effect of water vapor on oxide scale microstructure
TRANSCRIPT
Effect of Water Vaporon Oxide Scale Microstructure
B. A. Pint, K. L. More and P. F. TortorelliMetals & Ceramics Division
Oak Ridge National LaboratoryOak Ridge, TN 37831-6156
Research sponsored by theMicroturbines Program
U. S. Department of Energy (DOE)contract DE-AC05-00OR22725 with UT-Battelle, LLC.
IntroductionDOEGoal - improved efficiency microturbines
Various design possibilities, for example:- increase pressure- increase system temperature
New materials needed!
TO MUFFLER/STACK
HRSG/WATER HEATER
ELECTRICITYGENERATOR
COMBUSTOR
FUELSYSTEM
COMPRESSOR TURBINE
AIR
AIR
RECUPERATOR
Motivation:Heat exchangers are essential for improvingthe efficiency of “Microturbines”
(>1MW natural gas-fired engines)
A primary surface recuperator (PSR) is the best method ofimproving engine efficiency in terms of size and cost.
A PSR uses metal foil (75-125µm thick) which must becreep- and corrosion-resistant.
With a limited volume of Cr or Al, the alloy foil must be verycorrosion resistant in order to achieve a long PSR lifetime.
Water vapor in the exhaust gas can accelerate the corrosionrate and is a critical concern for candidate materials
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Tota
l M
ass
Gai
n (m
g/cm
2)
0 5000 10000 15000 20000 25000
Time (h) at 900°C (1652°F)
AusteniticStainless
Steel(20wt%Cr)
4mil foil
253MA 4mil foil
FeCrAl+Ce 2mil foil
FeCrAl+Y 2O3 2mil foil
Haynes 214 2mil foil
Protective Surface Oxideshigher growth rate for Cr2O3 limits temperature
Growth rate of Cr2O3 is much higher than that for Al2O3
Thin foils which form Al2O3 have a distinct advantage at higher temps
(all contain some type of reactive element: Y, Ce, Zr etc.)
900°C, 1652°F
PM2000(ODS FeCrAl)
Haynes 214(NiCrAl+Y/Zr)
Alloys and ExperimentsAlloy Strengthening Oxidation
Type 321 precipitate 17%Cr(Fe-17Cr-11Ni-Ti)
Alloy 625 solid sol.+γ”+MCx... 21%Cr(Ni-21Cr-9Mo-4Fe-4Nb)
Haynes 214 γ/γ’ 4.5%Al(Ni-16Cr-4Al-3Fe+Zr/Y)
Plansee PM2000 oxide dispersion 5%Al(Fe-19Cr-5Al-0.4Ti+Y2O3)
Foil specimens in as-rolled surface finishThicker specimens polished to 0.3µm finish10vol% H2O added to dried air by water atomizer
Creep Rupture Life100MPa, 4-5mil foils
Data at 100MPa: PM2000 - 5mil, 625 & 214 - 4mil(high stress used to accelerate testing schedule)
PM2000 has the longest creep rupture life of the three foils214 somewhat better than 625
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10
100
1000
10000
ruptu
re lif
e (
h)
700 800 900 1000 1100
Temperature (°C)
alloy 214
alloy 625PM2000
100 MPa
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Spe
cim
en M
ass
Cha
nge
(mg/c
m2)
0 200 400 600 800 1000
Time (h) in 100h cycles
800°C H2O
800°C Air
700°C H2O
4 milType 321
Water vapor effects on 321SSProblems with type 321 stainless steel
Rapid attack with 10%H20 compared to laboratory air
No clear mechanism explains water vapor attackTest temperatures - above corrosion and creep limits for 321
700°C, 1292°F800°C, 1472°Fair+10vol%H2O
plan view
plan view
epoxy
10µm 1µm
10µm 0.5µm
Scale formed on 321SSSEM secondary electron images after 100h, 700°C
With water vapor: fewer and smaller nodules (rich in Mn)thinner oxide (?)
AIR
AIR+10%H2O
TEM of Scale on 321 Stainless Steelbulk alloy after 100h at 700°C
Differences with water vapor: (from first observations)- areas without nodules (arrows) are generally thinner with H2O-> is breakaway due to evaporation in these areas?
