It was revealed that an AA5083 alloy had been severely sensitized after 40 years in service at ground atmosphere temperature. The degree of sensitization of the various regions through the alloy plate thickness is different, which is associated with the non-uniform distribution of Mg through the plate thickness. The alloy plate exhibits Mg-rich bands that promoted the heterogeneous distribution of Mg-containing precipitates. The composition of these precipitates ranges from that for low Mg content intermetallics to that of the β′ phase. Structure characterization revealed the β′ phase and a cubic Al-Mg phase that has lower magnesium contents compared to the β phase.