Density Functional Theory
of Iron Carbide and
Steel Surface Erosion
Chemistry
Wun C. Chiou, Jr
Emily A. Carter
University of California, Los Angeles
Dept. of Chemistry & Biochemistry
Thanks:
Funding: U. S. Army Research Office
Resources: Maui High Performance Computing Facility, Army
Research Laboratory MSRC
- Overview Research:
DFT calculations on bulk
and surfaces of Fe3Ccementite as a first step
towards understanding
the carburization
mechanism for steel
surface erosion.
Steel Erosion Issues
Carburization
Cementite
Approach
Bulk Fe3C
Surfaces of Fe3C
Conclusions
- Steel Erosion Issues Problem:
In a harsh operating environment, an exposed
steel surface can erode, leading to decreased
performance, safety concerns, and
repair/replacement costs.
Environment:
T ~ 1700 K
High Pressures
Chemicals (H2, O2, CO,
CO2, NO, NO2, and etc)
Mechanical Forces
Impact:
Petrochemical Industry
Processes
Steam Reformers
Gun Tubes
Industrial Furnaces
CO2-cooled Nuclear Reactors
Coal Gasification Processes
- Possible Erosion Mechanisms Oxidation
FeO, Fe2O3, Fe3O4
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 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 + Cn  FemCn
Consequences:
Phase-change stresses
Altered ductile properties, grain cohesion
MP-lowering: TM~1800K  TM<1500K
Metal Dusting: disintegration of the steel in a dust
of metal particles and C
- Fe3C - Cementite Partial Phase Diagram for the system Fe-C:
Raghavan, V. Phase Diagrams of Ternary Iron Alloys, pt. 1 (1987)
Cementite:
Fe3C
The most stable iron carbide, but still
metastable wrt/ a-Fe + C (gr)
Ferromagnetic
Orthorhombic
16 atoms / unit cell:
4C
8 Feg (2 Fe-C bonds)
4 Fes (3 Fe-C bonds)
- Approach Goals:
Understand the Problem:
Study the properties of Fe3C and Fe (to simulate steel)
bulk and surfaces
Study the surface chemistry interactions
Propose Solutions:
Explore possible corrosion-preventing surface coatings
Approach:
Density functional theory (DFT):
periodic, planewave-basis calculations
ultrasoft pseudopotentials to replace the
effect of core electrons
Generalized-gradient approximation to
exchange-correlation (GGA PW91)
this form of pseudopotential has proven to
be successful for bulk Fe
- Bulk Fe3C Similar to bulk Fe
(mostly d-state
contributions near EF)
Integrated DOS shows
some charge transfer
from Fe to C:
Cementite charge/atom:
C: 4.5
Fe: 7.3
(bcc-Fe: 5.267)
Geometry-optimized cementite properties
Property
a (Å)
b (Å)
c (Å)
E coh (eV/atom)
GGA USPP LDA LMTO Experiment
5.06
5.09
6.74
6.74
4.51
4.53
6.09
8.38
5.05
M 0 (B /Fe g)
1.95
1.74
(1.78)
M 0 (B /Fe s)
1.99
1.98
(1.78)
M 0 (B /C)
- 0.16
- 0.06
B0 (Mbar)
1.33
- Surfaces of Fe3C Investigate the relative stability of low-index
surfaces of Fe3C, cementite.
Narrowed search to four surfaces: stoichiometric,
high density, and minimal dangling bonds from
the cut
(100), (001), (110), (011)
Results: unrelaxed surfaces
Esurf = (Eslab - N*Ebulk) / 2A
Esurf (J/m2):
(110) 2.37
(001) 2.40
(011) 2.58
(100) 2.72
- Surfaces of Fe3C Relative stability correlates with surface
smoothness:
Surface
Es,unr(J/m2)
(100)
2.72
(011)
2.58
(001)
2.40
(110)
2.37
- Surfaces of Fe3C Relaxations into the surfaces:
Face
(100)
(100)
(100) relaxed
(001)
(001) relaxed
Surface energy
(J/m2)
relaxation (Å)
Static Relaxed
2.72
0.0844
(011)
2.58
0.1086
(001)
2.40
0.1082
(110)
2.37
Relaxation
energy (%)
2.47
9.2 %
2.37
8.1 %
2.05
14.6 %
2.20
7.2 %
rms
- Conclusion Iron carbide and carburization are
important aspects of steel erosion
Bulk Ecoh improves on earlier
predictions
Cementite DOS is similar to pure
Fe, but with some charge transfer
to C
Surface stability:
> (110) > (011) > (100)
(001)
Fe3C/Fe Interface calculations
Possible ceramic coatings