Erosion-oxidation of mild steel in a temperature gradient

Thinning of heat-exchanger tubes by erosion-corrosion has been a problem in fluidized bed combustors (FBCs), particularly at lower metal temperatures where thicker, mechanically protective oxide scales are unable to form. Many laboratory-scale tests have shown a decrease in material loss at higher temperatures, similar to that observed in FBC boilers, but also show a decrease in wastage at low temperatures (e.g., 200 C) which has not been detected in boilers. It has been suggested that this difference is due to the laboratory tests being carried out isothermally, whereas in a FBC boiler the fluidized bed is considerably hotter than the surface of the metal heat exchanger tubing. In this laboratory study the simulation was therefore improved by internally cooling one of the two low carbon steel specimens which were rotated in a horizontal plane within a lightly fluidized bed with relative particle velocities of 1.3 to 2.5 ms{sup {minus}1}. Tests were carried out at a bed temperature of 500 C and specimens were cooled to a wear scar surface temperature as low as 250 C. No significant difference in the wastage rate was detected between the cooled and isothermally exposed specimens when measured at the same, accurately determined, metal surfacemore » temperatures.« less