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Will a climbing ivy adversely affect a corten steel bridge?

Recently our community was the recipient of a "free" footbridge, courtesy of our state department of highways. When the bridge arrived and was erected, everyone was put off by its appearance since it looked like it had rusted. Many suggested painting the bridge but we are told that doing so would defeat the purpose of corten steel which is to resist degradation by oxidation. (I hope I have that right because I think it's a fancy way to say "it looks rusty because it is rusty.") Anyway, one suggested alternative to painting it was to allow a climbing ivy to cover the bridge. One of the city's engineers has rejected that idea because he says that the ivy would trap and hold moisture against the bridge. I'm no expert in metallurgy but it seems to me that if it's already rusted, what difference does it make? This whole thing has become a bit of a political issue so I'm wondering if someone can give me the straight and lowdown on this. Thank you.

David A. Feeback
attorney - Bluffton, Indiana, USA
2006

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2006

Regular steel rusts too, David. The problem with rust of non-Corten steel is that the rust is dusty, water absorbing, and poorly adherent: so rust just breeds more rust and corrosion continues unabated. The theory behind Corten is that by altering the formulation and processing, a rust can be generated that is adherent and does deter further rusting, somewhat like paint would do.

There is little doubt that ivy is vaguely bad for any construction material, whether metal, concrete, or wood. But I am delighted to allow Ivy to grow on my wood house anyway, balancing aesthetics against maintenance in a ratio that suits me, which is what your town may choose to do. Please contact U.S. Steel at www.ussconstruction.com and see if they have any specific experience and case histories on the effect of ivy or other vines on Corten. Although I suspect that your engineer is correct in principle, the problem may not be severe. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey