Galvanic Corrosion Between Coated Al Alloy Plate and Stainless Steel Fasteners, Part 1: FEM Model Development and Validation

Abstract

Aerospace structures often involve dissimilar materials to optimize structural performance and cost. These materials can then lead to the formation of galvanic couples when moisture is present. Specifically, noble metal fasteners (such as SS316) are often used in aluminum alloy load-bearing structures, which can lead to accelerated, localized corrosion attack of the aluminum alloy due to the cathodic current supplied by the SS316 fastener. This localized attack is difficult to predict, and tests are often expensive, so modeling of these galvanic couples could be of great utility. The work reported here focuses on the galvanic coupling between fasteners installed in a panel test assembly, and the resultant corrosion damage down the fastener holes. This arrangement is a common assembly geometry in aerospace applications. A specific sol-gel coating was applied to the fasteners, to determine its effectiveness on mitigating galvanic corrosion; bare fasteners were also tested, to investigate a worst-case scenario. Geometric constraints in the model were made to match those of an experimental test panel, which was exposed to ASTM B117 salt fog for 504 h. The electrochemical boundary conditions were generated in solutions appropriate to the material and environment to which it would be exposed. Anodic charge passed during exposure was calculated from image analyses of the corrosion damage in the experimental test, and the results were compared with the model. The Laplacian-based model provides a very good first approximation for predicting the damage within the fastener hole. Validation was provided by both experimental results generated in this study as well as comparison to results in the literature that used similar, but not identical, conditions.