Al-Mn CVD-FBR protective coatings for hot corrosion application

Ferritic steels are usually used in boiler or supercritical steam turbines which operate at temperatures between 600 and 650 °C under pressure. Protective coatings are often applied in order to increase their oxidation resistance and protect them against degradation. In this study, new Al-Mn protective coatings were deposited by CVD-FBR on two ferritic steels (P-92 and HCM12). The initial process parameters were optimized by thermodynamic calculations using Thermo-Calc software. Then, those parameters were used in the experimental procedure to obtain Al-Mn coatings at low temperature and atmospheric pressure. Co-deposition was achieved at moderate temperatures in order to maintain the substrates' mechanical properties. The coatings' microstructure and phase constitution was characterized. Fe-Al intermetallic coatings containing Cr and Mn were obtained. The phase constitution is discussed with reference to the Fe-Al-Mn ternary phase diagram. The effect of diffusion heat treatment on the phase transformations was investigated.