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Salt spray failures with type 2024-T3 aluminum

We have recently had salt spray failures on 2024-T3 material ran through a Alodine 1200S bath maintained at 2 oz/gal. We use a nitric acid based deoxidizer, and a caustic based etch. I personally think there is two parts to our problem on is material and the other is fluoride content in the Alodine bath. The fluoride content is running around 800-1000 ppm. To get a proper coating weight it takes about 1 minute 45 seconds in solution. Any suggestions or past experiences would be appreciated.

Steve Adams
- cedar city, Utah
1999

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Letter #3335 is quite similar to yours, Mr. Adams, and I think you'll find it informative. Regards,

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

All these letters I see concerning failure of chemical film of aluminum seem to always include a caustic etch. My question to these folks is why does industry always want to use an etch. For the most part the production I see come through our facility doesn't need that type of aggressive cleaning. I do agree with an etch you will get a better conversion from a cosmetic evaluation. Typically I limit the etch used and don't have too much problems with corrosion. Limiting the etch in a production mode may prove to be beneficial in the reduction of its own related problems. Good luck.

Tim Martin
plating shop - Springfield, Massachusetts
1999

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Tim, I believe you are correct that the purpose of the etch is mostly aesthetic. The matte finish can render fingerprints almost invisible whereas they can be a nuisance with a more specular finish.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
1999

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1999

Here we go again. The problems of bad material are discussed by myself and several others in #3335. The fluoride level sounds slightly low for a 2 oz/gal. solution, but is that the free fluoride or total fluoride? The immersion time sounds slightly high, although not knowing what coating weight you are shooting for, that's also hard to say. Accelerator level could be low, but baths with longer times, slightly low fluoride or accelerator don't necessarily fail salt spray. Those facts usually just mean that the bath is getting old and there is a likelihood of an accumulation of contaminants which could affect salt spray. It may be time to decant and recharge. I personally like to save 10% of the old bath when recharging, unless I know that there are contaminants in the bath.

I have also discussed the issue of etching in Question #3335. It has been shown by the U.S. Navy (the agency responsible for MIL-C-5541 ) and others that etching is detrimental to salt spray. period. Having said that, what happens when actual parts require an etch opens up the can of worms again about the actual practical value of salt spray testing. Tim is right in stating that sometimes we etch aluminum as a habit, and not because we need it. Most 2024 parts are machined parts with fresh surfaces, hopefully protected from atmospheric oxidation until they are conversion coated. A clean/deoxidize/conversion coat cycle should be sufficient. The best way to avoid fingerprint problems is to avoid handling the parts with bare hands. Remember that our sweat has sodium chloride in it, and chlorides are the most common cause of the corrosion of aluminum.

Happy Holidays to all.

Phil Johnson
- Madison Heights, Michigan

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Amiel Forshee (Boeing Co.) has published a couple of papers on this subject through AESF. One title that may be worth your review is: Salt Spray Failures - Recommendations for Troubleshooting Causes.

There are so many factors to consider: Alodine pH, dryer temperature, deoxidizer chemistry (Cl, Cu, Al content), rinses, conversion coating chemistry (Cl, SO4, Cr6/Cr3 ratio), test specimen preparation and so on. However, we have learned that Cr based conversion coatings are not especially compatible with Fe based deoxidizers.

Warren John Fullen
- Auburn, Washington
2000