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In this study, electroless nickel (EN) plating was performed with stabilizer concentration to improve cavitation erosion resistance of gray cast iron under marine environment. Cavitation erosion tests were carried out in natural seawater solution in accordance with modified ASTM G32 . The change of stabilizer concentration affected the plating rate, P content, and average crystal size of EN coating. The decrease of the plating rate with the stabilizer concentration and the flake-shaped graphite exposed to the gray cast iron surface negatively affected the cavitation erosion and corrosion prevention. In particular, cavitation erosion damage of EN coating was accelerated due to combined galvanic corrosion when the EN coating was destroyed and the substrate was exposed under the cavitation erosion environment.

Electroless nickel plating (ENP) was performed to prevent the cavitation erosion damage of gray cast iron in the marine environment. The effect of the changes in the pH of a plating bath on the ENP coating characteristics, corrosion resistance and the cavitation erosion damage behavior in natural seawater were examined. The results showed that the pH of the plating bath had a significant effect on the plating rate, crystallinity, composition, and microhardness of the ENP coating. The corrosion resistance of ENP coating was greatly affected by the plating rate and the microstructure of the substrate. The cavitation erosion damage behavior depended on the crystallinity of the ENP coating. Once the substrate is exposed by the cavitation erosion of ENP coating, it is accompanied by galvanic corrosion, which accelerates the damage. Furthermore, the cavitation erosion damage mechanism of ENP coating was investigated.

In this study, the characteristics of cavitation-erosion resistance of 316L stainless steel after plasma ion nitriding at various temperatures were investigated in natural seawater. Plasma ion nitriding was performed with N₂ and H₂ at a ratio of 1:4 from 400 to 500 °C for 10 hours. Micro-Vickers hardness measurement showed that the mechanical properties were improved by increasing hardness due to the formation of expanded austenite (S-phase) at 400 °C and 450 °C and γ'-Fe₄N and CrN phases at 500 °C. In the cavitation-erosion test, weight loss and damage rate decreased with increasing nitriding temperatures up to 450 °C, while the depth of damage was found to be increased at 500 °C. The lowest weight loss and damage depth were obtained at 450 °C, which measured the thickest layer of γN phase. At 500 °C, cavitation-erosion damage was significantly occurred due to the internal residual stress accumulated during the plasma ion nitriding.

In this study, plasma ion nitriding (PIN) was performed as a measure to prevent the cavitation damage of gray cast iron (GCI) which is a cylinder liner material of diesel engine and evaluated its applicability. GCI treated by PIN presented about 45.2% improvement in surface hardness. In addition, a thickness growth behavior of flake-shaped graphite was observed in the nitrogen diffusion layer. As a result of cavitation erosion test, the maximum surface damage depth increased greatly due to a crater growth behavior along the depth on the flake-shaped graphite distributed on the surface. Furthermore, the weight loss after cavitation test indicated poorer cavitation resistance compared to the as-received specimen. Consequently, the application of PIN to prevent the cavitation erosion damage of GCI requires minimization of defects by flake-shaped graphite.

Corrosion characteristics of SUS 304 and SUS 316L under the marine environment were investigated by plasma ion nitriding at 450 °C with a gas ratio of 25% N₂ and 75% H₂ for 10 hours. In the anodic polarization experiment performed in natural seawater, noble pitting potential and wide passive region were exhibited in SUS 316L due to the synergistic effect of molybdenum and nitrogen even after plasma ion nitriding compared with SUS 304. In the corrosion rate obtained by an extrapolation method after plasma ion nitriding, a similar result of corrosion current density represented in SUS 304 while lower corrosion current density was analyzed in SUS 316L.

In this work, corrosion resistance and cavitation-erosion characteristics were investigated by applying plasma ion nitriding technique to cast stainless steels used as materials of high speed rotors under seawater environment. Plasma ion nitriding was performed for 10 h at various temperature parameters with 25% N2 and 75% H2 gas ratio. The cavitation-erosion experiment was carried out under vibration amplitude of 30 °C and sea water temperature of 25 °C according to modified ASTM G32-92. The yN phase that improves corrosion resistance and mechanical properties was formed at the all of experimnetal temperatures after plasma ion nitriding treatment. The crystallite size of phases was calculated through the XRD patterns according to Scherrer formula and obtained smallest nano size of yN phase at 450 °C. Cavitation-erosion resistance was enhanced up to 450 °C but was deteriorated at 500 °C.

