Field test results for steam oxidation of TP347H FG - growth of inner oxide

A series of field tests have been conducted with TP347H FG in test superheater loops in coal-fired and biomass fired boilers of steam pressure 256 and 91 bar respectively. The exposure times are from 3,500 to 30,000 hours and the temperature range is from 450-630¢XC. The morphology, composition and the thickness of the inner oxide has been investigated by light optical and scanning electron microscopy equipped with EDS. Comparison of inner oxide pit thickness revealed that an increase in temperature from 455 to 525C increased the oxidation rate. A further increase in temperature did not result in thicker inner oxide presumably due to the formation of a more chromium rich Fe-Cr-Ni oxide at grain boundaries. At temperatures over 610¢XC, there is a decrease in inner oxide thickness. A thicker inner oxide is observed at the lower pressures compared to similar temperature ranges at higher pressures. The effect of temperature, exposure time and pressure are discussed.