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Page : 164 - 174
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- 전해연마한 슈퍼오스테나이트 스테인리스강의 해수온도에 따른 전기화학적 특성 연구
- Electropolishing technology uses an electrochemical reaction and improves surface roughness, glossiness, and corrosion resistance. In this investigation, electropolishing was performed to improve the corrosion resistance of super austenitic stainless steel. As a result of electropolishing, surface roughness (0.16 μm) was improved by about 76.5% compared to mechanical polishing (0.68 μm). In addition, the electropolished surface was smooth because the average and variance values of the depth histogram were small. Tafel analysis was performed after a potentiodynamic polarization experiment with seawater temperature, and the microstructure was compared and analyzed. The corrosion current density at 30 °C, 60 °C, and 90 °C was reduced by 0.083 μA/cm2, 0.296 μA/cm2, and 0.341 μA/cm2, respectively. Pitting occurred in the mechanical polished specimen at 30 °C, but partial intergranular corrosion was observed in the electropolished specimen, and pitting occurred predominantly at both 60 °C and 90 °C. In addition, the damage depths of the electropolished specimen were shallower than those of mechanical polishing at 30 °C and 60 °C, but the opposite result was seen at 90 °C.
- KEYWORDs : Electropolishing, Surface roughness, UNS N08367, Potentiodynimic polarization, Temperature
- AUTHORs : 황현규 · 김성종†
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Page : 175 - 186
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- Selective Catalytic Reduction (SCR) 환경에서 18% 크롬 스테인리스강의 부식 거동
- Effects of high-temperature environment and low-temperature environment on corrosion behaviours of 18Cr stainless steels (type 304L, type 441) in simulated selective catalytic reduction (SCR) environments were studied using weight loss test in each environment and rust analysis. With time to exposure to the hightemperature environment, type 441 was more resistant to corrosion than type 304L due to both higher diffusivity of Cr and lower thermal expansion coefficient in α-iron. The former provides a stable protective Cr2O3 layer. The latter leaded to low residual stress between scale and steel, reducing the spallation of the scale. With time to exposure to the low-temperature environment, on the other hand, type 304L was more resistant to corrosion than type 441. The lower resistance of type 441 was caused by Cr-depleted zone with less than 11% formed during the pre-exposure to a high-temperature environment, unlike type 304L. It was confirmed by results from the crevice corrosion test of sensitised 11Cr steel. Hence, to achieve higher corrosion resistance in simulated SCR environments, ferritic stainless steels having lower thermal expansion coefficient and higher diffusivity of Cr but containing more than 18% Cr are recommended.
- KEYWORDs : Diffusivity of chromium and oxygen, Thermal expansion coefficient, Cr-depleted zone, Spallation
- AUTHORs : 김희산†
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Page : 187 - 192
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- 우드칩을 연료로 하는 열병합발전소의 회분 퇴적 및 설비 고장 원인 분석
- The use of biomass is increasing as a response to the convention on climate change. In Korea, a method applied to replace fossil fuels is using wood chips in a cogeneration plant. To remove air pollutants generated by burning wood chips, a selective denitrification facility (Selective catalytic reduction, SCR) is installed downstream. However, problems such as ash deposition and descaling of the equipment surface have been reported. The cause is thought to be unreacted ammonia slip caused by ammonia ions injected into the reducing agent and metal corrosion caused by an acidic environment. Element analysis confirmed that ash contained alkali metals and sulfur that could cause catalyst poisoning, leading to an increase in the size of ash particle and deposition. Measurement of the size of ash deposited inside the facility confirmed that the size of ash deposited on the catalyst was approximately three times larger than the size of generally formed ash. Therefore, it was concluded that a reduction in pore area of the catalyst by ash deposition on the surface of the catalyst could lead to a problem of increasing differential pressure in a denitrification facility.
