| Abstract: | Tube specimens of TP347FG were exposed in a test superheater loop in a biomass plant in Denmark. The specimens were exposed to surface metal temperatures in the range of 455–568°C, steam pressure of 91 bar and exposure duration of 3500 and 8700 hours. The oxide thickness and morphology was investigated using light optical and scanning electron microscopy. The oxide present on the specimens is a duplex oxide with an inner chromium rich oxide and an outer iron rich oxide. The inner oxide consisted of a primary iron chromium nickel oxide in the original alloy grain and a chromium rich oxide, “healing layer”, at the grain boundaries. This gave the appearance of uneven inner oxide and it was clear that the varying subsurface grain size affected inner oxide thickness, especially after longer exposure times. Longer exposure times from 3500 to 8700 hours resulted in increased pit thickness. Comparison of pit thickness revealed that increase of temperature from 455 to 525°C increases the oxidation rate, however a further increase in temperature did not result in thicker inner oxide presumably due to the formation of a better healing layer at grain boundaries. These results are compared with the previous paper where the pressure and temperature was higher. A thicker inner oxide is observed at the lower temperatures and pressures compared with higher temperatures and pressures. Reasons for this behaviour are discussed. |
