Thermal Spray Coatings for Application in Petrochemical Field: A Comparison of Tungsten Carbide, Chromium Carbide and Inconel 625

Abstract An important problem for the petrochemical industry is the behaviour of materials in aggressive environments, when hydrogen sulphide, carbon dioxide and sand, which contribute to corrosion erosion of the surface, are present. Generally, the use of hard materials such as thermal sprayed tungsten carbide and chromium carbide reduces this problem. Cemented carbides are quite suitable for this purpose: they are composite materials of pure carbides with binder metal alloys of low melting point and high ductility; the selection of the binder metals depends mostly on its ability to wet the surface of the carbide particles to ensure secure coating adhesion. Among the cemented carbides, namely tungsten carbide cobalt-chromium based (WC/CoCr) is considered as the standard for application to ball valve bodies and seats in the petrochemical field, while chromium carbide nickel-chromium based (Cr3C2/NiCr) is suitable for particular applications. Inconel 625 is also used in this field and usually applied by welding. This paper addresses the characterization of corrosion behavior of HVOF coated samples of WC/CoCr, Cr3C2/NiCr and Inconel 625 in aggressive environments, and in particular ferric chloride test according to standard ASTM G48-92 and H2S/CO2 test based on NACE standards has been carried out. According to the test results, WC/CoCr based coatings show the best behavior both in terms of corrosion, thus confirming to be very versatile and useful for the application in petrochemical field.