- KEYWORDs : Woodchip, Biomass, Selective catalytic reduction, Ash deposition, Ammonia slip
- AUTHORs : 송민지 · 김우철 · 김희산 · 김정구 · 이수열†
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Page : 193 - 200
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- Titanium 양극산화시 TiO 2의 형상 및 특성에 미치는 전해질의 영향
- Titanium alloy (grade-4) is commonly used in industrial and medical applications. To improve its corrosion resistance and biocompatibility for medical use, it is necessary to form a titanium oxide film. In this study, the morphology of the oxide film formed by anodizing Ti-grade 4 using different electrolytes was analyzed. Wetting properties before and after surface modification with SAM coating were also observed. Electrolytes used were categorized as A, B, and C. Electrolyte A consisted of 0.3 M oxalic acid and ethylene glycol. Electrolyte B consisted of 0.1 M NH4F and 0.1 M H2O in ethylene glycol. Electrolyte C consisted of 0.07 M NH4F and 1 M H2O in ethylene glycol. Samples B and C exhibited a porous structure, while sample A formed a thickest oxide film with a droplet-like structure. AFM analysis and contact angle measurements showed that sample A with the highest roughness exhibited the best hydrophilicity. After surface modification with SAM coating, it displayed superior hydrophobicity. Despite having the thickest oxide film, sample A showed the lowest insulation resistance due to its irregular structure. On the other hand, sample C with a thick and regular porous oxide film demonstrated the highest insulation resistance.
- KEYWORDs : Anodizing, Electrolyte, Hydrophobic, Ti Nanostructure, Corrosion
- AUTHORs : 최예지 · 정찬영†
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Page : 201 - 213
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- 실험계획에 기반한 수돗물 성분(Chloride and Sulfate Ions)의 구조재료 부식 영향성 고찰
- Corrosion management of an aircraft and its engine relies on rinsing and cleaning using tap water. Few studies have reported effects of tap water species on corrosion behaviors of structural materials. In this study, a series of experiments were conducted based on the design of experiment. Solutions with different levels of chloride and sulfate ions were prepared using a full factorial design. Two structural materials (aluminum alloy and steel) were used for an alternate immersion test. Weight loss was then measured. In addition, a silver specimen was utilized as a sensor for chloride deposition measurement. The silver specimen was examined using the electrochemical reduction method, XPS, and SEM-EDS. Surface analysis revealed that levels of chloride and sulfate ions were sufficient for the formation of silver chloride and silver surface. Statistical analysis of weight loss and chloride deposition rate showed significant differences in measured values. Concentration of chloride ions greatly affected corrosion behaviors of structural materials. Sulfate ion hindered the adsorption reaction. These results emphasize the importance of controlling ion concentration of tap water used for cleaning and rinsing an aircraft.
- KEYWORDs : Structural materials, Cleaning, Rinsing, Full factorial design
- AUTHORs : 임동인 · 노흥수 · 권혁준 · 박승렬 · 조만식 · 이두열†
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Page : 214 - 219
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- 원자력발전소 증기발생기 Alloy 690 전열관 재료의 규칙화 반응
- Considering the case in the United States where most nuclear power plants with an initial design life of 40 years continue to operate until 60 or 80 years after undergoing material soundness evaluation, it is time to plan a more robust long-term operation strategy for nuclear power plants in Korea. There are some reports that SRO/LRO might be formed when Alloy 690 is heat treated for 10,000 hours to 100,000 hours at 360 to 450 °C. The possibility of LRO formation in Alloy 690 steam generator tubings of Kori nuclear power plant unit 1 (Kori-1) was investigated using existing research papers. The mechanism in which SRO/LRO occurred was also surveyed. Alloy 690 was found to be more likely to cause ordering than Alloy 600 in terms of alloy composition. The ordering could be evaluated through changes in material properties. However, it is difficult to evaluate it from a microstructural point of view. The likelihood of LRO in Alloy 690 of the Kori-1 plant operated at 320 °C for 19 years seemed to be low in terms of time and exposure temperature.
- KEYWORDs : Nuclear power plant, Long term operation, Alloy 690, Steam generator tubings, SRO/LRO
- AUTHORs : 황성식† · 최민재 · 김성우
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Page : 221 - 231
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- 알루미늄 합금의 수소취화 방지를 위한 경질양극산화 및 플라즈마이온질화의 영향
- Interest in aluminum alloys for the hydrogen valves of fuel cell electric vehicles (FCEVs) is growing due to the reduction in fuel efficiency by the high weight. However, when an aluminum alloy is used, deterioration in mechanical characteristics caused by hydrogen embrittlement and wear is regarded as a problem. In this investigation, the aluminum alloy used to prevent hydrogen embrittlement was subjected to surface treatments by performing hard anodizing and plasma ion nitriding processes. The hard anodized Al alloy exhibited brittleness in which the mechanical characteristics rapidly deteriorated due to porosity and defects of surface, resulting in a decrease in the ultimate tensile strength and modulus of toughness by 15.58 and 42.51%, respectively, as the hydrogen charging time increased from 0 to 96 hours. In contrast, no distinct nitriding layer in the plasma ion-nitrided Al alloy was observed due to oxide film formation and processing conditions. However, compared to 0 and 96 hours of hydrogen charging time, the ultimate tensile strength and modulus of toughness decreased by 7.54 and 13.32%, respectively, presenting excellent resistance to hydrogen embrittlement.
