Wu, xiaodan
(2015)
High-Temperature Corrosion Behavior of Alloys in Gaseous Environments with Low-Oxygen and High-Sulfur Potentials.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
To seek a better understanding of the compositional and environmental factors affecting the corrosion behavior of alloys in sulfur-rich atmospheres at temperatures above 600oC, relevant to advanced combustion systems for power generation, eight chromia-scale-forming commercial alloys were tested at 750oC in gases with a base composition of N2-15%CO-3%H2-0.12%H2S. This base composition was made more oxidizing by introducing two different levels of water vapor, 0.6% and 3%, into the reaction gas. Five model alloys were also prepared to study and verify the effects of major alloying elements, Cr, Co and Ni, on sulfidation resistance. The additional three model alloys were prepared to systematically study the effects of minor alloying elements Ti, Al and Mo. Finally, another group of three model alloys was made to study the individual effect of titanium on sulfidation resistance.
All alloys eventually exhibited breakaway behavior. A protective Cr2O3 scale formed initially and then broke down. The mechanistic process of the breakaway corrosion was assessed. As for the effects of major alloying elements, it was found that the alloys with a Ni/Co mass ratio near to unity had less weight gain and hence, superior sulfidation resistance. This is because when the Ni/Co ratio is near unity, Ni and Co availability are both sufficiently low to suppress their external sulfidation and consequently extend the incubation period. In the range of the alloys studied, 10-40wt.%Co, weight gain decreased with an increase in the alloy cobalt content. Similarly, weight gain decreased with an increase in alloy chromium content in the range of 19-28wt.%. Nickel was found to have a detrimental effect by increasing the tendency to form a liquid reaction product. This tendency increased with increasing nickel content in the alloy.
Besides major alloying elements, the judicious addition of minor alloying elements, Al, Ti and Mo, was found to have a significant effect to improve sulfidation resistance. This is because the minor additions promoted the formation of an oxide-enriched scale layer-comprised of Al2O3 and oxides containing Cr and Ti. This oxide-enriched scale layer was inferred to have inhibited the outward diffusion of base-metal elements, thus mitigating external sulfidation and consequently internal corrosion. It was found, based on the model alloys study, that remarkable sufidation resistance could be conferred to an alloy by maintaining an Ni/Co mass ratio near unity, and adding minor amounts of Al, Ti and Mo. A particularly sulfidation-resistant model alloy had the compostion of 33.1Ni-22Cr-34.9Co-2.5Al-2.5Ti-5Mo (in wt.%).
The individual effect of titanium was found to promote oxide-scale formation by shifting the kinetic boundary to a lower oxygen partial pressure. Titanium was observed to exist with Cr as complex oxides, which nevertheless acted as an effective barrier inhibiting the outward migration and subsequent external sulfidation of the base-metal elements.
In addition to the effects of major and minor alloying elements on sulfidation resistance, the mechanisms associated with the sometimes observed formation of whiskers, voids and nodules were assessed.
In the second part of this thesis, alloys Fe-30wt.%Mo-5wt.%Al and Fe-30wt.%Mo-10wt.%Al were studied in a simulated syngas atmosphere. It was found that alloy Fe-30Mo-10Al had better sulfidation resistance than conventional Ni-Cr-Co high-temperature alloys. The good corrosion resistance was attributed to the formation of a very thin Al2O3 layer on the surface of the alloy, identified by XPS analysis. The effect of Al level on sulfidation resistance was also studied and explained.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
29 January 2015 |
| Date Type: |
Publication |
| Defense Date: |
17 December 2013 |
| Approval Date: |
29 January 2015 |
| Submission Date: |
6 February 2014 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
251 |
| 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, high temperature, oxidation, kinetic boundary |
| Date Deposited: |
29 Jan 2015 15:28 |
| Last Modified: |
15 Nov 2016 14:17 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/20503 |
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