• Clean Technology
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• v.24 no.4
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• pp.339-347
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• 2018

In order to cope with environmental problems and climate change caused by fossil fuels, renewable energy supply is increasing year by year. Currently, waste energy accounts for 60% of renewable energy production. However, waste has a lower calorific value than fossil fuels and contains various harmful substances, which causes serious problems when applied to power generation boilers. In particular, the chlorine in the waste fuel increases slagging and fouling of boiler heat exchangers, leading to a reduction in thermal efficiency and the main cause of high temperature corrosion, lowering facility operation rate and increasing operating cost. In this study, the high temperature corrosion experiments of superheater materials (ASME SA213/ASTM A213 T2, T12 and T22 alloy steel) by alkali chlorides were conducted, and their corrosion characteristics were analyzed by the weight loss method and SEM-EDS. Experiments show that the higher the temperature and chloride content, the more corrosion occurs, and KCl further corrodes the materials compared to NaCl under the same condition. In addition, the higher the chromium content of the material, the better the corrosion resistance to the alkali chlorides.

• Journal of Welding and Joining
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• v.18 no.6
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• pp.48-54
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• 2000

The corrosion properties of Ni-Cr-W-Mo-B alloy sprayed by the high velocity oxy-fuel spraying (HVOF) was studied as a function of heat treatment by using both potentiodynamic polarization and immersion tests in the H₂SO₄ solution. The mechanical property was also evaluated by a microhardness tester. Microstructural characteristics of te as-sprayed and annealed coatings at 550, 750 and 950℃ have been analyzed by means of OM, XRD, SEM and TEM. The results showed that the corrosion resistance was improved by increasing the annealing temperature. As-sprayed coating had metastable and heterogeneous phases such as amorphous, nanocrystalline and very refined grain and precipitates, which induced a localized corrosion. The localized corrosion occurred preferentially at the unmelted particles which were composed of Ni matrix and Cr, W and Mo riched phase segregated in the boundaries. As annealing temperature was increased, the microstructure had shown some changes - reduction of porosity and s[plat boundary decomposition and crystallization of amorphous/nanocrystalline phases, grain coarsening,, formation and growth of precipitates such as {TEX}$M_{23}C_{6}${/TEX} and {TEX}$M_{7}C_{3}${/TEX}. In addition, the compositional difference between matrix and boundary phases gradually disappeared, which changed the corrosion type from localized corrosion to general corrosion and thus enhanced corrosion resistance.

• Korean Journal of Materials Research
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• v.12 no.1
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• pp.3-9
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• 2002

The corrosion behavior of alloys in a molten salt was investigated along with the oxidation characteristics in the air. The basic composition of alloys in the study was Fe-25Ni-7Cr with Si and RE(rare-earth metal) as additives. The corrosion rate of the alloys was low in a molten salt of LiCl while the rate was high in the mixed molten salt of LiCl and $Li_2O$. When Si is added to the base alloy of Fe-25Ni-7Cr, corrosion resistance was improved as the Si content is increased up to 3%, however, it was observed that the corrosion resistance was getting worse as the Si content is increased. The base alloy with 2.43% of Si and 0.9% of RE(KSA-65), showed higher corrosion rate compared to that of KSA-63 alloy with an equivalent amount of only Si. The corrosion resistance of KSA-65 was similar to that of the base alloy(KSA-60). The oxidation resistance of KSA-65 alloy was greatly increased even at $850^{\circ}C$ for a long term exposure.

• Journal of the Korean Electrochemical Society
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• v.27 no.1
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• pp.8-14
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• 2024

Austenitic stainless steel 316L has been used a lot of applications because of its high corrosion resistance and formability. In addition, copper brazing is employed to create complex shape of 316L stainless steel for various engineering parts. In such system, copper-based filler metals make galvanic cell at metal/filler metal interface, and it accelerates corrosion of stainless steel. Furthermore, Cu-rich region formed by diffused copper in austenitic stainless steel can promote a pitting corrosion. In this study, we used an ammonia (NH₃) gas to nitride the 316L stainless steel for improving the corrosion resistance. The thickness of the nitride (nitrogen high) layer increased with the treatment temperature, and the surface hardness also increased. The potentiodynamic polarization test showed the improvement of corrosion resistance of 316L stainless steel by enhancing the passivation on nitride layer. However, in case of high temperature nitriding, a chromium nitride was formed and its fraction increased, so that the corrosion resistance was decreased compared to the intact 316L stainless steel.

• Corrosion Science and Technology
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• v.5 no.5
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• pp.181-188
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• 2006

In this study, the characteristics of the experimentally produced high N-high Cr bearing stainless steels are discussed as a part of applications of materials for FGD (Fuel Gas Desulfurization) system of thermal power plants or for power plants using seawater as coolant. Corrosion resistance of developed alloys is especially investigated in detail. Corrosion characteristics of vacuum melted cast are shown to be superior to that of air melted one. From the viewpoint of CPT, It is estimated that the differences of corrosion resistance are $21.8^{\circ}C{\sim}24.6^{\circ}C$ at PRE 40 and $8^{\circ}C{\sim}12.4^{\circ}C$ at PRE 50, and the gaps becomes bigger as the PRE values are lower. In the evaluation of corrosion resistance in alloy A2501, Z3101, and A3301 according to Cr concentration, alloy A3301 shows a deviation from the general tendency in chloride solutions. It has relatively high PRE value as 48.6, but it has relatively poor pitting resistance. It is, however, difficult to observe a specific phase except ferrite in microstructure analysis and neither detects special phase such as sigma phase.

