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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.

Plasma electrolytic oxidation (PEO) coatings were fabricated on 5083 Al alloy in KOH electrolyte solution with adding various concentrations of Na2SiO3. Changes in voltage–time response and micro-discharge evolution were analyzed, and the surface and cross-section of the resulting coating layer were further characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results showed that discharge characteristics were evidently changed with different Na2SiO3 concentrations, particularly higher Na2SiO3 concentrations leading to lower dielectric breakdown voltages. It was found that porous surface structure became prevalent with increasing Na2SiO3 concentration. The EDS analysis confirmed the incorporation of Si element in the PEO coatings. The result of XRD analysis revealed that metastable phases such as γ- and η-alumina were produced as a result of PEO, while amorphous phases appeared with excessive Na2SiO3 concentrations (10 and 14 g/L). The coating thickness was significantly increased about 2–8 times with increasing Na2SiO3, almost depending on Na2SiO3 concentration.

The aim of this research is to investigate the optimum plasma ion nitriding temperature on corrosion resistance in natural sea water for plasma ion nitrided 316L stainless steel. Plasma ion nitriding was conducted at different temperatures of 350, 400, 450, and 500 °C with a mixture of 75% of nitrogen and 25% of hydrogen during 10 h. In conclusion of anodic polarization test, a wide passive potential region and a high corrosion potential were observed at a plasma ion nitriding temperature of 450 °C. Moreover, relatively less damage depth and clean surface micrographs were observed at 450 °C as results of observation of three-dimensional (3D) microscope and scanning electron microscope (SEM) after polarization experiments. In addition, higher corrosion potential and lower corrosion current density were indicated at plasma ion nitrided samples than the value of untreated substrate after Tafel analysis. Hence, plasma ion nitrided at 450 °C in sea water solution represented optimum corrosion resistance among the all the plasma ion nitriding temperature parameters.

Recently, plasma electrolytic oxidation (PEO) has emerged as a promising surface modification technique to improve surface properties of Al alloys. In this study, PEO coating process for Al–Mg alloy was conducted with two different electrolyte solutions under the same electrical parameters: one was potassium hydroxide (KOH) aqueous solution, and the other involved potassium hydroxide aqueous solution with sodium silicate (Na2SiO3). The surface morphology was observed with scanning electron microscope (SEM) and elemental compositions were identified with energy dispersive spectroscopy (EDS) analysis. The chemical structures of PEO coatings were identified by X-ray diffraction analysis. Cavitation experiment was performed using ultrasonic vibratory cavitation erosion testing apparatus. Cavitation damage of PEO coatings was characterized using SEM and three-dimensional (3D) microscope. The result indicated that the surface of Al–Mg alloy were successfully modified having complete different surface morphologies by changing electrolyte composition. It was found that the surface morphology had a great influence on the cavitation damage behavior of PEO coating.

Application of surface treatment has become common for protecting machine parts from oxidation, abrasion and corrosion induced by external environment. In particular, thermal spraying techniques are widely employed to improve wear, corrosion and thermal resistance. And compared to other methods they are simple and cost effective. However, the presence of porosity in the thermal spray coating can be highly detrimental because it provides access to penetration of corrosive matters, lowering corrosion resistance. Therefore, this research evaluate the electrochemical behavior under marine environment for aluminum–bronze alloy coated with MCrAlY and yttria-stabillized zirconia (YSZ) by atmospheric pressure plasma (APP) coating technology. Further application of carbon-based sealer removed voids and defects in the coating. The result reveled that, in case the voids and defects are completely removed, excellent corrosion resistance can be archived by application of good coating material along with formation of compact sealing layer.

The mechanical properties of aluminum alloy may be enhanced by modifying the microstructure of the metal by friction stir processing (FSP). Previous studies have demonstrated that the mechanical characteristics of 5456-H116 Al alloy subjected to FSP, at 250 rpm and 15 mm min−1 using a full screw probe, are similar to those of the original alloy. In the present work, the same alloy was processed under these optimal conditions, and the range of favorable protection potentials with regard to hydrogen embrittlement and stress corrosion cracking was determined to lie between −1.3 and −0.7 V (versus Ag/AgCl). The electrochemical behavior of the specimens subjected to FSP was superior to that of the original 5456-H116 Al alloy.

Offshore structures that are made of austenitic stainless steels are exposed to a severe corrosion environment, with fracturing of the passive film occurring by chloride ion intrusion, stress from dynamic external forces and fatigue due to wave and tidal forces.In this paper, we report our evaluation of the durability of STS 316L with respect to stress corrosion cracking and hydrogen embrittlement in natural seawater, which was carried out via electrochemical methods and slow strain rate tests (SSRTs). The effect of hydrogen on the material was assessed using a SSRT with an applied potential of ?0.95?V (versus Ag/AgCl). In addition, potentials below an applied potential of ?1.2?V indicate samples that are affected by atomic and molecular hydrogen. Theoretically, the optimum corrosion protection range possible without stress corrosion cracking and hydrogen embrittlement occurring is thought to be between?0.56 and ?0.9?V.