Three queries on corrosivity
Reluctant guinea pigs
Question: Our homeowners association along with the Department of Environmental Quality wants to put stannous chloride in our water to solve the corrosive water problem. I am against any more chemicals added to water. I think aeration would be safer. What other alternatives would you suggest for treating the water, and if we settle on the stannous chloride, will an RO water system filter it out of our drinking water? Have you any tests on the effects of stannous chloride on people? We are going to be the guinea pigs if the association agrees to it. Any info you could send my way would be greatly appreciated.
Frankie Avalon Wolfe, Ph.D., M.H.H., R.N.C.P.
Healing Feats Holistic Health Services
Answer: First of all, stannous fluoride is often used in toothpaste (as well as other salts containing tin) and is a proven aid in preventing tooth decay. Second, tin (stannous means tin), is generally considered to be non-toxic and, before it became too expensive, was used to make “tin” cans. While the two are not the same compound, a few parts per million (ppm) of stannous chloride added to your water isn’t likely to do you any harm.
I haven’t heard of anyone using this chemical for corrosion control and it seems like there may be less expensive alternatives. Yes, an RO system will remove most of the stannous chloride that’s added, along with other salts and minerals. A carbon prefilter provides sufficient prefiltration of suspended solids for most applications. A carbon postfilter should also be used, and you can either get a unit with the postfilter between the membrane element and the storage tank or between the storage tank and the faucet—you don’t need them in both places.
One thing for certain is that aeration, except in very unusual circumstances, isn’t going to make a water less corrosive, except inasmuch as it reduces hydrogen sulfide (H2S) which can be corrosive. Without knowing more about your water supply and the analysis, there’s no reason to second guess what your environmental engineers are telling you.
Wary of aluminum
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 near impossible) that additive substances like polyphosphates and silicates might help. Do you have any thoughts on these or other strategies?
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.
ROs and copper
Question: I read your publication every month and I’m designing two new houses in which I would like to install “under-the-sink” RO systems that would supply icemakers in refrigerators. I’ve heard that RO water cannot be used with copper water supply tubing due to leaching. Is this true with all RO systems?
Andrew Allocco, P.E.
Indian River Construction Services, Inc.
Vero Beach, Fla.
Answer: The issue with RO systems is that they reduce pH and—because of a reduction in the buffering afforded by alkalinity—increase corrosion (of most metal piping including copper). The typical RO unit will remove 90 percent of salts in water, so you can assume the water salts concentration of the purified water would be about 1/10th of the content in the feed water. The aggressiveness of this purified water toward the materials into which it comes into contact is a function of its purity—the higher the purity, the more its aggressiveness. All RO systems reduce pH (usually by 1 to 2 units).
One thought is to add a calcite filter (calcite is calcium carbonate and it elevates pH by gradually dissolving) to raise pH after the RO unit. Although in some respects, adding salt back to the water after going to all that trouble to take the salt out seems a little crazy. Can you use plastic or stainless tubing? This would eliminate the concern about corrosion. Brass tubing is significantly better than copper from the standpoint of resisting low pH corrosion, but you run the risk of lead leaching. As a rule of thumb, plastic tubing is your best recommendation in connecting RO to a fixture. And while there maybe some copper in a refrigerator, for example, it’s not likely to pose much of a problem.