A protective zinc patina, described as an adherent, dense layer of zinc carbonate, tends to form on galvanized zinc coatings exposed to alternating wet-dry exposure cycles. It has been reported that this protective layer develops in stages: Exposure to moisture leads to the formation of zinc hydroxide, "white rust". Subsequent chemical reaction of the zinc hydroxide with carbon dioxide in air produces the desirable, protective zinc carbonate layer.

To quote P.G. Rahrig (Executive Director, American Galvanizers Association) in the July 2002 issue of Materials Performance: "Simply put, galvanized coatings, in use, require wet and dry cycles in order to develop a natural occurring, protective zinc-carbonate patina ..."

White rust formation has been linked to a problem area known as "white storage stain" or "wet storage stain", where surface exposure conditions are not conducive to the formation of the protective zinc carbonate patina. Continuous wet, humid conditions without the benefit of freely circulating air are associated with such problems.

References/Literature:

P.G. Rahrig: "Zinc Coatings on Handrail Tubing: A Comparative Analysis", Materials Performance, July 2002.

Industrial Galvanizers Corporation: The "Z" Files, Data Sheet No. 11, "White Rust - Prevention and Cure".

Links: