This study aimed to address the limitations of traditional plasma nitriding methods by implementing a short-term plasma oxy-nitriding treatment on the surface of AISI 420 martensitic stainless steel. This treatment involved the sequential formation of nitride and oxide layers, to enhance surface hardness and corrosion resistance, respectively. The process resulted in the formation of a 20 μm-thick nitride layer and a 3 μm-thick oxide layer on the steel surface. Initially, the hardness increased by 2.2 times after nitriding, followed by a subsequent decrease of approximately 31 % after oxidation. While the nitriding process reduced corrosion resistance, the subsequent oxidation process led to the formation of a passive oxide film, effectively resolving this issue. The pitting corrosion of the oxide passive film started at 82.6 mVssc, providing better corrosion resistance characteristics than the nitride layer. Consequently, the trade-off between surface hardness and corrosion resistance in plasma oxy-nitrided AISI 420 martensitic stainless steel is anticipated to be recognized as an innovative and comprehensive surface treatment process for biomedical components.

In this study, defects generated in the YSZ coating layer of the IN738LC turbine blade are investigated using an optical microscope and SEM/EDS. The blade YSZ coating layer is composed of a Y-Zr component top coat layer and a Co component bond coat layer. A large amount of Cr/Ni component that diffused from the base is also measured in the bond coat. The blade hot corrosion is concentrated on the surface of the concave part, accompanied by separation of the coating layer due to the concentration of combustion gas collisions here. In the top coating layer of the blade, cracks occur in the vertical and horizontal directions, along with pits in the top coating layer. Combustion gas components such as Na and S are contained inside the pits and cracks, so it is considered that the pits/cracks are caused by the corrosion of the combustion gases. Also, a thermally grown oxide (TGO) layer of several μm thick composed of Al oxide is observed between the top coat and the bond coat, and a similar inner TGO with a thickness of several μm is also observed between the bond coat and the matrix. A PFZ (precipitate free zone) deficient in γ' (Ni3Al) forms as a band around the TGO, in which the Al component is integrated. Although TGO can resist high temperature corrosion of the top coat, it should also be considered that if its shape is irregular and contains pore defects, it may degrade the blade high temperature creep properties. Compositional and microstructural analysis results for hightemperature corrosion and TGO defects in the blade coating layer used at high temperatures are expected to be applied to sound YSZ coating and blade design technology.

Microbiologically Influenced Corrosion (MIC) occurring in underground buried pipes of API 5L X65 steel was investigated. MIC is a corrosion phenomenon caused by microorganisms in soil; it affects steel materials in wet atmosphere. The microstructure and mechanical properties resulting from MIC were analyzed by OM, SEM/EDS, and mapping. Corrosion of pipe cross section was composed of ① surface film, ② iron oxide, and ③ surface/internal microbial corrosive by-product similar to surface corrosion pattern. The surface film is an area where concentrations of C/O components are on average 65 %/ 16 %; the main components of Fe Oxide were measured and found to be 48Fe-42O. The MIC area is divided into surface and inner areas, where high concentrations of N of 6 %/5 % are detected, respectively, in addition to the C/O component. The high concentration of C/O components observed on pipe surfaces and cross sections is considered to be MIC due to the various bacteria present. It is assumed that this is related to the heat-shrinkable sheet, which is a corrosion-resistant coating layer that becomes the MIC by-product component. The MIC generated on the pipe surface and cross section is inferred to have a high concentration of N components. High concentrations of N components occur frequently on surface and inner regions; these regions were investigated and Na/Mg/Ca basic substances were found to have accumulated as well. Therefore, it is presumed that the corrosion of buried pipes is due to the MIC of the NRB (nitrate reducing bacteria) reaction in the soil.

Martensitic stainless steel is commonly used in the medical implant instrument. The alloy has drawbacks in terms of strength and wear properties when applied to instruments with sharp parts. 440C STS alloy, with improved durability, is an alternative to replace 420 J2 STS. In the present study, the carbide precipitation, and mechanical and corrosion properties of STS 440C alloy are studied as a function of different heat treatments. The STS 440C alloy is first austenitized at different temperatures; this is immediately followed by oil quenching and sub-zero treatment. After sub-zero treatment, the alloy is tempered at low temperatures. The microstructures of the heat treated STS 440C alloy consist of martensite and retained austenite and carbides. Using EDX and SADP with a TEM, the precipitated carbides are identified as a Cr23C6 carbide with a size of 1 to 2 μm. The hardness of STS 440C alloy is improved by austenitization at 1,100 oC with sub-zero treatment and tempering at 200 oC. The values of Ecorr and Icorr for STS 440C increase with austenitization temperature. Results can be explained by the dissolution of Cr-carbide and the increase in the retained austenite. Sub-zero treatment followed by tempering shows a little difference in the properties of potentiodynamic polarizations.

Hot-press forming(HPF) steel can be applied successfully to auto parts because of its superior mechanical properties. However, its resistances to aqueous corrosion and the subsequent hydrogen embrittlement(HE) decrease significantly when the steel is exposed to corrosive environments. Considering that the resistances are greatly dependent on the properties of coating materials formed on the steel surface, the characteristics of the corrosion and hydrogen diffusion behaviors regarding the types of coating material should be clearly understood. Electrochemical polarization and impedance measurements reveal a higher corrosion potential and polarization resistance and a lower corrosion current of the Al-coating compared with Zn-coating. Furthermore, it was expected that the diffusion kinetics of the hydrogen atoms would be much slower in the Al-coating, and this would be due mainly to the much lower diffusion coefficient of hydrogen in the Al-coating with a face-centered cubic structure. The superior surface inhibiting effect of the Al-coating, however, is degraded by the formation of local cracks in the coated layer under severe stress conditions, and therefore further study will be necessary to gain a clearer understanding of the effect of cracks formed on the coated layer on the subsequent corrosion and hydrogen diffusion behaviors.

Evaluation of the durability and stability of materials used in power plants is of great importance because parts or components for turbines, heat exchangers and compressors are often exposed to extreme environments such as high temperature and pressure. In this work, high-temperature corrosion behavior of 316 L stainless steel in a carbon dioxide environment was studied to examine the applicability of a material for a supercritical carbon dioxide Brayton cycle as the next generation power plant system. The specimens were exposed in a high-purity carbon dioxide environment at temperatures ranging from 500 to 800 oC during 1000 hours. The features of the corroded products were examined by optical microscope and scanning electron microscope, and the chemical compound was determined by x-ray photoelectron spectroscopy. The results show that while the 316 L stainless steel had good corrosion resistance in the range of 500-700 oC in the carbon dioxide environment, the corrosion resistance at 800 oC was very poor due to chipping the corroded products off, which resulted in a considerable loss in weight.

ber reinforced polymer (FRP) reinforcing bars for concrete structure have been extensively investigated and a number of FRP bars are commercially available. However, major shortcoming of the existing FRP bars is low elastic modulus compared to conventional steel bars. Because of these reasons, FRP Hybrid Bar which have the concept of material hybridization(synthetic resins, deformed bar, glass fiber etc) for concrete structures have been developed. One of main features of this bar is corrosion resistance and this is important to confirm this anti-corrosion capacity of FRP Hybrid Bar. In this study, galvanic corrosion current behavior of 3 types of specimens, including two types of FRP Hybrid Bars and one conventional steel rebar, was investigated under the 0%, 1.5%, 3% and 6% salt water ratio condition. The result of this study can be used to evaluate the applicability of FRP Hybrid Bar to concrete structures.

To overcome shortcomings of fiber reinforced polymer (FRP), a hybridized FRP rebar was developed by the authors. This hybrid bar herein called “FRP Hybrid Bar” was fabricated by adopting advantages from two different materials, including glass fiber reinforced polymer (GFRP). Corrosion resistant characteristics of FRP Hybrid Bar were evaluated and the test results were explained in this paper. The use of the alternative reinforcement could allow concrete structures to extend life-span, to save maintenance and repair costs, etc. if the FRP Hybrid Bar was applied to RC structures located in a very corrosive environment, such as marina or harbor areas.

티타늄은 표면에 형성되는 보호성 부동태 피막 때문에 일반부식과 해수에서 내식성이 강하지만 염산, 황산, 인산 등의 산에서는 보호성 산화물 피막이 파괴된다고 알려져 있다. 본 연구에서는 Ti에 Al 및 V등을 첨가한 α+β계에 대하여 1066℃와 966℃에서 5시간 용체화 열처리를 실시하고, 이 시편을 550℃, 600℃, 및 650℃에서 각각 1시간, 4시간, 8시간 및 16시간 시효열처리한 후 마이크로비커스 경도를 측정하고, 이 시편을 1N H₂SO₄ 용액에서 전기화학적 분극법으로 부식을 계측하였으며, 분극을 마친 시편의 표면을 현미경 조직사진으로 부식상태를 검토하였다. 시험 결과 용체화열처리한 시편이 모재와 시효열처리한 시편보다 높은 내식성을 나타내며. 용체화 온도가 높고 시간이 길어질수록 내식성은 증가하였다.

원자력 발전소 내·외부 계통의 표면에 침적된 방사성핵종은 원자로 구조재 및 핵분열생성물의 부식생성물 활성화에 의해 생성된다. 특히, 1차계통 내부에서 물과 부식된 표면 사이의 지속적인 마찰은 냉각재와 부식생성물을 혼합하게 만든다. 그 리고 이것들은 계통을 따라 순환한다. 본 논문에서는 설계단계에서 사용되는 1차 계통의 부식생성물과 방사성 핵종의 양을 예측하는 CRUDTRAN, DISER, MIGA-RT 및 CPAIR 코드를 분석하였다. 또한, CRUDTRAN을 이용하여 국내 경수로 1차계통 내 부식생성물 거동을 예측하였다. 본 연구목적은 웨스팅하우스형 원전의 실제 데이터로 계산된 값을 측정값과 비교하여 부 식생성물 평가 모델의 신뢰도를 향상시키는데 있다.

