Diffusion Coatings for Heat Exchanger Materials

Abstract

There is a need to develop cleaner electric power systems, with reduced emission of pollutants. This implies an increase in the maximum temperature of heat exchanger tubes, which is now, 600°C in modern power plants. For conventionally used ferritic steels, this increase is limited by the steels' mechanical properties and their corrosion resistance in coal, waste or biomass fired environments. Austenitic steels and nickel based alloys are potential candidates for increasing performance at higher temperature (700°C. Nonetheless, the corrosion resistance of these materials could be improved even further by applying a coating. Pack cementation is one of the easiest and cheapest coating processes. However, it requires a heating step which is presently performed at a minimum temperature of 750–800°C. The microstructure of the material may be significantly changed at these temperatures during the coating process, especially for ferritic–martensitic steels. Since the microstructure dictates the mechanical properties, these may also suffer from the coating process. As a consequence, there is a need to develop the pack cementation process at lower temperatures. In the present paper, ferritic steels P91 and HCM12A, and austenitic steels alloy 800H and a 347 type steel with 17Cr/13Ni, and the nickel based alloy IN617 were aluminised at temperatures below the above mentioned range. In order to facilitate the penetration of the protective elements during the coating process, different surface treatments were applied before the process to investigate the influence of an increase in the number of diffusion paths in the metal subsurface zone. The influence of surface treatments such as grinding or glass bead blasting on the efficiency of the process is discussed in detail.