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Page : 246 - 254
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- 유연운전에 따른 석탄화력보일러 수계통 튜브에서의 이상 유동가속부식 (Two-Phase Flow Accelerated Corrosion) 고찰
- Recently, coal-fired power plants are experiencing many problems that they have never experienced before due to an increase in flexible operation. In particular, a two-phase flow accelerated corrosion on water wall tubes in a boiler has not been detected overseas or domestically. There is no response plan to deal with such corrosion problem either. However, oxide film damage and tube material corrosion due to a two-phase flow accelerated corrosion are being discovered on water wall boiler tubes of domestic coal-fired power plants recently. If this situation is severe, it can cause enormous damage such as tube rupture. Therefore, in this paper, in order to prepare a response plan for a two-phase flow accelerated corrosion on water
wall tubes in the future, differences between a two-phase flow accelerated corrosion and a single-phase flow accelerated corrosion were investigated and an example of discovery of a two-phase flow accelerated corrosion on water wall tubes was presented. - KEYWORDs : Two phase flow accelerated corrosion, Flexible operation, Water wall tube, Oxide film, Power plant
- AUTHORs : 김상호† · 이승민 · 이재홍
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Page : 255 - 265
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- Effect of Seawater Concentration on Electrochemical Corrosion of Duplex Stainless Steel
- Duplex stainless steels (UNS S32205, UNS S32750) are used in various environments. The potentiodynamic polarization tests were conducted at 30 oC in order to study the electrochemical corrosion behaviors of duplex stainless steels under different seawater concentrations (fresh water, seawater, mixed water). The results of Tafel analysis in seawater showed that UNS S32205 and UNS S32750 had the highest corrosion current densities at 6.12 × 10 -4 mA/cm 2 and 5.41 × 10 -4 mA/cm 2 , respectively. The pitting potentials of UNS S32205 and UNS S32750 were comparable to or higher than the oxygen evolution potential in fresh water, mixed water, and seawater. The maximum damage depths and surface damage ratio caused by pit-
ting corrosion increased with chloride concentration. The synergy effect of molybdenum and nitrogen enhances the concentration of Mo, Ni, and Cr at the interface of the metal-electrolyte. In particular, in the +NH4 case of nitrogen, NH 3 and are formed to compensate for the pH drop in the pitting region, thereby strengthening the repassivation of the film. The excellent corrosion resistance of UNS S32750 is attributed to the strengthening effect of the chromium oxide film due to the presence of molybdenum and nitrogen. - KEYWORDs : Duplex stainless steel, UNS S32205, UNS S32750, Seawater concentration, Electrochemical corrosion
- AUTHORs : Ho-Seong Heo, Hyun-Kyu Hwang, Dong-Ho Shin, and Seong-Jong Kim†
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Page : 266 - 277
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- Improvement of Chloride Induced Stress Corrosion Cracking Resistance of Welded 304L Stainless Steel by Ultrasonic Shot Peening
- Due to its good corrosion and heat resistance with excellent mechanical properties, 304L stainless steel is commonly used in the fabrication of spent nuclear fuel dry storage canisters. However, welds are sensitive to stress corrosion cracking (SCC) due to residual stress generation. Although SCC resistance can be improved by stress relieving the weld and changing the chloride environment, it is difficult to change corrosion environment for certain applications. Stress control in the weld can improve SCC resistance. Ultrasonic shot peening (USP) needs further research as compressive residual stresses and microstructure changes due to plastic deformation may play a role in improving SCC resistance. In this study, 304L stainless steel was welded to generate residual stresses and exposed to a chloride environment after USP treatment to improve SCC properties. Effects of USP on SCC resistance and crack growth of specimens with compressive residual stresses generated more than 1 mm from the surface were studied. In addition, correlations of compressive residual stress, grain size, intergranular corrosion properties, and pitting potential with crack propagation rate were determined and the improvement of SCC properties by USP was analyzed.
- KEYWORDs : Stainless steel, USP, GTAW, SCC, Microstructure
- AUTHORs : Hyunhak Cho, Young Ran Yoo, and Young Sik Kim†
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Page : 278 - 282
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- Effect of Galvanizing Furnace Temperature on Material Property and Galvanized Surface of Hot Rolled Galvanized Steel
- Recently, hot rolled galvanized steel is widely used in automotive parts. As the paradigm of the automotive market has changed from fossil fuel vehicle to electric vehicle, the automotive industry needs more high-strength steels to reduce weights of automobiles. However, because high-strength steel contains high solute carbon, it is expected to have a risk of stretcher-strain on the surface due to dislocation trapping by solute [C] and [N]. Generally, galvanized steel is supposed to pass through a furnace around the temperature of Zinc pot to increase material temperature. Otherwise, the inhibition layer could not be formed. However, solute carbon and nitrogen are volatile enough to move around the furnace temperature. Moreover, the ratio of ferrite phase and precipitated Fe3C can be variable, resulting in yield point elongation related to the stretcher strain. Furthermore, the quality of the galvanized surface can be affected by a high temperature of the furnace. Although a relatively hot rolled galvanizing line furnace has a lower temperature than an annealing line furnace, it can affect various quality aspects. In other words, this paper aims to determine how these phenomena appear concerning furnace temperature.
