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.