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Density Functional Theory of Iron Carbide and Steel Surface Erosion Chemistry

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

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Density Functional Theory of Iron Carbide and Steel Surface Erosion Chemistry

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  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. • Petrochemical Industry Processes • Steam Reformers • Gun Tubes • Industrial Furnaces • CO2-cooled Nuclear Reactors • Coal Gasification Processes Environment: Impact: 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 + CnÞ FemCn 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

  6. Partial Phase Diagram for the system Fe-C: Raghavan, V. Phase Diagrams of Ternary Iron Alloys, pt. 1 (1987) - Fe3C - Cementite - 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 - N*Ebulk) / 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) relaxed rsurf C-Fe = 1.96 Å rsurfFe-Fe = 2.61 Å (100) rsurfC-Fe = 2.01 Å rsurfFe-Fe = 2.68 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) > (001) > (100), correlating to surface smoothness Fe3C/Fe Interface calculations Possible ceramic coatings

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