- KEYWORDs : Galvanizing furnace, Yield point elongation, Galvanized surface quality
- AUTHORs : Jong Chan Jeong†, Jae Joong Kim, and Seong Ho Han
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Page : 289 - 295
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- Characteristics of Hot-Dip Znmgal Coatings with Ultra-High Corrosion Resistance
- Zn-Mg-Al alloy hot-dip galvanized steel sheet has high corrosion resistance. Compared to conventional Zn coating with the same coating thickness, the high corrosion resistance Zn-Mg-Al coating is more corrosionresistant. Various coating compositions are commercially produced and applied in diverse fields. However, these steel sheets typically contain up to 3 wt% magnesium. In recent years, there has been a growing demand for higher corrosion resistance in harsh corrosive environments. Therefore, variations in Mg and Al contents were investigated while evaluating primary properties and performance. As a result, we developed new alloycoated steel with ultra-high corrosion resistance. A Zn-5 wt%Mg-Al coated steel sheet was evaluated for its corrosion resistance and various properties. As the amount of Mg added increased, the corrosion loss tended to decrease. The corrosion resistance of the coated steel sheet in a particular composition, the Zn-5 wt%MgAl coating sheet, was about 1.5 to 2 times higher than that of the conventional Zn-3 wt%Mg-Al coating sheet. Ultimately, this ultra-high corrosion-resistance coated steel sheet will provide a robust solution to conserve Zn resources and contribute to a low-carbon society.
- KEYWORDs : Zn-Mg-Al coating, Ultra-high corrosion resistance
- AUTHORs : Sungjoo Kim†, Seulgi So, Jongwon Park, Taechul Kim, Sangtae Han, Suwon Park, Heung-yun Kim, Myungsoo Kim, and Doojin Paik
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Page : 296 - 301
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- Innovative Development of Al-Zn-Si Coated Sheet Steels for Automotive Applications
- Steels have excellent mechanical properties and weldability. They are also economically producible. Thus, they are widely applied in various industries. However, they have a disadvantage in that rust can occur after a certain period of time. To compensate for this, Zn, which has excellent sacrificial corrosion resistance, can be coated on steels. With global zinc consumption increasing at the current rate, depletion is expected in the near future. Recently, POSCO has developed innovative Al-Zn-Si alloy coated steel sheets with better corrosion resistance than Zn coating. In this study, corrosion resistance, weldability, friction characteristics, and so on were evaluated compared to GI steel sheets to evaluate their applicability to automotive steel sheets. It showed excellent corrosion resistance even at a lower coating weight compared to GI steel sheet. It was also excellent in terms of galling and welding LME. Its spot welding life, electrodeposition coating, and bendability were equally excellent. This is presumed to be related to the formation of the Al-Zn-Si alloy phase at the interface of the coating layer.
- KEYWORDs : Al-Zn-Si coating, Corrosion resistance, Bendability, Spot welding, Galling
- AUTHORs : Jong-Sang Kim† , Suk-Kyu Lee, and Doo-Jin Paik
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Page : 302 - 309
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- Ni-Flash 코팅이 초고강도 전기아연 도금강재의 수소취화 및 액상금속취화에 미치는 영향
- The purpose of this study was to elucidate effects of a thin (tens to hundreds of nanometers) Ni-flash coating layer on hydrogen embrittlement (HE) and liquid metal embrittlement (LME) in ultra-high-strength electrogalvanized steel with a tensile strength of more than 1 GPa. Various experimental and analytical methods, including thermal desorption spectroscopy, slow strain rate testing, resistance spot welding, Xray diffraction, and metallographic observation, were employed. Results showed that an increase in Ni target amount for flash coating resulted in a decrease in diffusible hydrogen content during electrogalvanizing, resulting in a significant decrease in HE sensitivity. Moreover, a Ni target amount of more than 1000 mg/m 2 drastically reduced the occurring frequency and average depth of LME. This reduction could be primarily attributed to formation of Zn-Ni intermetallic phases during the welding process that could inhibit liquefaction of intermetallic phases in the heat-affected zone. This study provides a desirable Ni target amount for Ni-flash coating on ultra-high-strength steels conducted in a continuous galvanizing line or a high-speed batch line to achieve high resistance to both HE and LME.
