PS Advisor 02/01/01
Aluminum Tank Problem
My boat has aluminum freshwater storage tanks. I have access ports between all baffles. I am noticing large calcium-like deposits throughout the tank. When I scrape these off, I find large pits in the aluminum beneath the deposits. I assume the deposits are caused by a chemical reaction between the aluminum and chemicals in the water.
Can anything be done to prevent these deposits from occurring?
Many marine surveyors don’t approve of aluminum for water tanks, either because of uncertainty as to the quality of the alloy used, or because installation has to be immaculate in order to ward off corrosion. Despite its ability to protect itself in ideal conditions by forming a thin, uniform layer of oxide, aluminum is susceptible to corrosion due to stress, stray current, and particularly “crevice corrosion,” in which water is trapped in tiny spaces in the metal and, without oxygen or the ability to flow freely, becomes acidic and thus corrosive, creating an electrical current that eats into the metal.
As small pits form, more and more corrosion occurs, leading to bigger pits and gaps, and eventually to a failure in the tank. The problem is worse in areas with alkaline or hard freshwater supplies, and obviously if the tank is exposed to saltwater, inside or out. So the calcium-like deposits you mention are actually heavy oxidization forming in and over the corroded areas. It sounds as if the baffles in your tank may be contributing to the problem, because the welds and seams are providing more and more places for corrosion to occur.
The good news is that the deposits you see are much larger than the holes in the metal from which they’ve formed, and things often look a lot better after a thorough cleaning. So the answer to your second question is definite: Wire brush and clean the deposits out completely and expose the pitting so that you can evaluate it.
As for the first question, if the pits aren’t too extreme—that is, if they’re not weakening the structure of the tank itself—you might be able to dry the tanks completely, then reach inside your inspection ports and brush on some coats of epoxy sealant.
While epoxies generally become inert after curing, representatives at both Gougeon Brothers and 3M’s marine division noted that their epoxies are not officially approved for potable water systems.
There is, however, a resin-based product called Ceram-Kote-54, which is approved for potable systems in the U.S. According to company president Jack Mueller, your tank would be a good candidate for this application. For more information call or e-mail Ceram-Kote sales representative Jim Cummings in Pennsylvania: 570/287-4646, e-mail: firstname.lastname@example.org.
If the pitting has progressed too far, the best solution, unfortunately, will be to replace the tanks.
Ice a Threat?
Now that the sailing season has come to a close here in the Northeast, I’m faced with a storage problem. I’d like to keep my boat in the water for the winter. I can tent it (shrink wrap), leaving it available for me to work on various projects. My concern is the bay freezing over. I see many boats in the water each winter and always wonder if ice is truly a legitimate concern. Can the hull be damaged? Are there ways to prevent any possible damage?
Great Neck, New York
Yes, thick ice can damage a hull, though in our recent, milder winters the threat seems less serious. We guess we have global warming to thank for that. Most saltwater ice around here remains a bit punky until very, very cold. In our experiences living aboard in Newport Harbor, the ice only got a few inches thick around the boat and was continually breaking and cracking due to tide, wind, etc. We couldn’t stand on it. Depending on hull shape, ice squeezing the hull may simply push the boat upward.
Now, 6 inches or more of ice that stays in place for weeks would worry us, but we have not seen that here.
If you want to play it safe, buy a bubbler, specifically made for keeping ice from forming around boats…you see them in use at some marinas. Also called de-icers, they sell for $300-$400 at discount.