Task, Michael N
(2012)
Assessment of the Factors Affecting Protective Alumina Formation Under Hot Corrosion Conditions.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
In this study, the influence of microstructure, composition, and phase constitution on the Type I (900°C) and Type II (700°C) hot corrosion resistance of MCrAlY and β-NiAl base alloys was investigated. The Type II hot corrosion resistance of MCrAlY alloys is generally enhanced by microstructural refinement. This can be attributed to the more rapid establishment of a protective Al2O3-rich scale due to the higher density of short-circuit diffusion paths for Al (phase boundaries). However, it was shown that for a given bulk composition, the compositions of the individual phases is also extremely important. If one phase is lean in an element which is highly beneficial from a hot corrosion standpoint, e.g., Cr, Type II hot corrosion resistance is quite poor, regardless of the microstructural scale. In addition, coarse reactive-element-rich phases, which are commonly found in MCrAlY alloys, can be incorporated into the thermally grown Al2O3 scale and act as initiation sites for Type II attack. This stresses the importance of reactive element content and distribution in MCrAlY coatings.
During Type I hot corrosion exposure of β-Ni-36Al (at. %) base alloys, the incubation stage is greatly extended by the addition of 5% Pt, Co, or Cr. In each case, the beneficial effects can be linked to an enhanced ability to rapidly form a protective Al2O3 scale, and to heal this scale when it sustains damage during exposure. With regard to Type II hot corrosion, individual additions of 5 at. % Pt or Cr are beneficial, largely for the same reason; however, additions of 5 at. % Co and co-additions of 5 at. % Pt + 5 at. % Cr result in a decrease in the duration of the incubation stage. Subsurface phase transformations that occur in the latter systems prevent the alloy from maintaining the growth of the Al2O3 scale. This mechanism is discussed in detail.
Finally, the influence of alloy composition and exposure environment on the kinetics of the θ⇒α Al2O3 transformation in scales grown on β-NiAl alloys at 900°C was thoroughly investigated. The relative importance of the kinetics of this transformation during Type I hot corrosion exposure is discussed.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
26 September 2012 |
| Date Type: |
Publication |
| Defense Date: |
24 April 2012 |
| Approval Date: |
26 September 2012 |
| Submission Date: |
10 July 2012 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
240 |
| 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: |
hot corrosion; nickel aluminide; alumina; incubation stage; type I; type II |
| Date Deposited: |
26 Sep 2012 17:17 |
| Last Modified: |
19 Jul 2024 18:29 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/12841 |
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