Study of localized corrosion on 2198-T8 aluminum alloy welded by friction stir welding

In this study, the microstructure, the microhardness and the corrosion resistance of the AA2198-T8 alloy welded by the FSW process were investigated by microscopic techniques, immersion tests and electrochemical, global or local, in chloride containing solutions. Different types of localized corrosion were observed in the alloy which were correlated with the microstructure, mainly with the presence of T1 phase (Al₂CuLi) and Al-Cu-Fe micrometric precipitates. Thermal profile simulation, microhardness measurements, differential scanning calorimetry measurements and transmission electron microscopy were used to study the concentration and distribution of T1 phase along the zones affected by the weldment and surrounding. An inverse correlation between the microhardness and severe localized corrosion (SLC) resistance was obtained and related to the concentration of the T1 phase in the different zones of the welded alloy. SLC was predominant in the zone not affected by the weldment, or base metal (BM), when tested alone or galvanically coupled to the other zones. When galvanically coupled, the stir zone (SZ) behaves as a cathode in relation to neighboring zones. SLC was observed in the SZ only when this zone was tested in isolation from the others, however, it was associated with T2/TB phases. Differences in morphology and attack kinetics were observed between the SZ and BM. Local techniques proved to be appropriate for the study of corrosion of these alloys, while conventional electrochemical techniques did not present sufficient resolution to differentiate the typical localized attack types of these alloys. (author)