• 전기화학회지
• /
• 제10권3호
• /
• pp.219-228
• /
• 2007

The phase-shift method and correlation constants, i.e., the electrochemical impedance spectroscopy (EIS) techniques for studying linear relationships between the behaviors (${\varphi}\;vs.\;E$) of the phase shift ($0^{\circ}{\leq}-{\varphi}{\leq}90^{\circ}$) for the optimum intermediate frequency and those (${\theta}\;vs.\;E$) of the fractional surface coverage ($1{\geq}{\theta}{\geq}0$), have been proposed and verified to determine the Langmuir, Frumkin, and Temkin adsorption isotherms (${\theta}\;vs.\;E$) of H for the cathodic $H_2$ evolution reaction (HER) at noble and transition-metal/aqueous solution interfaces. At the Pt/0.1 MKOH aqueous solution interface, the Langmuir, Frumkin, and Temkin adsorption isotherms (${\theta}\;vs.\;E$), equilibrium constants ($K=5.6{\times}10^{-10}\;mol^{-1}\;at\;0{\leq}{\theta}<0.81$, $K=5.6{\times}10^{-9}{\exp}(-4.6{\theta})\;mol^{-1}\;at\;0.2<{\theta}<0.8$, and $K=5.6{\times}10^{-10}{\exp}(-12{\theta})\;mol^{-1}\;at\;0.919<{\theta}{\leq}1$, interaction parameters (g = 4.6 for the Temkin and g = 12 for the Frumkin adsorption isotherm), rates of change of the standard free energy ($r=11.4\;kJ\;mol^{-1}$ for g=4.6 and $r=29.8\;kJ\;mol^{-1}$ for g=12), and standard free energies (${\Delta}G_{ads}^0=52.8\;kJ\;mol^{-1}\;at\;0{\leq}{\theta}<0.81,\;49.4<{\Delta}G_{\theta}^0<56.2\;kJ\;mol^{-1}\;at\;0.2<{\theta}<0.8$ and $80.1<{\Delta}_{\theta}^0{\leq}82.5\;kJ\;mol^{-1}\;at\;0.919<{\theta}{\leq}1$) of OH for the anodic $O_2$ evolution reaction (OER) are also determined using the phase-shift method and correlation constants. The adsorption of OH transits from the Langmuir to the Frumkin adsorption isotherm (${\theta}\;vs.E$), and vice versa, depending on the electrode potential (E) or the fractional surface coverage (${\theta}$). At the intermediate values of ${\theta}$, i.e., $0.2<{\theta}<0.8$, the Temkin adsorption isotherm (${\theta}\;vs.\;E$) correlating with the Langmuir or the Frumkin adsorption isotherm (${\theta}\;vs.\;E$), and vice versa, is readily determined using the correlation constants. The phase-shift method and correlation constants are accurate and reliable techniques to determine the adsorption isotherms and related electrode kinetic and thermodynamic parameters. They are useful and effective ways to study the adsorptions of intermediates (H, OH) for the sequential reactions (HER, OER) at the interfaces.

• 한국표면공학회지
• /
• 제51권2호
• /
• pp.95-103
• /
• 2018

Recently, many types of constructional steels have been often exposed to under severe corrosive environments due to acid rain with increasing environmental contamination. In order to inhibit their corrosion in severe corrosive environments, a painting method has been widely applied to numerous constructional steels of land as well as marine. Therefore, development of paint having a good quality of corrosion resistance is considered to be very important. In this study, four types of anti-corrosive paints (AP: Phenol epoxy, AC: Ceramic epoxy, AT: Coal tar epoxy, AH: High solid epoxy) were coated to the specimens, and then, were immerged in various salt solutions (0.1, 0.3, 3, 6, 9 and 15% NaCl solutions) for 11 days. And, the corrosion resistance of these samples by effect of osmotic pressure with salt concentration was investigated with electrochemical methods such as measurement of corrosion potential, impedance and corrosion current density. The corrosion current densities of all samples (AC, AT and AH) submerged in 3% NaCl solution exhibited the smallest values compared to other salt solutions. However, in the case of lower values of salt solutions than 3% NaCl solution, the corrosion current density increased again because it makes easier for water, dissolved oxygen and chloride ion etc. to invade toward inner side of coating film due to increasing of the osmotic pressure than 3% NaCl solution, but in the case of higher values of salt solutions than 3% NaCl solution, the coating film is easily deteriorated due to high concentration of chloride ion rather than the osmotic pressure, which resulted in increasing the corrosion current density. In particular, the AC sample indicated the best corrosion resistance in 6% NaCl solution compared to other samples. Consequently, it is considered that the corrosion mechanism of the coated steel plate is completely different from bare steel plate, and the corrosion resistance of coating film by osmotic pressure and chloride ion depend on various types of epoxy of paint in NaCl solution.

