Transient stage oxidation of MCrAlY bond coat alloys in high temperature, high water vapor content environments
Introduction
Gas turbine engines are currently used in the production of approximately 30% of the electricity generated in the U.S. – a number that is expected to rise as the shale gas market matures – which dictates that the country's energy costs and policies will be significantly influenced by turbine efficiency and fuel flexibility. Integrated Gasification Combined Cycle (IGCC) power plants promise to improve both, as they transform coal, biomass and other carbon feedstock into clean, synthetic gas (syngas) that can either be reprocessed into liquid fuel for use in aero turbines, or converted into electricity at the plant – via gas turbine – with ~ 45% efficiency and full carbon capture. From a material standpoint, it is critical to understand how combusting syngas at IGCC turbine inlet temperatures (e.g. 1425 °C and rising) may affect degradation mechanisms of thermal barrier coating (TBC) systems that are used to protect hot section components [1]. In particular, it is important to understand how the degradation mechanisms might differ from ones associated with combusting natural gas, the conventional gas turbine fuel for which materials were originally optimized.
Higher water vapor levels in the combustion zone are one expected consequence of using high-hydrogen content fuels such as syngas. A study done by the National Energy Technology Laboratory (U.S. Department of Energy) [2] indicates that water vapor is expected to account for 12–14% of the turbine exhaust; industry use of steam injection to suppress NOx may raise the total to 25–30%, representing as much as a four-fold increase over that derived from natural gas combustion. Results of industrial turbine field tests with MCrAlY bond coats show increased non-ideal oxide formation in the thermally grown oxide (TGO) layer when syngas is used, compared to natural gas (Fig. 1) [3]. Instead of a thin (< 5 μm), continuous, thermo-chemically protective α-Al2O3 film that forms between the metallic bond coat and the yttria-stabilized zirconia (YSZ) top coat with natural gas, a spinel–alumina bilayer presents itself with syngas. The spinel (AB2O4) is susceptible to cracking and associated with an unwanted volume expansion between TGO and YSZ, factors that make for a weak interface with the top coat [4]. The thicker and more continuous spinel is – it measures as thick as 30 μm in Fig. 1 – the greater the threat to long-term TBC stability.
Spinels are known to manifest on MCrAlYs in the so-called transient oxidation period, the first several hours of high temperature exposure. Phases such as Cr2O3, CoO and NiO grow first, before α-Al2O3 has fully formed, then undergo solid state reaction with alumina as it develops beneath, ultimately yielding (Ni,Co)(Al,Cr)2O4 spinels at the TGO surface [5], [6]. Because the transient period is brief, spinel growth via this mechanism is observed to be limited [7], [8], [9], [10]; conventional wisdom would not expect transient oxidation to account for 30 μm thick layers seen in field-testing or laboratory experiments [11], [12].
Possible explanations for the surplus have been staked to long-term, steady state growth hypotheses. Since spinel is observed at the TGO surface, any such hypothesis must explain how Ni, Co and/or Cr in the bond coat might transport across the TGO, whereupon they oxidize. The most often cited mechanism postulates that Al-depletion of the bond coat leads to severe Ni, Co and Cr activity gradients that propel the species across alumina [13], but this was recently shown to be unlikely [14]. Another postulates that cracks in alumina, resulting from thermal cycling, provide the necessary short-circuit pathways for Ni, Cr and Co cations to cross the alumina layer [15], but experiments run under such conditions do not validate this mechanism [16], [17], [18]. Considering the efficacy of alumina as a diffusion barrier to spinel-forming cations – indeed, bond coats are specifically designed to take advantage of this property – a steady state mechanism of any kind is considered improbable.
The focus thus swings back to the transient stage. Could it be solely responsible for all spinel growth, even the most copious amounts? If the answer is yes, environmental conditions may be the key, specifically water vapor, which has been shown to increase the amount of spinel at the TGO surface of both NiCoCrAlY [19], [20], [21], [22] and the analogous alumina-former, FeCrAlY [23]. Enhanced oxidation kinetics have been observed in the transient stage, when less dense metastable aluminas (e.g. δ-, γ- and θ-alumina phases) develop before later transforming into stable α-alumina, a more robust diffusion barrier. One study attempted to connect these concepts for MCrAlY, postulating that water vapor extends the transient stage, thus allowing more diffusion of spinel-forming cations to the TGO surface [22]. The conclusions were drawn from limited exposures (24 h, 0–8% H2O) that do not narrowly target the transient stage, and on only one sample type (thermally sprayed NiCoCrAlY); additional study is needed to validate the claim and elaborate upon the mechanistic connections.
Such is the aim of the present study, which explores a wide range of sample types (NiCoCrAlY and CoNiCrAlY, each sprayed and cast) in a wide range of IGCC-relevant and environments (10–50 vol.%), using short exposures (0–5 h) that specifically target the transient stage. By emphasizing plan view microstructural analysis – and using the more traditional cross-section analysis as an occasional supplement – the extent to which spinel covers the TGO is evaluated in terms of area coverage (i.e. amount of interfacial weakness to the YSZ), not just thickness. This allows for better insight into how environmental conditions impact the kind of extensive transient stage growth that might, in only a few hours, present a serious threat to the long-term stability of a TBC system.
Section snippets
Material and methods
Metallic compositions of all materials investigated in this study are identified in Table 1. MCrAlY bond coat specimens were prepared from commercial powders in two ways — thermal spraying and chill casting. Spraying was conducted with Sulzer Metco powders using the high velocity oxy fuel method at the Forschungszentrum (IEK-1: The Institute for Energy and Climate Research, Materials Synthesis and Processing group) in Jülich, Germany; a 3 mm thick layer was coated onto – then cleanly removed
Microstructural observation
The “zero hour” time point was chosen for the majority of transient oxidation exposures. Past results indicated that, in wet environments with no YSZ top coat, spinel surface area coverage peaks between zero and 10 h before diminishing due to water-vapor-aided volatilization, which causes spinel to evaporate in the form of metal-hydroxides [14]. A median time point (e.g. 5 h) was not featured here, however, because it is observed that the longer samples are held at temperature, the more
Conclusions
In the early, transient stage of oxidation, development of MCrAlY bond coat TGO is influenced by the amount of water vapor in the environment. In general, higher and each promote the development of more (Ni,Co)(Al,Cr)2O4 spinel, measured by its percent area coverage of the TGO, but the most spinel coverage results from a combination of high and low . These results hold for both NiCoCrAlY and CoNiCrAlY, in both the cast and more commercially relevant sprayed bond coat morphology
Acknowledgments
The authors would like to acknowledge Praxair, Inc. for providing MCrAlY powders used in cast alloys, the Forschungszentrum (IEK-1) in Jülich, Germany for providing sprayed MCrAlY specimens, and the Laboratory for Electron and X-ray Instrumentation (LEXI) at UC-Irvine for access to supporting instrumentation. This work was supported by the U.S. Department of Energy under cooperative agreement DE-FE0004727. This report was prepared as an account of work sponsored by an agency of the United
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