Spreading of intergranular corrosion on the surface of sensitized Al-4.4Mg alloys: A general finding

► Spreading of intergranular corrosion on heat treated and sensitized Al-Mg alloys was investigated. ► Only a subset of all grain boundaries containing β-phase dissolve and form pits in NaCl solutions. ► Subsequently, attack of β-phase at other grain boundaries is triggered causing corrosion spreading. ► The “breakdown potential” in upward polarization scans is a potential associated with IGC spreading. ► The breakdown potential does not correspond to random pitting of less noble phases.