Chloride-induced Stress Corrosion Cracking Research Articles

Tolerance assessment and crack growth of chloride-induced stress corrosion cracking for Chinshan dry storage system

Evaluating the susceptibility of canister used in Chinshan Independent Spent Fuel Storage Installation (CSISFSI) to chloride-induced stress corrosion cracking (CISCC) has been an aging management plan in an initial license period of 20 years. The presence of concentrated chlorides, temperature and relative humidity, and sufficient weld residual stress are the three key environmental conditions for atmospheric CISCC. A single 2-D model was used to simulate natural exposure of austenitic stainless steel (SS) of SS304 and SS304L dry storage system (DSS) for transportable storage canister (TSC). This model will calculate the temperature on the TSC surface based on CFD method. This study estimated RH and DRH on the TSC surface by the realistic environmental conditions (e.g., ambient RH, ambient temperature, and the temperature on the TSC surface) after 1st, 10th, and 20th years, respectively. The flaw tolerance to part-depth (PD) and through-wall (TW) critical flaw were evaluated, and the two-phase CGR induced by CaCl2 or MgCl2 of the TSC surface under 0.18 m height were predicted. The results show that the fastest CISCC CGR was on the bottom induced by CaCl2 in the 20th year, and the slowest CGR was at the 0.18 m of TSC induced by MgCl2 in the 10th year. The two-phase CGR trends were very close if the deliquescence threshold had been considered under different environmental conditions.

Monitoring of helium gas leakage from canister storing spent nuclear fuel: Radiological consequences and management

Chloride-induced stress corrosion cracking (CISCC) of canisters has been identified as a potential degradation mechanism requiring ageing management — especially in a marine air environment. Methods for monitoring helium gas leakage from canisters have been developed on the basis of a phenomenon that the bottom temperature of the canister increases as the top temperature of the canister decreases when helium gas leaks during storage. In this paper, we consider the radiological consequences of CISCC based on two scenarios of helium gas leakage and propose effective management that includes a timeline for monitoring and mitigation. Monitoring helium gas leakage by measuring the difference in temperature between canister surfaces in the dry cask storage of spent fuel (SF) can detect leakage before SF undergoes oxidation as a result of air ingress and before country-specific regulatory limits of off-site doses are exceeded, thereby enabling mitigatory action.

Preliminary Modeling of Chloride Deposition on Spent Nuclear Fuel Canisters in Dry Storage Relevant to Stress Corrosion Cracking

This study presents a method that can be used to gain information relevant to determining the corrosion risk for spent nuclear fuel (SNF) canisters during extended dry storage. Currently, it is known that stainless steel canisters are susceptible to chloride-induced stress corrosion cracking (CISCC). However, the rate of CISCC degradation and the likelihood that it could lead to a through-wall crack is unknown. This study uses well-developed computational fluid dynamics and particle-tracking tools and applies them to SNF storage to determine the rate of deposition on canisters. The deposition rate is determined for a vertical canister system and a horizontal canister system, at various decay heat rates with a uniform particle size distribution, ranging from 0.25 to 25 µm, used as an input. In all cases, most of the dust entering the overpack passed through without depositing. Most of what was retained in the overpack was deposited on overpack surfaces (e.g., inlet and outlet vents); only a small fraction was deposited on the canister itself. These results are provided for generalized canister systems with a generalized input; as such, this paper is intended to demonstrate the technique. This study is a part of an ongoing effort funded by the U.S. Department of Energy, Nuclear Energy Office of Spent Fuel Waste Science and Technology, which is tasked with doing research relevant to developing a sound technical basis for ensuring the safe extended storage and subsequent transport of SNF. This work is being presented to demonstrate a potentially useful technique for SNF canister vendors, utilities, regulators, and stakeholders to utilize and further develop for their own designs and site-specific studies.

Chloride-Induced Stress Corrosion Cracking Tester for Austenitic Stainless Steel

Abstract In this paper, a new experimental tester was designed to improve the stability in load measurement and imposed environment for chloride-induced stress corrosion cracking (CISCC) testing of austenitic stainless steels that are used to construct spent nuclear fuel dry storage canisters. By fabricating compact tension specimens using the UNS S30400 stainless steel, the test is conducted by immersing the specimen in 5 % salinity artificial seawater at 50°C. From the measured load, the stress intensity factor is calculated and the crack length is measured by an optical microscope. The present experimental data confirm the existing findings that the CISCC growth rate is insensitive to the stress intensity factor but is dominantly affected by the specimen surface temperature. The crack growth rate also confirms the existing model.

