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합금원소 첨가가 구조용 강의 해변 내후성에 미치는 영향 연구(Ⅱ) : Ni, W, Mo의 첨가 효과 Ni, W and Mo Effects
정환교,유장용,김규영 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.12
The objective of this study is to understand the effect of W and Mo on the corrosion property of the Ca-modified weathering steel and to determine the optimum chemical composition to maximize the corrosion resistance at seaside corrosion environment. The effects of various alloying elements on the weathering steel were evaluated through a series of electrochemical test and analysis of the corrosion product formed on the steel surface. The polarization resistance of Ca-modified weathering steel can be increased by the addition of Ni, W and Mo, thus the corrosion resistance of Ca^(-)modified weathering steel containing Ni, Mo and W is increased at the initial stage of corrosion in a solution with high NaCl concentration. From the measurement on the permeability of Cl^(-) ions, the rust membranes formed from weathering steel of thin plate shape showed the anion-selectivity and the addition of various alloying elements (Ni, W and Mo) contributed to lower the permeability of Cl^(-) ions toward rust membranes. Particularly, Mo and W are more effective than Ni to suppress the permeation of Cl^(-) ions. It has an industrial significance for alloy design of the weathering steel since design of a lower cost weathering steel with higher corrosion resistance can be possible with addition of Mo or W instead of increasing the high cost Ni content to 2% or even higher. (Received May 19, 2004)
조선용 C-Mn 강의 제조방법에 따른 미생물 부식 특성 연구
鄭歡敎,兪張鎔,禹宗秀 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.6
The MIC(Microbiologically Influenced Corrosion) of lower deck steel plates in double hull VLCC(Very Large Crude Oil Carrier) cargo tanks has been focused because of severe localized corrosion. Recently, ship companies have reported that TMCP(Thermomechanical Control Process) steel plates have been showed more severe localized corrosion than conventional rolled steels. In order to elucidate the MIC resistance of TMCP steels by comparison with conventional rolled steels and normalized steels in environments of double hull VLCC cargo oil tanks, various measurements and corrosion tests were performed such as measurements of polarization curves, immersion tests in bacteria culture medium. All results revealed that three types of steels have almost the same corrosion resistance in bacteria culture medium. Three kinds of steel exhibit almost the same polarization behavior and the corrosion rate. The movement of the open-circuit potential first towards more negative values and later to more positive values is a phenomenon common to all kinds of specimens. This phenomenon resulted from an initial stimulation of the anodes by sulphide ions produced by the bacteria from the reduction of sulphate ions in the medium, followed by the formation of an insoluble partly protective film of ferrous sulphide on the surface of the electrode. The effect of SRB (Sulphate Reducing Bacteria) is clearly showed in the cathodic polarization curve. When the SRB is in a condition of rapid growth, there is a strong cathodic depolarization. However, as the sulphate is depleted and reaction products accumulate, the activity of the bacteria declines and the cathodic polarization curve returns the same form as in the inoculated culture.