Copper alloys have widely used for marine plants, ship building equipment, and renewable energy facilities under marine environment due to its excellent castability, durability, and corrosion resistance. In this study, the surface of ALBC3 alloy was modified with shot peening time to improve the strength and cavitation characteristics. For evaluation of cavitation resistance, cavitation experiments for the non-shot peened and shot-peened specimens were performed to investigate the extent of damage and determine the weight loss. As a result, the shot peened ALBC3 alloys improved cavitation resistance. In particular, the shot peening time of 3.5 min was considered to be the optimum shot peening condition since it presented an improvement of 35.7% in cavitation resistance.

Plasma electrolytic oxidation (PEO) is a promising coating process to produce ceramic oxide on valve metals such as Al, Mg and Ti. The PEO coating is carried out with a dilute alkaline electrolyte solution using a similar technique to conventional anodizing. The coating process involves multiple process parameters which can influence the surface properties of the resultant coating, including power mode, electrolyte solution, substrate, and process time. In this study, ceramic oxide coatings were prepared on commercial Al alloy in electrolytes with different KOH concentrations (0.5 ~ 4 g/L) by plasma electrolytic oxidation. Microstructural and electrochemical characterization were conducted to investigate the effects of electrolyte concentration on the microstructure and electrochemical characteristics of PEO coating. It was revealed that KOH concentration exert a great influence not only on voltage-time responses during PEO process but also on surface morphology of the coating. In the voltage-time response, the dielectric breakdown voltage tended to decrease with increasing KOH concentration, possibly due to difference in solution conductivity. The surface morphology was pancake-like with lower KOH concentration, while a mixed form of reticulate and pancake structures was observed for higher KOH concentration. The KOH concentration was found to have little effect on the electrochemical characteristics of coating, although PEO treatment improved the corrosion resistance of the substrate material significantly.

Anodizing is a technology to generate thicker and high-quality films than natural oxide films by treating metals via electrochemical methods. Electrochemical manufacturing method of nano structure is an efficient technology in terms of cost reduction, high productivity and complicated shapes, which receives the spotlight in diverse areas. Especially, artificial films generated by anodizing technology possess excellent mechanical characteristics including hardness and wear resistance. It is also easy to modify thickness and adjust shape of those artificial films so that they are mainly used in sensors, filters, optical films and electrolytic condensers. In this study, experiment was performed to observe the effect of current density on porous film formation in two-step anodizing for Al alloy. Anodizing process was performed with 10 vol.% sulfuric acid electrolyte while the temperature was maintained at 10℃ using a double beaker. and 10 ~ 30 mA/㎠ was applied for 40 minutes using a galvanostatic method. As a result, both pore diameters and distances between pores tended to increase as the local temperature and electrolysis activity increased due to the increase in applied current density.

Influence of Applied Frequency on Microstructural and Electrochemical Characteristics of Ceramic Coating Produced on Al-Mg Alloy by Pulsed Plasma Electrolytic Oxidation J.H. Lee, S.J. Lee and S.J. Kimc,∗ Korea Testing & Research Institute, Key Industry Division, Jongga-ro 15, Ulsan, 44412, Korea Kunsan National University, Department of Power System Engineering, Daehak-ro 558, Kunsan-si, Jeonbuk, Korea Division of Marine Engineering, Mokpo National Maritime University, Haeyangdaehak-Ro 91 Mokpo-si, Jeonnam, 530-729 Korea

Time and Temperature Dependent Damage Characteristics of 5083 Al Alloy under Cavitation-Corrosion Condition S.J. Lee, M.S. Han, J.Y. Jeong, M.J. Kim and S.J. Kimb,∗ Kunsan National University, Department of Power System Engineering, Daeha-ro 558 Gunsan-si, Korea Mokpo National Maritime University, Division of Marine System Engineering, Haeyangdaehak-Ro 91 Mokpo-si, Korea Mokpo National Maritime University, Division of Navigation Science, Haeyangdaehak-Ro 91 Mokpo-si, Korea

The cavitation damage characteristics of austenitic stainless steel with different concentrations of Ti were investigated. The microstructure of the alloys was observed with optical microscope to identify its correlation with cavitation resistance. Hardness of the alloys was measured to examine its contribution to cavitation damage. It was found that the microstructure played a more significant role in cavitation damage behavior of austenitic stainless steel with Ti than the hardness. The findings in this study revealed that Ti addition in austenitic stainless steel may present either a beneficial or detrimental effect on cavitation damage behavior, depending on the microstructural characteristics. In particular, Ti content of 1.0% represented the most deteriorated cavitation characteristics due to the formation of relatively coarse precipitates. Therefore, control of Ti concentration is essential for marine application of austenitic stainless steel.

