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Page : 49 - 54
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- Stress Corrosion Cracking of Heat Exchanger Tubes in District Heating System
- The purpose of this paper is to present failure analysis, of the heat exchanger tube in a district heating system. SS304 stainless steel is used, as material for the heat exchanger tube. The heat exchanger operates in a soft water environment containing a small amount of chloride ions, and regularly repeats operation and standstill period. This causes concentration of chloride ions on the outer surface of the tube, as well as repeat of thermal expansion, and shrinkage of the tube. As a result of microscopic examination, cracks showed transgranular as well as branched propagation, and many pits were present, at the initiation point of each crack. Energy disperstive spectroscopy analysis showed Fe and O peak, as well as Cl peak, meaning that cracks were affected by Cl ion. Failure of the tube was caused by chloride-induced stress corrosion cracking by thermal stress, high temperature, and localized enrichment of chloride ions.
- KEYWORDs : Pitting, Heat exchanger tube, Stress corrosion cracking, Corrosion fatigue cracking, Austenitic stainless steel
- AUTHORs : Sangwon Cho, Seon-Hong Kim, Woo-Cheol Kim, and Jung-Gu Kim†
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Page : 55 - 60
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- 지역난방 첨두부하보일러 대류부 부식 파손 분석
- Corrosion failure in the convection part of peak load boiler (PLB) of the district heating system led to water leakage. Herein, Internal Rotary Inspection System (IRIS) inspection was employed to examine wall thinning and the cause of leakage in the flue tube. The corrosive products of the turbulator and tube were investigated using scanning electron microscope combined with energy dispersive spectroscopy, X-ray
diffraction, and inductively coupled plasma (ICP). Majority of the serious corrosion damage was observed near the turbulator located in the upper flue tube. ICP analysis of the boiler water revealed oxide formation of sodium chloride in the lower end part of the flue tube. A cross-sectional view of the turbulator revealed
the presence of double-layers of the oxide film, indicating environmental change during operation associated with water leakage. The outer surface of the turbulator consisted of the acid oxides such as NOx and SOx along with sodium and chloride ions. Dew-point corrosion is hypothesized as the main cause for the formation
of acid oxides in the region of contact of the flue tube and the turbulator. - KEYWORDs : District heating system, PLB convection, Dew point corrosion, Turbulator, IRIS inspection
- AUTHORs : 김영수·채호병·홍민기·송민지·조정민·김우철·하태백·이수열†
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Page : 61 - 71
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- 기계학습을 이용한 유동가속부식 모델링: 랜덤 포레스트와 비선형 회귀분석과의 비교
- Flow-Accelerated Corrosion (FAC) is a phenomenon in which a protective coating on a metal surface is dissolved by a flow of fluid in a metal pipe, leading to continuous wall-thinning. Recently, many countries have developed computer codes to manage FAC in power plants, and the FAC prediction model in these computer codes plays an important role in predictive performance. Herein, the FAC prediction model was developed by applying a machine learning method and the conventional nonlinear regression method. The random forest, a widely used machine learning technique in predictive modeling led to easy calculation of FAC tendency for five input variables: flow rate, temperature, pH, Cr content, and dissolved oxygen concentration. However, the model showed significant errors in some input
conditions, and it was difficult to obtain proper regression results without using additional data points. In contrast, nonlinear regression analysis predicted robust estimation even with relatively insufficient data by assuming an empirical equation and the model showed better predictive power when the interaction between DO and pH was considered. The comparative analysis of this study is believed to provide
important insights for developing a more sophisticated FAC prediction model. - KEYWORDs : FAC, Statistical modeling, Machine learning, Random forest, Non-linear regression
- AUTHORs : 이경근†·이은희·김성우·김경모·김동진
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Page : 73 - 77
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- Micro-scale Observation of Corrosion of Hot-Dip Aluminized 11% Cr Stainless Steel
- Hot-dip aluminized coating has been widely used to protect steel substrate against corrosion. In this study, the corrosion behavior of hot-dip aluminized type 409L (11% Cr) stainless steel (SS) was investigated using macro- and micro-scale polarization tests. An Al-Fe-Si alloy layer that was formed due to inter-diffusion
