High Temperature Corrosion Behavior of Boiler Waterwall Tubes in Pyrite and Hematite Mixture Under Solid-Solid and Gas-Solid Reaction State

摘要

The slagging is a common problem of high temperature corrosion in fireside of waterwall tube of steam generator/ boiler of power plant component. The hematite (Fe_2O_3) layer very often exists on the fire side of the waterwall tube. The chemical interaction of hematite on the outer layer of the tube walls with molten pyrite (FeS_2) at high temperature (if the fuel contains sulfur) may cause slagging/fire side tube wastage due to the high temperature corrosion. This is closely similar to slagging/fire side corrosion of the waterwall tubes at high temperature when sulfur bearing fuels are used in boiler. Plain carbon steel ASTM SA210 Grade Al is widely used for the construction of the waterwall tubes in the boiler. This papers highlights the mechanism and extent of high temperature corrosion of plain carbon steel in 67% of hematite (Fe_2O_3) and 33% pyrite (FeS_2) in solid-solid and gas- solid reaction state. The samples are exposed in high temperature furnace in different reaction states as mentioned at a constant temperature of 923 K=750℃ for 5 hours to study the high temperature corrosion behavior. The corrosion rates are calculatedrnby weight gain method. The SEM and EDS study of the external scales of the samples are also carried out to examine the scale morphologies and constituent weight percentage of the elements of the external scales. It can be concluded from the results of the experiments that corrosion rate is much higher in solid-solid reaction state in comparison gas-solid reaction states with same proportion of hematite and pyrite The higher rate of corrosion in solid-solid reaction state is associated with external scale cracking/spalling and presence of appreciable amount of sulfur on the external scale.the lower corrosion rate in gas-solid reaction state is associated with the adherent corrosion resistant scale with finer grains of oxides and sulfides.