  Intermetallic phases in a recently developed Al-Li-Cu-Mg alloy have been
investigated to understand their roles in the initiation and propagation
processes of intergranular corrosion. Corrosion initiation involves trenching
formation in the Al matrix adjacent to the large particles of Al7Cu2(Fe, Mn)
phases. These phases containing Li are electrochemically active and susceptible
to self-dissolution via a de-alloying mechanism during corrosion process. The
subsurface particles of Al7Cu2(Fe, Mn) and Al20Cu2Mn3 phases act as the
internal cathodes for continuous corrosion propagation along the
particle-matrix interface and the associated grain boundaries. Corrosion
propagation along the particle-matrix interface was facilitated by the anodic
dissolution of the surrounding Al matrix due to the micro-galvanic interaction
with the cathodic intermetallic phases. In addition, T1 (Al2CuLi) precipitates
and the isolated particles of Al7Cu2(Fe, Mn) and Al20Cu2Mn3 phases were
dissolved along the path of corrosion propagation. The dissolved metal ions
were redeposited through the network of crevice.
