Metal dusting corrosion has been known for more than a 100 years as an industrial problem. As a result of extensive research over the last five decades several mechanisms have been evolved involving ferritic materials. However, a complete understanding is yet lacking. One of the most referred models, developed by Hochman-Grabke, suggests that formation of metastable cementite and its subsequent decomposition is the central aspect of the process. To verify this hypothesis, an Fe-Si model was designed based on silicon's ability to retard cementite formation. However, this strategy was unsuccessful because silicon oxidized and amount of silicon remaining after silica formation was not sufficient to suppress cementite. On the other hand, germanium does not form a stable oxide in the conditions employed. A alloying with germanium did prevent Fe3C formation, but not dusting, which resulted from an alternative mechanism. Dusted particles were confirmed to be cementite for pure iron specimens (where cementite scale formed) and ferrite for alloys that did not form cementite. These observations are inconsistent with the prior model. In addition, the general features of metal dusting corrosion have been characterized. Kinetics of coking and metal wastage for ferritic materials (Fe, Fe-Si, Fe-Ge and Fe-Ge-Ni) were found to be linear in nature, though respective rates may vary due to the differences in alloy catalytic activity and reaction morphologies. The carbon diffusion coefficient in cementite was evaluated from Fe3C scaling rates. Crystallographic orientations of different forms of cementite were established. Internal cementite precipitates in pure iron accounted for by a very high degree of supersaturation with respect to carbon, indicating a non-equilibrium situation. Coking and dusting rates were found to be strongly correlated and their gas composition dependence indicate the contribution of the Boudouard reaction. Reactions with fixed carbon activity gases demonstrated that kinetics rather than thermodynamics control the reaction rates. However, at a particular temperature, these rates increase with carbon activity. Activation energies for coking and dusting are equal for a given alloy, meaning that the same process controls them. For Fe-lOGe alloy, in the early stages of reaction, grains with near (001) surfaces were more susceptible to graphitization than grains having near (110) surfaces, but the underlying cause has not been revealed.
| Identifer | oai:union.ndltd.org:ADTP/258464 |
| Date | January 2008 |
| Creators | Al-Motin, Md. Abdulla, Materials Science & Engineering, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. Materials Science & Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Al-Motin Md. Abdulla., http://unsworks.unsw.edu.au/copyright |
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