Spatial distribution of Cr-bearing species on the corroded tube surface characterised by synchrotron X-ray fluorescence (SXRF) mapping and micro-XANES: exposure of tubes in oxy-firing flue gas

Synchrotron X-ray fluorescence mapping and micro-XANES were employed to characterize the spatial distribution of individual elements and the speciation of Cr on the cross section of various tubes that were exposed to oxy-fuel flue gas at 650 °C, 1 bar for 50 h. The gas composition tested is close to the flue gas produced from oxy-firing of low-rank coal in pilot-scale tests. Multi-layered scales with an uneven distribution were observed for individual elements on both the top surface and spalled layer of carbon steel SS400. Oxidation is the major reaction causing the scaling of the tube, whereas the other reactions such as sulphidation and chlorination led to the buckling of tube surface. The use of Cr, even at a low concentration of 1.2 wt% in 12Cr1MoVG, is essential and can considerably reduce the tube corrosion rate, as well as minimize the difference between oxy-fuel and air-firing flue gases on the tube mass loss rate. The CO 2 cycle with the involvement of oxidation (mainly of iron) and carburisation (of chromium) took place simultaneously for the Cr-bearing alloy, even under the coexistence of CO 2 and a number of oxidizers in the flue gas tested here. The fast diffusion of CO 2 and its derivatives facilitated a preferential occurrence of carburisation under the oxide scale. However, upon the closure of gas passage channels in the oxide scale of a high-Cr tube such as austenite SUS304, the reductants CO and carbon can flow back to tube top surface, causing the formation of carbide on the most outer scale that further fragments into fugitive pieces. Carburisation is also the major cause of corrosion of high-Cr tubes. In contrast, for a tube with medium Cr content, such as high-chrome T91 tube with 9 wt% Cr, it undergoes both oxidation and carburisation successively on the metal/oxide interface. The gas passage channels mostly remain open, and hence, the resultant carbide and carbon precipitate penetrated deep inside the tube.