Localized Corrosion Damage Morphology and Corrosion Electrochemistry for Al-Zn-Mg-Cu and Stainless Steel 316 Fastener Galvanic Couples in Simulated Marine Environments

Abstract

Introduction: Aluminum alloy 7050-T7451 (Al-Zn-Mg-Cu) is often used for aerospace applications because it provides an advantageous combination of strength, stress corrosion cracking (SCC) resistance, corrosion resistance, and fracture toughness1. 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 3-4. 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 matrix7-9. Defects in the corrosion protection system are commonly due to the harsh environment operating conditions of aircraft and are enhanced at complex joining/fastener locations which trap electrolyte into tight crevices leading to an occluded local environments where a galvanic cell is established between the aluminum alloy and a high strength steel fastener.12 While the danger of galvanic corrosion has been recognized, little work has been done to measure the extent of corrosion damage and characterization of the damage morphology of AA7050 and SS.1-13 However, there is a lack of literature on the unique factors and environments present in confined spaces such as a rivet hole and furthermore how these factors and environments effect the overall damage morphology. Previous work limits our understanding of damage morphologies to a few specific test environments such as marine seacoast atmospheric exposures, lab accelerated test environments, full immersion environment such as in 0.6 M NaCl, EXCO solution and 0.6 M NaCl + H2O2. There has been very little detailed investigation of crevice and rivet environments formed in a marine or simulated marine atmospheric solutions. The goal of this work is to utilize a 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. Experimental: Multiple techniques were used to understand the macro-scale and mirco-scale corrosion including utilizing a microelectrode array instrumented in a fastener geometry, synchrotron x-ray tomography, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and other diagnostic electrochemical methods in full immersion and under thin film conditions to elucidate key kinetic factors responsible for corrosion behavior. Results: 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. A horizontal slice through an X-ray tomograph is shown in Figure 1 (a) for the NaCl exposure. Scanning electron microscopy (Figure 1 (c)) and the reconstructed 3-D corrosion model (Figure 1 (d)) showed that the fissures in the simulated fastener were intragranular. This was further investigated using electron backscatter diffraction (Figure 1 (b)). The areas of corrosion for each individual fissure developed in the simulated rivet were tracked over time and depth enabling determination of damage volume and anodic charge. 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. 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. To test this hypothesis a multi-electrode array galvanically coupled was utilized to investigate sources of cathodic reactions and anodic sites continuously. Acknowledgments: This work was funded by ONR under the contract ONR: N00014-14-1-0012 with William Nickerson as contract manager. X-ray data were collected 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.