Metal finishing Q&As since 1989

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Our Aluminum Boats discolor in 24 hours in city water test tank

We submerge our raw 5052 H34 aluminum boats in our water filled testing tank, in as little as 24 hours the part of the aluminum that is under water is changing color, becoming a much darker, browner, dark grey color. The color change of the aluminum can be removed with a polish, so it is not penetrating to a great degree. We had emptied the tank at the beginning of January, and I had submerged some samples in the new water for various lengths of time, with no color change. Periodically since then, I have submerged aluminum samples, with the color change becoming more and more apparent the longer the water is in the tank. The water has been in the tank for 2 months now, and boats exiting the tank after being submerged for only 24 hours are in terrible condition. We use treated city water in our test tank, and I have test reports on the water supply, nothing seeming unusual. What should I do next?

Dave Noseworthy
- Winnipeg, Manitoba, Canada
2003

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2003

5052 is a very "active" (ready to corrode) alloy, just look at it cross eyed and it turns color.

5052 in "treated city water" in a plastic tank will chemically corrode.

5052 in treated city water in a metal (even stainless) tank will "electro"-chemically corrode.

5052 in DI water in a metal tank will electrochemically corrode.

Try distilled water in a plastic tank sealed from atmospheric (acid rain) contamination and probably the 5052 will last longer before corroding.

Even then, galvanic cells will set up in microscopic pits and near bends and near welds, and still, corrosion will take place.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

This is good information, but it still does not explain to me why the submerged samples of aluminum only discolor after the water has been in the tank for more than about 60 days. When I take some of the "old" water out of the tank and place it in a plastic container and submerge samples, there is no discoloration. If there is some kind of breakdown of my aluminum tank, and then over time the water's concentration of aluminum rises, is it possible for this aluminum to plate onto my submerged samples. If so, how can I stop this.

Dave Noseworthy
- Winnipeg, Manitoba, Canada
2003

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2003

Your metal tank is setting up a galvanic relationship with the aluminum. The plastic is not. (1) Try shorting the aluminum electrically to the tank (2) try insulating the aluminum from the tank, (3) Use a plastic lined tank.

Again, all you have to do is bend a piece of aluminum and you set up a galvanic site.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

The aluminum is corroding rapidly because city water contains a high level of chlorine. As a "science experiment" you can take a piece of aluminum foil and place it in a cup of pool water. It will fizzle and corrode before your very eyes.

David Yont
- Leonardtown, Maryland USA
March 8, 2011