Intergranular Corrosion Morphologies in Relation with Their Microscopic Mechanisms in the Artificially-aged 7055 Aluminum Alloy

The effect of artificial ageing on the intergranular corrosion behavior of 7055 aluminum alloy were investigated by accelerated corrosion test, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and EDS element mapping. The results show that when the alloy is aged at 120 degrees C, the grain boundary precipitates (GBPs) are the preferential corrosion sites; however; when the alloy is aged at 400 degrees C, the Al-matrix near the GBPs is the preferential corrosion site rather than the GBPs. According to the TEM and EDS results, when the alloy is aged at 120 degrees C, extended ageing leads to the coarsening and discontinuous distribution of GBPs as well as the increase in the Cu content in the GBPs. These factors result in the evolution of corrosion morphology from a typical intergranular corrosion morphology to a pitting morphology. When the alloy is aged at 400 degrees C, Cu elements will concentrate in the precipitates, especially in the GBPs, and the Al-matrix deplete Cu element. As such, the corrosion potential of GBPs becomes higher than that of the Al-matrix nearby, which leads to intergranular corrosion guided by the Al-matrix dissolution along the grain boundary.