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Proceedings of ASME Turbo Expo 2009: Power for Land, Sea and Ai rGT2009

June 8-12, 2009, Orlando, Florida, USA

GT2009-59041

COMPARISON OF THREE MICROTURBINE PRIMARY SURFACE RECUPERATOR ALLOYS

Wendy J. MatthewsCapstone Turbine Corporation

Chatsworth, California

Karren L. More and Larry R. WalkerOak Ridge National Laboratory

Oak Ridge, Tennessee

ABSTRACT

Extensive work performed by Capstone TurbineCorporation, Oak Ridge National laboratory, and variousothers has shown that the traditional primary surfacerecuperator alloy, type 347 stainless steel, is unsuitable forapplications above 650°C (~1200°F). Numerous studies haveshown that the presence of water vapor greatly accelerates theoxidation rate of type 347 stainless steel at temperatures above650°C (~1200°F). Water vapor is present as a product ofcombustion in the microturbine exhaust, making it necessaryto find replacement alloys for type 347 stainless steel that willmeet the long life requirements of microturbine primarysurface recuperators. It has been well established over the past

few years that alloys with higher Chromium and Nickelcontents than type 347 stainless steel have much greateroxidation resistance in the microturbine environment. Onesuch alloy that has replaced type 347 stainless steel in primarysurface recuperators is Haynes Alloy HR-1201, a solid-solution-strengthened alloy with nominally 33 wt.% Fe, 37wt.% Ni and 25 wt.% Cr. Unfortunately, while HR-120 issignificantly more oxidation resistant in the microturbineenvironment, it is also a much more expensive alloy. In theinterest of cost reduction, other candidate primary surfacerecuperator alloys are being investigated as possiblealternatives to type 347 stainless steel. An initial rainbowrecuperator test has been performed at Capstone to comparethe oxidation resistance of type 347 stainless steel, HR-120

and the Allegheny Ludlum austenitic alloy AL 20-25+Nb2.Evaluation of surface oxide scale formation and associatedalloy depletion and other compositional changes has beencarried out at Oak Ridge National Laboratory. The results ofthis initial rainbow test will be presented and discussed in this

paper.

1Haynes and HR-120 are trademarks of Haynes International, Inc.2AL 20–25+Nb is a trademark of ATI Properties, Inc. and islicensed to Allegheny Ludlum Corporation.

INTRODUCTION

A Primary Surface Recuperator (PSR) is a counterflow heat exchanger with a core manufactured from thincorrugated metallic foils. The addition of a PSR, used torecover the heat energy of exhaust gas by pre-heatingcombustion air, can almost double the thermal efficiency of asmall gas turbine such as the Capstone MicroTurbine3[1]. Alarge number of PSR designs have been investigated over theyears, with Capstone opting to use an all-welded annular PSRThis robust annular design, used on both the Capstone C30and C65 microturbines, has a significant impact on themicroturbine thermal efficiency [2]. The all-welded annulaPSR design has proven to be resistant to fatigue cracking due

to thermal and pressure cycling.When the PSR was first introduced as a microturbine

component, type 347 stainless steel (347SS) was the materialof choice. This Niobium-stabilized grade of 300 seriesstainless provided a good combination of creep and oxidationresistance with relatively low direct raw material cost. AsPSRs were put into production and began to accumulatesignificant operating exposure, stability issues associated withthe 347SS were discovered. Creep damage in certainapplications resulted in blockage of the hot gas flow throughthe recuperator, causing reduced recuperator effectiveness [3]In other applications, accelerated rates of oxidation wereobserved in the PSR thin foil material leading to blockage offlow channels (due to excessively thick oxide scale) and to

complete oxidation of the PSR foils, which both result inreduced PSR effectiveness [3,4,5].

At the same time, extensive laboratory research was beingcarried out to characterize the effect of water vapor on theoxidation rate of 347 stainless steel and other recuperatoralloys. A number of researchers have shown, at operatingtemperatures in excess of ~600°C (~1110°F), small amountsof water vapor will significantly increase the rate of oxidationof chromia-forming stainless steels such as 347SS, resulting inwhat is sometimes referred to as Accelerated Attack (AA) [6- 3 MicroTurbine is a registered trademark of Capstone Turbine Corporation.

