Soltanattar, Satia
(2018)
High Temperature Corrosion Behavior of Alloys in Mixed-Gas Environments.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
Recent efforts to increase the efficiency of and reduce NOx emissions from coal-fired boilers have led to higher operating temperatures and localized reducing conditions that favor sulfidation and carburization, together with possible formation of only the most stable oxides (e.g., Al2O3, SiO2 and Cr2O3). Under such conditions, the ability of a given alloy to develop a protective oxide scale is crucial for achieving extended corrosion resistance. This study assessed the effects of different major and minor alloying components on the corrosion resistance of several commercial alloys (all supplied by Haynes International), pure chromium and model alloys in environments having a relatively low oxygen, but high sulfur and carbon potentials. Such environments are relevant to low-NOx burner and coal gasification systems. Isothermal exposures were done for up to 100 h at 871°C (1600°F) and the exposed alloys were characterized in detail using some combination of XRD, SEM, EPMA, XPS, GDOES, EBSD and TEM.
It was found that the mode of growth and kinetics of chromia formation are affected by the presence of water vapor, sulfur and carbon in the atmosphere. The presence of these species was inferred to affect the grain-boundary diffusion. Under such conditions, chromia scales can grow much faster and have a finer grain structure than those formed in dry air. It was also shown that Ni-Co-base alloys exhibit much better sulfidation and carburization resistance to the mixed-gas environments studied compared to Ni- and Fe-Ni-base alloys. An optimum in sulfidation resistance can be achieved when the Ni:Co mass ratio is close to unity, due to the reduced risk of formation of relatively low-temperature metal/metal-sulfide eutectics. Resistance to carburization, however, could only be achieved by formation of a continuous alumina scale. The effect of Cr concentration on corrosion resistance of Ni-Co-base alloys was also studied. It was found that in the presence of specific minor alloying elements, higher Cr concentration is not necessarily better for sulfidation resistance. A combination of lower Cr and minor alloying elements can provide better resistance. Silicon additions, in combination with Al, Ti, and Mo, were found to be the most effective in providing the resistance to mixed gases owing to the establishment of a complex inner layer at the alloy surface.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
| Title | Member | Email Address | Pitt Username | ORCID |
|---|
| Committee Chair | Gleeson, Brian | | | | | Thesis Advisor | Gleeson, Brian | | | | | Committee Member | Meier, Gerald | | | | | Committee Member | DeArdo, Anthony | | | | | Committee Member | Morsi, Badie | | | |
|
| Date: |
20 June 2018 |
| Date Type: |
Publication |
| Defense Date: |
2 February 2018 |
| Approval Date: |
20 June 2018 |
| Submission Date: |
28 February 2018 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
252 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Materials Science and Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Sulfidation, Carburization, Mixed-Gas, Chromia |
| Date Deposited: |
20 Jun 2018 18:51 |
| Last Modified: |
20 Jun 2018 18:51 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/33842 |
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