Electrochemical Characterization of Copper-bearing Intermetallic Compounds and Localized Corrosion of Al-Cu-Mg-Mn Alloy 2024

摘要

The electrochemical behavior of the intermetallic compounds (IMCs) common in aluminum alloy 2024 (Al-4.4Cu-l.5Mg-0.6Mn) was characterized using a micro-electrochemical cell technique. IMCs studied included: Al_2Cu, Al_2CuMg, Al_7Cu_2Fe, and Al_(20)Cu_2Mn_3. The small cell size (< 50μm diameter) enabled measurements to be made on phase-pure, IMC crystals that were free of cracks and pores. This approach was used to collect many replicate polarization curves on a phase-by-phase basis to describe both the polarization behavior and the variability in the polarization response. The microcell technique was also used to measure the polarization behavior of small regions of matrix phase of the 2024-T3 that were free of large constituent particles. IMCs were found to exhibit two distinct types of behavior, which have been termed "passive" and "active". Passive behavior was characterized by low dissolution rates (< 10μA/cm~2) and a relatively noble open circuit potentials. This behavior was attributed to the presence of a Cu-rich surface layer that formed by dealloying. Active behavior was triggered by exceeding a critical breakdown potential during anodic polarization. After breakdown, IMCs exhibited much lower open circuit potentials and greater dissolution rates. This behavior was attributed to localized or general breakdown of the Cu-rich dealloyed layer. In terms of the median breakdown and corrosion potentials, these IMC phases decreased in activity in the order Al_2CuMg > Al_2Cu～Al_7Cu_2Fe > Al_(20)Cu_2Mn_3. Using information, variations in the pitting morphology in 2024-T3 associated with IMC particles is rationalized.