Currently, there is a need to develop cleaner electric-power systems, with reduced emission of pollutants. This implies an increase of the maximum temperature of heat exchanger tubes, which is now about 600 degreesC in modem power plants. For conventionally used ferritic steels, this increase is limited by their mechanical properties and their corrosion resistance in coal, waste or biomass firing environments. Austenitic steels and nickel-based alloys are potential candidates for increasing performance at higher temperatures of up to 700 degreesC, even if they are more expensive and have lower thermal conductivities. Nonetheless, the corrosion resistance of these materials could even further be improved 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 degreesC. Especially for ferritic-martensitic steels, the microstructure of the material may be significantly changed at these temperatures during the coating process. Since microstructure dictates the mechanical properties, these may also suffer by the coating process. As a consequence, there is a need to develop the pack cementation process to lower temperatures. In the investigations to be reported here the ferritic steel P91 was coated at 650 degreesC and/or by using the standard heat treatment of the material. In order to facilitate the penetration of the protective elements such as Al during the coating process, different surface treatments were applied before the process increasing the number of diffusion paths in the metal subsurface zone. Furthermore the metal activities in the powder pack were optimized using thermodynamic computer calculations.