Micro- and nano-scale intermetallic phases in AA2070-T8 and their corrosion behavior

Intermetallic phases on the as-polished surface of the Al-Cu-Li alloy AA2070-T8 were identified and characterized extensively with respect to microstructure and chemical composition using spectroscopic and microscopic techniques and atom probe tomography. This is the first comprehensive report of the intermetallic compounds in the 3rd generation of Al-Cu-Li alloys. A total of eleven compositional classifications and/or crystallographic phases and one GP zone were revealed, with Al7Cu2Fe(Mn) being the most abundant, comprising around 44% of all intermetallic particles. Seven types of large intermetallic compounds were found to be cathodic to the matrix and their nobility generally increases with their noble element content, e.g. Cu and Fe, and decreases with their Al content, which can be further correlated to the cathodic current densities supported by these particles. The intermetallic compound nobility is closely related to the Volta potential difference, which correlates with the extent of trenching around the particles during electrolyte exposure. Corrosion of this alloy in 0.1 M NaCl involves a series of chemical and electrochemical phenomena at the intermetallic compound/matrix/electrolyte interfacial region, including preferential dealloying of the intermetallic compound, galvanic dissolution of the peripheral matrix, trench development, hydrogen evolution at the bottom of trench, and pitting in the bulk alloy. However, after exposure, no clear corrosion attack on or around nm-scale precipitates was observed after exposure times comparable to that for large particles. These findings are essential to understand the material degradation mechanism of this alloy family and provide input into new alloy design.