The application of stainless steel has widely used in various industries due to an advantage of having excellent corrosion resistance characteristics. Nowadays, marine and offshore industries have sustained to evolve and to expand as a high value-added field. However, a passive film like Cr2O3 known for protecting corrosion of stainless steel is vulnerable to breakdown from chloride ions in seawater, leading to surface damages such as pitting corrosion, intergranular corrosion, stress corrosion cracking (SCC) and crevice corrosion, etc. Moreover, materials applied under cavitation environment can suffer cavitation-erosion caused by micro-jet. Hence, its surface damages are accelerated by synergistic effects of electrochemical corrosion caused by Cl- ions and cavitation-erosion excited by cavities.
Plasma ion nitriding technique is one of the most thermochemical surface hardening methods for stainless steels in order to improve mechanical properties by increasing hardness on surface of materials. That is a reason that plasma ion nitriding results less distortion compared to other surface modification treatment such as other nitriding process, carburizing, or sulfurizing, carbonitriding, etc. In addition, plasma ion nitriding treated at low temperature produces improvement of tribological properties and anti-corrosion characteristics due to a formation of nitrogen precipitation in surface hardening layer. Many researchers carried out with plasma ion nitriding variables such as process temperature, treatment time, N2-H2 gas mixture, etc. Through those various experiment, they found out a metastable phase, γN-phase and so called "S-phase", nitrided at a temperature of approximately 723 K or below which enhance the mechanical properties with sustaining of corrosion resistance for austenitic stainless steels. As mentioned earlier, stainless steels suffer crucial corrosion caused by Cl- ions and few papers demonstrated corrosion behavior of austenitic stainless steel in chloride solution such as HCl or NaCl or acid solution like H2SO4 solution, but there are quite few papers carried out in natural seawater. Based on different hypothesis, the reason of differences between artificial solution (HCl or NaCl) and natural seawater are still not clear, but some researchers proved different results of pitting potential between two medium. They regarded that results due to a different mechanism of electrochemical behaviors in terms of biological phenomenon.
Therefore, the characteristics of complex corrosion damage mechanism by corrosion and cavitation-erosion should be investigated in seawater to where stainless steel is practically applied in marin and offshore industries. Hence, in this investigation, experiment of plasma ion nitrided austenitic stainless steel was carried out in natural seawater.
The rolled austenitic stainless steels were used as specimens which are widely used to most of industries. Plasma ion nitriding process was conducted at different temperatures from 623 K to 773 K with a mixture of 75% of nitrogen and 25% of hydrogen during 10 h. Electrochemical experiment, cavitation-erosion test and hybrid test were performed to investigate corrosion characterization, enhancement of cavitation-erosion and synergistic damage behaviors, respectively. Furthermore, the optimum plasma ion nitriding temperatures were demonstrated for each experimental conditions in natural seawater.
In electrochemical experiment, less surface damage and damage depth at a plasma ion nitrided temperature of 723 K represented than those of other temperatures of specimen. As a result of cavitation-erosion test, specimens plasma ion nitrided at 723 K showed relatively less surface damage depth, weightloss and surface damage than the others. However, in hybrid test, different optimum temperatures were observed in different experiment, that was 773 K for hybrid anodic polarization experiment and 673 K for hybrid galvanostatic experiment, respectively. Hence, plasma ion nitriding temperature should be applied relying on the exposed electrochemical environment and application intended for either enhancement of cavitation-erosion or improvement of corrosion resistance, otherwise for both combined environments.