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Tuesday, March 16, 2010: 4:20 PM
214 C (Henry B. Gonzales Convention Center)
A new method involving an explicit 3D model of the oxide layer has been developed to simulate the passivation of FeCr alloys. It allows us to simulate the nucleation and growth of the passive film.
In the new model, the alloy is immersed in a virtual empty oxide lattice oriented with a given epitaxy. During the dynamic evolution, the metal cations generated by the oxidation of the alloy elements are injected into the virtual lattice where they are associated with oxygen ions coming from the solution, leading to formation of oxide nuclei, lateral growth of oxide islands and increase of the layer thickness. The dynamic evolution is based on the Kinetic Monte Carlo method. The KMC simulation takes into account the fundamental processes involved in the passivation mechanism: metal diffusion in the bulk and at the surface of the alloy, formation of metal cations and their injection in the oxide, nucleation and growth of the oxide layer, mass transfer through the oxide and oxide dissolution at the oxide-solution interface.The local value of the electric field is taken into account. The results reproduce qualitatively well the experimental data. For low concentrations in Cr (<14%) the oxide nuclei do not cover completely the alloy surface and the metal is corroded. For high chromium content (> 16%) the oxide layer grows, covers the whole surface and reaches a stationary thickness of the order of 9 Å. In the intermediate zone (14-16%) the transition from incomplete or no passivation to complete passivation is continuous. The passive film is enriched with chromium. For alloys with low Cr content, extensive iron dissolution is required to obtain passivation. This leads to increased surface roughness.
In the new model, the alloy is immersed in a virtual empty oxide lattice oriented with a given epitaxy. During the dynamic evolution, the metal cations generated by the oxidation of the alloy elements are injected into the virtual lattice where they are associated with oxygen ions coming from the solution, leading to formation of oxide nuclei, lateral growth of oxide islands and increase of the layer thickness. The dynamic evolution is based on the Kinetic Monte Carlo method. The KMC simulation takes into account the fundamental processes involved in the passivation mechanism: metal diffusion in the bulk and at the surface of the alloy, formation of metal cations and their injection in the oxide, nucleation and growth of the oxide layer, mass transfer through the oxide and oxide dissolution at the oxide-solution interface.The local value of the electric field is taken into account. The results reproduce qualitatively well the experimental data. For low concentrations in Cr (<14%) the oxide nuclei do not cover completely the alloy surface and the metal is corroded. For high chromium content (> 16%) the oxide layer grows, covers the whole surface and reaches a stationary thickness of the order of 9 Å. In the intermediate zone (14-16%) the transition from incomplete or no passivation to complete passivation is continuous. The passive film is enriched with chromium. For alloys with low Cr content, extensive iron dissolution is required to obtain passivation. This leads to increased surface roughness.
The oxidation process produces vacancies in the alloy which may form cavities at the oxide-metal interface or in the bulk of the alloy. For low chromium content, these cavities coalesce, leading to passivity breakdown and pit initiation.
See more of: Research in Progress Session - Corrosion Modeling: Fundamentals and Applications
See more of: Research in Progress
See more of: Research in Progress