United States Department of Transportation
i
Adhesion Criteria between Water-Based Inorganic Zinc Coatings and Their Topcoats for Steel
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1999-03-01
[PDF-2.33 MB]
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TRIS Online Accession Number:00763217
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Edition:Final Report, Oct. 1995 - Mar. 1998
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Abstract:This report presents the results of a test program designed to investigate, in the laboratory, possible causes for failure of water-based inorganic zinc (WBIOZ) coatings. Failures observed in the field have consisted of either topcoat delamination or the appearance of "freckle rusting" in the cured WBIOZ primer. There are many hypotheses regarding the cause of these failures, but no sufficient explanation exists. Furthermore, there are no proven means to ensure that failures do not occur. The general consensus of the literature is that the failures relate to improper cure of the WBIOZ primer. This improper curing is further hypothesized to relate to environmental conditions. The project initially focused on identifying what relationship, if any, existed between environmental conditions and the observed failure modes. Since a definitive relationship did not exist, further testing was conducted to investigate the effects of substrate salt contamination, improper zinc content in the mixed coating, and the effects of rinsing the primer before topcoating. This report presents the more practical results of the study. It demonstrates that environmental conditions may be a contributing cause of the failures but are not the primary cause. Substrate salt contamination and improper mixing of the coating were shown to cause freckle rusting. Acrylic topcoat delamination was related to residual alkaline compounds on the primer surface. The surface pH was reduced by applying a dilute acid solution or by weathering for 2 weeks in a northeastern marine atmosphere. Additional ways of reducing the surface pH (weathering or otherwise) should be investigated. See also PB97-146450. Sponsored by Federal Highway Administration, McLean, VA.;pg 70
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