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Title: Corrosion scale dynamics
Author: Tautschnig, Markus Peter
ISNI:       0000 0004 7657 9987
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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This work presents a methodology for simulating ionic and electronic grain boundary transport through thin films, scales or membranes with columnar grain structure. In the model developed the grain structure is idealized as a lattice of identical hexagonal cells - a honeycomb pattern. Reactions with the environment constitute the boundary conditions and drive the transport between the surfaces. Time-dependent simulations solving the Poisson equation self-consistently with the Nernst-Planck flux equations for the mobile species are performed. In the resulting Poisson-Nernst-Planck system of equations, the electrostatic potential is obtained from the Poisson equation in its integral form by summation. An object oriented C++ code has been implemented to solve the system of equations numerically. First, the model is used to interpret alumina membrane oxygen permeation experiments, in which different oxygen gas pressures are applied at opposite membrane surfaces and the resulting flux of oxygen molecules through the membrane is measured. Simulation results involving four mobile species, charged aluminum and oxygen vacancies, electrons, and holes, provide a complete description of the measurements. Second, the model is extended to simulate internal oxidation and stress generation within a thin film of alumina at conditions of high-temperature metal oxidation. The steady-state stresses predicted are compatible with experimental measurements of lateral growth stresses in alumina scale growth experiments. The hypothesized p-n ionic transition within the alumina grain boundaries is observed. In limiting cases the more general simulations are closely related to the Wagner theory of metal oxidation. The Wagner theory assumes local ionic equilibrium while the simulation results demonstrate the possibility of a significant deviation from local Schottky equilibrium within an oxide scale at conditions of steady-state growth. The deviation is related to stress generation, and limits the applicability of the Wagner theory.
Supervisor: Harrison, Nicholas M. ; Finnis, Michael W. Sponsor: Engineering and Physical Sciences Research Council ; British Petroleum Company
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral