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Density Functional Theory of Iron Carbide and Steel Surface Erosion Chemistry. Wun C. Chiou, JrEmily A. Carter University of California, Los Angeles Dept. of Chemistry & Biochemistry. Thanks: Funding : U. S. Army Research Office - PowerPoint PPT Presentation
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Density Functional TheoryDensity Functional Theoryof Iron Carbide andof Iron Carbide and
Steel Surface Erosion Steel Surface Erosion ChemistryChemistry
Wun C. Chiou, Jr Emily A. Carter
University of California, Los AngelesDept. of Chemistry & Biochemistry
Thanks:Funding: U. S. Army Research Office
Resources: Maui High Performance Computing Facility, Army Research Laboratory MSRC
- Overview -- Overview -
Steel Erosion Issues
Carburization
Cementite
Approach
Bulk Fe3C
Surfaces of Fe3C
Conclusions
Research: DFT calculations on bulk and surfaces of Fe
3C-
cementite as a first step towards understanding the carburization mechanism for steel surface erosion.
- Steel Erosion Issues -- Steel Erosion Issues -
Petrochemical Industry Processes
Steam Reformers
Gun Tubes
Industrial Furnaces
CO2-cooled Nuclear Reactors
Coal Gasification Processes
Problem: In a harsh operating environment, an exposed steel surface can erode, leading to decreased performance, safety concerns, and repair/replacement costs.
T ~ 1700 K
High Pressures
Chemicals (H2, O
2, CO,
CO2, NO, NO
2, and etc)
Mechanical Forces
Environment: Impact:
- Possible Erosion Mechanisms -- Possible Erosion Mechanisms -Oxidation
FeO, Fe2O
3, Fe
3O
4
Hydrogen Embrittlement
in H-rich atmospheres, solid grain ablation
Pyrolysis
melting of the surface
Spallation
cracking and loss of the coating and steel surface
Carburization
carbon diffusion into the surface
In-situ experiments are difficult:Which of these actually occur?Which is dominant?Picture: P.J. Cote, C. Rickard. Wear, 241, p.17-25 (2000).
- Carburization -- Carburization -Definition: A high-temperature corrosion phenomenon caused by carbon ingress from the environment into metal components, leading to internal carbide precipitation and changes to the mechanical properties of the materials.Grabke, H. J. Carburization: A High Temperature Corrosion Phenomenon. MTI, 1998.
Mechanism:C/CO transport to the steel surface
C diffuses into the surface
Solid state reaction with Fe in steel
Fem + C
n Þ Fe
mC
n
Consequences:Phase-change stresses
Altered ductile properties, grain cohesion
MP-lowering: TM
~1800K Þ TM
<1500K
Metal Dusting: disintegration of the steel in a dust of metal particles and C
- Fe- Fe33C - Cementite -C - Cementite -
Partial Phase Diagram for the system Fe-C:Raghavan, V. Phase Diagrams of Ternary Iron Alloys, pt. 1 (1987)
Cementite:Fe
3C
The most stable iron carbide, but still metastable wrt/ a-Fe + C (gr)FerromagneticOrthorhombic16 atoms / unit cell:
4 C8 Fe
g (2 Fe-C bonds)
4 Fes (3 Fe-C bonds)
- Approach -- Approach -
Goals: Understand the Problem:
Study the properties of Fe3C and Fe (to simulate steel)
bulk and surfacesStudy the surface chemistry interactions
Propose Solutions:Explore possible corrosion-preventing surface coatings
Approach: Density functional theory (DFT):
periodic, planewave-basis calculationsultrasoft pseudopotentials to replace the
effect of core electronsGeneralized-gradient approximation to
exchange-correlation (GGA PW91)this form of pseudopotential has proven to
be successful for bulk Fe
- Bulk Fe- Bulk Fe33C -C -
Similar to bulk Fe (mostly d-state contributions near E
F)
Integrated DOS shows some charge transfer from Fe to C:Cementite charge/atom:C: 4.5 Fe: 7.3
Geometry-optimized cementite properties
Property GGA USPP LDA LMTO Experiment
5.06 5.09
6.74 6.74
4.51 4.53
6.09 8.38 5.05
1.95 1.74 (1.78)
1.99 1.98 (1.78)
- 0.16 - 0.06
1.33
a (Å)
b (Å)
c (Å)E
coh (eV/atom)
M0 (
B/Fe
g)
M0 (
B/Fe
s)
M0 (
B/C)
B0 (Mbar)
- Surfaces of Fe- Surfaces of Fe33C -C -
Investigate the relative stability of low-index surfaces of Fe
3C, cementite.
Narrowed search to four surfaces: stoichiometric, high density, and minimal dangling bonds from the cut(100), (001), (110), (011)
Results: unrelaxed surfaces
Esurf
(J/m2):
(110) 2.37(001) 2.40(011) 2.56(100) 2.72
Esurf
= (Eslab
- N*Ebulk
) / 2A
- Surfaces of Fe- Surfaces of Fe33C -C -
(100) 2.72
(001) 2.40
(110) 2.37
(011) 2.56
Relative stability correlates with surface smoothness:
Surface Esurf
(J/m2)
- Surfaces of Fe- Surfaces of Fe33C -C -
(100)r
surfC-Fe = 2.01 Å
rsurfFe-Fe
= 2.68 Å
(001)r
surfC-Fe: 2.01 Å
(001) relaxedr
surfC-Fe = 1.88 Å
(100) relaxedr
surf C-Fe = 1.96 Å
rsurfFe-Fe
= 2.61 Å
Relaxations into the surfaces:
Relaxed Esurf
; DEsurf
(J/m2)*
(001) 2.1 ; 0.3(110) 2.2 ; 0.2(011) 2.5 ; 0.1(100) 2.5 ; 0.2
*estimated
- Conclusion -- Conclusion -
Bulk Ecoh
improves on earlier
predictions
Cementite DOS is similar to pure Fe, but with some charge transfer to C
Surface stability: (001) ~ (110) > (001) > (100), correlating to surface smoothness
Fe3C/Fe Interface calculations
Possible ceramic coatings
Iron carbide and carburization are important aspects of steel erosion
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