Multimodal Analysis of Corrosion for AA6016 Alloy at the Boundaries between Aluminum Matrix/Intermetallic Particles

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Aluminum alloys are attracting attention as lightweight materials for automobiles, and the fact that intermetallic particles indispensable for improving strength of the alloys can be harmful to the corrosion resistance has become an issue. However, the initial corrosion behavior at the interface of intermetallic particles for aluminum alloys has been hardly examined using a comprehensive analysis method. In this study, a nano-micro scale multimodal analysis of Energy Dispersive X-ray Analysis (EDS), Electron Backscatter Diffraction (EBSD), and Kelvin Force Microscopy (KFM) was performed to clarify the initial corrosion behavior for AA6016. AA6016 specimens (20 × 20 × 1 mm) were subjected to SEM-EDS/EBSD measurements after mirror polishing, followed by continuous KFM measurements after ion milling. The specimens were then immersed in 100 mL of 0.5 wt% NaCl solution for 1 hour at a corrosion test. The surfaces of the specimens were cleaned with distilled water, dried and subjected to subsequent KFM measurements. The measurement of SEM-EDS was conducted again after the series of KFM measurements prior and post corrosion test. Fig. 1 shows SEM images of the typical specimen surface for AA6016 before corrosion tests. As shown in Fig. 1, two types of intermetallic particles: Al-Fe-Si and Mg-Si were identified by EDS measurements. The KFM measurements of the potential distribution on specimen surfaces confirmed that both intermetallic particles were noble in potential relative to the aluminum matrix. However, less significant difference in the potential was detected at the grain boundaries identified by the EBSD measurements. After the corrosion test, some trenches were formed at the boundaries of the Al-Fe-Si type particle, and the noble potential area expanded around the particle. As for the Mg-Si type particles, a dissolution of Mg was detected at the particle, and the noble potential region disappeared at the particles immediately after the corrosion test but reappeared with time. Furthermore, the formation of thick oxide films was implied after the corrosion test, especially at the boundaries of Al-Fe-Si type particle as well as at the Mg-Si type particle. Thus, the initial corrosion behaviors of AA6016 related to the compositional/electrochemical modifications at and around intermetallic particles were demonstrated by using the nano-micro scale multimodal (EDS-EBSD-KFM) analysis. Although the modifications of compositions (Mg and O) as well as noble potential distribution were restricted within the particle for Mg-Si type particle, they were expanded at the boundaries and around the particle for Al-Fe-Si type particle. The Al-Fe-Si type intermetallic particles would play more important role than those of Mg-Si type particles in promoting localized corrosion for AA6016. Figure 1