Heat treatment effects on LPBF duplex stainless steel corrosion resistance

The LPBF-manufactured super duplex stainless steel (SDSS) was subjected to stress-relieving and solution annealing heat treatments and was tested in as printed conditions. The main objective of the study was to verify the corrosion resistance of SDSS by Tafel polarisation and electrochemical impedance spectroscopy in a chloride-rich solution. Structural analysis was performed by LOM/SEM and X-ray diffraction. The correlation between corrosion resistance, microstructure, and heat treatment was evaluated. As a result of the very fast cooling rate of the LPBF process, the SDSS reveals ferrite as the major phase in the as printed conditions, similarly after stress relieving at 300°C/5h. Ferrite grains are elongated in the building direction with some austenite precipitation along the grain boundaries. Solution annealing leads to a well-balanced phase composition, with austenite evenly distributed among ferritic grains. This transformation positively affects corrosion resistance. Notably, both the polarisation resistance (Tafel analysis) and the charge transfer resistance (EIS) consistently highlighted the improved corrosion resistance of solution annealed SDSS. The stress relieving at 300°C/5h of LPBF SDSS slightly improves electrochemical parameters when analysing anodic polarisation curves, while a more notable increase in charge transfer resistance was evidenced in the EIS test, compared to as-printed SDSS.