Influence of Oxidation Temperature on the High-Temperature Oxide Film Forming Behavior of T22 Heat-Exchange Tubes

Abstract

In this study, without changing the whole manufacturing process of T22 coiled tubes, high-temperature oxide films are formed through atmosphere adjustment by taking advantages of the cooling process after stress elimination heat treatment. The corrosion resistance of T22 heat-exchange tubes is improved, which are used in the steam generators (SG) of high-temperature gas cooled reactors (HTR). The surface microscopic morphology of the oxide films is observed using a scanning electron microscope, and the structure of these films is characterized using an X-ray diffractometer. The stability of the high-temperature oxide film forming process is investigated using TG-DTG, as well as the thermal expansion coefficient of the films. The results prove that: (1) The oxide film generated at 550 °C is uniform and dense; (2) The oxide films formed at various holding temperatures are mainly consisted by Fe2O3 and Fe3O4. When the holding temperature is 550 °C, the content of Fe3O4 is the highest as 70.1%; (3) The films are stable when placed in inert atmosphere (N2) below 900 °C, and there is not any change in the composition and structure of the films even after reacting with steam at 550 °C for 24 h; (4) The expansion coefficient of high-temperature oxide films is very close to that of the matrix of the heat exchange tube, and the difference between these two thermal expansion coefficients is 5.3×10-9 mm/°C.