Microstructural stability and oxidation behavior of Sanicro 25 during long-term steam exposure in the temperature range 600-750°C

The aim of the present study is the investigation of the microstructural features and the oxidation resistance of the austenitic steel Sanicro 25 during long-term steam exposure up to 10 000 h in the temperature range 600-750 degrees C. Steel microstructures of specimens after exposure times of 1000, 3000, and 10 000 h were analyzed by scanning and transmission electron microscopy as well as X-ray diffraction. The steam oxidation behavior was estimated by gravimetry in combination with microstructural investigations using the above mentioned analysis tools as well as depth profiling by secondary neutrals mass spectrometry. In the as-received condition, the only detectable precipitate was Z-(Nb,Cr)N phase. After exposures in the temperature range 600-750 degrees C various amounts of the following additional phases were identified: chromium nitride, mu-phase, and carbides with various compositions. Chromia base surface scales formed during exposure at 600-700 degrees C, whereas more rapidly growing Fe/Cr-rich oxides were found at 750 degrees C. The oxidation rates were only slightly higher than those of a typical nickel base alloy such as alloy 617. In the subsurface zones, depletion of scale forming elements was accompanied by enrichment of tungsten and copper-rich phases.