Density Functional Theory of Iron Carbide and Steel Surface Erosion Chemistry

1
Density Functional Theoryof Iron Carbide
andSteel 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
2
- Overview -
Research DFT calculations on bulk and surfaces
of Fe3C-cementite 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

3
- 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 Impact

• Petrochemical Industry Processes
• Steam Reformers
• Gun Tubes
• Industrial Furnaces
• CO2-cooled Nuclear Reactors
• Coal Gasification Processes

• T 1700 K
• High Pressures
• Chemicals (H2, O2, CO, CO2, NO, NO2, and etc)
• Mechanical Forces

4
- 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).
5
- 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 Þ FemCn

• Consequences
• Phase-change stresses
• Altered ductile properties, grain cohesion
• MP-lowering TM1800K Þ TMlt1500K
• Metal Dusting disintegration of the steel in a
  dust of metal particles and C

6
- 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
• 4 C
• 8 Feg (2 Fe-C bonds)
• 4 Fes (3 Fe-C bonds)

7
- 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

8
- 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
Geometry-optimized cementite properties
9
- 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 - NEbulk) / 2A

• Esurf (J/m2)
• (110) 2.37
• (001) 2.40
• (011) 2.56
• (100) 2.72

10
- Surfaces of Fe3C -
Relative stability correlates with surface
smoothness
Surface Esurf(J/m2)
(100) 2.72
(011) 2.56
(001) 2.40
(110) 2.37
11
- Surfaces of Fe3C -
Relaxations into the surfaces
(100) rsurfC-Fe 2.01 Å rsurfFe-Fe 2.68 Å
(100) relaxed rsurf C-Fe 1.96 Å rsurfFe-Fe
2.61 Å
(001) rsurfC-Fe 2.01 Å
(001) relaxed rsurfC-Fe 1.88 Å

• 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
12
- 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 (001) (110) gt (001) gt (100),
  correlating to surface smoothness
• Fe3C/Fe Interface calculations
• Possible ceramic coatings