Copyright © 2019 American Chemical Society. The aim of this study was to understand the kinetics of high-temperature tube corrosion and its rate-limiting step in oxy-fuel combustion mode. Four different types of ash deposits collected from low-rank coal combustion boilers were tested, with varying contents of sulfur and metallic species. A typical stainless steel, low chromium ferritic alloy, T23, was tested at a metal surface temperature of 650 °C for up to 200 h. Compared with air-fired flue gas which is lean in SO2 and steam, the aggressive flue gases formed under oxy-firing conditions enhanced T23 tube corrosion but to a marginal extent in a comparison with the S-rich ash deposit. The study found a positive correlation between the sulfur content within ash deposit and the thickness of the tube oxide layer. The sulfate in the ash deposit promoted the formation of molten and mobile sulfate complex and sulfides. The former species are predominant on the tube's outer surface, while sulfides are preferentially formed inside the oxide layer where oxygen is lean and/or even absent. In addition, the study confirmed that the rate of growth of tube corrosion is controlled by both diffusion and phase boundary reactions. Regardless of the combustion mode, the gas diffusion coefficient decreases with the increase of sulfur content in the ash deposit, due to the enhanced formation of molten sulfates on the oxide layer. In contrast, no change was observed for the phase boundary reaction rate constant with increased sulfur content in the ash deposit. This is mainly because the solid-to-solid reactions are independent of the solid ash properties during the formation of molten sulfates and sulfides.