Juntas de aço inoxidável super duplex UNS S32760 soldadas por fricção e mistura mecânica (FSW): microestrutura e comportamento de corrosão eletroquímica

Abstract

In this study, the corrosion resistance of the face and root surface of an AISD UNS S32760 friction stir welded (FSW) joint using optimized parameters was investigated. Microstructural changes were evaluated by MO, SEM and XRD and mechanical properties by Vickers microhardness mapping. The determination of the degree of sensitization was carried out by DL-EPR technique, while the localized corrosion resistance were evaluated by cyclic potentiodynamic polarization in neutral pH salt medium and potentiodynamic polarization in acid medium containing chloride ions. The results showed that the base metal (MB), retreating side (RS), stir zone (SZ), advancing side (AS), and root region presented distinct and heterogeneous microstructures. FSW welding promoted an increase in the ferrite phase volume fraction and grain refinement in the RSSZ, SZ, AS-SZ, AS-TMAZ and root regions, contributing to the increase in hardness. In the RT-ZM, ZM, AV-ZTMA and root regions, metallographic analyses after electrolytic attack indicated the formation, within the ferritic matrix, of Cr-rich precipitates, which were classified as out-of-equilibrium Cr nitrides (Cr2N). From the DL-EPR test, the highest degree of sensitization was observed at RT followed by AV, root, ZM and MB, where the corroded regions corresponded with the etched regions after electrolytic testing. through polarizations tests, the MB presented the highest localized corrosion resistance to localized corrosion and no pitting formation was observed while all welded regions presented pitting after the tests, which nucleation occurred preferentially in ferrite/ferrite grain boundaries, a fact that was related to the precipitation of Cr2N. It was concluded that the microstructural changes and the precipitation of Cr2N promoted by FSW welding were responsible for the reduction in the localized corrosion resistance of the welded regions, and this degradation was more pronounced in the RT, followed by the AV, root, and ZM.