Effect of Microstructure of Additively Manufactured Inconel 625 on Long-Term Corrosion Behaviour in Sulfuric Acid Media

Abstract

The aim of this work was to investigate the effect of the microstructure of Inconel 625 additively manufactured by laser powder bed fusion (LPBF) and laser-assisted directed energy deposition (LDED) on the resistance to corrosion in sulfuric acid solution. Corrosion tests lasted 30 days, 90 days and 180 days. Microstructure characterization was performed using light microscopy, scanning and scanning-transmission electron microscopy. Corrosion resistance measurements were conducted using polarization curves and impedance spectroscopy. After immersion in sulfuric acid, the corrosion pits were observed on the surface. The area fraction of corrosion pits increased with prolongation of the test and after 180 days was equal to 9.63% and 4.49% in LPBF and LDED specimens, respectively. The higher density of the pits occurred along the boundaries of the melt pools, grain boundaries and cell walls and next to precipitates. It was determined that higher drop in impedance and increase in corrosion current occurred in LPBF than LDED Inconel 625, which was mainly related to a more intensive propagation of corrosion pits in areas with higher free energy. Furthermore, the weaker corrosion resistance is related to a more refined microstructure with a higher density of the preferential sites of localized corrosion after long-term corrosion in sulfuric acid.