Analysis on High Temperature Corrosion Behaviors of Boiler Steels Under High-Chlorine Coal Ash

Chlorine is a harmful constituent in coal, contributing to severe high temperature corrosion on the super-heater and re-heater tubes in utility boiler firing high-chlorine coal (more than 0.3 wt.%). Characteristics of the corrosion contain not only the formed products on the metal surface, but also intergranular attack inner the alloy, resulting in great potential safety hazard and economic loss. The prevailing Cl-related mechanisms of high temperature corrosion involve active oxidation and fluxing, which mean both corrosive elements in the flue gas and deposits on the boiler metal surface can accelerate the corrosion. Cl2 as a catalyst in active oxidation can be released by sulfuration of alkali metal chlorides or reactivity by alkali metal chlorides with chromium/chromium oxide and iron/iron oxide or oxidation of HCl. However, the formation of low-melting eutectics (such as NaCl-Na2CrO4) in mechanism of fluxing can be an induction of severe corrosion because the rate of molten corrosion is much higher than chemical corrosion. Lab-scale experiments simulating the flue gas species, temperature gradient from hot flue gas (950 °C) to cold metal (610 °C), and deposit (four various Cl-containing coal ash) on the specimens were conducted in a tube furnace to investigate the corrosion of three common boiler steels (12Cr1MoVG, T91, TP347H). Furthermore, with the aid of the scanning electronic microscope associated with energy dispersive spectrometer (SEM-EDX) and X-ray diffraction instrument (XRD), the appearance and microstructure, the element contents, and composition of corrosion products on the specimens after corrosion have been analyzed. For high-chlorine coal, there existed white crystal on the surface of specimens (T91, TP347H) after corrosion test, and the XRD result showed NaCl, which can be explained by evaporation-condensation mechanism. However, no white crystal was detected for 12Cr1MoVG and it can be inferred that thick corrosion product layer with high thermal resistance was formed and 12Cr1MoVG suffered severe degradation. Through comparisons of alloy elements corroded in various oxidizers (Cl2, O2, and S), it can be seen that as the metal temperature increases, the negative value of Gibbs free energy for alloy elements corroded in Cl2 becomes higher, but the value is less corroded in O2 or S. Thus, alloy elements tend to be easier combined with Cl2, and Cl-induced corrosion is aggravated with the temperature increases. Similar results can be obtained by increased equilibrium vapor pressures of metal chlorides, evaporating easily and diffusing towards further to be oxidation. In comparison with high-chlorine coal, the corrosivity of low-chlorine coals on specimens were weak, especially for TP347H characterized with higher contents of Cr and Ni. Furthermore, the higher the ratio of Cl/2S or Cl/Na in the coal ash is, the more severe corrosion the specimens suffer.