Proceedings of ASME Turbo Expo 2009: Power for Land, Sea and AirGT2009

June 8-12, 2009, Orlando, Florida, USA

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11]. The increased rate of oxidation leads to an increased rateof Cr-depletion at the foil surface, which in turn leads to

breakaway oxidation. Breakaway oxidation is also referred toas chemical failure and is specifically identified asMechanically Induced Chemical Failure (MICF) when itoccurs in this temperature regime [12]. MICF occurs once theCr-depletion occurring across the thickness of the foil hasreached a level below the critical Cr-concentration required tomaintain and reform a healing chromia-scale. The effect ofwater vapor on the oxidation rate of 347SS was an important

discovery, since newer generation microturbines operate withturbine exit temperature (TET) or PSR inlet temperature above650°C (~1200°F). The Capstone C65 has a PSR inlettemperature of ~666°C (~1230°F) with ~3-4 vol.% watervapor (as a product of combustion) in the turbine exit gas.

At the same time that the effects of water vapor on theoxidation rate of 347SS were being evaluated, alternativematerials with greater oxidation resistance were also beingdeveloped and characterized [13,14,15]. Studies have shownthat increasing the amount of Cr and/or Ni present in an alloywill prevent or greatly reduce the rate of accelerated attack[16,17]. Alumina-forming austenitic alloys are also beinginvestigated as possible alternatives to 347SS [18].

Based on the published results of various researchers, and

the extensive characterization of 347SS field-operatedCapstone PSRs, the material used to manufacture the CapstoneC65 PSR was changed to Haynes alloy HR-120 [19,20].While the HR-120 PSR performs exceptionally well, and hasshown no signs of AA after extensive long-term elevatedtemperature exposures, the alloy is significantly moreexpensive than 347SS. The higher cost is primarily due to themuch higher Cr and Ni content of HR-120. The AlleghenyLudlum (AL) austenitic alloy AL 20-25+Nb has been

proposed as a lower cost alternative to HR-120. AL 20-25+Nb contains more Cr and significantly more Ni than347SS, though the content of both elements is not as high asthat of HR-120. The chemical compositions of HR-120, AL20-25+Nb and 347SS are given in Table 1. It is anticipatedthat the oxidation resistance of AL 20-25+Nb will be lowerthan HR-120, but significantly higher than that of 347SS.

In an effort to better understand the oxidation resistanceof AL 20-25+Nb when compared with HR-120 and 347SS,Capstone manufactured a “rainbow” PSR from the 3 alloys.This rainbow PSR was subjected to an elevated temperaturecyclic test, and samples were removed for analysis at OakRidge National Laboratory (ORNL). This paper presents theresults of the post-exposure characterization of the surfaceoxide scale and associated alloy depletion and othercompositional changes that have occurred in all 3 alloystested. It is not the intent of this paper to develop a life

prediction model for any of the alloys examined, but to

provide a direct comparison between the 3 alloys afterexposure to identical operating conditions.

Table 1: Typical Chemical Composit ion of HR-120, AL 20-25+Nb and 347SS (wt .%).

ElementHR-120

(N08120) [21]AL 20-25+Nb

[17]347SS (S34700)

[22]

Ni 37.0 25.5 10.5

Cr 25.0 20.5 18.0

Mn 0.7 1.0 2.00*

C 0.05 0.08 0.08*

Cu -- -- 0.75*Si 0.6 -- 1.00*

S -- -- 0.030*

Al 0.1 -- --

Nb 0.7 0.4 0.95

Mo 2.5* 1.5 0.75*

P -- -- 0.040*

W 2.5* -- --

Co 3.0* -- --

N 0.20 0.1 --

B 0.004 -- --

Fe As Balance(~33 wt.%)

As Balance(~50 wt.%)

As Balance(~66 wt.%)

* Maximum

NOMENCLATURE

AA Accelerated AttackBSE Backscatter ElectronEOH Equivalent Operating HoursEPMA Electron-Probe MicroanalysisMICF Mechanically Induced Chemical FailureORNL Oak Ridge National LaboratoryPSR Primary Surface RecuperatorTET Turbine Exit Temperature347SS Type 347 Stainless Steel

EXPERIMENTAL PROCEDURE

PSR Core Production

The heart of the PSR is the counter flow heat exchangercore, shown in Figure 1. Capstone Turbine Corporation has

been manufacturing 347SS PSR cores since 2001 (C30 andoriginal C60) [2], and HR-120 PSR cores since 2005(C60/C65 – replaced 347SS) [20,23]. A number of complexmanufacturing steps are involved in the production of a PSRcore.

