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There exists a need for environmentally benign coatings to eliminate cadmium and/or chromate coatings on high-strength fasteners. These alternative replacement coatings would provide high-strength, corrosion resistant fasteners for use in weapon systems.
There exists a need for environmentally benign coatings to eliminate cadmium and/or chromate coatings on high-strength fasteners. These alternative replacement coatings would provide high-strength, corrosion resistant fasteners for use in weapon systems. Traditionally, high-strength steels have been used with cadmium (Cd) electroplated coatings, followed by a hexavalent chromium (Cr6+) rinse. The environmental and personal hazards associated with both cadmium and chromates are well established. The elimination of hazardous materials will improve the life cycle costs and environmental / occupational health issues associated with the manufacture and maintenance of DOD weapon systems. Alternatives to both processes have been pursued with very promising results. This work describes a multi-layer approach to provide a protective coating system. These systems include a sacrificial metal base coat and may also include an organic topcoat technology that will provide the best combination of corrosion resistance. These approaches impart high end sacrificial corrosion resistance properties without use of hazardous Cr or Cd and without hydrogen embrittlement of fasteners.
Various types of zinc-rich coatings are available for corrosion protection. The challenges associated with the proper specification and application of zinc-rich coatings are briefly discussed. The paper then focuses on data from various studies, which quantify the corrosion protection benefits of various types of zinc-rich coatings.
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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.
The application of paints and other coatings over joints, gaps, and fasteners is universal in industrial applications. Where substrate flexing occurs, such as in aerospace and automotive use, an understanding of strains within such systems is essential. Results of finite-element analysis of various two- and three-dimensional models of coated joints are presented here.