Numerical simulation of internal corrosion at high temperatures

Aim of this thesis was the development of a simulation software for numerical modelling of internal corrosion at high temperature. The used model based on the cellular automata approach was validated by experimental results on nickel-based model-alloys and applied to complex systems. Generally, high temperature corrosion is a material degradation process occurring at the surface of engineering components. Considering internal corrosion, the corrosive species penetrates into the material by solid-state diffusion leading to the formation of internal precipitates, e.g., oxides (internal oxidation), nitrides (internal nitridation), and carbides (carburization). For nearly all metallic and ceramic materials, diffusion-controlled precipitation processes are of particular importance. Besides a significant mass transport by diffusion, chemical reactions and phase transformations are occurring at elevated temperatures. After all, corrosion results in a strong deterioration of the properties of a material, i.e., near-surface embrittlement or the dissolution of strengthening phases. In the framework of this thesis, a numerical tool based on the cellular automata approach for the description and prediction of high temperature corrosion processes was evaluated. Herefore, the diffusion area is subdivided into a system of cells which are assigned with different states. With every iteration step, these states may change according to transformation rules which depend on the interaction with the neighbouring cells. By a meaningful definition of the states and the transformation rules, it becomes possible to transfer diffusion processes to practical relevant complex situations. Starting with simple diffusion processes, the model was consequently evaluated and expanded. Up to now, it considers diffusion, nucleation and growth, the transition from internal to external scale formation, internal precipitation kinetics and grain-boundary diffusion. For verification, parameter studies on the nitridation of simple nickel-based model alloys were performed. In addition, the model was applied to complex materials such as the nickel-based superalloys Alloy 80a and Inconel 625 Si as well as the low alloy steel X60. Furthermore, the simulation results of the cellular automata were compared to those of other numerical approaches, critically regarded and the limitations were discussed. Finally, an outlook of possible enhancements and improvements of the model is presented.