Friction stir welding (FSW) is a new solid-state joining method which offers several advantages compared with conventional welding methods, including better mechanical properties, lower residual stress and reduced occurrence of defects. It has already been used for joining Al alloys in the aerospace and automotive industries. In spite of the advantages, FSW also has drawbacks, such as the risk of root flaws in single-side welds. Using a bobbin tool instead is a promising way to solve this problem since the root region is avoided. Compared with standard (single-side) FSW techniques, the bobbin tool FSW has an extra shoulder attached to the tip of the probe, namely the lower shoulder. This setup makes BTFSW capable of joining closed profiles like hollow extrusions. Furthermore, root flaws, such as lack of penetration, which occasionally occurred in standard FSWtechiques, can be completely avoided. In this work, 6061-T6 aluminum alloy was welded by using bobbin tool friction stir weld (BTFSW). The influence of BTFSW on the microstructure development and hardness distribution in the weldment has been investigated. The corrosion behaviors of the base metal and weld nugget in 3.5%NaCl (mass fraction) solution were investigated using SEM, XRD and electrochemical measurements. The results showed that the weld surface of 6061-T6 welded by BTFSW is of good quality. No welding defect was detected in the joints. Three microstructural zones, i.e., nugget zone, thermo-mechanically affected zone, and heat affected zone were discernible. The microstructural analysis indicates that the weld nugget region exhibited fine and equiaxed grain structure with an average grain size of ~8 μm, indicating the occurrence of dynamic recrystallization due to severe plastic deformation and thermal exposure. The thermo-mechanically affected zone underwent plastic deformation and recrystallization occured in this zone due to deformation strain and thermal input. The low hardness zone, determined by constructing the hardness distribution profile on cross-section of joint, located at thermo-mechanically affected zone of advancing side. Although 6061-T6 alloys are readily weldable, they suffered from severe softening in the heat affected zone because of the dissolution of Mg₂Si precipitates during the weld thermal cycle. BTFSW can improve the corrosion resistance of 6061-T6 aluminum alloy in 3.5%NaCl solution. The corrosion behavior results showed that both anodic dissolution and pitting were observed after the immersion test due to the inhomogeneous microstructure of 6061-T6 aluminum alloy. The corrosion products mainly composed of Al(OH)₃ and Al₂O₃. Furthermore, the corrosion process and mechanism were also discussed.