Erosion-oxidation of mild steel in a fluidized bed environment
This thesis is a contribution to research into erosion-corrosion investigating the rapid wastage of in-bed heat exchanger tubing in fluidized bed combustors. Two main new areas of research were investigated; erosion-corrosion of mild steel in a temperature gradient and erosion-corrosion modelling. It has been suggested that differences between the wastage behaviour measured in fluidized bed combustors and laboratory studies is due to laboratory tests being carried out isothermally (specimen and fluidized bed at the same temperature) whereas, in a FBC boiler, the fluidized bed is considerably hotter than the metal heat exchanger tubing. The fluidized bed test rig was modified to increase the temperature gradient between the specimen and the fluidized bed from initial tests conducted by Rogers (1992b). Tests were carried out over a range of bed temperatures (300 - 500° C) and cooled specimen surface temperatures (175 - 500° C) with a maximum temperature difference between the two of 250° C. It was discovered that the temperature of the wear scar during a test was up to 200° C hotter than the temperature at the back of the specimen where the specimen temperature was initially measured in tests by Rogers (1992b). After temperature calibration tests the wastage of the specimens in a temperature gradient were very similar to the wastage of specimens exposed isothermally at the same metal temperature. Short term oxidation experiments were conducted on mild steel to obtain oxidation kinetics for erosion-corrosion modelling. It was found that the initial apparent parabolic rate constant was an order of magnitude larger than at longer time. Erosion studies were conducted with the aim to obtain quantitative data on the particle flux and the erosive behaviour of the bed with temperature. Results were not accurate enough to yield quantitative data but provided an estimate of the particle flux in the fluidized bed test rig. Results obtained from the short term oxidation and erosion studies were used in simple erosion-oxidation models to construct erosion-corrosion regime maps which tended to predict metal erosion to higher temperatures than observed experimentally. Predictions of material wear from the combination of an oxide removal and spalling mechanism predicted sensible wastage rates which agreed with experimental results.