Steam oxidation of ferritic steels - laboratory test kinetic data

The oxidation resistance of 9-12% chromium steels in steam-containing environments simulating the service conditions of steam power plant has been investigated for exposure times ranging from I h up to 10 000 It. In the long-term experiments the steels were evaluated by the mass changes determined in exposure tests, which were interrupted every 250 h, thus providing information concerning the scale growth and spalling characteristics. The morphologies of the oxide scales were examined using optical microscopy, scanning electron microscopy, secondary neutrals mass spectrometry and Raman spectroscopy. Different mechanisms of oxidation were observed for the various steels in different temperature regimes and exposed for different durations. For some steels, the steam oxidation resistance increased with increasing exposure temperature. The scale thickness at which spalling was observed varied according to the exposure temperature and the interval between thermal cycles, so that a critical scale thickness for spalling cannot be derived. The cracking and spallation of scales was correlated with the type, morphology and growth of pores and voids in the scale and could be influenced by the steel microstructure.