Grain size effects on grain boundary and precipitation characteristics and their combined influence on corrosion behavior of extruded 2196 Al-Cu-Li alloy plates

Peripheral coarse grain (PCG) is a common defect in aluminum alloy extrusions. In Al-Cu-Li alloy, corrosion behavior is governed by coupled grain-size-related microstructural features, including grain boundary density, intragranular precipitation, grain boundary precipitate (GBP) continuity, and precipitate free zone (PFZ) width. However, the relationship between these coupled features and corrosion resistance in heterogeneous structures remains insufficiently understood. In this study, extruded 2196 alloy plate containing a PCG layer were investigated. Coarse, medium, and fine grained surface microstructures were prepared to evaluate the effects of grain size, precipitates, GBPs, and PFZ on intergranular, exfoliation, and electrochemical corrosion behavior. The results show that corrosion resistance increases with increasing grain size under the T6 aged condition. Fine grained microstructures contain dense intragranular T 1 and σ precipitates, continuous GBPs, negligible PFZs, and high grain boundary density, which together provide interconnected micro-galvanic corrosion pathways and result in deeper intergranular corrosion penetration and more severe exfoliation corrosion. In contrast, coarse grained microstructures contain relatively sparse intragranular precipitates, discontinuous GBPs, and wider PFZs, which interrupt continuous corrosion channel formation and reduce corrosion damage. Based on electrochemical measurements, a quantitative relationship between grain boundary length and charge transfer resistance was established to predict corrosion resistance within the investigated grain-size range. These findings provide guidance for evaluating corrosion susceptibility and optimizing microstructures in extruded Al-Cu-Li alloy components.