1. 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

2. - 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:

3. - 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 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

4. DFT Results - 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 (bcc-Fe: 5.267) Geometry-optimized cementite properties

5. - 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

6. - 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 s,unr (J/m 2 )

7. - Surfaces of Fe 3 C - (100) (001) (001) relaxed (100) relaxed Relaxations into the surfaces: FaceSurface energyRelaxationrms (J/m 2 )energy (%) relaxation (Å) StaticRelaxed (100)2.722.4710 %0.012 (011)2.582.55 1 %0.007 (001)2.402.0515 %0.016 (110)2.372.34 1 %0.013

8. - 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) > (011) > (100) Fe 3 C/Fe Interface calculations Carbide formation Iron carbide and carburization are important aspects of steel erosion