Improved oxidation resistance of ferritic-martensitic steels in water vapour containing environments via diffusion coatings

D. Schmidt, M.C. Galetz, M. Schütze

Ferritic-martensitic steels are of high interest as superheater materials or as materials for interconnectors in solid oxide fuel cells. In comparison to austenitic steels and nickel base alloys they offer much better heat transfer behaviour and a lower coefficient of thermal expansion, as well as lower costs. Modern 9% Cr-steels have sufficient creep strength however their corrosion resistance particularly in H2O containing oxidising environments (e.g. up to 25% H2O in the combustion of biomass or the oxyfuel process) needs further improvement.
A large number of studies have shown that the corrosion resistance of 9% Cr-steels in water vapour containing combustion environments is inferior to that in dry atmospheres, due to the formation of a volatile Cr-species of the type CrO2(OH)2. This investigation starts from the idea of a shift of partial pressure of CrO2(OH)2 to lower values if the solid oxide phase on the alloy surface is a manganese –chromium – spinel scale, so that at 630°C the formation of volatile chromium species can be suppressed. Therefore, the metal subsurface region was enriched with manganese and chromium via a diffusion coating (pack cementation) to promote the formation of Cr –Mn– spinel during oxidation. In the course of the project it was observed that the diffusion treatment can lead to the formation of a chromium carbide surface layer. The surface treated alloys show extreme stability under oxidising water vapour containing environments, as illustrated by oxidation exposures in a simulated combustion environment (N2 + 1%O2 + 10%H2O) at 650°C for 800 h.
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