Painting peels off of hot-dip galvanized pipe
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We have installed some hot dip galvanized pipes in outdoor, which carries water at about 100 Deg C. We used a primer coat followed by an epoxy paint. But we notice that the paint peels off. So can you advise us on the painting to be used. Srini vasanEngineering - Singapore ++++ There many reason of peeling of paint in duplex coating after galvanizing if you want to do duplex coating you need to do proper pretreatment Remember you should do application with in required weather environment Syed Kazmi- Dubai, U.A.E ++++ Adhesion of paint to galvanized steel is very difficult. Without a good etch primer is is almost a hopeless case. You said you used a primer, but was it an etch primer? Some galvanizers passivate the surface after galvanizing, and this too can have an effect.
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Have you considered powder coating over galv as a better way to do this?
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Geoff Crowley |
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Galvanized pipe that is sold as galvanized pipe has been passivated by the producer. Passivated galvanized does not take paint it will peel off.
You can check for passivation through one of the following methods.
1) Make up a 10% solution of copper sulphate. If a drop is placed on the surface of the galvanize and turns black, it is not passivated. With time it may turn black even if passivated.
2) Add 0.2% diphenol cabazide to a 50/50 by volume acetone [linked by editor to product info at Rockler] water mixture. Make a second 10% sulfuric acid solution. Mix 1 part of the first solution to 10 parts of the second solution. Place a drop on the galvanized sample. If it turns purple it is passivated. When part 1 and 2 are mixed they are only good for a few days, so only mix what you need.
Epoxy and urethane primers will stick well to unpassivated galvanized surfaces.
Ronald ZeemanCoil coating - Brampton, Ontario, Canada
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As you did not elaborate on paints used it is somewhat difficult to assess your problem from a professional point of view.
But here goes. You appear to be getting Embrittlement, possibly you used an Inappropriate undercoat/ 100 degrees Centigrade is also going to be a major problem unless you use a High tech system. Luke in Oz
Luke Speer- SAFETY BEACH, NSW, Australia
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Dear Mr. Speer, I did not understand the embrittlement part of your answer. Does that mean, Zn coating is embrittled? Please advise. With Best Regards Pankaj Gupta- Baton Rouge, Louisiana ++++++ My husband is building our granddaughter a swing set using new galvanized pipe and now we want to paint it-is there a certain kind of paint that must be used on new galvanized pipe and is there any preparation that must be used? Thank-you, Connie RobinsonDavid's Welding - Yoakum, Texas April 14, 2009 Hi, Connie. Yes, you should scrub it with a brush and a bucket of
trisodium phosphate Regards,
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, rinse it, then prime it with a