• Corrosion Science and Technology
• /
• 제8권4호
• /
• pp.162-169
• /
• 2009

Pure titanium and its alloys are drastically used in implant materials due to their excellent mechanical properties, high corrosion resistance and good biocompatibility. However, the widely used Ti-6Al-4V is found to release toxic ions (Al and V) into the body, leading to undesirable long-term effects. Ti-6Al-4V has much higher elastic modulus than cortical bone. Therefore, titanium alloys with low elastic modulus have been developed as biomaterials to minimize stress shielding. For this reason, Ti-30Ta-xZr alloy systems have been studied in this study. The Ti-30Ta containing Zr(5, 10 and 15 wt%) were 10 times melted to improve chemical homogeneity by using a vacuum furnace and then homogenized for 24 hrs at $1000^{\circ}C$. The specimens were cut and polished for corrosion test and Ti coating and then coated with TiN, respectively, by using DC magnetron sputtering method. The analyses of coated surface were carried out by field emission scanning electron microscope(FE-SEM). The electrochemical characteristics were examined using potentiodynamic (- 1500 mV~+ 2000 mV) and AC impedance spectroscopy(100 kHz~10 mHz) in 0.9% NaCl solution at $36.5{\pm}1^{\circ}C$. The equiaxed structure was changed to needle-like structure with increasing Zr content. The surface defects and structures were covered with TiN/Ti coated layer. From the polarization behavior in 0.9% NaCl solution, The corrosion current density of Ti-30Ta-xZr alloys decreased as Zr content increased, whereas, the corrosion potential of Ti-30Ta-xZr alloys increased as Zr content increased. The corrosion resistance of TiN/Ti-coated Ti-30Ta-xZr alloys were higher than that of the TiN-coated Ti-30Ta-xZr alloys. From the AC impedance in 0.9% NaCl solution, polarization resistance($R_p$) value of TiN/Ti coated Ti-30Ta-xZr alloys showed higher than that of TiN-coated Ti-30Ta-xZr alloys.

• 한국재료학회지
• /
• 제9권11호
• /
• pp.1075-1082
• /
• 1999

50/50 vol% LSM-YSZ의 복합공기극(LSM=$\textrm{La}_{1-x}\textrm{Sr}_{x}\textrm{MnO}_{3}$(0$\leq$x$\leq$0.5))이 콜로이드 증착법에 의해 준비으며 주사전자현미경과 임피던스 분석기에 의해 연구되어졌다. 재현성있는 임피던스 스펙트럼들이 LSM-YSZ/YSZ/LSM-YSZ로 구성된 향상된 셀을 사용함으로써 얻어졌다. 이러한 셀들의 임피던스 스펙트럼들은 작동온도에 강하게 영향을 받았으며, 가장 안정된 조건은 $900^{\circ}C$에서 도달되어졌다. $900^{\circ}C$에서 공기//공기 셀에 대해 측정된 전형적인 임피던스 스펙트럼들은 2개의 불완전한 호(depressed arc)로 구성되었다. LSM전극에 대한 YSZ의 첨가는 LSM-YSZ 공기극의 저항 감소를 가져왔으며, 전해질 표면의 불순물의 영향을 제거하기 위한 연마는 공기극 저항을 더욱 감소시켰다. 또한 촉매층(Ni 혹은 Sr)을 가진 LSM-YSZ 전극은 촉매층이 없는 경우에 비해 공기극 저항의 감소를 가져왔다. LSM-YSZ 공기극 저항은 전극조성, 전해질의 형태, 인가 전류에 의해 영향을 받았다.

