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Fe-Al-Cr계 합금의 부식거동에 미치는 Al 및 Cr 합금원소의 영향
최한철(Han-Cheol Choe) 한국표면공학회 2005 한국표면공학회지 Vol.38 No.6
Effects of Al and Cr alloying elements on the corrosion behavior of Fe-Al-Cr alloy system was investigated using potentiodynamic and cyclic potentiodynamic polarization tests(CPPT) in the H₂SO₄ and HCI solutions. The corrosion morphologies in Fe-Al-Cr alloy were analysed by utilizing scanning electron microscopy(SEM) and EDX. It was found that the corrosion potential of Fe-20Cr-20Al was highest whereas the critical anodic current density and passive current density were lower than that of the other alloys in 0.1 M H₂SO₄ solution. The second anodic peak at 1000 ㎷ disappeared in the case of alloys containing high Al and low Cr contents. Pitting potential increased with increasing Cr content and repassivation potential decreased with decreasing AI content in 0.1 M HCI solution. Fe-Al-Cr alloy containing high Al and Cr contents showed remarkably improved pitting resistance against Cl? attack from pit morphologies.
최한철(Han-Cheol Choe) 한국표면공학회 2005 한국표면공학회지 Vol.38 No.6
Electrochemical characteristics of welded stainless steels containing Ti have been studied by using the electrochemical techniques in 0.5 M H₂SO₄ + 0.01 M KSCN solutions at 25℃. Stainless steels with 12 ㎜ thickness containing 0.2-0.9 wt% Ti were fabricated with vacuum melting and following rolling process. The stainless steels were solutionized for 1 hr at 1050℃ and welded by MIG method. Samples were individually prepared with welded zone, heat affected zone, and matrix for intergranular corrosion and pitting test. Optical microscope, XRD and SEM are used for analysing microstructure, surface and corrosion morphology of the stainless steels. The welded zone of the stainless steel with lower Ti content have shown dendrite structure mixed with γ and δ phase. The Cr-carbides were precipitated at twin and grain boundary in heat affected zone of the steel and also the matrix had the typical solutionized structure. The result of electrochemical measurements showed that the corrosion potential of welded stainless steel were increased with higher Ti content. On the other hand, reactivation(Ir), passivation and active current(Ia) density were decreased with higher Ti content. In the case of lower Ti content, the corrosion attack of welded stainless steel was remarkably occurred along intergranular boundary and γ/δ phase boundary in heat affected zone.
최한철(Han-Cheol Choe),이호종(Ho-Jong Lee),정용운(Yong-Woon Jeong) 한국표면공학회 2005 한국표면공학회지 Vol.38 No.3
SKD11 steel has been widely used for tools, metallic mold and die for press working because of its favorable mechanical properties such as high toughness and creep strength as well as excellent oxidation resistance. The ion nitrided tool steel containing Mo results in improvement of corrosion resistance, strength at high temperature and pitting resistance, especially in Cl? contained environment. But the Mo addition causes a disadvantage such as lower oxidation resistance at elevated temperature. In this study, several effects of ion-nitriding on the oxdation characteristics for SKD11 steel with various oxidation temperature were investigated. SKD11 steels were manufactured by using vacuum furnace and solutionized for 1 hr at 0,050℃, Steel surface was ion nitrided at 500℃ for 1 hr and S hr by ion nitriding equipment. Ion nitrided specimen were investigated by SEM, OM and hardness tester. Oxidation was carried out by using muffle furnace in air at 500℃, 700℃ and 900℃ for 1 hr, respectively. Oxidation behavior of the ion nitrided specimen was investigated by SEM, EDX and surface roughness tester. The conclusions of this study are as follows: It was found that plasma nitriding for 5 hr at 500℃, compared with ion nitriding for 1 hr at 500℃, had a thick nitrided layer and produced a layer with good wear, corrosion resistance and hardness as nitriding time increased. Nitrided SKD11 alloy for 1 hr showed that wear resistance and hardness decreased, whereas surface roughness increased, compared with nitrided SKD11 alloy for 5 hr. The oxidation surface at 900℃ showed a good corrosion resistance.