• Journal of the Korean institute of surface engineering
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• v.56 no.6
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• pp.401-411
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• 2023

This study investigates the synthesis, characterization, and application of zinc oxide (ZnO) nanoparticles for corrosion resistance and stress corrosion cracking (SCC) mitigation in high-temperature and high-pressure environments. The ZnO nanoparticles are synthesized using plasma discharge in water, resulting in rod-shaped particles with a hexagonal crystal structure. The ZnO nanoparticles are applied to Alloy 600 tubes in simulated nuclear power plant atmospheres to evaluate their effectiveness. X-ray diffraction and X-ray photoelectron spectroscopy analysis reveals the formation of thermodynamically stable ZnCr₂O₄and ZnFe₂O₄ spinel phases with a depth of approximately 35 nm on the surface after 240 hours of treatment. Stress corrosion cracking (SCC) mitigation experiments reveal that ZnO treatment enhances thermal and mechanical stability. The ZnO-treated specimens exhibit increased maximum temperature tolerance up to 310 ℃ and higher-pressure resistance up to 60 bar compared to non-treated ZnO samples. Measurements of crack length indicate reduced crack propagation in ZnO-treated specimens. The formation of thermodynamically stable Zn spinel structures on the surface of Alloy 600 and the subsequent improvements in surface properties contribute to the enhanced durability and performance of the material in challenging high-temperature and high-pressure environments. These findings have significant implications for the development of corrosion-resistant materials and the mitigation of stress corrosion cracking in various industries.

• Corrosion Science and Technology
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• v.6 no.4
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• pp.159-163
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• 2007

Micron size Co-alloy 800 (T800) powder is coated on the high temperature, oxidation and corrosion resistant super alloy Inconel 718 substrate by the optimal high velocity oxy-fuel (HVOF) thermal spray coating process developed by this laboratory. For the study of durability improvement of high speed spindle operating without lubricants, friction and sliding wear behaviors of the coatings are investigated both at room and at an elevated temperature of $1000^{\circ}F(538^{\circ}C)$. Friction coefficients, wear traces and wear debris of coatings are drastically reduced compared to those of non-coated surface of Inconel 718 substrate both at room temperature and at $538^{\circ}C$. Friction coefficients and wear traces of both coated and non-coated surfaces are drastically reduced at higher temperature of $538^{\circ}C$ compared with those at room temperature. At high temperature, the brittle oxides such as CoO, $Co_{3}O_{4}$, $MoO_2$ and $MoO_3$ are formed rapidly on the sliding surfaces, and the brittle oxide phases are easily attrited by reciprocating slides at high temperature through oxidation and abrasive wear mechanisms. The brittle solid oxide particles, softens, melts and partial-melts play roles as solid and liquid lubricants reducing friction coefficient and wear. These show that the coating is highly recommendable for the durability improvement coating on the machine component surfaces vulnerable to frictional heat and wear.

• Corrosion Science and Technology
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• v.15 no.3
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• pp.129-134
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• 2016

The effects of Ti on the high temperature oxidation of Ni-based superalloys were investigated by cyclic oxidation at $850^{\circ}C$ and $1000^{\circ}C$. The oxide scale formed at $850^{\circ}C$ consists of $Cr_2O_3$, $Al_2O_3$, and $NiCr_2O_4$ layers, while a continuous $Al_2O_3$ layer was formed at $1000^{\circ}C$. The oxidation rate of the alloy with higher Ti content was higher than the alloy with less Ti content at $850^{\circ}C$, possibly due to the increase in the metal vacancy concentration in the $Cr_2O_3$ layer involved by incorporation of $Ti^{4+}$. However, Ti improved the oxidation resistance of the superalloy at $1000^{\circ}C$ by reducing oxygen vacancy concentration in $Al_2O_3$ layer.

• Journal of Environmental Science International
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• v.18 no.2
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• pp.187-195
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• 2009

This study was investigated to control the corrosion and scale at the cooling water system in steel works. Laboratory and field tests were performed for the indirect cooling water system of plate mill. Throughout the experiment, various factors such as leakage of pipes, heating rate and capacity, and the reaction between existing and substitute inhibitors were carefully monitored. The results showed that the harmful effect of high temperature could be minimized, and satisfactory corrosion/scale controls were effectively achieved using inhibitor, even at the increased temperature of $80^{\circ}C$. The batch and field tests in the gas scrubbing cooling water system of blast furnace and cooling water system of corex plant indicated that the new inhibitor was more effective for the prevention of corrosion and scale than the existing one.

• Corrosion Science and Technology
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• v.5 no.1
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• pp.1-4
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• 2006

Martensitic stainless steel is used when mechanical properties such as high tensile strength and hardness are required. Medium carbon-contained martensitic stainless steel which contains more than 0.2 wt% of carbon should be heat-treated and quenched at the temperature where undissolved carbides are totally dissolved into the matrix. In particular, the dissolution and reprecipitation behaviors of various forms of carbides are affected by such parameters as heating rate, heating temperature, duration time and cooling rate. This study is to investigate the effects of heat treatment parameters of 14Cr-3Mo martensitic stainless on corrosion resistance and phase transformation in relation to the dissolution and reprecipitation of carbides.