Suppression of oxidation of high chromium steels at elevated temperatures in steam atmosphere by alumina-based coating film grown by chemical solution deposition process

抄録

The surface roughness in moving and stationary blades of steam turbines is known to become rougher through steam oxidation during plant operation, resulting in the degradation of the turbine performance. The formation of an Al₂O₃–SiO₂ based coating film on high chromium steel, which is used in moving and stationary blades, through chemical solution deposition (CSD) is attempted to suppress the increase of surface roughness. The arithmetic average roughness R_a and maximum height R_z after an oxidation test in a steam atmosphere at 873 K for 1,000 h were equal to 3.0 and 19.4 µm, respectively, for high chromium steels without coating, while those for high chromium steel with an Al₂O₃–SiO₂ based coating were 0.10 and 1.0 µm, respectively. Therefore, the formation of a coating film is proven to significantly suppress the roughening of the surface. The observation of the cross-section texture near the high chromium steel substrate surface of both specimens showed that a two-layer oxide film forms on the surface of the non-coated specimen; the outer layer consists of Fe₃O₄, whereas the inner layer comprises (Fe,Cr)₃O₄ and CrO₂. This film forms a very coarse texture. On the other hand, the oxide film does not form on high chromium steel with an Al₂O₃–SiO₂ based coating film deposited using CSD; the Al₂O₃–SiO₂ based coating film remains as the topmost surface after testing. A thin intermediate layer with a thickness smaller than 0.1 µm forms between the high chromium steel substrate and the coating film. Its thickness increases with increasing the temperature and reaches approximately 0.5 µm with heat treatment at 973 K for 1,000 h. This layer consists of Cr, Mn, Al, Si, and O.