700°C, 1292°F
1µm
321
TEM Bright Field ImagesW plating
321 AIR
AIR+10%H2O
W plating
both cases: large (Cr, Fe,Mn)Ox particles
TEM of Scale on 321 Stainless SteelEDX maps of scale after 100h at 700°C
Nodules are Mn:Cr:Fe 4:2:1 from EDX spectraTi more associated with Cr in scale (W signal overlap)
also some Ti-rich particles
W plating
Fe, Cr oxide
321SS
STEM dark field image
AIR+10%H2O
Fe
0.25 m
Ni
Ti
Mn
Cr
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Spe
cim
en M
ass
Cha
nge
(mg/c
m2)
0 1000 2000 3000 4000 5000 6000
Time (h) in 100h cycles
800°C Air
800°C H2O700°C H2O
900°C H2O
rapid attack 100µm (4mil)Alloy 625
Oxidation Results: 625100h cycles at 700°-900°C on 100µm foi l
No rapid attack for water vapor observed (mass loss due to CrO 3 evap.)Higher Cr in 625 (21 vs. 17wt%) than 321Low mass gains at 700°C, very high mass gains at 900°CProbable use at ≈800°C depending on life requirements
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Spec
imen
Mas
s G
ain
(mg/c
m2)
0 1000 2000 3000 4000 5000
Time (h) in 100h cycles
800°C Air (4mil)800°C H2O (4mil)
900°C H2O (4 mil)
900°C air (2 mil)
50 and 100µm(2-4mil)
Haynes 214
Oxidation Results: 214100h cycles at 800°-900°C on 50-100µm foi l
800°C: low mass gain in air or air+water900°C: 500h cycles in air, 2mil material, lower transient mass gain
100h cycles in air+H2OMaximum temperature near 900°C with little observed H2O effect
Mass gain x 5.4 = scale in µm
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1.0
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Spe
cim
en M
ass
Cha
nge
(mg/c
m2)
0 1000 2000 3000 4000 5000 6000
Time (h) in 100h cycles
700°C H2O
800°C H2O
900°C H2O
800°C air
800°C air
125µm (5 mil)PM2000
Oxidation Results: PM2000100h cycles at 700°-900°C on 125µm foi l
700°C: low mass gains800°C: variable results, somewhat lower rates in air + H2O900°C: large initial mass gain of some concernMaximum temperature also near 900°C depending on H2O effect
Mass gain x 5.4 = oxide in µm
Haynes 214
PM2000
epoxy
10µm 10µm
10µm
10µm
Metallography of Oxidation Productseffect of water vapor
Higher temperatures so that there is something to see
Both cases, metal-oxide interface appears rougher with water vapor
In some areas on PM2000 the scale appeared thicker with H2O
214100h
1100°C
AIR AIR+10%H2O
PM2000100h
1000°C
epoxy
Haynes 214
PM2000
PM2000
epoxy
epoxy
10µm 2µm
10µm 2µm
Scale formed on PM2000SEM backscattered electron images
after 100h, 1000°C
Without water vapor: Cr-rich outer layer (bright layer in BSE image)With water vapor: rougher and occasionally thicker oxide
rougher interface with protrusions (arrows)no Cr-rich layer observed
AIR
AIR+10%H2O
epoxy
epoxy epoxy
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Mas
s C
hang
e (m
g/cm
2 )
Time (h)
PM2000, 1000°C, 10% H2O
PM2000, 1000°C
PM2000 Foil1000°C
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0 20 40 60 80 100 120
Mas
s C
hang
e (m
g/cm
2 )
Time (h)
214, 1100°C, 10% H2O
214, 1100°C
214 Foil, 1000°C, 10% H
2O
OxidationKinetics
foil and 1mmthick specimens,air & air+10%H2O
Main effect of H2O
appear to be ontransient stageoxidation for aluminaforming alloys
TEM of Scale on Haynes 214bulk alloy after 100h at 1100°C
Some differences with water vapor:- thicker oxide- more voids in outer Ni-rich oxideDifficult to make TEM cross-section of scale on foil material
1100°C, 2012°F
500nm 214
alumina
TEM Bright Field Images
NiAl2O4
W plating
NiAl2O4
214
AIRAIR+10%H2O
TEM of Scale on PM2000bulk alloy after 100h at 1000°C
Some differences with water vapor:- no Cr-rich oxide at gas interface -> evaporated (?)- finer grain size (more diffusion paths leads to faster oxidation?)- no increase in void concentration (which contradicts a possible model)
1000°C, 1832°F
PM2000
alumina
TEM Bright Field Images
W platingFe, Cr oxide
PM2000
AIR AIR+10%H2O
TEM of Scale on PM2000bulk alloy after 500h at 1000°C
Longer-term observations with water vapor:- again no Cr-rich oxide at gas interface -> evaporation?- grain size looks similar to that in dry air- similar void concentration as observed in dry air
1000°C, 1832°F
PM2000
alumina
TEM Bright Field Images
W platingFe, Cr oxide
PM2000
AIR AIR+10%H2O
W plating
0.5 m
Oxidation Limited Temperature Predictions
Typical oxidation models give time to rapid(base metal) attack: “time to breakaway”Mechanistic models: supply vs. consumption rate
For 100µm foil, can’t tolerate large loss incross-sectional thickness
* More concerned with time to ≈20% metal consumption
For Cr2O3: on 625 at 800°C, predict life >50,000h
For Al2O3: less certainty about prediction >800°C
experimental results on 2mil foil:
PM2000/Haynes 214: ≈1000h life at 1050°C>22,000h life at 900°C
Some uncertainty regarding water vapor effecte.g. at 1100°C:10-40% lower life with 10%H2O
Acknowledgments
Research sponsored by the Microturbines Program,U. S. Department of Energy (DOE) contractDE-AC05-00OR22725 with UT-Battelle, LLC.
Experimental worked assisted at ORNLby:George GarnerMike HowellShawn Trent