an Iridite and or in the same category as a colored dye. If there are any fluctuations in electricity carrying properties of this process I am unaware. Most of the parts that are chem filmed never reach over 180 degrees in the process but are subject to exterior metal impurity removal.

Hope that helped you out some.. ACE

Steven T. Howard
chem film plating per military spec - Kerrville, Texas

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