Cold-rolled ~80 µm (3.2 mils) thick foil in the brigh

annealed condition is formed into what is called the fin-foldwhich is a wavy corrugated pattern [5,19,20]. Individuaaircell sheets, comprised of both fin-folded and crushedregions, are fabricated after subjecting the fin-folded stockmaterial to crushing and trimming operations. These sheetsknown as the primary sheets, are made with a slight offset inthe fin-fold pattern resulting in “A” and “B” primary sheetsVarious spacer bars are welded together with the two primarysheets to create an individual aircell, as shown in the inset oFigure 1. The aircells are bent into a curved shape and ~165

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170 aircells are stacked together for welding to become thePSR core. The turbine exit gas flows between the aircells ofthe PSR core, pre-heating the compressor discharge airflowing inside the aircells before it exits to the combustor.

Figure 1: Primary Surface Recuperator (PSR) Core withIndividual Aircell (Inset)

Rainbow PSR Engine Testing

Individual aircells were manufactured using HR-120, AL20-25+Nb and 347SS. A rainbow PSR core was then stackedand welded containing 1/3 HR-120, 1/3 AL 20-25+Nb and 1/3347SS aircells. A recuperator was built using this rainbow

core and installed on a Capstone C65 MicroTurbine. Theengine was used to perform an elevated temperature cyclictest.

The test engine ran with a TET set-point ~55C° (~100F°)above the normal operating TET set-point, resulting in anaverage PSR inlet temperature of ~720°C (1330°F). Thecyclic testing was performed using an aggressive profile ofone cycle per hour at the elevated TET set-point with aminimum temperature drop on cool down of 500°C (~900°F)

between cycles. The aggressive cycle and elevated TET set- point were chosen in an effort to accelerate the rate ofoxidation experienced by the rainbow PSR core. The testengine accumulated a total of 1,642 hours and 1,551 cycles.

Previous Capstone experience has shown that for 347SS,

every cycle with a temperature drop on cool down similar toor greater than that used in the elevated TET test is equivalentto ~1.5 hours of steady-state operation [5]. This means thatthe Equivalent Operating Hours (EOH) experienced by 347SSat the elevated TET set-point was 1,642 hours plus 1.5 times1,551 cycles, or ~3,969 EOH. Previous experience has alsoshown that a 347SS PSR core operating at normal TET set-

point has ~3.2 times the oxidation life of a 347SS PSR coreoperating at the TET set-point used for the elevated TETtesting [20], thus ~3,969 EOH at the elevated TET set-point is

considered to be equivalent to ~12,700 EOH for 347SS atnormal operating TET set-point.

RESULTS

Once the elevated TET cyclic test was completed, therainbow core was removed from the PSR. Sections were cufrom the hot turbine-exit gas-inlet side of the core. Samples oHR-120, AL 20-25+Nb and 347SS aircells were examined

using EPMA. The ORNL JEOL JXA-8200 CombinedMicroanalyzer, equipped with both Wavelength and EnergyDispersive Spectrometers (W/EDS), was used to performmicrostructural and compositional characterization of therainbow PSR core samples.

With reference to the samples removed from actual PSRcore aircells, the inside of the aircell (exposed to compressorair) is called the “air side” and the outside of the aircell(exposed to exhaust gas) is called the “gas side”. All figureshown are labeled to identify the air side and the gas side ofthe image.

Cross-section samples were prepared from the hot gasinlet side of the engine-exposed rainbow PSR aircells, withsamples being taken from both the crushed and fin-folded

regions of the aircells. All of the samples were examined withthe same types of analyses being performed from both regionsof the aircells. The oxidation damage was more severe in thecrushed region for all of the samples. The crushed regioncorresponds to the region of highest temperature exposure atthe recuperator inlet and thus, all data presented herein are forsamples from the crushed regions.