합금원소 첨가가 구조용 강의 해변 내후성에 미치는 영향 연구 (Ⅰ) : Ca, Si의 첨가 효과 Ca and Si Effects
정환교,유장용,김규영 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.11
The objective of this study is to understand the effect of Si, on the corrosion property of the Ca-modified weathering steel and to determine the proper range of such alloying elements to maximize the effect of Ca on the corrosion resistance of the steel. The pH of a thin water film covering the steel was measured to evaluate the effect Ca addition on pH value. The electrochemical impedance spectroscopy (EIS) was employed to evaluate the corrosion resistance of the Ca-modified weathering steel. The characteristics of rust formed on the weathering steels having different alloy chemistry were examined. Analysis on the inclusion morphology and composition showed that the Ca-containing inclusions were mainly composed of Ca-Al-Mn-O-S,and Si was not founded in them except the specimen containing 0.6%Si. In distilled water, the Ca-modified specimens with or without Si showed higher pH values as Ca content was increased. The effect of Ca addition was not greatly affected by the addition of Si when the steel contained a proper Ca content of 50-70 ppm. In the impedance test, the specimens containing higher amounts of Ca showed higher polarization resistance, meaning better corrosion resistance. The analysis on the rust layers formed on the Ca-modified steel surface showed that the dense inner layer was mainly composed of a-FeOOH and y-FeOOH, whereas the loose outer layer was composed of ν-FeOOH and β-FeOOH. Fe₃O_(4) was distributed randomly throughout the rust layer. The specimens containing a high amount of Ca showed a dense and compact morphology, and the specimens containing Si also showed a dense and compact one. The result of Mossbauer analysis showed that more α-FeOOH was formed as Ca content was increased. (Received May 24, 2004)
Si 첨가가 Fe-14Cr-1A1 페라이트계 스테인리스 합금의 고온 내산화성에 미치는 영향
김규영,정환교,안장규 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.9
A study has been made to investigate the effects of silicon(Si) addition on the oxidation properties of Fe-14Cr-1Al ferritic stainless steels at 1173K. Four steels containing different amount of Si (0.5, 1, 2, 5 wt% Si) were arc melted and homogenized at 1473K for 24 hours and oxidized at 1173K. Both the isothermal and cyclic oxidation tests revealed that the addition of Si increased the oxidation resistance of Fe-14Cr-1Al alloy. With 0.5w%Si, Cr oxide was formed as surface oxide, and Si, Al oxides were formed internally below Cr oxide layer. With 1w%Si, Al oxide was mixed with Cr oxides on the surface. However, with Si content higher than 2w%, a stable Al₂O₃ layer was formed rapidly. From theoretical evaluation using ChemSage^(TM) program, the Al activity in Fe-Al-5Si steel is about 5 times higher than that in Fe-1Al alloy at 1173K, which is the most probable reason for the rapid formation of Al oxide in the high Si steels.
TiAl 의 고온내산화성 향상을 위한 EB-PVD 방법을 이용한 Al+Y 혼합코팅
김규영,정환교,최태완 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.1
A method for "Al+Y codeposition" was developed to improve the resistance of high temperatrue oxidation of TiAl using a single process of EB-PVD. Isothermal and cyclic oxidation tests at 950℃ showed that the Al+Y codeposition improved the oxidation resistance of TiAl significantly. During oxidation, the oxides formed in the Al+y codeposition layer are composed of two parts ; the outer (Y,Al)O layer and the inner Al₂O₃, layers containing small amount of Y-rich oxide. Both the outer (Y,Al)O and inner Al₂O₃ layers have a fine-grained structure which prevents effectively inward diffusion of oxygen and consequently prohibits formation of porous TiO₂. During cyclic oxidation, these (Y,Al)O and Al₂O₃, layers relieve easily both the growth and thermal stresses by permitting easy plastic deformation. However, when exposed to high temperature for an extended period of time, an Al₂O₃, layer having a columnar structure is developed underneath the existing fine-grained Al₂O₃, layer by reaction with Al from the substrate TiAl. This columnar Al₂O₃, is responsible for degradation of the oxide layer.
이온플레이팅 방법으로 코팅된 이트륨이 Ni3Al 의 고온내산화성에 미치는 영향 (2)
김규영,정환교 대한금속재료학회(대한금속학회) 1997 대한금속·재료학회지 Vol.35 No.8
The Y-coated Ni₃Al with post heat treatment shows much better oxidation resistance than the Y-coated Ni₃Al without post heat treatment. In order to explain the effect of post heat treatment at low oxygen pressure, post heat treatment after Y-ion plating is performed in flowing hydrogen as a function of time. During post heat treatment, the Y-coated layer is modified into (Y, Al)O-type oxide by reaction between Y₂O₃, and Al₂O₃. Thickness of the modified Y-layer is related to oxygen pressure and time. The Y-modified layer formed during post heat treatment acts as a barrier to the transport of oxygen. The fine (Y,Al)O-type oxide can relieve easily growth stress by permitting easy plastic deformation, and can absorb the thermal stress developed in the Al₂O₃ layer. The tensile stress generated by the difference in thermal expansion coefficient of the (Y,Al)O-type oxide and Al₂O₃, layer compensates the large compressive stress generated by the difference in thermal expansion coefficient of the Al₂O₃layer and Ni3Al alloy.