JTTEE5 20:621–629 DOI: 10.1007/s11666-010-9602-0 1059-9630/$19.00  ASM International

Philip Puetz, Xiao Huang, Q. Yang, and Z. Tang (Submitted August 9, 2010; in revised form November 2, 2010) In this study, the transient surface oxide formation on APS NiCrAlY samples was examined after oxidation heat treatment at temperatures between 1000 and 1100 °C. The surface oxides observed on the NiCrAlY surface included a sporadic top layer of NiO and a continuous layer of alumina immediately adjacent to the NiCrAlY coating. Cr-rich oxide in smaller quantities than alumina was also found surrounding the NiO and dispersed within alumina. The alumina assumed whisker-shaped morphology when being observed from the surface but formed continuous film along the NiCrAlY surface. Although the formation of alumina has been observed on all the samples examined in this study, the NiCrAlY sample heat treated at 1050 °C for 5 h generated more continuous a-alumina layer and also contained less surface NiO and Cr-rich oxide. Based on the results, it is believed that NiO developed first upon exposure to an oxidizing environment at high temperature, and stable alumina began to form with the increase in heat treatment temperature and time.

Keywords

alumina, heat treatment, NiCrAlY, surface oxide formation

1. Introduction To enhance the thermodynamic efficiencies, the use of supercritical water as coolant has been considered in the design of the Gen-IV Supercritical Water-Cooled Reactor (SCWR) (Ref 1, 2). The identification of appropriate materials to store the SCW fluid is one of the major challenges for the development of the SCWR. Among the candidate materials tested so far, Fe-Cr-based ferriticmartensitic (F/M) steels showed higher corrosion rate at high temperature than that of the nickel-based alloys and austenitic steels (Ref 3, 4). Austenitic stainless steels, such as 304, 316, and 800H (Ref 5), were observed to have an increased rate of corrosion in subcritical water (Ref 6) and spallation of oxide scale (Ref 7). Nickel-based alloys in general are relatively immune to corrosion in SCW but corrosion can be high in subcritical water (Ref 8, 9). They also exhibit greater susceptibility to localized pitting and intergranular corrosion cracking than that of F/M steels (Ref 10). Based on information available in the literature, most of the alloys tested suffer from one or many modes of corrosion, depending on test conditions (temperature, pressure, water chemistry, and stress). Application of

Philip Puetz and Xiao Huang, Department of Mechanical Engineering, Carleton University, Ottawa, ON, Canada; Q. Yang, Institute of Aerospace Research, National Research Council Canada, Ottawa, ON, Canada; and Z. Tang, Northwest Mettech Corp., North Vancouver, BC, Canada. Contact e-mail: xhuang@ mae.carleton.ca.

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coatings to these structural materials is being considered in this research with the objective being to improve the corrosion resistance of stainless steels and solid solution nickel-based alloys. For alloys that rely on the formation of protective chromia to provide corrosion resistance, the formation of soluble hexavalent chromium (CrO42 (chromate), HCrO4, or H2CrO4) (Ref 11, 12) under an oxidizing supercritical water environment can render the oxide layer non-protective. On the other hand, Al2O3 has shown superior corrosion resistance to most of the aqueous conditions, whether supercritical (Ref 13) or steam (Ref 14). Also, in a high temperature oxidation environment, the oxidation rate of material is not affected by the presence of water vapor if a-Al2O3 is exclusively formed on the surface (Ref 15). As such, this study focuses on the examination of oxide formation on one of the aluminaforming MCrAlY coating compositions. Extensive corrosion and oxidation coating development in gas and steam turbines has resulted in the wide of use of MCrAlY, where M represents Ni-, NiCo-, CoNi-, or Fe-based coating compositions (Co will be excluded in this study because of 60Co concerns (Ref 16). The formation of a thermally grown alumina layer provides oxidation and corrosion resistance under an isothermal and cyclic thermal environment (Ref 17). In practice, coating materials that have shown the most promise are those that form a slow-growing, dense, and adherent a-aluminum oxide layer (Ref 18, 19). Upon first contact with the oxidizing environment, oxides will begin to form on the MCrAlY surface. The nature of each oxide formed depends on coating composition and microstructure, coating application method, and post-coating heat treatment condition. The coating composition must be formulated in such a way as to ensure that the primary oxidation product is the desired protective and thermodynamically stable aluminum oxide, instead of a transition metal oxides or spinels

