Nickel alloying in laser powder bed fusion processed duplex stainless steel: Its effect on microstructure and pitting corrosion response

The effect of nickel alloying on the surface and interface microstructure and pitting corrosion behavior of laser powder bed fusion (LPBF)-produced 2205 duplex stainless steel (DSS) was studied. Ni addition promoted austenite formation (>20 %), refined grains (from 117 to 181 µm to 28–36 µm), and reduced low-angle grain boundaries (LAGBs) (60–15 %). In addition, the critical pitting temperature (CPT) was increased by 10–20 °C in 0.2–1 M NaCl, demonstrating a significant enhancement of pitting resistance. Unlike LPBF 2205 DSS, LPBF 22Cr DSS (nickel over alloyed) exhibited building direction dependent pitting corrosion resistance: in 0.2 M NaCl, the XOY plane showed a lower CPT than XOZ/YOZ, but this ranking was reversed at >0.6 M NaCl. Two pit nucleation sites were identified in LPBF 2205 DSS, as opposed to five in LPBF 22Cr DSS. Pit nucleation susceptibility was driven by austenite content, grain boundary length, and LAGB proportion. The high Cl^- sensitivity of Type IV sites on XOZ/YOZ planes in 22Cr DSS resulted in their lower CPT compared to the XOY plane in >0.6 M NaCl.