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Current IssueOctober 02, 2014
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Ask The Expert: Three Questions on Corrosivity: Wary of Aluminum
11/01/2000 
 
Question: I was referred to you folks as possible sources of information about how to inhibit corrosion in aluminum potable water tanks. I have been told, aside from painting them (which, because they are in a sailboat and integral to the hull, thus is nearly impossible) that additive substances like polyphosphates and silicates might help. Do you have any thoughts on these or other strategies?

Tim Liveright 

Answer: Answer: Aluminum doesn't corrode very rapidly in fresh water. When it does, the oxide film on the surface usually protects against further corrosion. Anodized aluminum resists corrosion because the anodizing process deliberately creates the protective oxide film on the surface. It may be possible to anodize the tank (this is done by soaking in chromic and/or sulfuric acid). The potential problems include getting the inside thoroughly coated and removing the anodizing solutions (which are poisonous) well enough to safely use the tank for drinking water.

You can certainly add a little polyphosphate to the water, assuming you don't mind drinking water that contains polyphosphates. You would have to add it to every tankful of water; otherwise, the coating on the aluminum surface would gradually dissolve, thus exposing the aluminum to renewed corrosion. I'm not familiar with silicates, but they likely have the same tendency to re-dissolve if not added continuously. Personally, I have never liked the idea of adding anything to my drinking water.

You should be aware that many drinking water supplies contain aluminum at concentrations much higher than what you can expect to get from aluminum leaching out of your storage tank. This is because alum is deliberately added as a coagulant and some dissolved aluminum remains in the water after filtration. A link between aluminum and Alzheimer's disease has been suggested, but never demonstrated. And traces of aluminum are not considered hazardous -- but I'll admit I don't use aluminum cookware.

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