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Steam Effects on Oxidation Behavior of Alumina-Scale Forming Nickel-Based Alloys and a Kinetics Analysis of Complex Scale Evolution during Isothermal Oxidation

Zhao, Wei (2012) Steam Effects on Oxidation Behavior of Alumina-Scale Forming Nickel-Based Alloys and a Kinetics Analysis of Complex Scale Evolution during Isothermal Oxidation. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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The first part of this study investigated steam effects on the oxidation behavior of Al2O3-scale forming Ni-based alloys. This detrimental effect was assessed in the context of Giggins and Pettit’s oxidation map for the Ni-Cr-Al system, which was determined under dry oxidizing conditions. Of particular relevance to the design of high-temperature alloys and coatings is the location in the oxide map of the kinetically-dictated critical Al content (NAl*) in a given alloy to form a continuous Al2O3-scale. It was found that NAl* increases when the air contains 30% steam. A rigorous quantitative analysis based on Maak’s modification of Wagner’s theory was carried out to determine the key parameter affecting an increase in NAl* under wet oxidizing conditions. By checking the sensitivity of NAl* with respect to each parameter in the theory, it was deduced that the only parameter which can give the necessarily large change in NAl* found in wet-air oxidation is the critical volume fraction of internal oxide, fv*, a parameter that is usually assumed to be a constant and independent of the reacting conditions.
From experimental observation, it was found that the surface of NiO scale became unstable in wet oxidizing conditions. Unlike the dense and uniform surface formed scale in dry air, finger-like protrusions or a powder-like porous structure could form under wet conditions. The experimental conditions and possible mechanisms for this observed instability were assessed.
In the second part of this study, a novel kinetics analysis method was established to quantify the kinetics of scale evolution based on analyzing the instantaneous growth rate constant ki and the time exponent ni determined from thermogravimetric (TG) data. A methodology is proposed to ascertain whether the oxidation kinetics has come to a steady state from a transient stage, and to accurately determine the growth rate constant. Both a fundamental analysis and an experimental validation of this methodology are presented. This kinetics analysis method was further extended to quantify the scale evolution during the initial oxidation stage. Specifically, a method to track the extent and kinetics of the metastable to stable Al2O3 phase transformation in thermally formed alumina scales was established.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Zhao, Weiwez29@pitt.eduWEZ29
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairGleeson, Brian M.bmg36@pitt.eduBMG36
Committee MemberMeier, Gerald H.ghmeier@pitt.eduGHMEIER
Committee MemberDeArdo, Anthony J. deardo@pitt.eduDEARDO
Committee MemberVeser, Götzgveser@pitt.eduGVESER
Date: 4 June 2012
Date Type: Publication
Defense Date: 24 January 2012
Approval Date: 4 June 2012
Submission Date: 20 March 2012
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 210
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: Steam Effect, Alumina Scale, Surface Instability, Critical Concentration, Transient Oxidation, Instantaneous Kinetics
Date Deposited: 04 Jun 2012 20:28
Last Modified: 15 Nov 2016 13:56


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