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Transient Oxide Formation on APS NiCrAlY After Oxidation Heat Treatment

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since they are less protective and stable oxides compared with alumina (Ref 18). MCrAlY coatings are in fact secondary alumina formers that generally develop chromium oxide scales on the external surface of the coating and subsequently form an alumina layer underneath the chromium oxide layer during exposure to an oxidizing environment (Ref 20). However, at temperatures above 900 C, Cr2O3 has the tendency to react further with oxygen to form volatile CrO3 (Ref 21), leaving alumina on the exposed surface. While the thermodynamically stable hcp a-phase of alumina is preferred as the primary thermally grown oxide (TGO) constituent (Ref 22), there are several transient phases of alumina with different crystal structures that have also been shown to exist: c-Al2O3 (cubic spinel structure), d-Al2O3 (tetragonal structure), and h-Al2O3 (monoclinic structure) (Ref 23). These metastable aluminum oxide structures form blade or whisker-type surface morphologies and tend to grow at a significantly faster rate than a-Al2O3. All transient aluminum oxide phases can be converted to the a-phase through heat treatment (Ref 21); however, this phase change can be accompanied by a significant volume change (Ref 18) that may lead to cracking. This study will examine the oxide formation on the high velocity air plasma-sprayed NiCrAlY coatings after carrying out air furnace heat treatments at different temperatures and periods.

2. Materials and Experimental Procedures NiCrAlY coating was applied using Axial III Plasma Spray System (Northwest Mettech Corp., North Vancouver, Canada). The torch employed in this system injects powder axially, among the three electrodes, ensuring that virtually all of the powder injected passes through the hottest part of the plasma plume. The high velocity plasma was generated using the processing parameters listed in Table 1, achieving a particle velocity in excess of 400 m/s. Nickel-based Hastelloy 9 substrate was grit blasted with aluminum oxide of #24 mesh size prior to the coating application. After coating, the samples were polished to 1200 grit finish with silicon carbide paper. This was done to level the surface and remove any oxide(s) formed during the APS coating process. Heat treatments were carried out in an atmospheric furnace at temperatures and

Table 1 Plasma spray parameters for NiCrAlY coating Total nozzle flow rate, L/m N2 proportion, % H2 proportion, % Ar proportion, % Carrier gas flow rate, L/m Powder feed rate, g/min Nozzle diameter Spray distance, cm Turntable rotational speed, rpm Powder specification Nominal coating thickness, lm

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260 10 25 65 12 50 ¼00 175 365 Praxair Ni-211-4 (Ni—22 wt.% Cr—10 wt.% Al—1 wt.%Y) 200-250

durations given in Table 2. Decreased heat treatment time was selected for heat treatment at higher temperature to observe the transient state of oxide formation. Microscopic examination was carried out using a scanning electron microscope (SEM). A Philips XL30S FEG SEM with a Phoenix EDS detector system and Hitachi S-570 SEM with a Link Systems LX-5 model 5697 EDS detector were used to generate secondary electron (SE) and backscattering electron (BSE) images, and energy dispersive x-ray spectrometry (EDS) data. Since the oxide films have thicknesses of a few microns, it is probable that the EDS spectra obtained from surface oxides contained information pertinent to both the oxide layer as well as the underlying material. Oxygen and yttrium were not reported in the results because of the limited accuracy in detecting oxygen and low concentration of yttrium with EDS. XRD analysis was used to provide information in terms of the crystal structure of the phases present in the selected samples. XRD analysis was performed using a Burker AXS D8 Advance diffractometer controlled with General Area Detector Diffractometer Software (GADDS).