Pitting behavior of friction stir repair-welded 304L stainless steel in 3.5% NaCl solution at room temperature: role of grain and defect structures

Stainless steel (SS) canisters used for storing spent nuclear waste are prone to chloride-induced stress corrosion cracking. Friction stir welding (FSW), being a low heat input process, can be used for repair welding of these canisters. In this study, an artificial crack was introduced on the 304L SS coupons by an electric discharge machining process. The artificial crack was repair-welded by FSW by maintaining the tool temperature a constant at two levels (725 and 825 °C). Friction stirring significantly reduced the grain size of the stirred zone (SZ) from about 47 to 2–4 μm. The fraction of low-angle grain boundaries increased in the SZ from 2 to 37–43%. On the other hand, the fraction of special grain boundaries (Σ3 and Σ9) that was ~ 50% in the base metal reduced to < 10% in the SZ. During friction stirring, the oxide layer of artificial crack was broken and aligned to form a spiral defect called lazy-S structure. All these microstructural changes affected the corrosion behavior of the FSW specimens when tested in 3.5% NaCl at room temperature using cyclic polarization, chronoamperometry, and electrochemical impedance spectroscopy. The FSW specimens showed lower polarization resistance and lower transpassive potential than those of the base metal specimens. However, the pitting protection potentials of the FSW specimens were higher than that of the base metal. The pitting behavior of the FSW specimens was influenced more by the preferential attack on the lazy-S region than by passive film breakdown. The flat band potentials of the passive film formed on the FSW specimens were more positive than that of base metal. The charge carrier density of passive film formed on the FSW specimens was higher than that of the base metal. The higher fraction of low-angle grain boundaries present in the FSW specimens could supply more number of misorientation dislocations at the metal/film interface which could form anion vacancies by a climb process leading to formation of oxide at these locations without stressing the substrate. Therefore, low-angle boundaries are considered helpful for formation of stress-free passive films that will be highly stable and enhance both pitting and stress corrosion cracking resistance.

Characterization of Physical and Chemical Conditions for Atmospheric Corrosion of Spent Nuclear Fuel Dry Storage Canisters

For long-term interim storage, spent nuclear fuel (SNF) is currently stored in stainless steel canisters at reactor sites. At near-marine sites, deposition and deliquescence of chloride-rich sea-salt aerosols may lead to localized canister corrosion and potentially, to penetration via chloride-induced stress corrosion cracking (CISCC). To assess the potential for CISCC, and to aid in development of appropriate corrosion testing protocols, expected physical and chemical conditions have been characterized for canisters at four near-marine independent spent fuel storage installations (ISFSIs) (Diablo Canyon, Maine Yankee, Calvert Cliffs, and Hope Creek).First, appropriate diurnal temperature and relative humidity (RH) cycles for corrosion testing were determined. Diurnal weather fluctuations result in variations in brine volume and solute concentration that may affect initiation, rate, or extent of canister surface corrosion and CISCC. To determine appropriate testing conditions, daily cycles in canister surface environment were estimated by taking local weather data and applying an equation of state for water [1] to calculate RH values at elevated canister surface temperatures. Results indicated that even slightly elevated temperatures (5-10°C) greatly reduced the diurnal range of RH experienced on the canister surface, and that previous experimental studies had used simulated diurnal RH ranges that were much too large.Second, dust samples collected from in-service canister surfaces were used to evaluate the composition of soluble salts present. While sea-salt aerosols were dominant at one ISFSI, at other sites, the soluble salts contained elevated concentrations of nitrate, sulfate, and calcium relative to sea-salts. Despite storage at near-marine locations, measured nitrate:chloride ratios in the dusts were as high as 1:1. Deliquescent brine compositions, calculated using the geochemical solubility and speciation code EQ3/6 [2], differed greatly from those produced by sea-salt aerosols. The importance of other anions (i.e. NO3 - and SO4 2 -) as possible corrosion inhibitors will be evaluated experimentally, using brine formulas based on the measured salt compositions.Third, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) analysis of the dust samples indicated that salt aerosols comprised only a small fraction of the dust, with the largest fraction consisting of detrital silicate mineral grains. Such grains would largely be inert but may affect corrosion processes by spreading deliquesced brine over the surface through capillary effects. Larger particles may also act a crevice-formers, providing occluded regions for corrosion initiation. Inert dust particle size distributions have been determined from the SEM images and are being used to develop experimental testing protocols to evaluate these effects. Median particle sizes were in the 4-7 µm range, with the largest particles generally less than 100 µm. For laboratory testing, silicate particles with diameters of 5 µm and 74 µm (200 mesh) will be used, as well as a mixture to assess combined effects.Diurnal variations in temperature and RH, and the presence of other anionic species, may significantly affect the corrosiveness of near-marine locations by impacting brine corrosivity (composition and concentration) and rates of corrosion initiation. Inert dust particles may promote corrosion initiation by creating crevices on the metal surface and will certainly affect the brine distribution on the surface and the resulting cathode size and cathode kinetics. These data provide the basis for developing realistic test matrices to evaluate these effects. Acknowledgements: SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. This document is SAND2020-5449 A.

Cathodic and Anodic Stress Corrosion Cracking of a New High-Strength CrNiMnMoN Austenitic Stainless Steel

A new high-nitrogen austenitic stainless steel with excellent mechanical properties was tested for its resistance to stress corrosion cracking. The new conventional produced hybrid CrNiMnMoN stainless steel combines the excellent mechanical properties of CrMnN stainless steels with the good corrosion properties of CrNiMo stainless steels. Possible applications of such a high-strength material are wires in maritime environments. In principle, the material can come into direct contact with high chloride solutions as well as low pH containing media. The resistance against chloride-induced stress corrosion cracking was determined by slow strain rate tests and constant load tests in different chloride-containing solutions at elevated temperatures. Resistance to hydrogen-induced stress corrosion cracking was investigated by precharging and ongoing in-situ hydrogen charging in both slow strain rate test and constant load test. The hydrogen charging was carried out by cathodic charging in 3.5 wt.% NaCl solution with addition of 1 g/L thiourea as corrosion inhibitor and recombination inhibitor to ensure hydrogen absorption with negligible corrosive attack. Slow strain rate tests only lead to hydrogen induced stress corrosion cracking by in-situ charging, which leads to total hydrogen contents of more than 10 wt.-ppm and not by precharging alone. Excellent resistance to chloride-induced stress corrosion cracking in 43 wt.% CaCl2 at 120 °C and in 5 wt.% NaCl buffered pH 3.5 solution at 80 °C is obtained for the investigated austenitic stainless steel.

Evaluation of crack growth of chloride-induced stress corrosion cracking in dry storage system under different environmental conditions

Detailed information of temperature and humidity on the welded transportable storage canister (TSC) is the most valuable data available for investigating the chloride-induced stress corrosion cracking (CISCC) of long-term storage. Chloride salts turn deliquescent in specific situations, depending on the site condition (e.g., the environmental temperature and relative humidity (RH)) and cask condition (e.g., loading pattern and thermal load). This study aimed to evaluate the susceptibility to CISCC of the candidate TSC material using a computational fluid dynamics (CFD) simulation of a marine environment containing chloride sea salts (e.g., CaCl2 and MgCl2) under different environmental conditions. After validation using CFD simulation, the temperature deviation between 2-D and 3-D models was found to be minimal. Thus, a highly accurate 2-D model was developed to accelerate the simulation, mainly because the storage time of CISCC is up to 20 years, and the whole license period lasts 40 years. Results showed only the long-term environmental temperature had a considerable impact on the components. RH decreases as the temperature of TSC increases by decay heat from SNFs. The maximum accumulated crack depth on the bottom edge of TSC will not be over than 0.25 mm/year in the original 20-year licensing period. The location of CISCC occurring on the TSC surface was lower than 0.18 m in height and that inspecting the surface once a decade is sufficient to ensure the integrity of the TSC. As well as supporting our re-inspection method of long-term structural integrity, this study also provide an alternative way to reduce re-inspection load.

Cold spray technology in nuclear energy applications: A review of recent advances

Cold spray technology is a promising solid-state, powder-based deposition methods for fabrication of coatings, near-net-shape manufacturing, and component repair. In recent years, this technology has been investigated for many applications in the nuclear energy sector. For example, it has been explored for the deposition of coatings of corrosion and oxidation materials on zirconium-alloys fuel claddings in light water reactors (LWR) for achieving more accident-tolerant fuel (ATF) cladding. Other examples of its use for nuclear energy applications include coatings for enhancements in wear resistance and heat transfer, near-net shape manufacturing of next-generation fuel claddings of oxide dispersion-strengthened (ODS) steels, and for mitigation and repair of potential chloride induced stress corrosion cracking in used fuel storage canister systems. This paper reviews published literature on the application of cold spray process for nuclear energy applications, while pointing to gaps that have yet to be addressed.

EXPERIMENTAL STUDY OF CHLORIDE-INDUCED STRESS CORROSION CRACKING (CISCC) FOR AUSTENITIC SUS 304L UNDER THERMAL INSULATION

Chloride-Induced Stress Corrosion Cracking (CISCC) has been frequently reported in the petrochemical industries where it usually causes failure to the austenitic stainless steel structures encased with thermal insulation in a chloride-containing environment. This study aims to analyze and evaluate the risk of chloride concentration and different types of insulation materials on CISCC of Austenitic SUS 304L. The experimental study was carried out for 14 days using U-bend specimens through the Drip Test apparatus, as per ASTM G30 and ASTM C692. U-Bend specimens were installed onto the test rig and shielded with different insulation materials. Sodium chloride (NaCl) salt solutions were periodically dripped onto the specimens, to simulate the wet and dry cycle. Insulation materials used were rockwool, calcium silicate and perlite, whereas the concentrations of NaCl solutions were set at 0.1, 1.0 and 3.5 wt.%. The specimens were inspected usingdye penetration tests (DPT), stereo microscope and optical microscope throughout the study to determine the CISCC susceptibility. Results showed that cracking was observed on the specimen with rockwool insulation at 3.5 wt% NaCl and 90°C. Rockwool has a high water absorption capacity at which saltwater will evaporate when in contact with a hot metal surface, resulting in the build-up of salt deposits of high concentration over time. SCC was not observed on other specimens with different conditions, but salt deposits, general corrosion and pitting corrosion were found. From the study, the thermal insulation of rockwool was found to have the highest tendency to cause SCC, followed by calcium silicate and perlite.Keywords: SUS304L, chloride-induced stress corrosion cracking, insulation materials, U-Bend specimen, drip test

Cold spray deposition of 304L stainless steel to mitigate chloride-induced stress corrosion cracking in canisters for used nuclear fuel storage

A feasibility study of the cold spray deposition process of 304L stainless steel on 304 stainless steel substrates as a mitigation method for chloride-induced stress corrosion cracking (CISCC) has been investigated under various substrate conditions. The study is aimed at the application of this technology to mitigate CISCC that may potentially occur in or near the fusion welded regions of stainless steel canisters in Dry Cask Storage System (DCSS) for used nuclear fuels. Spherical gas-atomized 304L stainless steel powder, in the size range of 25 μm–44 μm, was used as feedstock powder for the cold spray process. The powder was deposited on 304 substrates with various surface conditions: as-polished, oxidized, cold-rolled, and plates with prototypical CISCC. The effects of cold spray parameters on quality of cold spray coatings were investigated. Thickness, porosity, and phases in the as-deposited materials were evaluated using scanning electron microscopy (SEM) and X-ray diffraction (XRD), and correlated with microhardness and adhesion strength measured via micro-indentation and ASTM C633 pulling test, respectively. XRD analysis of the coatings was also conducted to examine the effects of cold spray condition on residual stress state in the coating. Detailed cross-sectional examination of coating/substrate interfaces was performed with transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS). Dense and continuous coatings with good adhesion strength, high hardness, and high degree of compressive stress were produced for the various substrate conditions by adjusting cold spray parameters. The results demonstrate that a cold sprayed stainless steel coating is a viable option to provide a physical barrier against CISCC in fusion weld regions of stainless steel in corrosive chloride salt-bearing environments.

