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.

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.

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).

The likelihood of failure by the corrosion of high temperature H2S/H2, which affect to a risk of facilities, was analyzed through the risk based-inspection using API-581 BRD. We found that the corrosion rate was increased as temperature and H2S concentration were increased. Also, the technical module subfactor(TMSF) was increased as an used you increased, material thickness decreased, inspection number decreased, and inspection effectiveness increased. In these conditions, the maximum value of TMSF was not varied, but the TMSF was sensitively varied at low temperature for high concentration of H2S.

The likelihood of failure for the thinning of high temperature sulfide and naphthenic acid corrosion, which affect to a risk of facilities, was analyzed through the risk based inspection using API-581 BRD. We found that the corrosion rate was increased with increasing temperature and total acid number(TAN). And maximum value of the technical module subfactor(TMSF) was not varied with operating condition, but the TMSF was sensitively changed at the range of low temperature, low flow rate, and high TAN. Also, the TMSF was increased as an used year and inspection effectiveness increased, but it was increased as thickness, inspection number, and over design decreased.

발전소 내 방사화 부식생성물의 대부분을 차지하고 있는 니켈 페라이트계 부식생성물을 모사 발생시키기 위한 고온 고압용 장치를 제작하여 연구를 수행하였다. 배관형 포집기를 이용한 부식생성물 발생장치로부터 방사화 부식생성물과 가장 유사한 부식생성물을 얻을 수 있었다. 발전소에서 입자성 부식생성물이 발생되는 원리인 온도에 따른 용해도 차이를 구현하기 위하여 270C에서 부식반응이 일어나 상대적으로 높은 온도를 가진 포집용 장치에 부식생성물이 포집되도록 장치를 제작하였으며 , 발생된 부식생성물은 주사전자현미경 관찰과 EDAX를 통한 조성분석으로 그 특성을 관찰하였다. 부식생성물은 포집 된 위치 에 따라서 침상 형태의 산화물과 결정 형태의 산화물로 나뉘었으며, 조성 분석 결과 결정 형태의 부식생성물이 니켈 페라이트로서 발전소에서 발생되는 입자성 부식생성물과 유사한 것을 알 수 있었다.