Abstract: The effects of metal inert gas (MIG) welding and friction stir welding (FSW) on the microstructure of 6061 aluminum alloy were studied by OM. Meanwhile, combined with the polarization curves test, the corrosion behaviors of the base metal and welded joints of pre-deposited NaCl after exposure under ultraviolet light (UV) irradiation and in the dark were compared. The results showed that the uniformity of the MIG welded joint microstructure was poor, and the content of Mg in weld zone was much higher than that in base metal. So the MIG welded joint showed the poor thermodynamic stability. UV irradiation could accelerate the corrosion rate of the base metal and the welded joints, in particular, the MIG welded joint was significantly affected. The effect of 30 days UV irradiation on the FSW joint was similar to that of the base metal. These effects are closely related to the corrosion resistance of the material and the corrosion products of semiconductor properties formed on the surface of the aluminum alloy.