Role of microstructure on the varying corrosion behavior across the UNS S32750 super duplex stainless steel friction stir weld

The microstructure and corrosion behavior of the three regions, namely advancing side (AS), retreating side (RS), and center of the stir zone (CSZ) of friction stir welded UNS S32750 super duplex stainless steel (ferrite + austenite), were investigated. There is a significant grain refinement in these zones due to severe plastic deformation. Electrochemical corrosion behavior of the alloy was studied in freely exposed 0.5 M H2SO4 + 3.5 wt.% NaCl solution at room temperature. Perceptible difference in critical current and passivation current densities among the three zones and the base metal was noticed. These differences were mainly due to the variation in passive film thickness shown by these zones in the alloy. The studies show that the passive film thickness variation was due to significant differences in grain refinement seen and kernel average misorientation (KAM) in these zones of the alloy. It is postulated that the larger the grain boundary density and dislocation cells, the better is the ability of the alloy to form thick passive film.