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A five-year coatings research project was initiated by the Pennsylvania Department of Transportation in 1986 to evaluate the performance of an organic zinc rich coating system. The coating system consisted of a moisture cured urethane zinc rich primer, an epoxy intermediate, and an aliphatic polyester urethane finish coat. The system was applied to the Windgap Bridge located in McKees Rocks, Pennsylvania (near the City of Pittsburgh) in 1986 and 1987.
A five-year coatings research project was initiated by the Pennsylvania Department of Transportation in 1986 to evaluate the performance of an organic zinc rich coating system. The coating system consisted of a moisture cured urethane zinc rich primer, an epoxy intermediate, and an aliphatic polyester urethane finish coat. The system was applied to the Windgap Bridge located in McKees Rocks, Pennsylvania (near the City of Pittsburgh) in 1986 and 1987. Coating system performance was tracked for the next five years using a preestablished protocol of visual inspections and physical testing. The coating system, which has now reached a twenty-year milestone, was re-inspected in 2007. Even though no coating maintenance work was performed since the initial installation, the coating system continues to provide good corrosion protection and has maintained a good appearance. This paper presents the results of the 2007 inspection and compares those results with data obtained during the original five-year research project.
Zinc-rich coatings have long been known to provide excellent corrosion resistance in highly corrosive environments, in general,inorganic zincs for new construction and organic zincs for maintenance. A recent trend has been toward zinc-rich coatings with reduced levels of zinc dust. An SSPC committee formed to revise SSPC Paint 29, Zinc Dust Sacrificial Primer, Performance Based, to reference performance only, removing reference to minimum zinc dust level.
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Zinc rich coatings have long been used in the protective coatings industry as one of the primary means of steel substrate protection against corrosion. The primary protection mechanism has historically been galvanic sacrificial loss of zinc metal and the simultaneous formation of protective zinc oxides and salts. Various standards and customer specifications exist to ensure that the coating will provide the necessary corrosion protection for the life of the asset.
Zinc-rich primers, with zinc dust loadings of 80-85% by weight in the dry film, are often the preferred primer during new construction of assets placed in environments with high atmospheric corrosivity. Coating standards such as SSPC-Paint 20 and ISO 12944 demand that zinc-rich primers contain at least 65% and 80% zinc dust by weight in the final dry film, respectively. Traditional zinc rich primers need this high zinc loading to achieve galvanic protection of steel. New technology allows us to develop zinc primers with a lower content of zinc and/or different zinc morphology than dust to provide similar or better corrosion protection to the steel.