Qualitative Analysis

Elemental maps for each of the HR-120, AL 20-25+Nband 347SS samples were acquired using W/EDS and areshown in Figure 2, Figure 3, and Figure 4, respectively. Thesemaps provide a relative measure of the difference in the extenof surface oxidation and Cr-depletion experienced by the HR-120, AL 20-25+Nb, and 347SS elevated TET aircells from therainbow PSR core, after identical engine-exposure conditionsHigher relative concentrations of the element being mappedare indicated by the brighter regions of the images.

Figure 2a is a Backscatter Electron (BSE) image of thelocation chosen for element mapping on the HR-120 sampleAs has been shown in previous analyses [19,20,23], a thickerCr-rich oxide scale has formed on the gas side of the foil(bottom of image). Figure 2b-d are element maps showing thedistribution of Cr, Fe, and O.

The element maps in Figure 2 show a very thin Cr-oxide

layer on both the air and gas sides of the sample. The gas sideof the sample also has a thin outer layer of Fe-(Ni,Mn) oxide.Cr depletion is occurring below the oxide scale on the gas sideof the sample. The extent of Cr-depletion appears to beconsistent within the grains and along the grain boundaries ofthe HR-120 material, i.e., there does not appear to be anyselective Cr-depletion occurring along the grain boundariesSuch non-selective Cr-depletion has been observed on severalong-term elevated TET HR-120 samples [23] and on field-operated PSR cores [24]. Work is on-going to establish a

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correlation between the oxidation damage experienced at theelevated TET set-point and that at the normal operating TETset-point.

Figure 3a is a BSE image the AL 20-25+Nb sample, andFigure 3b-d are element maps showing the distribution of Cr,Fe, and O.

The element maps in Figure 3 show a very thin Cr-oxidelayer on both the air and gas sides of the sample. The gas sideof the sample also has a relatively thin intermittent outer layerof Fe-oxide in the form of small nodules similar to those

observed on AL 20-25+Nb material examined by otherresearchers [15,25]. Cr-depletion is occurring below the oxidescale, primarily on the gas side of the sample, and selectivelyalong grain boundaries to a depth of ~1/3 of the thickness ofthe foil sample.

Figure 4a is a BSE image of the 347 SS sample, with

element maps for Cr, Fe, and O shown in Figure 4b-d. Thesemaps indicate a thick, multi-component oxide scale present on

both the air and gas sides of the sample, typical of theaccelerated attack observed in 347 SS PSRs [5,20].

The innermost oxide layer, immediately adjacent to theregion of Cr-depletion, is predominantly Cr-rich oxide, whilethe thick, outermost oxide scale is Fe-oxide, typical of thenodules that form upon breakdown of the Cr-rich layer andspread across the outer surface [5]. A further oxide layer

between the Cr-rich and Fe-rich oxides contains Cr, Fe, and

Ni. Cr depletion extends through the thickness of the sampleselectively along the grain boundaries. The thick multicomponent oxide scale on both the air and gas sides of the 347SS sample is typical of the AA that has been observed in 347SS PSRs [5,20]. The extent of Cr-depletion visible is verysimilar to that measured for a 12,400 EOH 347 SS sample [5].

Figure 2: HR-120 Sample from rainbow PSR - a) BSE image, b) Cr map, c) Fe map, and d) O map.

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Figure 3: AL 20-25+Nb Sample from rainbow PSR - a) BSE image, b) Cr map, c) Fe map, and d) O map.

Figure 4: 347SS Sample from rainbow PSR - a) BSE image, b) Cr map, c) Fe map, and d) O map.

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Quantitative Analysis

Compositional analyses of the three alloys includeestimates of Cr loss from the starting (bulk) Cr-reservoir.Methods for predicting the extent of Cr-loss in thin foilsections based on chromium volatilization rates, have been

proposed and developed [26]; however, these requiremeasurement of mass change over time, making themimpractical for use with samples removed from field-exposedPSRs. In this work, the starting Cr contents of the HR-120,

AL 20-25+Nb and 347 SS are based on the nominalcompositions given in Table 1, i.e., 25.0, 20.5, and 18.0 wt%,respectively. Estimation of the amount of Cr-loss from thestarting Cr reservoir is based on the amount of the starting foilthickness consumed by oxidation, and the depth and level ofCr-depletion. This procedure for estimating the Cr-loss fromthe starting reservoir has been described elsewhere [5,23].While this method is somewhat simplistic, it has provensufficient for purposes of alloy comparison and componentlife prediction [5,23].

