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Powder coating vs. galvanizing exterior steel I-beams

Our contractor is asking us change our plans: specifically switch to powder coating instead of galvanizing on exterior steel I-beams; understand the difference in the process but what is the difference with the longevity, rust, and repainting? will we be painting the I-beams 5 - 10 years from now if we go with the powder coating? thanks!

Nancy Wells
homeowner - Tahoe City, California
September 6, 2008

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September 12, 2008

Polyester powder with a zinc phosphate pretreatment could last 5-15 years - depending upon film weights, baking conditions plus many more factors. Powder coating without pretreatment may last 5 years (if no damage occurs to the coating). Damage to the powder finish could lower this to 6 months. A galvanized finish may last 25 years, and combined with powder coating may last >35 years.

Terry Hickling
Birmingham, United Kingdom

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September 15, 2008

Powder coating is a rather poor corrosion protection, offering only barrier protection, but a fantastic decorative system.
As soon as you scratch, score, mark, puncture the PC finish, you'll get rust in external conditions.
You get to choose from 300+ colours, several textures and a few different types of powder. You can get good chemical resistance, or good UV resistance.
Life expectancy until need for a re-coat (if done on ungalvanized steel and for external installation) perhaps 5 years.

Galvanizing is excellent corrosion protection, but a rather dull decorative effect. You can scratch, score mark it, and you'll still have protection. (up to 5 mm width score is protected), and this protection is electrolytic and barrier.
You get a choice of only one colour, and that a somewhat inconsistent one.
Chemical resistance is good with alkalis, poor with acids. UV not a problem.
Life expectancy until rust? Perhaps 75 years, maybe 150 years.

So, real question is: what do you want any coating for? What do you want it to do?
Protect? against what? Look beautiful?

If you want the best of both worlds, do both. Galvanize it, then PC over the top (but ensure you choose a competent powder coater for this), and get perhaps 35 years until need to re-paint.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

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