Aluminum alloys & corrosion
I am trying to research the corrosion resistance of aluminmum alloys for architectural applications. Specifically, we want to use sanded aluminum plate (a #4 sanding with a mop or broom finish) on the facade of a building. This is the rough finish we are looking for, and do not wish to use an anodized or powder-coated plate. So far, fabricators have recommended 5056 or 5083. The alloy would have to be commercially available in 0.190 and 48 x 96 plate. We accept that any aluminum will form corrosion, but wish to minimize it. The site is in an urban but not an industrial area. Any advice would be greatly appreciated.
architects - NYC NY
Aluminum is a very reactive metal. Magnesium is even more reactive. The alloys you cited have over 4% Magnesium. Both metals have a natural tendency to oxidize, and in an urban environment where acid rain and soot and road salt are inevitable, any unfinished aluminum plates are bound to show some corrosion as part of the re-oxidation within a short time. I mention magnesium especially because its reaction products are white, and the surface is bound to end up with white pits or streaks. The rough surface you desire will compound the problem, as dirt and moisture will tend to stick in the grooves of the sanded finish.
I would reconsider the decision not to anodize them. An anodize coating is a thick, practically inert alumnum oxide, many times more corrosion resistant than the bare metal.The alloys you mentioned have excellent corrosion resistance when anodized. The anodizer would have to use caution not to etch away the sanded finish, and yet not leave embedded sanding particles in the surface, but many anodizers do finishes like yours without problems. The cost of anodizing is normally a small percentage of the total material and fabrication costs, especially considering possible replacement a few years later. Paint is an option, but the beauty of a clear anodize is that the coating is clear, and the aluminum surface will retain its metallic look.
P.S. If I sound a little prejudiced toward anodize, it's only because I've been around it for some 29 years, mostly in job shops where I guess I've seen most types of aluminum corrosion.
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