The purposes of this research are (1) evaluating the importance of voids existing in oxide scales to exfoliation of oxide scales formed in superheated steam, (2) discussing possible role of Cr-rich layer in void formation in the oxide scales. First, superheater tubes, which are made of 9Cr-1MoVNb ferritic steel and were experienced with scale exfoliation, were removed from a fossil power plant and oxidation interfaces of the boiler tubes were examined in terms of structure, void distribution, elemental profile of the oxide scales and exfoliation characteristics. Then, exfoliation behaviors of oxide scales were evaluated on actual superheater tubes and on the steam oxidation specimens experimentally oxidized in autoclaves by tensile tests in terms of exfoliation strength and of position of exfoliation and its relation with void distribution in the scales. The results indicated an important role of void formation and growth in exfoliation of scale exfoliation. Void formation mechanism was also discussed based on the observed characteristics of the oxide scales.

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