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 Resources: Maui High Performance Computing Facility, Army Research Laboratory MSRC

- Overview - Steel Erosion Issues Carburization Cementite Approach Bulk Fe 3 C Surfaces of Fe 3 C Conclusions Research: DFT calculations on bulk and surfaces of Fe 3 C- cementite as a first step towards understanding the carburization mechanism for steel surface erosion.

- Steel Erosion Issues - Petrochemical Industry Processes Steam Reformers Gun Tubes Industrial Furnaces CO 2 -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 (H 2, O 2, CO, CO 2, NO, NO 2, and etc) Mechanical Forces Environment:Impact:

- Possible Erosion Mechanisms - Oxidation FeO, Fe 2 O 3, Fe 3 O 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 (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, Mechanism: C/CO transport to the steel surface C diffuses into the surface Solid state reaction with Fe in steel Fe m + C n  Fe m C n Consequences: Phase-change stresses Altered ductile properties, grain cohesion MP-lowering: T M ~1800K  T M <1500K Metal Dusting: disintegration of the steel in a dust of metal particles and C

- Fe 3 C - Cementite - Partial Phase Diagram for the system Fe-C: Raghavan, V. Phase Diagrams of Ternary Iron Alloys, pt. 1 (1987) Cementite: Fe 3 C The most stable iron carbide, but still metastable wrt/  -Fe + C (gr) Ferromagnetic Orthorhombic 16 atoms / unit cell: 4 C 8 Fe g (2 Fe-C bonds) 4 Fe s (3 Fe-C bonds)

- Approach - Goals: Understand the Problem: Study the properties of Fe 3 C 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 Fe 3 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: 4.5Fe: 7.3 Geometry-optimized cementite properties

- Surfaces of Fe 3 C - Investigate the relative stability of low-index surfaces of Fe 3 C, cementite. Narrowed search to four surfaces: stoichiometric, high density, and minimal dangling bonds from the cut (100), (001), (110), (011) Results: unrelaxed surfaces E surf (J/m 2 ): (110) 2.37 (001) 2.40 (011) 2.56 (100) 2.72 E surf = (E slab - N*E bulk ) / 2A

- Surfaces of Fe 3 C - (100)2.72 (001)2.40 (110)2.37 (011)2.56 Relative stability correlates with surface smoothness: Surface E surf (J/m 2 )

- Surfaces of Fe 3 C - (100) r surfC-Fe = 2.01 Å r surfFe-Fe = 2.68 Å (001) r surfC-Fe : 2.01 Å (001) relaxed r surfC-Fe = 1.88 Å (100) relaxed r surf C-Fe = 1.96 Å r surfFe-Fe = 2.61 Å Relaxations into the surfaces: Relaxed E surf ;  E surf (J/m 2 )* (001) 2.1 ; 0.3 (110) 2.2 ; 0.2 (011) 2.5 ; 0.1 (100) 2.5 ; 0.2 *estimated

- Conclusion - Bulk E coh 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 Fe 3 C/Fe Interface calculations Possible ceramic coatings Iron carbide and carburization are important aspects of steel erosion