Copper enriched by dealloying as external cathode in intergranular corrosion of aluminium alloy AA6005

•The Cu nanofilm along the grain boundaries is the internal cathode during IGC.•External cathodes are created by dealloying of the grain surfaces.•The α-phase corrodes in acidified chloride solution.•The principle of separation of anodes and cathodes is satisfied. The purpose of this work is to understand the role of Cu in the propagation mechanism of IGC on extruded alloy AA6005-T5. The constituent Cu segregates as a Cu-rich nanolayer, together with the formation of a solute depleted zone, along the grain boundaries during thermomechanical processing of the alloy. While the α-phase particles corrode in acidified chloride test solution, additional Cu becomes enriched as nanoscale particles and layer segments by dealloying on the external surface and fissure walls formed by IGC. While the grain-boundary layer functions as the internal cathode, enriched Cu acts as effective external cathodes in IGC propagation.