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Galvanized but unpainted doors rusted in one year

My question concerns the limitations that galvanizing provides. I am involved with a job on the South Texas Coast. The hollow metal frames were provided as galvanized and also some as galvannealed. The frames were onsite and installed for over a year prior to being painted. There is much rust showing now on the frames and it is even coming through on frames that have been painted. These frames were not provided with bituminous coating because it was not specified. My question is does galvanizing alone provide assurance against rust or is there some standard as far as how soon it needs to have a coating applied ?
On this same project there is widespread rust surfacing including on stainless steel items.

Michael Rouse
- Spring, Texas
April 15, 2009

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April 17, 2009

Rust forming on galvanized steel within a short time like that suggests something anomalous.

You didn't mention (maybe don't know?) if this is pregalvanized sheet made into doors or post fabrication hot dip galvanizing.
The former is very smooth, but has a thin zinc coating compared to the latter, but the latter lasts longer.

Under "normal" atmospheric conditions, galvanizing should last many decades. In low pollution areas perhaps 80-100 years, in coastal or higher pollution areas, at least 20.

But galvanizing has a weakness (don't we all?), and that's acid. It cannot withstand acid, and acidic conditions can erode it within weeks at worst.
Acid conditions can occur with a few specific conditions, including (but not limited to) cow excrement, moist peat soils, acid from chemical plants, certain fertilizers, heavily polluted moist conditions (acid rain), certain plant leaves accumulations, etc.

Other than galvanizing in these conditions, its hard to see how it would rust quickly.
Painting galvanized steel will make it last longer (you need to pretreat fresh galv. to get the paint to stick).

You mentioned galvanneal. This is a much thinner coating, closer in zinc thickness to zinc plating than hot dip galv. The thinner the zinc the less corrosion protection duration you'll get. With plating, you might get 10-12 microns thickness. Galvanneal 15-25? Hot dip galv. 100 microns. Life expectancy is directly proportional to thickness.

That you have stainless rusting suggests a highly corrosive environment. (Guess that would be 304 stainless grade?)

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK