In Korea, borated stainless steel (BSS) is used as a storage rack in spent fuel pools (SFP) to maintain the nuclear criticality of spent fuels. As the number of nuclear power plants and the corresponding amount of spent fuels increased, the density in SFP storage rack also increased. In this regard, maintaining subcriticality of spent nuclear fuels became an issue and BSS was selected as the structural material and neutron absorber for high density storage rack. Since it is difficult to replace the storage rack, corrosion resistance and neutron absorbency are required for long period. BSS is based on stainless steel 304 and is specified in the ASTM A887-89 standard depending on the boron concentration from 304B (0.20-0.29% B) to 304B7 (1.75-2.25% B). Due to the low solubility of boron in austenitic stainless steel, metallic borides such as (Fe, Cr) 2B are formed as a secondary phase. Metallic borides could cause Cr depletion near it, which could decrease the corrosion resistance of the material. In this paper, the long-term corrosion behavior of BSS and its oxide microstructures are investigated through accelerated corrosion experiment in simulated SFP conditions. Because the corrosion rate of austenitic stainless steel is known to be dependent on the Arrhenius equation, a function of temperature, the corrosion experiment is conducted by increasing the experimental temperature. Detail microstructural analysis is conducted using a scanning electron microscope, transmission electron microscope and energy dispersive spectrometer. After oxidation, a hematite structure oxide film is formed, and pitting corrosion occurs on the surface of specimens. Most of the pitting corrosion is found at the substrate surface because the corrosion resistance of the substrate, which has low Cr content, is relatively low. Also, the oxidation reaction of B in the secondary phase has the lowest Gibbs free energy compared to other elements. Furthermore, oxidation of Cr has low Gibbs free energy, which means that oxidation of B and Cr could be faster than other elements. Thus, the long-term corrosion might affect the boron content and the neutron absorption ability of the material. Using boron’s high cross-section for neutrons, the neutron absorption performance of BSS was evaluated through neutron transmission tests. The effect of the corrosion behavior of BSS on its neutron absorption performance was investigated. Samples simulated to undergo up to 60 years of degradation before corrosion through accelerated corrosion testing did not show significant changes in the neutron shielding ability before and after corrosion. This can be explained in relation to the corrosion behavior of BSS. Boron was only leached out from the secondary phase exposed on the surface, and this oxidized secondary phase corresponds to about 0.17% of the volume of the total secondary phase. This can be seen as a very small proportion compared to the total boron content and is not expected to have a significant impact on neutron absorption performance.

Severe wall thinning is found on the tube of a low-pressure evaporator(LPEVA) module that is used for a heat recovery steam generator(HRSG) of a district heating system. Since wall thinning can lead to sudden failure or accidents that lead to shutdown of the operation, it is very important to investigate the main mechanism of the wall thinning. In this study, corrosion analysis associated with a typical flow-accelerated corrosion(FAC) is performed using the corroded tube connected to an upper header of the LPEVA. To investigate factors triggering the FAC, the morphology, composition, and phase of the corroded product of the tube are examined using optical microscopy, scanning electron microscopy combined with energy dispersive spectroscopy, and x-ray diffraction. The results show that the thinnest part of the tube is in the region where gas directly contacts, revealing the typical orange peel type of morphology frequently found in the FAC. The discovery of oxide scales containing phosphate indicates that phosphate corrosion is the main mechanism that weakens the stability of the protective magnetite film and the FAC accelerates the corrosion by generating the orange peel type of morphology.

LiOH-H3BO3 용액중에서의 Zircaloy-4 핵연료 피복관의 부식가속과 억제현상을 조사하고 이러한 부식특성에 미치는 Li 및 B의 영향을 해석하기 위하여, 여러 조건의 LiOH-H3BO3</TEX> 용액을 사용하여 350˚C, 165bar의 고온, 고압 조건에서 Zircaloy-4 피복관의 노외 부식시험을 수행하였다. 원전 수화학 모의조건에 대응되는 용액 중에서의 부식속도의 천이는 물 분위기에서 보다 빨리 발생되고 천이후 물 분위기와 거의 유사한 부식속도를 나타내는 천이적 후의 부식거동을 보였다. 한편 pH의 변화는 부식특성에 큰 영향을 미치지 않았다. 부식가속과 억제 모의실험으로부터, 산화막내로 침투하는 Li의 양이 용액중 Li 농도에 크게 의존하며, Li 농도가 일정하게 정해진 용액의 경우 B 첨가에 관계없이 산화막내에 일정량의 Li이 농축될수 있다는 가정을 제시하였다. 또한 B 첨가에 의한 부식억제가 B 또는 B-(OH) 화합물의 산화막내 Li 침투 억제에 의한 것이 아니라 일들에 의해 산화막내로 산화성 성분의 이동이 억제되는데 기인할 수 있음을 제시하였다. 부식가속 개시점에 대응되는 산화막 두께측정 결과와 용액내 Li 농도간의 관계로부터, 용액중 Li 농도가 높을수록 부식가속이 얇은 산화막 두께에서 시작됨을 알았다. 특히 노내조건에서의 핵연료 피복관의 부식가속이 산화막내 Li 농축에 의해 일어나는 부식특성으로 해석될 수 있음을 보였다.

본 연구는 50mm 길이의 SD400 12mm 철근을 수돗물, DW + 10% CaCl2와 금속 + DW + 10% CaCl2 총 세종류의 액체에 넣어 가속 부식을 시킨 후 가속화에 따른 철근 산화물의 변화를 주기적으로 관찰하여 각각 철근의 부식으로 인해 생겨나는 화학물질을 분류하고, 변화하는 실험 조건이 주는 영향도 같이 비교하였다. 실험 결과 가속 부식 시킨 철근 산화물의 구성 요소들은 Iron Carbon – CFe15.1, Lepidocrocite – FeO(OH), Magnetite – Fe3O4, Wustite – Fe0.942O 등이라는 것을 확인할 수 있었으 며 그 변화를 그래프로 나타내었다.

There structures are build up due to marine concrete development. These marine concrete long-term exposure to water occurs when the rebar corrosion or cracks destroyed. We study to improve this phenomenon. We had the Salt durability assessment that accompanies.

There are various method for evaluating the durability life of concrete structures due to salt damage . The best way is to perform a corrosion test for a rebar embedded in concrete specimen was exposure to marine environment. However, this method has the disadvantage that it takes a long period of time. Also, accelerated corrosion test which was complemented complements the time-consuming weakness is limited to apply because it could not reveal a correlation between long-term exposure test. Accordingly, the purpose of this study is to derive a correlation coefficient between cycle drying -wetting accelerated corrosion test and long-term exposure test. Corrosion initiation time was measured in four types of concrete samples, i.e., two samples mixed with fly ash(FA) and blast furnace slag(BS), and the other two samples having two water/cement ratio(W/C = 0.6, 0.35) without admixture(OPC 60 and OPC 35). The accelerated corrosion test was carried out by two case, i.e., one is a cyclic drying-wetting method(case 1), and the other is a artificial seawater ponding test method(case 2). Whether corrosion occurs, it was measures using half-cell potential method. The results indicated that case 1 is to accelerated the corrosion of rebar about 24~36% as compared with case 2, then the corrosion of rebar embedded in concrete occurred according to the order of OPC60, FA, BS, OPC35. Correlation coefficient between accelerated corrosion test and long-term exposure test, case 1 is 4.23 to 5.42, and case 2 is 6.54 to 7.82.

This study, depending on the concrete mix material by performing the test on the basis of the wet and dry condition and freeze-thaw, and to elucidate the difference in the corrosion initiation time of there steel, in both environments, it is an object of correlation coefficient derived through the results of the accelerated corrosion test in a laboratory corrosion period of the structure of the marine environment.

해수중 환경에서 콘크리트 구조물 내에 매립된 철근은 용존산소의 부족으로 부식이 잘 발생하지 않는다. 이 때문에 해수중 환경의 부식촉진시험은 전기화학적인 방법으로 실시되어, 실제 부식 메커니즘과 맞지 않고 장기거동과의 상관성 도출도 어려운 실정이다. 본 연구에서는 해수중 환경에서의 부식촉진시험법을 정립하기 위해 온도와 염화물농도를 주된 변수로 부식촉진시험을 실시하였다. 부식의 발생 유무는갈바닉 전위측정법과 반전지전위법을 통한 철근부식모니터링 결과로 판단하였다. 부식촉진시험 결과 온도의 영향이 가장 지배적이라고 평가되었다. 염화물량은 시험 시편의 깊이별 염화물 농도를 측정하였다. 동일한 조건으로 FEM 내구성 해석 프로그램인 DuCOM을 통해 염화물침투 해석을 실시하여 입증하였다. 또한, 인공해수 침지 조건에 따른 용존 산소량은 실험을 통해 구했으며 이를 통해 부식촉진시험 결과의 타당성을 검증하였다.