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In Field Aluminum Siding in Contact with ACQ Lumber

2004

Dear Sir or Madam:

The question is: A contractor placed aluminum finish siding on ACQ (copper impregnated) treated lumber for a portion of a job. He used standard zinc coated metal brad type fasteners. These have already corroded and the aluminum around these fasteners look as if it has been burned. Which makes sense.

Now we could use stainless steel fasteners to re attach the aluminum siding to the wood, but there will still be a reaction between the aluminum and the copper, since this is exterior siding.

1- Will the aluminum corrode fully or pit over time in the presence of the ACQ?

2- Are there any means of protecting the less noble (anodic) aluminum from the cathodic copper? e-g. with some sort of copper wire to magnesium, etc.

3- Replacing the siding will be a big expense since the entire siding job is in essence attached to these materials.

Thanks

Tom Jensen
ARCHITECT - Floral Park, New York, USA

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2005

Ah, for the good old days when change was gradual rather than mandated nationally or internationally for overnight implementation. For the good old days, when there was a chance to learn from the best teacher, experience. This is a metal finishing website, with only tangential relationship to ACQ lumber, yet we're drowning under postings of foibles and missteps as millions of people concurrently make the same mistakes on ACQ lumber because there was no time to implement it gradually and learn from the mistakes of others :-)

This stuff, due to its large mass of noble copper content, is apparently viciously reactive with virtually every metal, but especially zinc and aluminum.

1. Unfortunately, I don't see any reason to think the reaction with the aluminum siding will stop, and stainless fasteners will not help. But if you can somehow insulate the fasteners, I think it's entirely possible that the casual touching of aluminum siding to ACQ lumber will not carry destructive galvanic currents because of the natural oxidation layer on aluminum. I don't know that it will be okay, but I think it will be. 2. I doubt that magnesium sacrificial anodes can protect the aluminum because there could be an ionic path from the copper to the aluminum (a wet fastener hole for example) when there is not an ionic path from the aluminum to the magnesium.

If possible please let us know how this works out as I know that others face similar quandaries.

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