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Aluminum 6061 Deteriorating?

I am doing research on possible causes of deteriorating aluminum 6061 plates. We have a steel deck on top of two aluminum columns. The problem is that the plates on the underside of the deck are completely deteriorated while the plates on top of the deck are completely fine. Other than salt water coming from the ocean, what other possibilities can contribute to this?

Jerry Turek
- Vancouver, BC, Canada
2003

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If this is important, you would have to have a metallurgical or corrosion consultant investigate it. All you can get over the Internet from people who haven't seen it, and probably aren't even picturing it right, is wild guesses. But my wild guess is the top quickly dries in the sun while dew and rainwater cling to the bottom and cause the corrosion.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

It is possible that the steel is corroding, even slightly, and the corrosion products are getting onto the lower aluminum parts of the structure. These corrosion products can contain ferrous/ferric ions that will galvanically attack the aluminum. The aluminum will become oxidised, while the ferrous/ferric ions will be reduced.

Tony Mazza
- Whitby, Ontario, Canada
2004