3. Results and Discussion The surface morphology of the air plasma sprayed NiCrAlY coating, Fig. 1, shows the typical constituents that are present on a thermally sprayed coating surface: coating splats, unmolten particles, porosity, and glazed areas indicating remelting and solidification. Table 3 summarizes the compositional analysis results from the as-sprayed coating surface and cross section. The results are consistent with the original powder material composition. Notable among them are the variations in Y content. Owing to its low concentration and limited detection capability of EDS, Y is not being included in the analysis of heat-treated samples.

3.1 Microstructure After Heat Treatment at 1000 °C for 10 h The examination of the NiCrAlY sample, subjected to a heat treatment in an air furnace at 1000 C for 10 h, shows the formation of a number of distinct phases with different morphologies: light cluster phases in a matrix of whisker-shaped phase A (Fig. 2a). Figure 2(b) gives a more detailed view of the sample surface morphology under BSE mode, clearly illustrating the spatial distribution of the three phases (A, B, and C). In particular, the light contrasted phases, when they are more closely

Table 2 Heat treatment parameters Temperature, °C Air furnace 1000 1050 1100

Time, h 10 5 1

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Peer Reviewed Fig. 1 SEM Images showing the as-sprayed NiCrAlY coating surface (a, SE mode) and cross sections (b, c, BSE mode) of the APS NiCrAlY coating

Table 3 EDS analysis results of the as-sprayed NiCrAlY (wt.%) Elements Powder nominal composition Coating surfaces (area scan) Coating cross section (area scan)

Ni

Cr

Al

Y

67 68.32 67.93

22 18.59 17.74

10 11.96 12.37

1 1.13 1.96

examined, exhibit two different shades: a lighter globular phase (B in Fig. 2b) and a more grayish surrounding phase (C in Fig. 2b). A significant compositional difference was detected between them, with the lighter phase B being enriched in Ni and the grayish phase C in Cr, in addition to the presence of Al and Ni. Figure 2(c) gives a more detailed view of light globular phase B, typical of NiO. The whisker-shaped matrix phase can clearly be visualized

Fig. 2 NiCrAlY surface features after heat treatments at 1000 C for 10 h. (a) General view of the heat-treated surface. (b) Cluster of globular phase B surrounded by grayish phase C in a matrix of whisker-shaped phase A. (c) Enlarged view of globular phase B, and (d) whisker-shaped phase A

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in Fig. 2(d). The composition for the whisker-shaped phase A, summarized in Table 4, indicates a significant amount of aluminum in comparison with the coating composition. The presence of an elevated amount of oxygen in the whisker-shaped phase A, shown on the left side of the EDS spectrum (Fig. 3a), confirms that this phase is alumina in nature. The whisker-shaped alumina resembles

Table 4 EDS analysis results for various phases shown in Fig. 2 and 4 (O and Y are not analyzed) Elements, wt.% Whisker-shaped phase A on surface Phase A on cross section Bright globular phase B on surface Phase B on cross section Grayish phase C on surface

Ni

Cr

Al

16.76 11.02 91.51 95.69 21.19

8.44 7.74 5.27 4.31 47.31

74.80 81.24 3.22 … 31.5

the metastable h-Al2O3 as reported in the literature (Ref 24). The EDS composition analysis also suggests that the globular phase is primarily that of NiO, based on the high Ni concentration and the presence of oxygen on the EDS spectrum (Fig. 3b). The grayish phase C is termed as Cr-rich oxide since it cannot be identified based on EDS analysis results given in Table 4 and Fig. 3(c). Possible Cr-rich oxides found on NiCrAlY include Cr2O3 and Ni(Cr, Al)2O4 (Ref 25, 26). A cross-sectional view of the NiCrAlY sample heat treated in air furnace (Fig. 4a) shows noticeable change in the coating surface microstructure in comparison to the as-deposited coating microstructure (Fig. 1). The formation of a dense surface oxide layer, primarily as alumina (Table 4), can be seen with an average thickness of approximately 3 lm. Occasionally, protruded surface layers, as shown in Fig. 4(b), are observed. Several phases corresponding to that observed in Fig. 2 are found within the protruded area. These include a light phase B on top