• 한국건설순환자원학회논문집
• /
• 제9권3호
• /
• pp.268-275
• /
• 2021

콘크리트의 강도를 결정하는 3가지 요인은 시멘트 페이스트의 강도, 골재의 강도, 골재와 시멘트페이스트 계면영역의 강도가 있다. 이 중 계면영역의 강도가 가장 취약하다. ITZ(Interfacial Transition Zone)는 10~50㎛로 형성되며, 수산화칼슘의 비율은 높아지고, CSH는 낮은 비율을 나타낸다. 높은 수산화칼슘 비율은 ITZ의 부착강도 저하의 원인이 된다. 이로인해 ITZ는 더 약한 영역이 된다. ITZ의 문제점은 경량골재를 활용할 때 더 불리한 요소로 나타난다. 계면특성의 기존연구는 계면파괴인성을 측정하고, 계면에 영향을 주는 인자들을 파악했고, 굵은 골재를 사용하지 않은 시멘트 경화체에 SEM과 XRD분석을 진행했다. 또한 EMPA-BSE장비를 활용하여 미세구조를 파악하였다. 하지만 기존의 연구에서는 미세구조와 역학적 성질 파악에 어려움이 있다. 따라서 본 연구에서는 천연골재와 경량골재 계면을 파악하기 위해 EIS측정 장비를 활용하여 전기저항을 측정하는 방식을 채택하였고, 경량골재 겉면을 고로슬래그 코팅을 통해 계면상태의 변화를 실험하였다. 실험결과, 천연골재와 경량골재의 압축강도는 밀도가 높은 천연골재 높은 강도를 나타냈고, 경량골재 표면 코팅 시 천연골재 이상의 강도를 나타냈으며, 골재 종류별 전기저항의 차이를 보였다.

• 한국표면공학회지
• /
• 제34권5호
• /
• pp.465-474
• /
• 2001

Type 304SS coatings were performed at 200$\square$ onto AISI 1045 carbon steel substrate using unbalanced magnetron sputtering (UBMS) with an austenitic AISI 304 stainless steel (SS) target of 100mm diameter. The total deposition pressure in the active Ar gas was 2$\times$10$^{-3}$ Torr. Coatings were done at various target power densities and bias voltages. Chemical compositions of metallic elements of the coatings were measured by energy dispersive X-rays spectroscopy (EDS). The structure and the morphology of Type 304SS coatings were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Corrosion properties of the coated specimens were examined using electrochemical polarization measurements and electrochemical impedance spectroscopy in a deaerated 3.5% NaCl solution. The porosity rate was obtained from a comparison of the dc polarization resistance of the uncoated and coated substrates. Scratch adhesion testing was used to compare the critical loads for different coatings. XRD results showed that the sputtered films exhibit a ferritic b.c.c. $\alpha$-phase. Potentiodynamic polarization curves indicated that all samples had much higher corrosion potential and better corrosion resistance than the bare steel substrate. The corrosion performance increased with increasing power density and the adhesion was enhanced at the bias voltage of -50V. An improvement in the corrosion resistance can be obtained with a better coating adhesion. Finally, an optimized deposition condition for corrosion protection was found as $40W/cm^2$ and -50V.

• Corrosion Science and Technology
• /
• 제18권6호
• /
• pp.300-311
• /
• 2019

The corrosion behaviors of twinning-induced plasticity (TWIP) steels with different alloying elements (Cu, Al, Si) in a neutral aqueous environment were investigated in terms of the characteristics of the corrosion products formed on the steel surface. The corrosion behavior was evaluated by measuring potentiodynamic polarization test and electrochemical impedance spectroscopy. For compositional analysis of the corrosion products formed on the steel surface, an electron probe x-ray micro analyzer was also utilized. This study showed that the addition of Cu to the steel contributed to the increase in corrosion resistance to a certain extent by the presence of metallic Cu in discontinuous form at the oxide/steel interface. Compared to the case of steel with Cu, the Al-bearing specimen exhibited much higher polarization resistance and lower corrosion current by the formation of a thin Al-enriched oxide layer. On the other hand, Si addition (3.0 wt%) to the steel led to an increase in grain size, which was twice as large as that of the other specimens, resulting in a deterioration of the corrosion resistance. This was closely associated with the localized corrosion attacks along the grain boundaries by the formation of a galvanic couple with a large cathode-small anode.

