Advanced ultrasupercritical (A-USC) steam power plants are designed to operate at temperatures and pressures over which conventional steels require additional corrosion protection. The fireside tubes undergo extensive corrosion while the internal tubes suffer from excessive steam oxidation, in particular when ferritic-martensitic (FM) P92 is employed. This paper overviews the formation of Al_xFe_y-based diffusion coatings on FM P92 and on austenitic stainless steels (ASS) from aqueous slurries containing Al microparticles. The temperature and time of annealing will be shown to clearly influence the formation of different intermetallic Al_xFe_y coating phases through concomitant oxidation reactions of the Al microspheres, melting of Al and dissolution of Fe (and alloying elements) in the melt resulting in self-propagating high temperature synthesis (SHS) followed by final solid state diffusion. Sharp interfaces between the intermetallic layers appear when the coatings are elaborated in the ASS (> 20 wt.% Cr) due to segregation of Cr and/or formation of Al_xCr_y that act as diffusion barriers to Al. In contrast, Cr dissolves in the Al_xFe_y phases when the coating grows in P92 (<9 wt.%Cr).
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