Microstructural Evolution and Anti-Corrosion Characteristics of As-Cast and Lpbf Printed Inconel Plates in Seawater Environment

Inconel 718 (IN 718) belongs to the category of nickel-based superalloys with outstanding mechanical characteristics such as high strength, better corrosion resistance, greater hardness, fatigue and wear resistance. This alloy is mostly implemented in manufacturing the components of aeroengines, nuclear reactors, space shuttles, pressurized boilers and so on. In most cases, the parts made from this alloy work in severe aggressive environments. A prolonged exposure of this alloy in such environments will pave the way for corrosion phenomenon which will minimize the service life of the components. It is reported that the parts made via additive manufacturing will exhibit better mechanical characteristics than the casted and forged parts. This work is aimed to investigate the anti-corrosion characteristics of both as-cast and LPBF printed IN 718 samples at various time intervals in the seawater environment. By means of FESEM micrographs, the microstructural evolution in both as-cast and LPBF printed IN 718 samples was analyzed. The anti-corrosion characteristics of both samples were investigated in the environment containing 3.5 wt.% NaCl (seawater) in terms of EIS and potentiodynamic polarization curves along with the corroded morphology. The results showed that the LPBF printed IN 718 samples exhibit fine cellular and columnar grains decorated with the NbC precipitates along their boundaries, whereas the casted samples have larger and coarser grains with uneven segregation of NbC precipitates. From the Tafel and EIS (Nyquist and Bode) plots, the LPBF IN 718 sample showed greater corrosion resistance in terms of current density and polarization resistance values. In particular, the 15 h LPBF sample has a strong, compact passive film formed on it. This compact passive film paved the way for this sample to exhibit lower current density (3.03 × 10−7 A/cm²) and higher polarization resistance (3678 Ω · cm²) when compared with the LPBF and cast samples at other intervals.