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"Pontoon Pipes for floating tank cover pitted in water"

December 19, 2015

Q. Hi

I have a pontoon pipe which is a part of internal floating roof of a tank. The whole system was placed in the carbon steel tank.

The pontoons got corroded when the tank was hydrotested. Areas immersed in the water are corroded with severe pitting.

Is it due to galvanic corrosion? Chloride content was also relatively high.

George Micheal
- Montgomery, Alabama, United States

December 2015

A. Hi George. I'm not familiar with the methods and the usual materials of construction of floating covers. Are the pontoons steel or aluminum? Are they painted/powdercoated? Is there ever actual metal-to-metal contact between the tank and the pontoons?


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

December 28, 2015

A. That sure sounds like galvanic corrosion to me.

Sadly, no type of coating, that I know of, is going to inhibit that kind of rust. Any kind of coating is going to have a certain porosity. The water/chlorides are going to zero in on that.

My best, perhaps less than helpful, suggestion is to fabricate the pontoons out of well passivated stainless steel.

Or, you might install some sacrificial anodes of zinc or magnesium in electrical contact with your pontoons.

Sadly, we cannot repeal thermodynamics.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

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