of alloying elements between Al coating and SS substrate was observed between Al coating and 409L SS substrate. In both macro- and micro-scale polarization tests, the corrosion potential (Ecorr) of the 409L SS substrate was much nobler than that of the Al coating and alloy layer. Ecorr of the alloy layer was between that of Al coating and 409L SS substrate. This indicates that the alloy layer can act as a buffer between the more active Al coating and the nobler SS substrate for pit growth in aluminized SS. The presence of the alloy layer appears to be helpful in hindering pitting corrosion of aluminized SS. - KEYWORDs : Aluminized stainless steel, Alloy layer, Micro-scale polarization, Pitting corrosion
- AUTHORs : Min-Seung Cho, Choong-Nyeon Park, and Chan-Jin Park†
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Page : 78 - 85
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- Corrosion Behavior of Carbon Steel in Diluted Sulfuric Acid based on Seawater
- The International Maritime Organization (IMO) will administer a new 0.5% global sulfur cap on fuel content from 1 January 2020, lowering from the present 3.5% limit. Seawater SOx (sulfur oxide) scrubbing is especially spray scrubbing and a promising alternative to complying with the IMO regulation. However, the ionization
of SO2 (sulfur dioxide) and the H2SO4 (sulfuric acid) formed from SO3 (sulfur trioxide) is proposed to accelerate corrosion of the internal seawater pipe. Apparently, the corrosion of the scrubber seawater piping system occurs in a severe and frequent manner. Hence, in this study, electrochemical measurement and
weight loss of carbon steel (used as seawater pipe in most of the ships) in diluted sulfuric acid solution were investigated to determine corrosion rate, corrosion current density, corrosion potential, electrochemical behavior, and impressed-current density. Accordingly, the corrosion rate of carbon steel sheet in various
diluted sulfuric acid solutions was observed to be greater than that in natural seawater, thus suggesting the fundamental data to deal with corrosion problems in scrubber seawater pipe. - KEYWORDs : Corrosion, Electrochemical polarization, Sulfuric acid, Seawater, SOx Scrubber
- AUTHORs : Mun Su Kim and Jin-A Jeong†
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Page : 86 - 91
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- Accelerated Prediction Methodologies to Predict the Outdoor Exposure Lifespan of Galvannealed Steel
- Generally, atmospheric corrosion is the electrochemical degradation of metal that can be caused by various corrosion factors of atmospheric components and weather, as well as air pollutants. Specifically, moisture and particles of sea salt and sulfur dioxide are major factors in atmospheric corrosion. Using galvanized
steel is one of the most efficient ways to protect iron from corrosion by zinc plating on the surface of the iron. Galvanized steel is widely used in automobiles, building structures, roofing, and other industrial structures due to their high corrosion resistance relative to iron. The atmospheric corrosion of galvanized steel shows complex corrosion behavior, depending on the plating, coating thickness, atmospheric environment, and air pollutants. In addition, corrosion products are produced in different types of environments. The lifespans of galvanized steels may vary depending on the use environment. Therefore, this study investigated the corrosion behavior of galvannealed steel under atmospheric corrosion in two locations in Korea, and the lifespan prediction of galvannealed steel in rural and coastal environments was conducted by means of the potentiostatic dissolution test and the chemical cyclic corrosion test. - KEYWORDs : Galvannealed steel, Outdoor exposure test, Atmospheric corrosion, Lifespan prediction
- AUTHORs : Ki Tae Kim, Young Ran Yoo, and Young Sik Kim†
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Page : 92 - 101
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- Critical Influence of Rivet Head Height on Corrosion Performance of CFRP/Aluminum Self-Piercing Riveted Joints
- This study investigates how rivet head height affects the corrosion performance of carbon fiber reinforced plastic (CFRP) to aluminum alloy self-piercing riveted joints. Specimens with two different head heights were prepared. A rivet head protruding out of the top CFRP laminate forms the proud head height while a rivet head penetrating into the top CFRP generates the flush head height. The salt spray test evaluated corrosion performance. The flush head joints suffered from severe corrosion on the rivet head. Thus, the tensile shear load of flush head joints was substantially reduced. Electrochemical corrosion tests investigated the corrosion mechanisms. The deeper indentation of the flush head height damaged the CFRP around the rivet head. The exposure of damaged fibers from the matrix increased the cathodic potential of local CFRP. The increased potential of damaged CFRP accelerated the galvanic corrosion of the rivet head. After the rivet head coating material corroded, a strong galvanic couple was formed between the rivet head base
