Development of a new out-of-pack diffusion coating treatment for high corrosion resistance of steam power plants

Modern 9-12% Cr ferritic-martensitic steels are used in conventional power plants for steam lines, boiler piping, superheaters and steam turbines. Since the use of X20CrMoV121 new improved Cr-steels like the P91 and the tungsten and boron alloyed Cr-steel NF616, the HCM12A or E911 have been developed. To improve the efficiency of power plants, supercritical steam parameters up to 650°C and 300 bar are required. Under these conditions martensitic steels have a good creep rupture strength which is an advantage for their use as heat exchanger materials, but the corrosion resistance should be improved. Austenitic steels and nickel-based alloys are potential candidates for increasing the steam temperatures up to 700°C. In order to protect the martensitic steels by diffusion coating treatments, the coating process has to fit the substrate requirements concerning the process temperature and time. Indeed, above a certain temperature, the martensite transforms into ferrite and the material thus looses its mechanical properties. For P91 martensitic steels, the removal of the martensite occurs above 650°C. As a consequence, a new out-of-pack process will be developed. The process consists in enriching the substrate surface with elements that are expected to form a protective oxide layer under service conditions. These elements are: Mn, and Si and a combination of both, which are introduced at high temperature by diffusion into the substrate surface. A high Mn content in the surface zone forms protecting MnCr-spinels. Si is expected to form a diffusion barrier, which could reduce the Cr diffusion to the substrate surface. A diffusion treatment of martensitc Cr-steels with both elements suppose a synergetic effect.