Microstructure and corrosion behaviors of friction stir-welded Q235 low-carbon steel joint

Friction stir welding (FSW) was used to prepare Q235 low-carbon steel joint, and the microstructure of different zones of the joint was characterized. The electrochemical corrosion behavior of different macroscopic zones of the joint was evaluated in 3.5 wt.% NaCl solution. The results showed that the retreated-side heat-affected zone (HAZ RS ) and the advanced-side heat-affected zone (HAZ AS ) did not undergo phase transformation during FSW, and their microstructures were similar to those of the base material (BM), which was mainly composed of blocky ferrite and pearlite. The retreated-side thermo-mechanical affected zone (TMAZ RS ), the stirring zone (SZ), and the advanced-side thermo-mechanical affected zone (TMAZ AS ) underwent phase transformation, and the microstructure was mainly composed of proeutectoid ferrite and pearlite. The order of the corrosion resistance of different micro-zones from high to low was: HAZ RS > BM > HAZ AS > TMAZ RS > SZ > TMAZ AS . The corrosion mechanism for BM, HAZ RS , and HAZ AS was mainly the dissolution of ferrite. By contrast, the corrosion mechanism for TMAZ RS , SZ, and TMAZ AS was mainly galvanic corrosion between proeutectoid ferrite and pearlite.