Investigations of the Effects of Electrochemical, Environmental and Geometric Parameters on Galvanic Coupling through Finite Element Modeling
Authors: Skelton Rebecca Joan , Kelly Robert G. 
Published: February 2020

Experimental studying of the galvanic coupling between different alloys can quickly become a difficult task, because of the many parameters to consider. Many of these parameters are dependent on each other, forcing researchers to create a web of test matrices in order to isolate one parameter at a time. Both electrochemical and geometric parameters must be considered, along with a constant environmental storage of samples before and after tests to guarantee that there has been no change in the system. This complexity makes experimental testing slow, however it can produce thorough results. Finite element modeling (FEM) has become a useful tool in studying galvanic corrosion[1] mainly because of its ability to both efficiently assess the effects of specific parameters and to isolate changes in parameters in a way that may not be possible experimentally.

A computational study using FEM can allow a vast portion of parameter space to be assessed, guiding experimental tests to regions of interest. This approach narrows the daunting sea of parameters which experimentalists swim in, especially in the field of galvanic corrosion. In real applications, these parameters can be changed in a variety of ways. For example, different coatings can affect the electrochemical kinetics of the alloys, humidity changes can affect the concentration of the electrolyte solution, and atmospheric pressure or environmental changes can affect the temperature of the solution.

The focus of this research therefore was to study the effects of different parameters on the galvanic coupling of AA7075 and SS316. These two materials are both common in aerospace applications, particularly with AA7075 as the structural material and SS316 as the fastener holding the assembly together. SS316 has been shown to severely couple with AA7075[2], leading to the formation of intergranular corrosion fissures of 2mm in length inside of the fastener hole[3]. Knowing which parameters create a large impact on this galvanic corrosion, and which do not, is valuable information when trying to maintain the safety of structures.

A detailed computational matrix was run in the model, analyzing the effects of cathodic and anodic kinetics, conductivity, and temperature on the overall current distributions. These effects were quantified to determine which parameter made the largest contribution to the galvanic current, and which were negligible. A study of the interactions between these parameters was also conducted, in order to determine which parameter would be easily isolated by experimental technique.

The computations were first run on a simple geometry in the model with one cathode surface and one anode surface, to get a generalized idea of the importance of the parameters. Afterwards, an occluded cell geometry was utilized in the model, to determine how the geometric change effects the contributions of the different parameters on the current distributions. A comprehensive analysis of these parameters will be presented, along with ideas of how to use this analysis for practical applications.

[1] C. Liu, V. N. Rafla, J. R. Scully, and R. G. Kelly, “Mathematical modeling of potential and current distributions for atmospheric corrosion of galvanic coupling in airframe components,” Corros. 2015, no. May 2016, 2015.

[2] C. A. Matzdorf, W. C. Nickerson, B. C. Rincon Tronconis, G. S. Frankel, L. Li, and R. G. Buchheit, “Galvanic test panels for accelerated corrosion testing of coated al alloys: Part 1 - Concept,” Corrosion, vol. 69, no. 12, pp. 1240–1246, 2013.

[3] R. J. Skelton and R. G. Kelly, “Investigation of Galvanic Coupling on AA7075-T6 Navair Plate inside Fastener Hole,” in ECS Meeting Abstracts, 2018.