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The effect of friction stir welding (FSW) on the pitting corrosion resistance of a lean duplex stainless steel was investigated in chloride-containing and chloride-free electrolytes. FSW was carried out by employing a constant rotation speed of 800 rpm at the welding speeds of 50, 100, and 150 mm/min using WC-based tool. Cyclic polarization results showed the same corrosion resistance of the base metal and welded joints in 0.1 M H 2 SO 4 . The optical microscopy observations after cyclic polarization tests revealed that no signs of pitting corrosion were evident for the base metal and different FS welds in chloride-free 0.1 M H2SO4 solution. In contrast, a wide positive hysteresis loop indicative of pitting corrosion susceptibility was displayed for the base metal and FS welds in 0.1 M H 2 SO 4 +0.1 M NaCl solution. The optical micrographs of pitting corrosion in the base metal and various FS welds demonstrated that the size of pits was decreased and the number of pits was increased after FSW. It was also found that increasing the welding speed led to an increase in the number of pits and a decrease in the size of pits. In other words, the resistance to pit growth was improved by decreasing the heat input during FSW. These results were interpreted with respect to the noticeable grain refinement that occurred during FSW.