Microstructure and mechanical properties evolution during long-term natural aging has been studied by means of optical and transmission electron microscopy as well as tensile tests and corrosion resistance measurements. It was found that the sharp decrease of plasticity and corrosion resistance during aging results from film like grain - boundary β' - and β - phases precipitation. Special thermomechanical and thermocyclic treatments have been worked out to produce a desired grain - boundary structure that affect the grain - boundary precipitation and alloys properties. The results have been demonstrated that grain - boundary precipitation of Al - Mg alloys can be controlled by prior deformation and recrystallization. The creation of special boundaries with a high density of coincident sites or low - defect faceted boundaries remain free of precipitation during subsequent aging. Pure grain boundaries provide stable ductility and corrosion resistance.