In the briquet-burning hot water boiler the $SO_x$-dew point is calculated and found to be between $130^{circ}C;and;154^{circ}C$. The corrosion rate depends on the surface temperature and the concentration of the condensate on the surface. The concentration of the condensate is decided acoording to the difference detween $SO_x$-dew point and the surface temperature. When the surface temperature is $80^[circ}C$, the concentration of the condensate is also high (0.15N). Therefore the high concentration and high temperature promote the high corrosion rate of $14{ imes}13^{-3}g/100cm^2{cdot}hr$ on the SS41 material. On the other hand, when the surface temperatures are $60^{circ}C;and;40^{circ}C$, the concentrations and the co..sion rates are reduced dramatically to $0.11;N,;8.6{ iems}10^{-3}g/100cm^2{cdot}hr$ and $5{ iems}10^{-4}g/100cm^2{cdot}hr$ respectively.