Metal finishing Q&As since 1989
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Pontoon Pipes for floating tank cover pitted in water
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.
- Montgomery, Alabama, United States
December 19, 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?
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
December 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
Consultant - The Bronx, New York
December 28, 2015
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