Steam oxidation and mechanical performance of a ferritic-martensitic steel with slurry aluminide coating

A key route to advancing power plant efficiency is to enhance current steam operating parameters, namely, increase in steam temperature and pressure from the current levels of 540 degrees C and 180 bar to the proposed 700 degrees C (EU) and 300 bar. Conventional ferritic-martensitic steels, while having good mechanical properties and fatigue resistance, do not possess the high-temperature corrosion resistance for long-term use, with a temperature limit of similar to 620 degrees C. Adding aluminide coatings to the surfaces of the steels can improve the oxidation resistance of this class of material through the growth of an Al2O3 scale. One of the major concerns in using coating is the potential deleterious effects on the mechanical properties of the substrate, either through the precipitation of brittle phases or through cracks in the coating. This study compares the microstructural features of slurry aluminide coatings applied to Grade 91 and the change in mechanical performance. The tensile, thermomechanical fatigue and creep test results showed that while the coated samples entered the tertiary creep zone earlier than the uncoated ones, cracking of the coating remained confined to the surface of the specimen. Nanoindentation mapping showed a hardness increase in the coating and within the aluminium diffusion zone, which is related to the exposure time.