- KEYWORDs : Ni-flash coating, Electro-galvanizing, Ultra-high-strength steel, Hydrogen embrittlement, Liquid metal embrittlement
- AUTHORs : 오선호·박진성·김성진†
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Page : 283 - 288
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- Effect of Heat Treatment on Corrosion Resistance of Zn-Mg-Al Alloy Coated Steel
- Hot-dip Zn-Mg-Al coatings have a complex microstructure consisting of Zn, Al, and MgZn 2 phases. Its crystal structure depends on alloy content and cooling rates. Microstructure and corrosion resistance of these coatings might be affected by heat treatment. To investigate effect of heat treatment on microstructure and corrosion resistance of Zn-Mg-Al coatings, Zn-1.5%Mg-1.5%Al coated steel was heated up to 550 oC at a heating rate of 80 oC/s and cooled down to room temperature. At above 500 oC, the ter-nary phase of Zn-MgZn 2 -Al was melted down. Only Zn and MgZn 2 phases remained in the coating. Heat- and non-heat-treated specimens showed similar corrosion resistance in Salt Spray Test (SST). When a Zn-3.0%Mg-2.5%Al coated steel was subjected to heat treatment at 100 oC or 300 oC for 200 h and compared with GA and GI coated steels, the microstructure of coatings was not significantly changed at 100 oC. However, at 300 oC, most Al in the coating reacted with Fe in the substrate, forming a Fe-Al compound layer in the lower part of the coating. MgZn 2 was preferentially formed in the upper part of the coating. As a result of SST, Zn-Mg-Al coated steels showed excellent corrosion resistance, better than GA and GI.
- KEYWORDs : Zn-Mg-Al coating, Heat treatment, Corrosion, Phase transformation, MgZn2
- AUTHORs : Il Ryoung Sohn†, Tae Chul Kim, Sung Ju Kim, Myung Soo Kim, Jong Sang Kim, Woo Jin Lim, Seong Mo Bae, Su Hee Shin, and Doo Jin Paik
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Page : 310 - 314
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- 산성 용액에서 전기화학적 특성과 침지 시간에 따른 고크롬 백주철의 부식 거동
- The corrosion behavior of high chromium white cast iron was studied in 0.5 mol dm -3H2SO4 + 0.01 mol dm-3 HCl solution over time through electrochemical and immersion experiments. Potentiodynamic and potentiostatic polarizations revealed active-passivation transition behavior, with critical current densities observed at -0.27 V SSE and 0.00 V SSE , repectively. The former potential showed preferential dissolution of primary γ phases, while the latter one showed preferential dissolution of eutectic γ phases. Immersion tests showed an exponential increase in corrosion rate, with significant acceleration observed around 1000 seconds due to the onset of eutectic γ phase dissolution. Over a 24-hour immersion period, both γ phases exhibited extensive corrosion, leaving carbides largely intact. These findings elucidate distinct corrosion behaviors of high chromium white cast iron in acidic environments, providing critical insights into material performance evaluation. Understanding these mechanisms is essential for predicting the longevity and durability of materials in corrosive conditions, thereby informing better material design and application strategies.
- KEYWORDs : High chromium white cast iron, Corrosion, Acidic solution
- AUTHORs : 이준섭†·바산자프 오취개럴·오준석·이재현
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Page : 324 - 333
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- 상대 습도, 염화물 누적률, 표면 입자를 고려한 탄소강의 대기부식 모델
- Atmospheric corrosion poses a significant threat to durability of metallic materials and safety of structures, making precise prediction of corrosion rates crucial in industrial and engineering settings. Understanding the exact rate of corrosion is essential. However, accurate inclusion of various environmental factors that can influence atmospheric corrosion in the calculation of corrosion rate is a complex challenge. This study introduces a physics-based model that incorporates electrochemical methods and considers active surface area affected by surface contaminants to estimate atmospheric corrosion rate of carbon steel. The model can evaluate corrosion levels using key factors such as chloride deposition rate, relative humidity, and the presence of surface particles. By integrating these considerations, this model moves beyond empirical estimations, providing a more stable prediction of corrosion rate that is less susceptible to environmental variations. This model provides a robust tool for defense applications, offering precise insights into the dynamics of atmospheric corrosion that could enhance the maintenance and safety of weapon systems.
- KEYWORDs : Atmospheric corrosion, Corrosion prediction, Physics-based model, Active area
- AUTHORs : 신진수·권혁준·김홍석·이두열†
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