Estimating the Throwing Power of SS316 when Coupled with AA7075 Through Finite Element Modeling

Abstract

Galvanic corrosion is common in applications involving a fastener and panel assembly. Often, the fastener is made from a more noble metal and the panel is made from a less noble metal, selected for their respective mechanical properties. The ability for the more noble material to galvanically couple to the panel’s surface as a function of distance is referenced to as “throwing power,” and was the main subject of this research. In this work, SS316 and AA7075 were investigated as the fastener and panel material, respectively. A Ti-6Al-4V fastener and a sol-gel coated SS316 fastener were also considered to determine the impact of different materials on the galvanically driven throwing power. Along with different fastener materials, different fastener geometries were considered as well. Raised fasteners are generally used in tandem with washers, while countersunk fasteners are not in order to remain flush with the surface. The difference between these two geometries on the throwing power was investigated. It was determined that the SS316 washer was the largest contributor to the galvanic current in the raised fastener assembly, due to its large surface area. At distances of two inches away, the SS316 fastener and washer were able to double the natural corrosion rate of AA7075. A countersunk SS316 fastener, with the same total surface area as that of the raised fastener and washer assembly, was seen to lower the throwing power which forced a large amount of current down the fastener hole. Throughout all of the computational tests, the model relies on the generation of accurate electrochemical kinetics measured in solutions of appropriate composition.