A Study on the Location of the Cathode in Galvanic-Induced Crevice Corrosion

Abstract

Fastener-in-panel systems are commonly used in aerospace construction. The geometries are very complicated, especially in the context of corrosion. Generally, fasteners are made of more noble materials, such as stainless steels, due to their good mechanical properties. Panels can represent the structural components of an aircraft, which are generally built with aluminum alloys, due to their high strength-to-weight ratio. The combination of these two metals when in electrolytic contact, as could occur from a light rain or morning dew, makes the system susceptible to galvanic corrosion. The fastener also forms an occluded region within the panel, adding in the possibility of crevice corrosion. Furthermore, aircraft are often exposed to cyclic wet and dry environments, leading to an atmospheric water layer which can exacerbate corrosion.The complicated and interacting mechanisms involved in the fastener/panel design have been investigated through finite element modeling (FEM) in this work. FEM has been advancing in recent years, and specifically has become a useful tool in studying galvanic corrosion [1]. The complexities surrounding a fastener-in-panel system are difficult to isolate individually experimentally. Modeling allows deconvolution of the specific parameters, efficiently assessing the effects and isolating changes in parameters in a way that is not possible experimentally.Previous work has compared experimental analysis with FEM in a large-scale fastener geometry [2]. This paper describes a similar investigation, with a more prominent focus on the fastener hole. Specifically, galvanic-induced crevice corrosion was studied in order to determine which location on the cathode was producing the main driving force. Therefore, the geometry was representative of a cross-section of one fastener installed in a panel, in order to negate the interaction of multiple fasteners. Knowledge of the location driving the cathodic reactions would aid in finding mitigation strategies for crevice corrosion, such as whether coating the head of the fastener or the shaft of the fastener is more important.Different locations of the active cathode were tested systematically, such as variations between an inert fastener shaft (that is, the crevice former) or an active shaft coupled with an inert or active fastener head. The transport of aluminum ions was tracked within the model as well. Both steady-state and time-dependent models are described, in order to determine if the cathode inside of the fastener hole acts as a driving force before being stifled through the production of hydroxyl.SS316 and AA7075 are the materials of focus for the fastener and panel, respectively. SS316 has been shown to form a severe galvanic couple with AA7075 [3], leading to the formation of intergranular corrosion fissures of 2mm in length inside of the fastener hole [2]. A sol-gel coated SS316 fastener was also investigated, to determine the level of mitigation achieved. Boundary conditions for these materials were generated experimentally, through potentiodynamic scans.[1] C. Liu and R. G. Kelly, “A Review of the Application of Finite Element Method (FEM) to Localized Corrosion Modeling,” CORROSION, 2019.[2] R. S. Marshall, R. Kelly, A. Goff, and C. Sprinkle, “Galvanic Corrosion Between Coated Al Alloy Plate and Stainless Steel Fasteners, Part 1: FEM Model Development and Validation,” Corrosion, vol. 75, no. 12, pp. 1461–1473, 2019.[3] 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.