Corrosion Resistance of Alodine Treated Aluminum

Letter 4044

Corrosion Resistance of Alodine Treated Aluminum

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I am building a home built airplane constructed primarily of 2024 T3 Alclad. Most of the parts get an acid wash ( Alumiprep-33 [link is to product info at BoatersWorld]) followed by a dip in Alodine (Iridite?) and finally a coat of primer. My question is: What happens to the corrosion resistance of the alclad if some of the parts don't get a good coat of primer? Does the Alodine increase or decrease the rate of corrosion?

Patrick Armstrong
- El Granada, California

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Chromate conversion coatings ("Alodine") have a dual purpose. They act as a good paint base. They also protect the material from corrosion (theory is that a cut in the film may allow hex-chromium to 'bleed' into the scratch in a wet environment and protect the exposed material - true? I don't know). Alclad is much more corrosion tolerant than bare material, in that the cladding typically contains less in the way of alloying elements. Parts inside the airplane are often left in Alodine, only. However, I would endeavor to get good primer coverage on the parts, just in case the Alodine application is not what it should be - it is a lot more difficult to do properly when you cannot immersion process the details.

Mark McGlynn
- Oklahoma City, Oklahoma

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Hi, l looking for the same info for clear Iridite.

Joseph Chow
- Bayan Lepas , Penang , Malaysia

March 1, 2009

Hi, Joseph. Alodine and Iridite are two trade names for Mil-DTL-5541 [link is to spec at TechStreet] conversion coating. So Mark's answer applies as well to Iridite and to Alodine. Good luck.

Regards,

Ted Mooney, P.E.
finishing.com
Brick, New Jersey

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