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| Home > Publications database > Effect of water vapour on growth and adherence of chromia scales on pure chromium |
| Dissertation / PhD Thesis/Book | PreJuSER-58936 |
; ;
2007
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-486-2
Please use a persistent id in citations: http://hdl.handle.net/2128/2628
Abstract: The oxidation behaviour of chromium was studied in the temperature range 950 to 1050$^{\circ}$C. A number of atmospheres such as Ar-O$_{2}$, Ar-H$_{2}$-H$_{2}$O, and more complex N$_{2}$-O$_{2}$-H$_{2}$O and N$_{2}$-H$_{2}$-H$_{2}$O were used, to allow the effects of oxygen and water vapour partial pressures to be determined. It was shown that the oxide scale formed in Ar-O$_{2}$ environments was dependent on the oxygen partial pressure. Decreasing the pO$_{2}$ in such gas mixtures lowered the oxidation rate and improved scale adherence. Different behaviour was found in Ar-H$_{2}$-H$_{2}$O. Scale adherence remained good in all low pO$_{2}$ gases and decreasing the pO$_{2}$ in the gas (this was done by using gases of higher H$_{2}$/H$_{2}$O ratio) led to an increase in the oxidation rate. The main finding of the work was that the oxide scale formed on chromium in Ar-H$_{2}$O and Ar-H$_{2}$- H$_{2}$O gases significantly differs from that formed in Ar-O$_{2}$. It appeared that oxide grains were much smaller in low pO$_{2}$ gases containing water vapour than in dry high pO$_{2}$ gases. It is believed that this effect of water vapour on the oxide morphology development is due to the faster adsorption and dissociation of H$_{2}$O at the outer surface compared with O$_{2}$, which enhance the inward transport of oxygen ions to the metal/oxide interface. It is proposed that increased oxygen grain boundary diffusion is the dominant process but molecular transport of H$_{2}$ and H$_{2}$O through the scale could also operate and contribute to the inner scale growth. The tracer study combined with SNMS techniques showed that in low pO$_{2}$ gases a significant amount of the scale was formed at the metal/oxide interface and this supports the observations that smaller oxide grains promote inward oxygen transport. It was also found that the extent of inward growth was higher in Ar-H$_{2}$-H$_{2}$O than in Ar-H$_{2}$O gas. This may indicate that the oxygen partial pressure of the gas or the hydrogen partial pressure were also factors determining the oxidation behaviour in low pO$_{2}$ gases. [...]
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