최한철(Han-Cheol Choe),고영무(Yeong-Mu Ko) 한국표면공학회 2004 한국표면공학회지 Vol.37 No.5
The defects of partial denture frameworks are mainly shrinkage porosity, inclusions, micro-crack, particles from investment, and dendritic structure. In order to investigate a good casting condition of partial denture frameworks, the three casting alloys and casting methods were used and detected casting defects were analyzed by using electrochemical methods. Three casting alloys (63Co-27Cr-5.5Mo, 63Ni-16Cr, 63Co-30Cr-5Mo) were prepared for fabricating partial denture frameworks with various casting methods; centrifugal casting (Kerr, USA), high frequency induction casting (Jelenko Eagle, USA), vacuum pressure casting (Bego, Germany). The casting temperature was 1,380℃ (63Co-27Cr-5.5Mo and 63Ni-16Cr) and 1,420℃ (63Co-30Cr-5Mo). The casting morphologies were analyzed using FE-SEM and EDX. The corrosion test of the dendritic structure was performed through potentiodynamic method in 0.9% NaCI solutions at 36.5℃ and corrosion surface was observed using SEM. The defects of partial denture frameworks improved in the order of centrifugal casting, high frequency induction casting, and vacuum pressure casting method, especially, pore defects were found at part of clasp in the case of centrifugal casting method. The structure of casting showed dendritic structure for three casting alloys. In the 63Co-27Cr-5.5Mo and 63Co-30Cr-5Mo, α-Co and ε-Co phases were identified at matrix and γ-Ni₂Cr second phase were shown in 63Ni-16Cr. Also, the corrosion resistance of cast structure increased in the order of vacuum pressure casting, high frequency induction casting, and centrifugal casting method.
Sol-Gel법으로 HA코팅된 치과용 임플란트 합금의 표면특성
최한철(han-Cheol Choe),고영무(Yeong-Mu Ko) 한국표면공학회 2005 한국표면공학회지 Vol.38 No.4
Surface characteristics of HA(hydroxyapatite) coated dental implant alloy by Sol-Gel method were investigated using potentiostat, ICP, SEM, EDX, EPMA and surface roughness tester. Surface roughness of HA coated specimen by Sol-Gel showed higher than that of PVD coated specimen. Corrosion resistance increased in the order of 1% lactic acid, artificial saliva, 0.5% HCI and 0.9% NaCI solution. Amount of Ca element release was higher than that of V and P in the 0.5% HCI and 0.9% NaCI solution.
임상에서 사용된 치과용 Ni-Ti합금 파일의 피로파괴현상
최한철 ( Han Cheol Choe ),배성철 ( Seong Chul Bae ),고영무 ( Yeong Mu Ko ),황호길 ( Ho Keel Hwang ) 대한금속재료학회 ( 구 대한금속학회 ) 2006 대한금속·재료학회지 Vol.44 No.6
The purpose of this study was to investigate the fatigue fracture phenomena of clinically used dental Ni-Ti alloy files. ProTaper(R)(Maillfer, Dentsply, Ballaigues, Switzerland) were used, respectively, for experiment. In order to observe the fatigue surface of clinically used file, the fractured files were classified into three groups: fractured files at the upper, middle and lower part after clinical use. The fractured surface and chemical composition were studied with energy dispersive X-ray spectroscopy(EDS) and field emission scanning electron microscope(FE-SEM) to analyze elements on the surface of fractured file. Fatigue fracture crack of Ni-Ti file was initiated and propagated at defects such as scratch formed by machine under repeated load. Fractured surface of Ni-Ti file showed the brittle fracture behavior at the exterior and ductile fracture behavior at the interior of specimen. The ductile fracture behavior appeared predominantly at the lower part of Ki-Ti file compared with at the upper part of Ni-Ti file.
최한철 ( Han Cheol Choe ),홍국선 ( Kook Sun Hong ),고영무 ( Yeong Mu Ko ),김병옥 ( Byung Ock Kim ) 대한금속재료학회 ( 구 대한금속학회 ) 2006 대한금속·재료학회지 Vol.44 No.6
The aim of this investigation was to analyze surface of clinically failed dental implant fixture in order to determine possible causes of failure. The failed implants were classified into three groups; 3 and 5 month used implant, 19 and 60 month used implant, and fractured implant, after implantation for 33 month and 120 month in patient jaws. The topography and surface chemical composition were studied with energy dispersive x-ray spectroscopy(EDS) and field emission scanning electron microscope(FE-SEM) to analyze elements on the surface of flank and bottom of thread. Evaluations were performed at the upper, middle and lower implant fixture. From surface observation of 3 and 5 month used implant, contaminants were detected in the middle and lower part of implants. Overall, foreign elements such as silicon, carbon, oxygen, zinc, sulfur, zirconium, and chloride were detected in failed implants. From surface observation of 19 and 60 month used implant, the corrosion was accelerated at the bottom of thread that was accumulated chloride. Corrosion products showed in the bottom of thread including dissolution elements, such as Au, Cu, Ag, from suprastructure of dental implant system. From surface observation of fatigue fractured implants, the crack were nucleated in the bottom of thread and the crack of corrosion fatigue was predominantly propagated. The corrosion products including chloride were found opposite side of starting point of corrosion fatigue crack. We could find the fatigue striation and estimate the dimple fracture of implants.