The influence of welding flash on the corrosion resistance of friction stir spot welded (FSSW) AZ31B was examined by mass loss testing complimented with the scanning reference electrode and microcapillary polarization techniques. The microstructure of the flash was characterized by optical microscopy and the use of tungsten carbide tracer and correlated with the corrosion morphology of the joints. It was observed that the flash increased the corrosion rate of the welds, and its removal can reduce the corrosion rate by 20%. The increase in susceptibility for corrosion was explained by examining the electrochemical characteristics of the flash, and in particular to show the presence of a second stir zone (SZ) region in the flash. The electrochemical properties of the flash were correlated to second phase particle dissolution using a detailed microscopy analysis.The coupling of the two regions resulted in the formation of a local galvanic cell in the flash, leading to accelerated corrosion. Increases in dwell time and/or rotational speed of the tool during FSSW resulted in the formation of a larger SZ region in the flash, and produced a greater cathode to anode ratio. Materials and Corrosion 2017, 68, No. 4 Effect of welding flash on the corrosion of FSSW Figure 6. Influence of welding flash and exposure time on the location and morphology of corrosive attack. Important weld surfaces are labeled with letters that are consisted with the schematic in Fig. 1b. Abottom of pin hole surface, Cshoulder penetration surface, Fupper sheet surface, Iwelding flash www.matcorr.com