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