Fig. 3 EDS spectra of the three phases observed on the heat-treated NiCrAlY surface. (a) Alumina, (b) NiO, and (c) Cr-rich oxide(s) (Note the strong oxygen peak on the left side of each spectrum)

Fig. 4 SEM images taken under BSE mode showing the surface features typical of the NiCrAlY coating surface after heat treatment at 1000 C for 10 h. (a) Image showing largely continuous Al2O3 layer. (b) Occasional occurrence of mixed oxides. Locations where EDS spot analysis are labeled

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Peer Reviewed Fig. 5 (a) NiCrAlY coating surface after heat treatment at 1050 C for 5 h, (b) Globular NiO distributed in whisker-shaped alumina matrix, (c) Whisker-shaped alumina, and (d) Globular NiO

surface with grayish phase C and alumina. EDS spot analysis confirms that the light phase is NiO and the grayish constituent contains Cr-rich oxide. From the spatial location of these three phases, it seems that at the protruded locations the NiO has formed first followed by the formation Cr-rich oxide. The alumina phase subsequently grows between these two phases through outward diffusion of Al.

3.2 Microstructure After Heat Treatment at 1050 °C for 5 h The microstructure of the NiCrAlY coating after heat treatment at 1050 C for 5 h (Fig. 5a, b) shows the formation of two distinct phases: a globular phase B that forms in a matrix of a whisker-shaped phase A. Close-up views of the whisker-shaped matrix phase A and the globular phase B are further illustrated in Fig. 5(c) and (d). The composition of the whisker-shaped phase, summarized in Table 5, indicates a significant amount of aluminum in addition to a strong oxygen peak observed on the EDS spectrum and as such it is identified as alumina. The globular phase B, similar to that observed in the sample heat treated at 1000 C for 10 h, is determined to be NiO based on the significant proportion of nickel in its composition (Table 5). In contrast to the one observed on

Journal of Thermal Spray Technology

Table 5 EDS analysis of region shown in Fig. 5 (O and Y are not analyzed) Elements, wt.% Whisker-shaped phase A on surface Phase A on cross section Globular phase B on surface Phase B on cross section Cr-rich phase C on cross section

Ni

Cr

Al

19.61 14.04 95.84 96.35 44.59

11.83 14.45 4.15 3.64 31.25

68.54 71.51 … … 24.16

the sample after heat treatment at 1000 C for 10 h, no apparent Cr-rich phase is observed on the coating surface after heat treatment at 1050 C for 5 h. This suggests the possible evaporation of Cr2O3 via the formation of volatile CrO3 under this heat treatment condition. A cross-sectional view of the heat-treated NiCrAlY sample (Fig. 6) shows a thin layer of oxide formation on the surface, along with some internal oxidation in the coating microstructure when compared to the as-sprayed coating microstructure. The formation of the oxide scale is largely continuous with occasional protrusion of mixed oxides. EDS spot analysis indicates the presence of alumina, NiO, and Cr-rich oxide corresponding to the three types of oxides observed previously in the sample heat treated at 1000 C for 10 h.

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3.3 Microstructure After Heat Treatment at 1100 °C for 1 h

Fig. 6 Cross section of the NiCrAlY coating surface after heat treatment at 1050 C for 5 h. The average coating thickness of the largely continuous aluminum-rich surface scale is ~2 lm. Phases A, B, and C are identified as alumina, NiO, and Cr-rich oxide, respectively

The surface of the NiCrAlY sample subjected to an air furnace heat treatment of 1100 C for 1 h (Fig. 7a, b) shows the formation of three different phases: a larger, globular phase B that forms on top of a smaller, nodular phase C in a matrix of a whisker-shaped phase A. A detailed view of the whisker-shaped phase A is illustrated in Fig. 7(c). As summarized in Table 6, a significant proportion of aluminum (and oxygen) was found in phase A. Compared to the as-deposited coating composition, it suggests the formation of alumina. Figure 7(d) shows enlarged views of the larger globular oxide B and the surrounding nodular-shaped phase C. Their compositions (Table 6) indicate a significantly higher nickel (and oxygen) in the globular phase and chromium (and oxygen) in the nodular oxide phase. This supports the determination of these phases being NiO and Cr-rich oxide. The cross section of the NiCrAlY sample after air furnace heat treatment, shown in Fig. 8, demonstrates porous oxide formation at this temperature. The average thickness of TGO is approximately 1 lm, much thinner

