B. Rammer, M. Schütze
Sewage sludge is a practically endless source of the essential plant nutrient phosphorus. However, removal of their heavy metal contents is obligatory to enable the application of sewage sludge ashes as fertilizer material. A thermochemical process accomplishes this separation in high chlorine containing environments at temperatures of up to 1000°C. Unfortunately there are no metallic furnace materials commercially available that can withstand such conditions over longer periods of time. Previous experiments have shown – in accordance with thermodynamic calculations – that corundum is resistant to oxidizing high-temperature chlorine corrosion conditions. Although it offers outstanding corrosion resistance, its mechanical properties – in particular its brittleness and low thermal shock resistance – make it inapplicable as a material for large moving components such as rotary furnace tubes.
A new coating concept combines the chemical resistance of corundum with the mechanical properties of a metallic alloy. It consists of a two-layer thermal spray coating system with a ceramic top coat and a corrosion resistant metallic NiAlMo bond coat that is specially designed for the use in reducing environments as can be expected under the ceramic top coat. Results from the coating development and their performance under the testing conditions will be reported in the paper.