Synergistic damage mechanism of corrosion and cavitation-erosion for plasma ion nitrided 18Cr–8Ni–1.1Mn–0.43C stainless steel in seawater

Abstract

In this work, it was investigated that resistance of combination damages of electrochemical corrosion and cavitation-erosion for plasma ion nitrided 18Cr–8Ni–1.1Mn–0.43C stainless steel at various process temperatures under cavitation environment in seawater. The plasma ion nitriding treatment was performed with a gas mixture ratio of 25% N2 and 75% H2 at a temperature of 623, 673, 723, and 773 K for 10 h, at a chamber pressure of 250 Pa. Expanded austenite (γN) was produced at whole plasma ion nitriding temperatures and especially phases of CrN and γ′ were observed at 773 K in X-ray diffraction (XRD) analysis. In Tafel analysis, the highest corrosion potential and corrosion current density were analyzed on the untreated (non-cavitation) specimen among specimens under the cavitation condition. Under cavitation environment, the highest corrosion potential and the lowest corrosion current density presented at 673 K. As a result of observation of surface morphologies after anodic polarization experiment under cavitation environment, uniform corrosion was observed on the untreated specimen and plasma ion nitrided specimens, while localized corrosion was shown on the untreated substrate under non-cavitation environment. Furthermore, greater damage depth and severe morphologies were observed at plasma ion nitrided specimens than untreated substrate, whereas the greatest weight loss at 723 K was indicated due to the high pitting corrosion potential and wide passivity potential region.

Effects of Plasma Ion Nitriding Temperatures on Cavitation-Erosion Resistance of STS 304 in Seawater

“…Especially, when plasma ion nitridation treatment is performed at low temperature, the corrosion resistance is improved by the formation of expanded austenite (S-phase) [4]. However, when stainless steel is used under seawater environment, physical damage due to cavity impact pressure and electrochemical damage due to chlorine ion are superimposed to accelerate damage [5]. Espitia and Allenstein et al have studied cavitation-erosion resistance in aqueous solutions and artificial chloride solutions after plasma ion nitriding for stainless steels [6,7], but little research has been done in seawater solutions.…”