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topic 1546

Verification of proper Iridite Finish


Do you know how to test/verify an Iridited aluminum surface to determine if the finish has been correctly applied? We've received some parts in that are started to corrode and trying to determine what might be the problem.

I appreciate your help,

Gary Guilbeaux


Iridite is a Macdermid brand name for Mil-C-5541 [link by ed. to spec at TechStreet] conversion coating. I believe that if you check that spec, and the specs it references, it will tell you how to check for the coating.

Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


The 2024 aluminum alloy "chem film" class 1a has not been passing the mil spec. The pH we hold it about 1.6-1.7; about the concentration 1.0-1.1 oz/gal. The only thing we can think of is the improper cleaning procedure. We have a alkaline non-etch cleaner, rinse, deox, rinse and to see no water break, then Alodine. However, we passed it once and that one time we had ScotchBrited the parts.

With the 6061 we do give it Alumate etch. However, we were told not to etch the 2024 because of the high copper. However, when you etch the 6061 and Alodine it its darker and has better corrosion resistance.

Do we need to use an etch for the 2024 -- or give me some parameters for pretreatment

julio espinoza
- san jose California

October 2011

Hi, Julio.

Etching will not improve the corrosion resistance of 2024 aluminum. However, if what you are salt spray testing is sample panels rather than actual parts, the one approach that will work is throwing away the test panels and buying new ones. The alloy changes with time, the copper concentrating at grain boundaries, and there is no way to get the panels to pass the test. Please patiently search this site with relevant search terms and you will find a number of threads about this issue. Letter 4140 is a good starting point. Good luck!


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey



- HOPKINS, Minnesota

October 24, 2011

Hi, Vern. Mil-DTL-5541 [link is to free spec at Defense Logistics Agency,] Chromate Conversion Coating, sometimes called "chem-film", and often called Alodine or Iridite (these are brand names), is a conversion process rather than a built-up coating. The thickness will be perhaps a few millionths of an inch, not thousandths of an inch.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

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