In this paper, the characteristics of a cavitation-erosion damage behavior on the STS 304 and hot-dip aluminized STS 304 under cavitation environment in sea water solution was investigated. The electrochemical experiments were carried out by potential measurement, anodic/cathodic polarization test, Tafel analysis, and also galvanostatic experiment in current density variables for the samples. The apparatus of cavitation-electrochemical experiment was manufactured in compliance with modified ASTM G-32 standard, with the conditions of sea water temperature of and the measurement, amplitude of . The damage behavior was analyzed by an observation of surface mophologies and a measurement of damage depth by a scanning electron microscope(SEM) and a 3D microscope, respectively, after electrochemical test. After polarization experiment under cavitation environment, much higher damage depths for the hot-dip aluminized STS 304 were observed comparing to the untreated STS 304. In addition, higher corrosion current density in hot-dip aluminized STS 304 presented than that of untreated STS 304 as a result of Tafel analysis.

In this study, Al and Zn arc thermal spray coatings were carried out onto the substrate of SS400 steel to improve corrosion resistance and durability of hull structural steel for ship in marine environment. Therefore cavitation-erosion test was conducted to evaluate the durability of painted and thermal spray coated specimens. And then the damaged surface morphology and weight loss were obtained to compare with each other, respectively. As a result, the painted specimen was the poorest cavitation resistance characteristics because surface damage behavior appeared to be exfoliated in bulk shape during the cavitation experiment. And Zn thermal spray coating layer presented the significant surface damage depth due to relatively low surface hardness and local cavitation damage tendency. On the other hand, as a result of the weight loss analysis, the painting layer presented the poorest cavitation resistance and the Al thermal spray coating layer relatively showed the best results after cavitation experiment.

Aluminum alloy is a very strong reactivity material, but it has excellent corrosion resistance due to protective oxide film created in air. However, it is not practical because the film thickness is uneven and varies depending on the generation condition. Therefore, aluminum anodizing was performed to form film with commercially applicable hardness, corrosion resistance, and wear resistance. This offers such advantages as commercial applicability to large areas and low prices. In this study, the electrochemical characteristics with concentration of sulfuric acid electrolyte were compared with the two-step anodizing method which is widely used. A surface observation revealed regular structures and pores with the size of several tens of nm, and the anodized film presented excellent corrosion resistance with considerably low corrosion current density in sea water.

In this study, aluminum titanium nitride (AlTiN) coating was deposited by arc ion plating onto mirror finish STS 304 plate. The surface and cross-section of the coating was characterized by SEM and EDX analysis. Several electrochemical corrosion experiments were performed including rest potential measurement, potentiodynamic polarization experiment and Tafel analysis. The result of the experiments indicated that the AlTiN coating presented lower corrosion current density than the substrate material (STS 304) under uniform corrosion environment. It was also observed that AlTiN coating may have a risk of being attacked by localized corrosion attack such as pitting when pores or micro/nano particles in the coating are exposed to chloride ion containing corrosion environment, especially marine environment.

Machinery Technology Research Team, Korean Register, 36, Myeongji ocean city 9-ro, Gangseo-gu, Busan, Republic of Korea, 46762(Received June 16, 2015; Revised October 08, 2015; Accepted October 13, 2015)Corrosion is of greatest concern for metallic materials exposed to corrosive seawater or aggressive marine atmospheres. Marine structures and components made of metallic materials incur an initial cost and additional large costs for corrosion control and maintenance. There have been worldwide efforts to minimize marine corrosion and extend service life of the materials. It is believed that various factors are associated with corrosion of marine grade metallic materials, particularly the temperature of the solution affecting the corrosion rate by changing dissolved oxygen solubility and concentrations of chloride. In the present study, the electrochemical characteristics of S355ML steel are investigated to identify corrosion acceleration tendencies with changes in solution temperature under marine environments. It was found that increasing seawater temperature, promoted not only activation of chloride ion transfer, but also the formation of porous Fe(OH)

