Assessment of the Utility of Laplace's Equation for Modeling of Localized Corrosion

Connection between dissimilar materials are frequently encountered in complex structures involved in aviation and aerospace industry. In a typical aircraft structure, aluminum-alloy-based airframe component and noble fasteners such as stainless steel is one of the most common couples encountered. When the aircraft is exposed in an aggressive moisture environment such as rainfall, fog, splash, or salt-enabled deliquescence, a thin layer of electrolyte or droplet can be established on the dissimilar materials assembly surface. If the assembly has a fastener-hole geometric configuration, it can assist trapping or wicking of surface electrolyte into the tight crevice between the fastener and the airframe component once breakdown happens on the protective coating above the assembly, resulting in a localized galvanic corrosion environment in which Al-alloy based component is the corroding anode, whereas the stainless steel fastener is the cathode. This phenomenon can further lead to pit-to-fatigue crack initiation on the Al-alloy based component which deteriorates its structural integrity and shortens the lifespan. The main goal of this dissertation is to investigate the galvanic-coupling induced localized corrosion distribution along AA7050-T7451 with a fastener-hole configuration between AA7050-T7451 and SS316L in a simulated environment representative of atmospheric corrosion. This goal was fulfilled by the following tasks: (1) develop a robust finite-element-method (FEM) based modeling approach based on Laplace’s Equation to accurately predict corrosion distributions and bound the application of this modeling framework within its limitation by providing reasonable assumptions; (2) utilize FEM modeling approach, electrochemical techniques, and pertinent instrumental characterization tools to systematically investigate the effect of electrolyte layer thickness, solution chemistry, materials surface properties, and galvanic coupling geometry on the electrochemical and corrosion distributions in the galvanic coupling between AA7050 and SS316L, and capture the underlying mechanism for the dependence of corrosion distributions on each external variable.