Abstract: In this work, the corrosion behavior of friction stir welding (FSW) joints of 2A12 aluminum alloy with and without ultrasonic impact treatment (UIT) was investigated by immersion corrosion test and polarization curve test in 3.5% NaCl aqueous solution. The results showed that the average corrosion rate of the joints with UIT was about half of that without UIT. The corrosion potential of the heat affected zone (HAZ) without UIT was −0.629 V (Ag/AgCl), indicating the worst corrosion resistance. And the average corrosion depth of HAZ induced by pitting corrosion and intergranular corrosion was about 125 μm. After UIT, the corrosion potentials of each sub-area of the FSW joints increased, and the corresponding corrosion current density decreased. The maximum depth of corrosion pit did not exceed 40 μm. The whole surface of the joint exhibited a form of uniform corrosion. Ultrasonic impact makes the surface grains of the material finer and denser, and the bond between the strengthening phase and the base metal is tighter, which is the main reason for the improvement of the corrosion resistance of the material. The main reason for the improvement of the corrosion resistance of the joints was that the grains of the material were refined and densified after UIT.