Division of Marine Engineering, Mokpo National Maritime University, Haeyangdaehak-ro 91, Mokpo-si, Jeonnam, Korea, 58628(Received June 16, 2015; Revised October 14, 2015; Accepted October 16, 2015)Stainless steel is generally known to have characteristics of excellent corrosion resistance and durability, but in a marine environment it can suffer from localized corrosion due to the breakdown of passivity film due to chloride ion in seawater. Furthermore, the damage behaviors are sped up under a cavitation environment because of complex damage from electrochemical corrosion and cavitation-erosion. In this study the characteristics of electrochemical corrosion and cavitation erosion behavior were evaluated on 16.7Cr-10Ni-2Mo stainless steel under a cavitation environment in natural seawater. The electrochemical experiments have been conducted at both static conditions and dynamic conditions inducing cavitation with different current density parameters. The surface morphology and damage behaviors were compared after the experiment. After the cavitation test with time variables morphological examinations on damaged specimens were analyzed by using a scanning electron microscope and a 3D microscope. the galvanostatic experiment gave a cleaner surface morphology presented with less damage depth at high current density regions. It is due to the effect of water cavitation peening under the cavitation condition. In the cavitation experiment, with amplitude of 30 ㎛ and seawater temperature of 25 ℃, weight loss and cavitation-erosion damage depth were dramatically increased after 5 hours inducing cavitationKeywords: stainless steel, cavitation, corrosion, galvanostatic experiment, seawater

In spite of highly advanced paint coating techniques, corrosion damage of marine metal and alloys increase more and more due to inherent micro-cracks and porosities in coatings formed during the coating process.Furthermore, flowing seawater conditions promote the breakdown of the protective oxide of the materials introducing more oxygen into marine environments, leading to the acceleration of corrosion. Various corrosion protection methods are available to prevent steel from marine corrosion. Cathodic protection is one of the useful corrosion protection methods by which the potential of the corroded metal is intentionally lowered to an immune state having the advantage of providing additional protection barriers to steel exposed to aqueous corrosion or soil corrosion, in addition to the coating. In the present investigation, the effect of flow velocity was examined for the determination of the optimum corrosion protection current density in cathodic protection as well as the corrosion rate of the steel. It is demonstrated from the result that the material corrosion under dynamic flowing conditions seems more prone to corrosion than under static conditions.

In this study, the current density requirements of zinc sacrificial anode for corrosion protection of AA5083-H321 aluminum hull for the ship was determined through various electrochemical tests. Surface observation and chemical composition analysis were performed after the galvanostatic test for various current density requirements determined by the cathodic polarization experiment. As a result, the most appropriate current density requirement range was determined as 3.7 × 10−6–5.8 × 10−6 A/cm2. Excessive current density over 7.6 × 10−6 A/cm2 caused local surface damage due to the effect of Mg-rich and Fe-rich intermetallic phases. In addition, the self-consumption of zinc sacrificial anode was determined through Tafel's extrapolation method.

This study investigated the cavitation erosion resistance of electroless Ni-P (EN) coated gray cast iron (GCI) in seawater solution. Furthermore, the optimum coating design parameters were examined to minimize cavitation erosion damage through analysis of variance (ANOVA) based on the L9 orthogonal array. In this study, four coating design factors were used: concentration of source of nickel (A), concentration of reducer agent (B), deposition temperature (C), and pressure of shot peening (D). In accordance with the regulation of the modified ASTM G32, the cavitation erosion experiment was conducted for 1 hour in a seawater solution to find the optimum design parameters which can minimize the cavitation erosion damage. Besides, ANOVA was performed to verify the contribution of each coating design parameter. As a result, the concentration of reducer agent among the EN process parameters was determined as the most significant factor in the cavitation erosion behavior.

Division of Marine Engineering, Mokpo National Maritime University, Haeyangdaehak-ro 91, Mokpo-si, Jeonnam, Korea, 530-729(Received June 10, 2015; Revised June 25, 2015; Accepted June 26, 2015)The commercialization of aluminum had been delayed than other metals because of its high oxygen affinity. Anodizing is a process in which oxide film is formed on the surface of a valve metal in an electrolyte solution by anodic oxidation reaction. Aluminum has thin oxide film on surface but the oxide film is inhomogeneous having a thickness only in the range of several nanometers. Anodizing process increases the thickness of the oxide film significantly. In this study, porous type oxide film was produced on the surface of aluminum in sulfuric acid as a function of electrolyte temperature, and the optimum condition were determined for anodizing film to exhibit excellent cavitation resistance in seawater environment. The result revealed that the oxide film formed at 10 represented the highest cavitation resistance, while the oxide film formed at 15 showed the lowest resistance to cavitation in spite of its high hardness.Keywords: aluminum, anodizing, oxide film, cavitation, seawater

Arc thermal spray coating using Al-3%Mg thermal spray wire was carried out to prevent steel from corrosion damage under the marine environment. Post-sealing was applied to Al-3%Mg spray coating treatment using organic/inorganic composite ceramics in order to improve the corrosion resistance of the as-sprayed coating. The results of various electrochemical experiments with sealing treatment indicated that the improvement in corrosion resistance was observed due to low current density in all applied potential range during anodic and cathodic polarization experiments. Futhermore, the natural potential measurement exhibited severe potential fluctuation due to influence of micro-crack presence on the surface of sealed thermal spray coating layer. In addition, the sealed layer was easily eliminated during anodic polarization. Nevertheless, Al-3%Mg spray coating layer improved corrosion resistance by sealing treatment because the sealed coating efficiency was determined to be 92.11%, indicating the exterior environment barrier effect which is based on the Tafel analysis.

