Development of diffusion coatings on nickel base alloys for the use in chlorine-containing high temperature processes

To open up the possibility of using sewage sludge ashes as fertilizers the removal of their heavy metal contents is obligatory. A process newly developed at the BAM Berlin executes this separation in highly chlorine-containing atmospheres at temperatures of up to 1000 degrees C [1]. Unfortunately there are no materials available which can withstand such conditions over longer periods of time. This project deals with the development of materials that allow the operation in highly corrosive environments. The corrosion resistance of nickel base alloys against chlorine-induced high-ternperature corrosion will be optimized by application of aluminum and/or silicon-containing diffusion coatings. As coating method the pack cementation process was selected. In this process, the metal to be coated is embedded in a powder, consisting of the coating metal, a halogen-distributor (e.g. ammonium chloride) and aluminum oxide as filler material. During an annealing process of several hours at temperatures of 800 to 1000 degrees C, gaseous metal halides form. They diffuse through the powder pack and decompose at the substrate surface, thereby depositing the coating metal. Subsequent solid phase diffusion results in the formation of a protective diffusion layer. From the thermodynamic point of view, materials with a high content of aluminum and silicon show best prerequisites to build up slow-growing, stable oxide layers with a high potential to protect the material against corrosive attacks. The actual performance of the materials will be examined in long-time tests under simulated field conditions (high temperatures and chlorine-containing atmospheres).