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December 12 - December 18, 2011

What is the service life of a painted weathering steel bridge, compared to a painted carbon steel bridge in the same service environment?

Selected Answers

From ishita Bhattacharya of Berger Paints India Ltd on December 14, 2011:
     A painted weathering steel bridge will certainly perform better than a painted carbon steel bridge in the same service environment. Weathering steel can serve without a coating under normal humid environmental conditions but fails in a severe saline condition. It is then when the coating will serve as a protection against corrosion. Moreover, weathering steel needs to be exposed to an alternating wetting and drying cycle to develop the protective layer. But to conclude, a painted weathering steel will perform better than painted carbon steel bridge in any environment. As paint can fail in both conditions, weathering steel will have better performance compared to carbon steel.

From James Albertoni of CA Department of Water Resources on December 13, 2011:
     Ultimately, it depends on the environment. Theoretically, the painted weathering steel should have a longer service life because after the paint has failed, the weathering steel will corrode at a slower rate than plain steel. This, of course, is assuming that the weathering steel can be exposed to alternating wetting and drying cycles to form its protective corrosion product. Also, if you are too close to the ocean, weathering steel does not perform well. If you have the proper conditions for weathering steel, I don't see why you would want to paint it. It is a look that is attractive to some architects and capable of meeting most design lifes. As always, be aware that weathering steel produces rust-colored runoff that will stain everything it touches.

From Tom Schwerdt of Texas Department of Transportation on January 18, 2012:

     James, I think the requirement of a "wet/dry cycle" is bunk. We have 30+-year-old weathering steel girders here in Austin. The ones under the deck never get a wet cycle and still have most of the original millscale in place. Nowhere do they have pitting or section loss.

     Yes, you never form the "protective patina" - because it never experiences any significant corrosion. Concerns about exposures which are too salty and/or wet are valid. However, I expect a properly designed weathering steel structure in the correct environment (and I think 95+% of Texas qualifies)  should see over 100 years without any maintenance other than occasional washing/cleaning to remove debris.

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Tagged categories: Carbon Steel; Steel; Weathering

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