Progress on Environmentally Compliant Aluminum Ceramic Compressor Coatings

During the last decade there has been an increasing emphasis on compliance to ever stricter environmental laws as well as compliance to regulations that have been designed to protect workers from exposure to toxic or otherwise harmful substances or processes. This world-wide emphasis has forced a continuing review of materials and processes used in the manufacture and protection of compressor materials from corrosion. Turbine compressors have been coated with silicone aluminum paint, diffused nickel cadmium and aluminum pigmented ceramic coatings that contain hexavalent chromium. These three processes utilize various chemicals including toxic substances, carcinogens and volatile organic compounds (VOC). All three of the coating processes need to be either made compliant or eliminated from use. This paper will review efforts that have been made to develop compliant aluminum ceramic compressor coating materials as applied to various steel and stainless steel substrates. In all cases the new materials that have been developed are free of toxic or carcinogenic materials. Test results will be compared to specification requirements for chrome containing compressor coatings in the area of physical properties including surface finish, thickness and adhesion. Additionally, environmental test data will be presented based on standard test methods that compare new compliant coatings with conventional chrome containing materials. Finally, process steps and conditions will be described for these new coatings.