Oxidation under pure steam: Cr based protective oxides and coatings

At temperatures of 900 °C and higher, the formation, transformation and failure of protective oxides in air have been deeply studied. However, there is significantly less available information of these processes when they take place under pure steam and in the lower temperature range pertinent to steam power plants. New designs for these plants are expected to operate at 625-700 °C, at which the candidate ferritic/martensitic steels exhibit very low steam oxidation resistance. In this paper, available knowledge of the behavior of Cr based protective oxides formed under steam at 650 °C will be presented. It is already known that on ferritic/martensitic steels with a Cr content lower than ~ 9 wt.% such as P92, a nonprotective, thick, dual layer composed of Fe3O4 and (Fe, Cr)3O4 forms. However, significantly higher steam oxidation resistance has been recently found when exposing NPM, a 9 wt.% Cr martensitic steel rich in W and Co, to pure steam at 650 °C. In this case a protective, very thin multilayer forms, with alternating Fe3O4 and (Fe, Cr, Mn)3O4 layers. Different oxides formed after 10,000 h of exposure to steam at 650 °C, on Cr containing coatings. In the case of Fe based, Cr rich coatings, both diffusion and overlay, a protective spinel was observed. However, Cr containing coatings based on Ni develop a very stable, protective thin Cr2O3 layer. Results show that along with the Cr content, other factors such as the grain size below the scale appear to determine the formation of thin protective scales. The steam pressure was also found to significantly and negatively affect the stability of protective Cr based oxides. Chromia former steels and coatings may not be the best solution for 650 °C new generation steam power plants.