Metal finishing Q&As since 1989

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Aluminum Corrosion

2006

I have had 20 sheets of .025 6061T6 aluminum sit out in the rain for 8 months. They are for my airplane project. There is 10-20% corrosion on either side of most of these 12 x 4 sheets. I have moved them out of the weather, dried them off and stacked them 10 degrees from vertical.

I've spent some time emailing and talking with people on the phone. [Ed. note: name deleted] of Henkel, makers of Alodine, talked with me for a good 15 minutes. Here's what he had to say:

The corrosion that occurred in my stacked sheets of 6061 left outside in the rain will continue to occur, even though they are now dry and sitting indoors. He suggested cleaning them with Alumiprep 33 [on eBay or Amazon] and then immediately treating with Alodine [on eBay or Amazon]. I asked if I couldn't just clean w/ Alumiprep and not Alodine for now. Later Alumiprep again and then Alodine right after making the part. Sounded good to me.

But he said corrosion would still occur, even after cleaning off the old corrosion. He mentioned mills put an anticorrosive finish on, which has now been degraded. What do you say?

I also called [Ed. note: name deleted] at Macdermid, makers of Iridite, which is a conversion coating similar to Alodine. He didn't think corrosion would continue after cleaning/etching, but was only going on personal experience.

I hate to go through all the trouble of Alodining all 20 sheets, especially all at once, if it's not needed. So I am thinking of a middle of the road approach: clean them all up now with Alumiprep then check every few months for corrosion.

I appreciate your help.

Dan Morehouse
- Bellingham, Washington

Ed.note: We deleted the individual's names, Dan, because it's probably not fair to publicly quote people's comments that were made in private. Aluminum is an active metal which will corrode if not given a corrosion proofing treatment; cleaning a part is not corrosion proofing it.

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2006

Would cleaned sheets corrode in storage?
-- Surface inhomogeneities that contributed to the initial corrosion will still be present. Unless the final cleaning step is a hot DI water rinse, the storage area is heated, and the sheets are not in contact with the floor or walls (I suggest polypropylene slats), expect corrosion to recur.

Get price quotes on acid cleaning (+ light caustic etch + deox.) + chem film of all 20 sheets as a single job & in small sets. My guess is that the latter may cost 4-5 times as much.

Don't chem film at home; it could be even more expensive.
--- Pacific Gas & Electric of Erin Brockovich [affil link to Amazon] fame on Feb. 3 announced a settlement of $295 Million to claimants exposed to hex. chrome. This is in addition to prior settlements and the actual soil and aquifer clean-up costs.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

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If I recall my college chemistry, what makes aluminum corrosion resistance is its affinity for oxygen molecules which tightly bond to the surface of the aluminum. This makes a relatively impermeable layer of oxygen molecules leaving the aluminum in a relatively unaltered state. Similar to flash rusting of steel, but not a unsightly. If simply exposed to the air, I do not think corrosion the extent you saw when exposed to water and the like would continue.

I found pieces of airframe in 40 feet of water off Koror, Palau from a F4F that coated with zinc chromate were as pristine as they were the day they were made, whereas the iron Pratt and Whitney engine (detached from the airframe by the crash)looked like a coral head.

Keith Patton
- Texas
August 10, 2009