CoatingsPro Magazine

JAN 2013

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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Never AgAiN AvoidiNg CoAtiNg FAilures oN Hot dip gAlvANized MetAl By Mike o'Brien A coating applied over hot-dip galvanizing (HDG) is commonly referred to as a Duplex Coating System. According to literature published by the American Galvanizing Association, "A duplex coating system affords greater corrosion protection than paint or hot-dip galvanized can provide alone. In fact, many tests have shown that a duplex system lasts from 1.5 to 2.5 times the normal combined life of both zinc and paint systems.1" Each year, numerous, massive, and oftentimes expensive premature coatings failures (PMCF's) occur with duplex coating systems. In one failure of which 1 Duplex Systems: Painting Over Hot-Dip Galvanized Steel, American Galvanizers Association, Centennial, OH, 2005, p. 5. the author is aware, the costs associated with investigating, litigating, and resolving the duplex coating system failure exceeded several million dollars. Reality simply does not seem to substantiate the extended life claims made in reference to the duplex system. Why? If hot-dip galvanizing is such a great corrosion-resistant product, why is it regularly painted over with coatings? Why do so many PMCF's occur every year when coatings are applied over HDG? What is the proper surface preparation for HDG when applying a duplex system? Is it better to apply a duplex coating system in the galvanizing shop or to wait until it arrives in the field? This article and a companion article slated for the March 2013 edition of CoatingsPro Magazine will provide answers to these and other each year, numerous, massive, and oftentimes expensive premature coatings failures (pMCF's) occur with duplex coating systems. 80 Coatingspro g January 2013 Above This surface shows significant corrosion occurring on HDG, beneath the epoxy primer. This corrosion eventually resulted in delamination of the entire coating system from the HDG. relevant questions regarding painting over HDG. The focus of these articles is batch hot-dip galvanizing, not continuous sheet galvanizing. Why Apply CoAtings on hDg? Liquid coatings are applied on HDG for the following purposes: (1) to enhance appearance (aesthetics), (2) to match an existing color or provide a regulatory color code, and (3) to lengthen the life of HDG by providing barrier protection from attack by acids or salts in the surrounding environment. Coating failures occur on galvanized steel for a variety of reasons. Before discussing typical conditions and circumstances that cause PMCF's with duplex systems on batch HDG, it is necessary to familiarize readers with the galvanizing process and with the transformation of zinc after it is applied on steel. This foundation will greatly assist readers'

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