Introduction

Generally, high-temperature coatings are used on sutfaces to provide corrosion protection and/or for aesthetic reasons. A good understanding of the sutfaces to be coated and the service conditions typically aids in the selection of the coatings. Sutfaces are composed of carbon steel, various grades of stainless steel and, in some cases, nonmetallic materials. Considerations normally include whether the coating selection is based upon "operating" or "design" conditions, whether the sutfaces are in continuous or cyclic high-temperature service, and whether or not the sutface is insulated.

This report addresses high-temperature coatin s for uninsulated surfaces. NACE Publication 6H189 discusses coatings for use under insulation.

Corrosion usually does not occur on carbon steel when moisture (electrolyte) has evaporated from the sutface. This normally occurs above 100°C (212°F); however, as a factor of safety the threshold level is normally considered to be 107 to 120°C (225 to 250°F). Stress corrosion cracking (SCC) of some 300 series stainless steels can occur above the temperature range of 60 to 66°C (140 to 150°F) when chlorides are present. These threshold temperatures can change when other chemical environments are involved. High-temperature coatings often provide protection from corrosion during cyclical temperature variations that allow the sutface temperatures to drop below the stated threshold temperature levels. Coatings also protect the sutfaces from corrosive contaminants associated with environmental conditions (e.g., salt air, acid rain).