The Article “Pressure-Treated Wood: The Next Generation (FHB #160, December 03/January 04) cautions against aluminum fasteners due to the corrosive effects of ACQ AND CBA preservatives. Does anyone know if these preservatives will affect other aluminum outdoors such as deck furniture, grills, stands for umbrellas, etc? Is paint on these objects sufficient to protect them or will they need additional insulation from the wood?
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welcome to BT Bill, and what a good question! Geez I hope my folding webbed chairs dont disintergrate! Lets see what pops out of the woodwork..
Aluminum is more chemically active in the general scheme of things than is steel, and steel has problems with the copper in ACQ. Thus, putting aluminum in direct contact with ACQ and moisture would seem to possibly be inviting some problems. However, most aluminum chairs I have seen have plastic feet, so that should provide protection unless you tend to spend a lot of time tipped over in your chairs... Aluminum exposed to moist air corrodes fairly quickly but the oxide that forms is "self healing" and tends to minimize further corrosion of the aluminum, although, of course, it does continue under a number of circumstances. I have no idea if this same effect provides any degree of corrosion resistance with aluminum and ACQ. This article on ACQ and aluminum would tend to make me think any protective effect of aluminum oxide might be short lived in real life:
From http://www.housingzone.com/topics/pb/cmaterials/pb03ia009.asp :
"As the industry phases out wood pressure-treated with chromated copper arsenate (CCA) in preparation for the Jan. 1, 2004, ban, keep in mind that the high level of copper in the alternatives - alkaline copper quarternary (ACQ) and copper azole lead the field - reacts with aluminum, causing corrosion. "
From http://www.stainless-fasteners.com/pressure_treated_ACQ.htm :
"Newly developed Arsenic Free treatments including alkaline copper quaternary (ACQ) and copper azole (CA) contain approximately 6 times more copper than CCA. In long term corrosion testing conducted by the Forest Service division of USDA on fasteners used in CCA treated wood, only stainless steel nails and screws exhibited virtually no visual signs of corrosion and negligible weight loss over a fourteen year period while fasteners made of galvanized steel and aluminum showed significant degradation. These results illustrate that stainless steel is your best fastener choice."