Development of novel diffusion coatings for 9–12 % Cr ferritic-martensitic steels

Eventhough 9-12% Cr steels are mechanically designed for power plant applications up to 650 °C, their effective use is limited by the corrosion resistance at this temperature. Therefore, the present paper addresses the development of diffusion coatings on 9% Cr ferritic-martensitic steels. The difficulty of coating these materials with conventional diffusion processes arises from the temperature limit above which the conversion of the martensite is accelerated and the mechanical properties would be deteriorated. Aluminide coatings consisting of Fe 2 Al 5 or FeAl phases were thus developed for deposition temperatures between 650 and 715 °C by the conventional pack cementation technique. As the addition of boron was expected to improve the oxidation properties of the coating, the influence of B on the aluminide coating was investigated. The precedent diffusion of Cr as an interdiffusion barrier before switching to the Al diffusion step was also investigated. As a further technique, the fluidised bed chemical vapour deposition (FBCVD) method allowed the development of Fe 2 Al 5 coatings at 550 °C. Furthermore, Si or codiffusion Al-Si coatings were developed at temperatures as low as 550 °C.