• Journal of Electrical Engineering and Technology
• /
• 제12권1호
• /
• pp.317-328
• /
• 2017

This paper presents design and specific absorption rate analysis of a 2.4 GHz wearable patch antenna on a conventional and electromagnetic bandgap (EBG) ground planes, under normal and bent conditions. Wearable materials are used in the design of the antenna and EBG surfaces. A woven fabric (Zelt) is used as a conductive material and a 3 mm thicker Wash Cotton is used as a substrate. The dielectric constant and tangent loss of the substrate are 1.51 and 0.02 respectively. The volume of the proposed antenna is $113{\times}96.4{\times}3mm^3$. The metamaterial surface is used as a high impedance surface which shields the body from the hazards of electromagnetic radiations to reduce the Specific Absorption Rate (SAR). For on-body analysis a three layer model (containing skin, fats and muscles) of human arm is used. Antenna employing the EBG ground plane gives safe value of SAR (i.e. 1.77W/kg<2W/kg), when worn on human arm. This value is obtained using the safe limit of 2 W/kg, averaged over 10g of tissue, specified by the International Commission of Non Ionization Radiation Protection (ICNIRP). The SAR is reduced by 83.82 % as compare to the conventional antenna (8.16 W/kg>2W/kg). The efficiency of the EBG based antenna is improved from 52 to 74 %, relative to the conventional counterpart. The proposed antenna can be used in wearable electronics and smart clothing.

• 전기화학회지
• /
• 제6권2호
• /
• pp.119-129
• /
• 2003

The Langmuir adsorption isotherms of the over-potentially deposited hydrogen (OPD H) fur the cathodic $H_2$ evolution reaction (HER) at the poly-Au and $Rh|0.5M\;H_2SO_4$ aqueous electrolyte interfaces have been studied using cyclic voltammetric and ac impedance techniques. The behavior of the phase shift $(0^{\circ}{\leq}{-\phi}{\leq}90^{\circ})$ for the optimum intermediate frequency corresponds well to that of the fractional surface coverage $(1{\geq}{\theta}{\geq}0)$ at the interfaces. The phase-shift profile $({-\phi}\;vs.\;E)$ for the optimum intermediate frequency, i.e., the phase-shift method, can be used as a new electrochemical method to determine the Langmuir adsorption isotherm $({\theta}\;vs.\;E)$ of the OPD H for the cathodic HER at the interfaces. At the poly-Au|0.5M $H_2SO_4$ aqueous electrolyte interface, the equilibrium constant (K) and the standard free energy $({\Delta}G_{ads})$ of the OPD H are $2.3\times10^{-6}$ and 32.2kJ/mol, respectively. At the poly-Rh|0.5M $H_2SO_4$ aqueous electrolyte interface, K and ${\Delta}G_{ads}$ of the OPD H are $4.1\times10^4\;or\;1.2\times10^{-2}$ and 19.3 or 11.0kJ/mol depending on E, respectively. In contrast to the poly-Au electrode interface, the two different Langmuir adsorption isotherms of the OPD H are observed at the poly-Rh electrode interface. The two different Langmuir adsorption isotherms of the OPD H correspond to the two different adsorption sites of the OPD H on the poly-Rh electrode surface.

• 한국신재생에너지학회:학술대회논문집
• /
• 한국신재생에너지학회 2006년도 추계학술대회
• /
• pp.352-355
• /
• 2006

As the stainless steel has good corrosion resistance, mechanical property and ease of manufacture, it has been studied as the candidate material of metallic bipolar plate for automotive PIMFC. But, metal is dissolved under fuel cell operating conditions Dissolved ions contaminate a membrane electrode assembly (MEA) and, decrease the fuel cell performance. In addition, metal oxide formation on the surface of stainless steel increases the contact resistance in the fuel cell. These problems have been acted as an obstacle in the application of stainless steel to bipolar plate. Therefore, many kinds of coating technologies have been examined in order to solve these problems. In this study, stainless steel was coated in order to achieve high conductivity and corrosion resistance by several methods. Contact resistance was measured by using a tensile tester and impedance analyzer Corrosion characteristics of coated stainless steel were examined by Tafel-extrapolation method from the polarization curves in a solution simulating the anodic and cathodic environment of PEMFC. Fuel cell performance was also evaluated by single cell test. We tested various coated metal bipolar plate and conventional and graphite were also tested as comparative samples. In the result, coated stainless steel bipolar plate exhibited better cell performance than graphite to bipolar plate.