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

Environmental and regulatory constraints in the global energy market are placing increasing pressure on conventional power plant to seek improvements in operational efficiency. A key route to advancing plant efficiency lies with the enhancement of current steam operating parameters, namely increases in steam temperature and pressure from the current levels of 540 °C and 180 bar to the proposed 700 °C (EU), 750 °C (US) and 300 bar. The combined effect of increasingly demanding operating conditions and cycling, place current conventional materials in extremely aggressive chemical and thermo-mechanical environments. Current ferritic-martensitic steels whilst having good mechanical properties and fatigue resistance do not possess the high temperature corrosion resistance necessary for long term use, with a temperature limit of ~620 °C. The application of coatings to the surfaces of the steels offers a method to improve the oxidation resistance of this class of material. Aluminide coatings provide good high temperature oxidation resistance through the growth of an Al2O3 scale. One of the major concerns in the use of such coatings is the potential deleterious effects on the mechanical properties of the substrate, either through the precipitation of brittle phases or through cracking in the coating. The work reported here compares the microstructural features of slurry and pack cementation aluminide coatings applied to Grade 91, and the change in mechanical performance. Results show that the grain size was not affected by the pack aluminisation process, and whilst the content of V and Nb in the MX precipitates differed from that before the pack aluminisation process, it was close to the equilibrium condition. Oxidation tests showed that the coatings prevented breakaway oxidation as expected. The tensile, TMF and creep test results showed that whilst 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.