Electrochemical properties in natural seawater of super austenitic stainless steel with plasma ion nitriding and electropolishing

In this investigation, electropolishing and plasma ion nitriding were applied to super austenitic stainless steel for the purpose of improving corrosion resistance and hardness. Electrochemical experiments were performed in natural seawater, which is used as scrubber wash water. Further, various experiments and analyses—such as FE-SEM, EDS, XRD, 3D laser microscopy, and potential measurement—were performed to evaluate both surface and electrochemical characteristics with the surface treatments. The hardness of electropolished and plasma ion nitrided stainless steel increased by 284.06 Hv and 1548.3 Hv, respectively, compared to mechanical polishing. After electropolishing, the surface roughness (Sa) decreased by approximately 78.2 % compared to mechanical polishing. And after plasma ion nitriding, CrN(precipitate) Fe4N(compound) and γN(expanded austenite) were observed on the surface. The thickness of the layer formed on the surface was expressed as 10 to 12 μm In addition, the surfaces observed after the cyclic potentiodynamic polarization experiment revealed that local corrosion occurred in the mechanical polishing and electropolished specimens, whereas the plasma ion nitrided specimens presented a tendency toward general corrosion.