Oxide Scale Characterization and Study of Oxidation Kinetics in T91 Steel Exposed to Dry Air at High Temperatures (873–1073 K)

In this study, the oxidation response of a T91 steel in dry air environment has been investigated by performing cyclic oxidation tests at 873, 973 and 1073 K for a total duration of 1000 h with intermediate intervals. The temperature range for this study is chosen to be higher than the operating temperature range of the alloy to accelerate the oxidation kinetics. Developing a comprehensive understanding of the mechanisms and kinetics involved at such accelerated oxidation environments would help to formulate better strategies for countering oxidation related failure events. The results of this study showed that the oxidation rate is greatly influenced by temperature as well as exposure time, and follows a near-parabolic rate law. Characterization of the oxide scales revealed formation of dual oxide layers where the inner layer is primarily rich in Fe and Cr, and the outer layer in Fe only. At 1073 K, formation of cracks can be observed in the oxide scales after 24 h, which became more severe after 1000 h. The oxidation kinetics, when fitted to an Arrhenius type equation, can be associated with activation energy (Q) and parabolic rate constant (k_p) values. The Q values lie within 104–109 kJ/mol, whereas the k_p values increase with temperature due to increase in the reaction rate. The kinetic equation developed based on the Q and k_p values can be used to predict the thickness (inner, outer and overall) of the oxide scales in T91 steel at any temperature and time within the studied ranges.