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Corrosion of marine infrastructure is relentless. The continual exposure to moisture in the presence of chlorides can rapidly degrade steel structures. The use of paint as a barrier coating for corrosion control requires near constant maintenance. Conversely, a duplex zinc coating, that is, a metallic zinc coating with a paint top-coat, offers both barrier and sacrificial corrosion protection mechanisms, with much longer lifetimes between maintenance compared to paint only coatings. A paint top-coat over a metallic zinc base layer protects the zinc from initial corrosion. The metallic zinc base layer similarly protects the paint from underfilm corrosion at scratches and holes. In this way, duplex zinc coatings provide more than twice the life of the corrosion protection provided by one coating system alone. The synergy between the zinc and the paint coatings provides protection far superior to either system used independently. Duplex zinc coating systems have decades of proven performance protecting marine structures from corrosion.
Key Words: Zinc Coatings, Hot-Dip Galvanizing, Thermal Sprayed Zinc, Duplex Coatings
The polarity of the zinc-steel galvanic couple in hot aqueous solutions was published more than 20 years ago. It used an inorganic zinc primer coating that was applied under thermal insulation at elevated temperatures [30C-60C (86F-140F)]. Since the year 2000, industrial practices or standards do not recommend using inorganic zinc rich coatings under thermal insulation. Research has showed over the years that good practice of corrosion prevention under insulation is to apply an additional layer of a heat resistant modified epoxy or inorganic polymer coating as an additional barrier.
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Hot dip galvanizing steel articles after fabrication is one of the most widely used methods to provide corrosion protection. As a final step in the process, the hot dip galvanized coating is inspected for compliance with specifications. Correct measurement techniques and accurate interpretation of the results are necessary to provide an acceptable hot dip galvanized coating.