A BSE image of a typical profile scan location for theHR-120 sample is shown in Figure 5, and the correspondingelement profile scan for Cr, Fe, Ni, and O is shown in Figure6. The various measurements indicated that this sample has

experienced ~4.4% Cr-loss from the starting Cr reservoir to~23.9 wt.% Cr, that is, it retained 95.6% of the originalstarting Cr content.

Figures 7 and 8 are the BSE image and element profilescan, respectively, for the AL 20-25+Nb sample. This sampleexperienced ~8.9% Cr-loss from the starting Cr reservoir,resulting in a net Cr level of ~18.7 wt.%, or ~91.1% of theoriginal starting Cr content. .

Similar data for the 347SS sample are shown in Figures 9and 10, and indicate a Cr-loss from the starting Cr reservoir of~51%, to ~8.8 wt.% Cr. Previous analyses have shown thatonce the Cr level in 347SS is depleted to below ~11-13 wt.%,

breakaway oxidation occurs with rapid consumption of the foil[5].

A comparison of the post-exposure Cr profiles of thethree PSR alloys examined is shown in Figure 11. The

profiles are for the full foil thickness between the oxide layersand include the Cr-depleted layers. It is clearly observed thata significant amount of the starting 347SS foil thickness has

been lost to oxidation, since the Cr profile covers only ~46 µm(~1.8 mils) of material. In addition, the thickness of the foilthat still retains approximately the starting nominal Cr content

(18 wt.%) is only ~16 µm (~.6 mils). The HR-120 and AL 20-25+Nb profiles show similar Cr-depletion depths and similarthicknesses of foil retaining the starting nominal Cr content,

but it is clear that the HR-120 retains significantly more Crthan the AL 20-25+Nb. The higher Cr retention of the HR-

120 foil is directly related to the much higher starting nominalCr content. It should be noted that the Cr-depletion along thegrain boundaries of the AL 20-25+Nb has not been accuratelycaptured by the Cr profiles shown in Figure 11.

Figure 5: HR-120 Profile Scan Location

Figure 6: HR-120 - EPMA O, Cr, Fe, and Ni Prof iles

Figure 7: AL 20-25+Nb Profi le Scan Location

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Figure 8: AL 20-25+Nb - EPMA O, Cr, Fe, and Ni Prof iles

Figure 9: 347 SS Profile Scan Location

Figure 10: 347 SS - EPMA O, Cr, Fe, and Ni Profi les

Figure 11: Comparison of Post-Exposure Cr-Profi les ofHR-120, AL 20-25+Nb, and 347 SS

SUMMARY

The elevated TET cyclic rainbow test has provided adirect comparison of three PSR alloys: 347 SS, AL 2025+Nb, and HR-120. The 347SS experienced excessiveoxidation, with breakaway oxidation either occurring orimminent, while the HR-120 alloy exhibited minimaoxidation damage. The AL 20-25+Nb alloy experiencedsignificantly less oxidation damage than 347SS, though itsresistance was not as great as HR-120. The comparativeoxidation resistance of the three alloys was characterized byestimating the total Cr loss during the test; the results aresummarized in Table 2.

Table 2: Total Cr Loss from Bulk Starting Reservoir andRemaining Cr Content

MaterialTotal Cr LossFrom StartingReservoir (%)

Remaining FoilCr Content

(wt.%)

HR-120 4.4 23.9

AL 20-25+Nb 8.9 18.7

347 SS 51.0 8.8

The onset of accelerated attack in 347 SS is generally

signaled by the formation of Fe-oxide nodules, whichultimately leads to breakaway oxidation [6,8,9,16]. Fe-richoxide nodule formation apparently is associated with surfaceexposed Cr-depleted grain boundaries, or to localized Cr 2Oevaporation and/or rapid Fe diffusion through the Cr-richoxide [5,15]. In service, 347SS PSRs have exhibited Fe-oxidenodule formation after relatively short times of less than 2,000hours [5]. With an estimated 12,700 EOH for the 347SS foiin this rainbow test, the sample progressed well beyond theFe-oxide nodule formation phase.