Surface modification is a technology to form a new surface layer and overcome the intrinsic properties of the base material by applying thermal energy or stress onto the surface of the material. The purpose of this technique is to achieve anti-corrosion, beautiful appearance, wear resistance, insulation and conductance for base materials. Surface modification techniques may include plating, chemical conversion treatment, painting, lining and surface hardening. Among which, a surface modification process using electrolytes has been investigated for a long time in connection with research on its industrial application. The technology is highly favoured by various fields because it provides not only high productivity and cost reduction opportunities, but also application availability for components with complex geometry. In this study, an electrochemical experiment was performed on the surface of 5083-O Al alloy to determine an optimal electrolyte temperature, which produces surface with excellent corrosion resistance under marine environment than the initial surface. The experiment result, the modified surface presented a significantly lower corrosion current density with increasing electrolyte temperature, except for 5 °C of electrolyte temperature at which premature pores was created.

Aluminum acts as sacrificial anode and corrosion protection with Al₂O₃ formation. If the same current on material for Al ships with steel ships supplies, the more hydrogen would be occurred, that result is bring about over-protection. For this reason, the damage by hydrogen embrittlement leads to the serious accident. In this study, we evaluate electrochemical behavior with rotation speed of 5083-H116 Al alloy material for Al ship in seawater. To examine the electrochemical characteristics with rotation speed and its effects on performance, experiments were conducted at four rotation speed. Results of experiments, the corrosion current density and damage were increased by applying the rotation speed compared to static state.

Aluminum anode of marine growth prevention system for ship is installed in seachest or sea water strainer. The Al anode is connected to a control panel that feeds a current to the anode. The dissolved io)ns produced by the anode are transferred in sea water, spreads through the sea water pipe system and creates a protective film in the pipelines. Thereby, corrosion in pipeline system significantly is reduced. In application on condition as a steel ship, the big accident can be caused by the corrosion. Accordingly, in this research, we evaluated influence of applied current and flow velocity on electrochemical characteristics of Al anode for marine growth prevention system (MGPS). Based on the results of the erosion-cavitation experiments, cavitation rate increased greatly until 120 min. of the experimental time and decreased a little at the point of 180 min. where pit grew and merging occurred but showed a tendency of steadily increasing consumption rates. Based on the results of the Tafel analysis, compared to static states, corrosion current densities show a rapidly increasing tendency when flow occurred.

The offshore structure industry, which is emerging as a higher value-added business, has continued to evolve and expand. Much of this is contributed to development of new alloys with high corrosion resistance. Stainless steel, in particular, has significantly improved the corrosion characteristics of carbon steel. However, the protective film for corrosion protection of stainless steel is vulnerable to Cl− ions in seawater, leading to damages such as pitting corrosion and crevice corrosion. Especially, application of the material for underwater rotating body can cause physical erosion damage due to cavitation. This paper investigated the electrochemical characteristics of 316L stainless steel exposed to cavitation environment in seawater. The results revealed that the cavitation condition exhibited relatively high corrosion current density due to the breakdown of the protective film by either the impact pressure during bubble collapse or the micro-jet, suggesting that acceleration of corrosion damage was achieved compared to the static condition.

The atmospheric pressure plasma coating technology serves the function for enhancing the durability of products damaged by cavitation, corrosion, and erosion. However, it has many shortcomings compared to the vacuum plasma method such as porosity, low adhesive strength, and residual internal stress. Nevertheless, this technology has been frequently used recently because it has advantages of being helpful in simplifying processes and being applicable to diverse product shapes and sizes. In general, in the case of ceramic coating, dual structures are formed by adding MCrAlY metal bond coating between the base metal and ceramic top coating. In the present study, an aluminum-bronze alloy was applied with atmospheric pressure plasma coating of CoNiCrAlY/ZrO 2 –Y 2 O 3 materials and the cavitation characteristics of the coated material were experimented in seawater. According to the results of the experiment, atmospheric pressure plasma coating showed much more excellent hardness and cavitation characteristics than the base metal.