metal (boron steel) and the damaged CFRP, further accelerating the flush rivet head corrosion. The results of this study suggest that rivet head flushness should be avoided to enhance the corrosion performance of CFRP to aluminum alloy self-piercing riveted joints. - KEYWORDs : CFRP, Rivet, Proud head, Flush head, Corrosion potential
- AUTHORs : Md Abdul Karim, Jin-Hee Bae, Dong-Hyuck Kam, Cheolhee Kim, and Yeong-Do Park†
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Page : 102 - 109
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- 인장응력과 H2S 부식의 복합조건 하에서 고강도 강재의 수소확산 거동 분석을 위한 Numerical 확산모델과 이론적 고찰
- The hydrogen diffusion and trapping model with a numerical finite difference method (FDM) was modified and extended to accommodate H2S corrosion and scale forming processes of high-strength steel under tensile stress condition. The newly proposed diffusion model makes it possible to clearly understand combined effect of tensile stress and H2S corrosion process on hydrogen diffusion behaviors. The core concept of this theoretical approach is that overall diffusion behavior is separated into diffusion process through two respective layers: an outer sulfide scale and an inner steel matrix. Diffusion coefficient values determined by curve-fitting permeation data reported previously with the newly proposed diffusion model indicate that the application of tensile stress can contribute to continual increase in the diffusivity in the sulfide scale with a high density of defect. This suggests that the scale with a lower stability under the stress condition can be a key parameter to enhance hydrogen influx in the steel matrix. Consequently, resistance to hydrogen assisted cracking of the steel under tensile stress can be decreased significantly.
- KEYWORDs : Steel, Hydrogen diffusion, Hydrogen sulfide, Tensile stress, Finite difference method
- AUTHORs : 김성진†
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Page : 110 - 116
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- NaCl과 NaBr수용액에서 적층가공으로 제조된 Ti-6Al-4V 합금의 공식 저항성 비교
- Resistances to pitting corrosion of additive manufactured (AM) Ti-6Al-4V alloys in 0.6 M NaBr and 0.6 M NaCl aqueous solutions were compared using micro-droplet cell techniques. With respect to the pitting corrosion resistance, this study focused on two different types of halide anions in aqueous solutions, i.e. Br- and Cl-. The differences between Br- and Cl- halide anions for breakdown on passive films of AM
Ti-6Al-4V alloy were explained using Langmuir adsorption model with their equilibrium adsorption coefficients. The results of the analysis showed that the lower resistance to pitting potential of AM Ti-6Al-4V alloy in Br- aqueous solution was attributed to the higher equilibrium adsorption coefficient of Br-. In addition,
micro-electrochemical test results showed that the pitting corrosion resistance of dark grains in additive manufactured Ti-6Al-4V alloy was lower as compared to that of bright grains due to the larger volume of α\' phase that caused the susceptibility to pit initiation. - KEYWORDs : Pitting corrosion, Additive manufacturing, Halide anion, Polarization, Langmuir adsorption model
- AUTHORs : 서동일·이재봉†
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Page : 117 - 120
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- A Research on Stray-Current Corrosion Mechanism of High Voltage Cable Connector on Electrification Vehicles
- Considering the tendency of development of electrification vehicles, development and verification of new evaluation technology is needed because of new technology applications. Recently, as the battery package is set outdoors of an electric vehicle, such vehicles are exposed to corrosive environments. Among major components connected to the battery package, rust prevention of high-voltage cables and connectors is considered the most important issue. For example, if corrosion of high voltage cable connectors occurs, the corrosion durability assessment of using an electric vehicle will be different from general environmental corrosion phenomena. The purpose of this study is to investigate the corrosion mechanism of high voltage cable connectors of an electric vehicle under various driving environments (road surface vibration, corrosion environment, current conduction by stray current, etc.) and develop an optimal rust prevention solution. To improve our parts test method, we have proposed a realistic test method to reproduce actual electric vehicle corrosion issues based on the principle test.
- KEYWORDs : Stray current corrosion, High voltage cable, Electrification vehicle, Fretting
- AUTHORs : Hwi Yong Lee, Seung Ho Ahn†, and Hyun Taek Im
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