- KEYWORDs : Hydrogen embrittlement, Aluminum alloy, Hard anodizing, Plasma ion nitiridng, Tensile test
- AUTHORs : 신동호 · 김성종†
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Page : 232 - 241
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- 수소연료전지 자동차의 수소밸브용 알루미늄 합금의 수소취화에 의한 기계적 특성에 미치는 CrN과 TiN 코팅의 영향
- The mechanical properties of the hydrogen valve responsible for supplying and blocking hydrogen gas in a hydrogen fuel cell electric vehicle (FCEV) were researched. Mechanical properties by hydrogen embrittlement were investigated by coating chromium nitride (CrN) and titanium nitride (TiN) on aluminum alloy by arc ion plating method. The coating layer was deposited to a thickness of about 2 ?m, and a slow strain rate test (SSRT) was conducted after hydrogen embrittlement to determine the hydrogen embrittlement resistance of the CrN and TiN coating layers. The CrN-coated specimen presented little decrease in mechanical properties until 12 hours of hydrogen charging due to its excellent resistance to hydrogen permeation. However, both the CrN and TiN-coated specimens exhibited deterioration in mechanical properties due to the peeling of the coating layer after 24 hours of hydrogen charging. The specimens coated at 350oC presented a significant decrease in ultimate tensile strength due to abnormal grain growth.
- KEYWORDs : Hydrogen embrittlement, Aluminum alloy, CrN, TiN, Arc ion plating
- AUTHORs : 허호성 · 신동호 · 김성종†
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Page : 242 - 251
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- 고분자전해질 연료전지 분리판용 304 스테인리스 강재의 유기습식및 건식코팅에 따른 내식성 비교연구
- The electrochemical corrosion behaviors of 304 stainless steels (STSs) with various coatings (organic coating and dry coating) were examined, and their applicability as bipolar plates in polymer electrolyte membrane fuel cells (PEMFCs) was validated. The results showed that the organic-coated samples had a significant decrease in anodic and cathodic current density compared to the uncoated sample. However, an increase in carbon black content in the organic coating or additional heat treatment at 700o C resulted in a decrease in corrosion resistance. In addition, improvements in corrosion resistance achieved by adding TiO 2 powder to the organic coating were found to be limited. In contrast, dry coating with TiC and CrC exhibited higher corrosion potential, significantly lower current density, and reduced contact resistance compared to the organic coatings. Notably, the TiC-coated sample showed a comparatively lower current density and more stable behavior than the CrC-coated sample. Based on a series of experimental results, a thin TiC coating without defects is proposed as a promising surface treatment strategy for STS bipolar plates in PEMFC.
- KEYWORDs : Bipolar plates, Stainless steel, Corrosion, Organic coating, PVD
- AUTHORs : 김용현 · 박진성 · 김성진†
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Page : 252 - 256
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- 모사 슬러리 용액에서 정전류 분극을 활용한 고크롬 주철 부식 시험
- The galvanostatic polarization technique was used to accelerate corrosion in high chromium cast iron (HCCI) immersed in a simulated slurry solution of 0.1 mol dm -3 H2SO4, 0.05 mol dm -3HCl, and 10 wt% SiC. The HCCI contained 27 wt% of Cr and 2.8 wt% of C,and its microstructure mainly comprised austenitic and carbide phases. A two-electrode system using a dense carbon rod and the HCCI sample was employed for the galvanostatic polarization by applying an anodic current for 24 hours. The corrosion rate increased upon applying the anodic current, but the increase was not significant, particularly for current densities higher than 10 ?Acm-2. Following polarization, the corrosion morphology revealed that the anodic current accelerated surface corrosion in the HCCI; however while the depth of the corroded area increased, the increase was not substantial. The propagation behavior of the anodic current and its impact on corrosion were further discussed.
- KEYWORDs : High chromium cast iron, Corrosion, Acidic solution, Galvanostat
- AUTHORs : 바산자프 오취개럴 · 이준섭† · 이예진 · 오준석 · 이재현
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