Galvanic Corrosion of AA7050-T7451 and 316L Stainless Steel in a Simulated Fastener
Authors: Rafla Veronica N , Glanvill Sarah , Parsons Aaron , Davenport Alison J , Scully John 
Published: February 2020

Introduction:
Aluminum alloy (AA) 7050-T7451 (Al-Zn-Mg-Cu, UNS A97050) is often used for aerospace applications because it provides an advantageous combination of strength, stress corrosion cracking (SCC) resistance, corrosion resistance, and fracture toughness 1. Grain boundary precipitation of MgZn2- η influences the corrosion behavior by locally depleting Zn, Mg, and Cu, especially in the T6 temper2, which can enable intergranular corrosion (IGC), intergranular stress corrosion cracking (IGSCC) and exfoliation corrosion 3. The T74 temper has been overaged to suppress IGC and SCC on grain boundaries4 5. Cu content is important, as the composition of both the matrix and η changes with aging, decreasing the potential window between Mg(Cu)Zn2 dissolution and the matrix pitting or repassivation potential; this decreases IGC and exfoliation susceptibility6. However, incongruent dissolution of Cu-rich phases may leave a Cu-rich surface that is cathodic to the matrix7. This may increase general and local corrosion. 8

Fastener holes and complex joining locations are particularly vulnerable towards corrosion because their geometry inherently aids trapping/wicking of the electrolyte into tight crevices and produce thin films with high cathodic reaction rate9. Few studies have incorporated all factors present at a rivet hole that could affect corrosion damage morphology evolution and studied by operando observation including attempts to quantify sources of cathodic current and charge. The importance of micro-galvanic coupling, induced by the constituent particles compared to meso or macro-scale galvanic coupling provided by the stainless steel fastener towards the total accumulated corrosion damage observed is unclear. Such damage can transition to fatigue. 10

The goal of this work is to utilize a simulated fastener configuration to interrogate the electrochemical, microstructural, and physical factors that govern galvanic induced local corrosion pit morphology development at both the meso- and micro-length scales. The macro and micro topography of corrosion and controlling microstructural factors are examined in a separate study.

Experimental:
Synchrotron x-ray tomography (XCT) was used to monitor real time operando corrosion measurements on a simulated rivet geometry under a NaCl droplet to enable 3D assessment of corrosion damage and the associated anodic charge 11
12.

Results:
A horizontal slice through an X-ray tomograph is shown in Figure 1 for the NaCl exposure. Scanning electron microscopy and the reconstructed 3-D corrosion model showed that the fissures in the simulated fastener were intragranular (Figure 1). The areas of corrosion for each individual fissure developed in the simulated rivet were tracked over time and depth allowing determination of damage volume and anodic charge (Figure 2). This enabled a comparison of the theoretical maximum available cathodic charge associated with the cathodic reactions on the stainless steel and other sources over the range of galvanic couple potentials found in potential distribution modeling (Figure 3)13. The stainless steel pin and exposed constituent particles, uncovered during corrosion, both supported and enabled the growth of multiple fissures. This suggests that cathodically limited fissure growth is not so severe of a constraint as to confine corrosion to the rivet hole mouth as well as the growth of a single fissure during the exposure.

Acknowledgement:

This work was funded by the Office of Naval Research under the contract ONR: N00014-14-1-0012 with William Nickerson as contract manager. X-ray data were collected and analyzed with assistance from Sarah Glanvill, Andrew du Plessis and Weichen Xu of the University of Birmingham and Aaron Parsons and Trevor Rayment of Diamond Light Source.
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Figure 1