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Fe-oxide nodule formation did not occur on the HR-120samples, nor has nodule formation been observed in any of theHR-120 samples analyzed to date [23,24]. The lack of noduleformation on HR-120 samples may be due to a number offactors: (i) the higher Ni content of HR-120 may be impedingor preventing nodule formation; (ii) the presence of both Niand Mn in the outward growing Fe-rich oxide also may slowdown or eliminate nodule formation (347SS PSR samples,with much lower Ni content, all developed an outwardgrowing Fe-oxide that did not contain Ni or Mn [5,23]); and

(iii) non-selective Cr-depletion observed in these and in previously-characterized HR-120 samples [23] also may playa role in the prevention of Fe-oxide nodule formation. Sinceanalysis of laboratory-exposed HR-120 samples indicated thatnodule formation occurred at locations of grain boundary Crdepletion [15], the lack of preferential grain boundary Cr-depletion in engine-exposed HR-120 PSR foils also may be acontributing factor.

Fe-oxide nodule formation was observed on the AL 20-25+Nb rainbow samples, and has been observed by otherresearchers [15,25], although the rate of growth of suchnodules on this alloy apparently slows after initial formation[27]. In this study, Fe-oxide nodules on AL 20-25+Nbappeared to form over regions of Cr-oxide and Cr-depleted

grain boundaries. These samples also exhibited selective Cr-depletion along the grain boundaries, similar to the behaviorof 347SS, but not as extensive.

Breakaway oxidation is the greatest concern for PSR life.The length of time that the PSR foil retains sufficient Crcontent to continue to form and reform the protective oxidelayer, thereby healing any flaws at the oxide surface, willdetermine the PSR operating life. Once the Cr content in thePSR foil falls below this critical value, extremely rapid(breakaway) oxidation will occur. Previous analysis hasshown that the critical Cr content below which breakawayoxidation will occur in 347SS is ~11-13 wt.%. The Cr levelsin the 347SS samples from this test fell below this critical Crcontent, and breakaway oxidation ensued. The critical Crcontent for a 20Cr-25Ni austenitic stainless steel, similar toAL 20-25+Nb, has been shown to be ~16 wt.% [12,25,27].The AL 20-25+Nb samples from this test had 18.7 wt.% Crremaining and no breakaway oxidation was observed. Thecritical Cr content for HR-120 is unknown, but is likely to besimilar to that of AL 20-25+Nb. The HR-120 samples fromthis test retained 23.9 wt.% Cr, and showed no sign of

breakaway oxidation.The estimates of total Cr-loss summarized in Table 2

suggest that in this test the rate of loss of Cr from AL 20-25+Nb was approximately twice that for HR-120, suggestingthat AL 20-25+Nb may provide only half the oxidation life ofHR-120 in the PSR application. Additional testing in the

microturbine environment under service conditions isnecessary to provide confirmation of these life estimates.

ACKNOWLEDGMENTS

Part of this research was sponsored by the AssistantSecretary for Energy Efficiency and Renewable Energy,Office of FreedomCAR and Vehicle Technologies, as part ofthe High Temperature Materials Laboratory User Program,Oak Ridge National Laboratory, managed by UT-Battelle,

LLC, for the U.S. Department of Energy under contracnumber DE-AC05-00OR22725.

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Capstone Turbine Corporation.25. Rakowski, J. M., Stinner, C. P., Lipschutz, M., andMontague, J. P., “Environmental Degradation of Heat-Resistant Alloys During Exposure to Simulated andActual Gas Turbine Recuperator Environments”,Proceedings of ASME Turbo Expo 2007, May 14-17,Montreal, Canada, GT2007-27949

26. Young, D.J. and Pint, B.A., “Chromium VolatilizationRates from Cr 2O3 Scales into Flowing Gases ContainingWater Vapor”, Oxidation of Metals, 66 (2006), pp. 137-

153.27. Rakowski, J. M., Stinner, C. P., Lipschutz, M., and

Montague, J. P., “Environmental Degradation of HeatResistant Alloys During Exposure to Simulated andActual Gas Turbine Recuperator Environments”Proceedings of ASME Turbo Expo 2008, June 9-132008, Berlin, Germany, GT2008-51336

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