The effects of Nb and Cr addition on the microstructure, corrosion and oxide characteristics of Zr based alloys wereinvestigated. The corrosion tests were performed in a pressurized water reactor simulated-loop system at 360oC. Themicrostructures were examined using OM and TEM, and the oxide properties were characterized by low-angle X-ray diffractionand TEM. The corrosion test results up to 360 days revealed that the corrosion rates were considerably affected by Cr contentbut not Nb content. The corrosion resistance of the Zr-xNb-0.1Sn-yCr quaternary alloys was improved by an increasing Nb/Cr ratio. The crystal structure of the precipitates was affected by a variation of the Nb/Cr ratio. The Zr-Nb beta-enrichedprecipitates were mainly formed in the high Nb/Cr ratio alloy while Zr(NbCr)2 precipitates were frequently observed in the lowNb/Cr ratio alloy. The studies of oxide characteristics revealed that the corrosion resistance was related to the crystal structureof the precipitate.

A study on the corrosion behavior of Inconel alloys and Incoloy 800H in molten salt of LiCl-Li2O was investigated at 650˚C for 24-312 hours in an oxidation atmosphere. The order of the corrosion rate was Inconel 600< Inconel 601< Incoloy 800H< Inconel 690. Inconel 600 showed the best performance suggesting that the content of Fe, Cr and Ni are the important factor for corrosion resistance in hot molten salt oxidation conditions. The corrosion products of Inconel 600 and Inconel 601 were Cr2O3 and NiFe2O4, In case of Inconel 690, a single layer of Cr2O3 was formed in the early stage of corrosion and an outer layer of NiFe2O4 and inner layer of Cr2O3 were formed with an increase of corrosion time. In the case of Incoloy 800H, Cr2O3 and FeCr2O4 were observed. Most of the outer scale of the alloys was observed to be spalled from the results of the SEM analysis and the unspalled scale which adhered to the substrate was composed of three layers. The outer layer, the middle one, and the inner one were Fe, Cr, and Ni-rich, respectively. Inconel 600 showed localized corrosion behavior and Inconel 601, 690 and Incoloy 800H showed uniform corrosion behavior. Ni improves the corrosion resistance and too much Cr and/or Fe content deteriorates the corrosion resistance.

The electrolytic reduction of a spent oxide fuel involves liberation of the oxygen in a molten LiCl electrolyte, which is a chemically aggressive environment that is too crosive for typical structural materials. Therefore, it is essential to choose the optimum material for the process equipment for handling a molten salt. In this study, the corrosion behavior of pyro-carbon made by CVD was investigated in a molten LiCl-Li2O salt under an oxidation atmosphere at 650˚C and 750˚C for 72 hours. Pyro-carbon showed no chemical reactions with the molten salt because of its low wettability between pyro-carbon and the molten salt. As a result of XRD analysis, pyro-carbon exposed to the molten salt showed pure graphite after corrosion tests. As a result of TGA, whereas the coated layer by CVD showed high anti-oxidation, the non-coated layer showed relatively low anti-oxidation. The stable phases in the reactions were C(S), Li2CO3(S), LiCl(l), Li2O at 650˚C and C(S), LiCl(l), Li2O(S) at 750˚C. Li2CO(S) was decomposed at 750˚C into Li2O(S) and CO2(g).

염분분위기에서의 부식은 사용후핵연료의 중간저장 기간 동안 304 스테인레스 강재 건식저장용기의 주 열 화기구들 중 하나다. 본 연구에서는 감소정도가 서로 다른 냉연 304 스테인레스 강 시편들에 0.5wt.%의 염화 나트륨 연무를 분사시키면서 느린 변형속도시험(SSRT)과 중성염 분사시험(NSS)을 85℃와 200℃에서 수행하 였다. 85℃에서 2000 시간 동안 시험한 NSS시편의 무게 변화는 200℃에서 시험한 시편의 무게 변화와 크게 달 랐다. NSS 시편의 85℃에서 무게 감량은 미미하였지만, 냉연 감소율이 증가함에 따라서 무게 변화는 점진적으 로 감소하였다. 85℃와 200℃에서 그리고 염분분사 환경에서 가볍게 냉연 가공된 시편의 SSRT 시험으로부터얻은 항복강도와 극한 인장응력의 값은 공기 중의 값보다 약간 낮았다. 그러나 염분 분위기에서 부식으로 인 한 20% 감소 냉연시편의 강도는 더 이상 변화하지 않았다. 예비결과는 냉연 304 스테인레스 강의 질과 성능이 건식저장용기의 제작을 위한 조건에 맞는다는 것을 증명하였다. 그러나 냉연 스테인레스 강의 장기적인 성능 을 더 잘 이해하기 위해서는 염분분위기에서 이 재질의 부식거동에 관한 더 많은 연구가 필요하다.