In-Situ optical Microscopy Corrosion Experiments for Correlating Microstructure to Corrosion Susceptibility in Aluminum Alloy

Abstract

Aluminum-Magnesium alloys (5xxx series) are widely used in marine applications, structural components in transportation, and military due to their high corrosion resistance, low density, and good mechanical properties. However, at moderate and high temperatures (>50oC) these alloys are vulnerable to sensitization, or precipitation of β phase (Al3Mg2) preferentially on grain boundaries, resulting in susceptibility to intergranular corrosion and environmentally induced stress corrosion cracking. The primary research theme of this talk is to understand how microstructure variations dictate local corrosion susceptibility of aluminum 5456 in corrosive environments. To explore microstructure effects, we corroded sensitized aluminum 5456 in a dilute nitric acid solution. Prior to corrosion, electron backscatter diffraction (EBSD) was used to characterize the microstructure and deformation state of the alloy in terms of the grain structure, geometrically necessary dislocation density, constituent particles, and grain boundary character. In situ optical microscopy corrosion experiments were then used to directly observe corrosion initiation and propagation behavior, with fiducial markers used to correlate the observed corrosion behavior with the characterized microstructure. Image recognition algorithms facilitated the quantification of corrosion sites and corrosion propagation rates as a function of time and grain boundary path. Obtaining this information is crucial as the microstructural features that govern corrosion initiation may be different from the features that dictate propagation pathways. My talk will focus on the microstructure characteristics that dictate corrosion susceptibility to initiation as well as propagation and the relevance of these data for developing corrosion- and sensitization-resistant microstructures.