Computational and Experimental Analyses of Galvanic Corrosion of AA7075-T6 Induced By Dissimilar Metal Fasteners and Mitigated By Sol-Gel Coatings

Abstract

In modern aircraft, there is a need to protect aluminum structures from galvanic corrosion due to dissimilar metals used in complex aerospace structures. Galvanic corrosion between corrosion resistant fasteners (e.g., stainless steel or titanium alloys) and aluminum alloy structures represent a recurring problem. One means of mitigating such attack is via the application of surface treatments to the fasteners designed to limit their cathodic current density. Detailed measurements and modeling of these effects are important to optimization of the surface treatment and estimates of the level of improvement likely to be achieved. The present work combines experimental measurements of galvanic corrosion damage and computational modeling of galvanic current distributions. The goal is quantitatively connect observations of decreased galvanic corrosion damage on AA7075-T6 coupons due to the presence of stainless steel fasteners under thin electrolyte films as would be present in aerospace service. In particular, sol-gel surface treatments were applied to stainless steel and titanium alloy fasteners which were then placed in AA7075-T6 panels before exposure to salt spray testing. Experimental analyses of corrosion damage were performed using white light interferometry generated depth profiles. These data were used to create data for total volume mass loss (TVML). Computational modeling of galvanic current distributions was performed using a commercial FEM package for thin electrolyte film conditions with experimentally-derived electrochemical kinetics as boundary conditions. The polarization curves used and the current distributions for a 0.1 mm electrolyte layer are shown in Figure 1. Integration of the anodic current densities on the AA7075 over the surface produces a corrosion damage metric, the total anodic current (TAC). The effects of sample geometry, including spacing between fasteners and their location relative to the edges of the sample, as well as water layer thickness and solution concentration on the TAC will be reported. The TVML and TAC can be correlated to one another and to total measured mass loss via Faraday’s Law. Comparisons between the TVML and the TAC will be discussed in detail. Of particular importance will be analyses of the effective throwing power of the fasteners as a function of fastener material and the presence or absence of sol gel surface treatments. Figure 1: (a) Polarization curves in 0.6 M NaCl of the materials of interest. (b) False color plot of anodic (positive) and cathodic (negative) current distribution (A/m2) for a 0.1 mm electrolyte layer of 0.6 M NaCl Acknowledgements: The financial support of the Office of Naval Research through Contract # N68335-16-C-0121 (William Nickerson, PM) via Luna Innovations (Adam Goff) is gratefully acknowledged. Figure 1