Fig. 7 Surface features of the NiCrAlY coating surface after heat treatment at 1100 C for 1 h. (a) Image showing the distribution of the globular phase B and nodular phase C clusters in a matrix of whisker-shaped phase A. (b) An enlarged view of the clusters (phases B and C), (c) Whisker-shaped alumina, and (d) Globular NiO and Cr-rich nodular phase C

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Elements, wt.% Whisker-shaped phase A on surface Large globular phase B on surface Smaller nodular phase C on surface

Ni

Cr

Al

20.95 87.91 33.30

12.08 9.93 55.00

66.97 2.16 11.70

Fig. 8 High-magnification SEM images showing the typical cross-sectional features of the NiCrAlY surface after heat treatment at 1100 C for 1 h

than that on the sample heat treated at 1050 C for 5 h. EDS spot analysis confirms the presence of the three oxide phases (alumina, NiO, and Cr-rich oxide) that were previously observed. In comparison to the other heat treatment conditions in the series, this heat treatment condition yields a higher proportion of Cr-rich oxide in the whisker-shaped oxide matrix. Although the sample was heat treated at a temperature higher than the two previous samples, the shorter heat treatment time may not have provided sufficient time for stable alumina to form, and Cr-rich oxide to become further oxidized into volatile CrO3. Also, the high heat treatment temperature contributes to the formation of a more porous surface scale shown in Fig. 8.

3.4 XRD Results The XRD analysis results are given in Figure 9 for samples in the as-sprayed condition and after heat treatment. The XRD spectrum for the as-sprayed sample reveals the main diffraction peaks for c phase (Ni-based solid solution) and c0 -Ni3Al. Also presented is the b-NiAl in the as-sprayed sample. For the heat-treated samples, a-Al2O3 was found to be present on all the three samples; the sample heat treated at 1050 C for 5 h exhibited the strongest a-Al2O3 peaks, suggesting denser alumina formation. Some indication of NiO was also observed on the heat-treated samples although the diffraction peaks were extremely weak for the sample heat treated at 1100 C for 1 h. The presence of Cr2O3 (and possibly Ni(Cr,Al)2O4 spinel) was evident on the sample subjected to heat treatment at 1000 C, while for the samples heat treated at

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Table 6 EDS spot scan data for various locations (O and Y not analyzed)

Fig. 9 XRD spectra for NiCrAlY in the as-sprayed condition and after heat treatment

1050 C for 5 h and 1100 C for 1 h, there seemed to be several weak peaks associated with Cr2O3. b phase, found in the as-deposited sample, was not present on any of the heat-treated samples because of the depletion of Al caused by the formation of alumina and the inward diffusion of Al into the substrate during the heat treatment.

4. Discussion The primary goal of this study was to examine the transient oxides formation on NiCrAlY during oxidation heat treatment. The sequence and morphology of phase evolution in the transient stage (first 5-10 h) can have an impact on the stable oxide formation and long-term coating stability. As such, it is important to have an understanding of these features before designing an oxidation heat treatment procedure for a given coating system. From the results presented in this study, it is apparent that alumina has formed under all the three air furnace heat treatment conditions. In addition to alumina formation, other types of oxides, such as NiO and Cr-rich oxide (Cr2O3 or Ni(Cr,Al)2O4 spinel based on the literature (Ref 27) and XRD analysis), also developed. NiO was observed to be the first oxide formed upon heat treatment at some protruded locations, followed by the formation of Cr-rich oxide (Cr2O3) and alumina as the secondary oxide. From the observation of the coating structure on cross sections, Cr-rich oxide was observed to be also dispersed within alumina. The results also showed that the sample that had been exposed to 1050 C for 5 h seemed to have the most Al2O3 (from XRD) and the least amounts of NiO and Cr-rich

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oxide (SEM surface observation). This observation suggests that vacuum heat treatment condition at 1050 C for 5 h is more beneficial than the other two conditions examined in this study in terms of generating an alumina layer. With regard to the general impact of heat treatment on oxide formation, it seems that a critical exposure period exists at each temperature. Upon reaching this exposure period, a continuous layer of Al2O3 may form, and yet Al is not depleted to allow spinel formation. The effect of temperature on oxide formation under constant time is more complicated as both diffusion rate and oxide stability play their respective roles. This study illustrated that a higher heat treatment temperature reduced the tendency for Cr2O3 formation. Besides, our earlier study of vacuum heat-treated NiCrAlY showed that increasing temperature from 1000 to 1050 C resulted in more Al2O3 formation; however, reduced amount of Al2O3 was observed when increasing temperature from 1050 to 1100 C (Ref 28). There are several common types of alumina, h, c, or, a-Al2O3, observed in the transient oxidation stage of MCrAlY coatings. The exact type being formed is dependent upon heat treatment temperature, time, alloy composition, and the surface preparation method. Needleshaped h-Al2O3 has been found to be formed during heat treatment of VPS and HVOF MCrAlYs in the temperature range of 850-1050 C (Ref 29, 30). A study of a HVOF nano-NiCrAlY coating showed that after 24-hour thermal exposure at 1000 C, a-Al2O3 was detected (Ref 25). Furthermore, on a sputtered Ni-30Cr-12Al-0.3Y coating, needle-like h-Al2O3 was observed after 5 h at 900 C while complete a-Al2O3 (with wrinkled ridges) were formed on the same coating after 1 h of oxidation heat treatment at 1100 C (Ref 31). When heat treatment was carried out at 1000 C, h-Al2O3 was initially formed and after 10 h of oxidation treatment, a-Al2O3 began to form. At the end of 50 h, a-Al2O3 became the dominant oxide with only traces of h-Al2O3. In this study, although XRD analysis revealed the presence of a-Al2O3 in all the heat-treated NiCrAlY samples, the presence of the h-Al2O3 phase cannot be excluded because of the limitation in the detection capability of XRD of a phase with a small quantity. Indeed, from the surface observation of the oxide scale, all the samples studied showed whiskershaped alumina, a characteristic form of h-Al2O3. It is therefore more likely that the alumina that develops immediately above the NiCrAlY assumes a-Al2O3 form, but the alumina exposed to the surface is of h-Al2O3 phase. Further study is needed to evaluate the exact nature of alumina formation on these samples and to determine the critical conditions (temperature, duration, oxygen activity, and localized compositions) when transition from h fi a-Al2O3 occurs.

5. Conclusion In this study, the surface oxide formation on APS NiCrAlY samples was evaluated. The oxides observed on NiCrAlY coating samples heat treated at various conditions included NiO, Cr-rich oxide, and alumina. Although

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the formation of alumina was observed on samples heat treated at all three temperatures, the NiCrAlY sample heat treated at 1050 C for 5 h generated a denser and more continuous alumina layer. An examination of the samplesÕ cross sections showed that NiO formed on top of nodular-shaped Cr-rich oxide in areas protruded from the otherwise smooth alumina layer. NiO apparently formed first during heat treatment, and as heat treatment time increased, a more stable alumina developed beneath the NiO. Cr-rich oxide was found to be in the vicinity of NiO and interspersed within the alumina. A reduced amount of Cr2O3 was observed with increasing heat treatment temperature (and reduced time). Heat treatment at 1000 C for 10 h or 1050 C for 5 h yielded an alumina layer with an average thickness of 2-3 lm. Decreased alumina thickness was observed for the sample heat treated at 1100 C because of the shorter duration of the heat treatment. The oxide scale generated at this temperature was also more porous than the others. Based on this study of transient oxide formation on NiCrAlY coating samples, it is believed that a slightly extended heat treatment period at 1050 C would ensure complete alumina formation on the NiCrAlY surface in the form a-Al2O3.

Acknowledgment We are thankful to the NSERC for providing financial support for this research. The Research Associateship for Philip Puetz was provided by the NSERC/CRD funding program.

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