On residual stress analysis and microstructural evolution for stainless steel type 304 spent nuclear fuel canisters weld joint: Numerical and experimental studies

Chloride-induced stress corrosion cracking (CISCC) in the weldments of austenitic stainless steel canisters is one of the primary safety concerns during the dry storage of used nuclear fuel at Independent Spent Fuel Storage Installations in coastal areas. In order to evaluate the CISCC behavior in the canister, a 3D sequentially coupled thermo-mechanical finite element model was built to simulate the residual stresses induced by two intersecting longitudinal and circumferential multi-pass welds in austenitic stainless steel mockup canister. Weld-induced residual stresses from simulation were in good agreement with the experimental measurement results by deep-hole drilling and contour methods. Through-wall high tensile axial and hoop stresses were observed in both the fusion zone and heat affected zone (HAZ) in the longitudinal and circumferential weld, respectively. On the other hand, the microstructure within the weldments of 304 stainless steel mockup was characterized using optical microscopy, scanning electron microscopy, electron back-scattered diffraction and energy dispersive X-ray spectroscopy methods. A few amount of MnS inclusions in both the base metal and HAZ, and a lot of (Mn,Si)xOy inclusions in the fusion zone were found. Meanwhile, there is some α′-martensite in the base metal, while the martensite was reversed in the HAZ. In the fusion zone, there is a lot of δ-ferrite. The martensite reversion phenomenon was also verified by the combination of Feritscope test and thermal simulation, which could predict a martensite reversion temperature as 400C°. The welding residual stresses and microstructure analysis results could offer the instructive information for the prediction of CISCC behavior of the spent nuclear fuel canisters.

Accelerated crack growth experiments of SS304H for dry storage canister in substitute ocean water – Effect of temperature

Spent nuclear fuels (SNFs) are expected to be stored in dry storage canisters (DSCs) for much longer time than earlier anticipated due to a lack of permanent deep geological repository. As a result, DSCs located in marine environment may undergo chloride induced stress corrosion cracking (SCC) during long-term storage of SNFs. We report here the effect of test temperature on accelerated crack growth experiments carried out on sensitized austenitic stainless steel SS304H in substitute ocean water using fracture mechanics approach. Wedge-opening loading specimens were selected for measurement of crack growth rates using direct-current potential drop technique. Average crack growth rates of 0.975 × 10−10 ± 9.528 × 10-12, 3.258 × 10−10 ± 9.551 × 10-11 and 1.580 × 10-9 ± 2.593 × 10−10 m/s were obtained for 22, 37 and 60 °C, respectively resulting in an activation energy of 60.9 kJ/mol corresponding to diffusion of hydrogen in steel. Intergranular crack propagation was noted along with formation of chromium-rich carbide precipitates at grain boundaries.

The effect of sensitization treatment on chloride induced stress corrosion cracking of 304L stainless steel using U-bend test

Chloride induced stress corrosion cracking (CI-SCC) commonly occurs for 300 series of austenitic stainless steels that are widely used in the petrochemical industry. The occurrence of CI-SCC requires certain circumstances such as corrosive environment, tensile stress, and susceptible material. Heat treatment temperature between 550 °C–650 °C followed by cooling process causes the sensitization of austenitic type stainless steel in which a chromium-depleted band is formed adjacent to the grain boundary. In this study, the impact of sensitization on CI-SCC was examined using austenitic stainless steel grade 304L (SS 304L). The specimens were shaped into U-bend as per ASTM G30 and exposed for long-term immersion test. U-bend samples were heat sensitized at 600 °C for 2 h followed by air cooling. Samples were immersed in 0.1 wt%, 1.0 wt% and 3.5 wt% NaCl into the water bath at temperature of ambient (baseline), 70 °C and 90 °C for 42 days. They were monitored weekly for visual examined for crack. Stereo and optical microscopy were used to identify the CI-SCC and morphology of U-bend specimens. In the beginning, pit corrosion was observed and followed by cracks growth initiated from the localized pit. The results indicated that Cl-SCC was observed for all sensitized SS 304L specimens at elevated temperature conditions which was more susceptible than the as-received type specimens. The trend indicated that the higher the temperature or chloride concentration, the shorter the time for crack initiation. There was no CI-SCC for both types of specimens at ambient condition. Two CI-SCC likelihood graphs with respect to chloride concentration versus temperature are plotted based on the test data.

Factors Involved in Stress Corrosion Cracking of Tubes from a Nuclear Power Plant Feedwater Heater

This study has been carried out to identify the operating factors involved in the failure of two tubes made of 304 stainless steel removed from a high pressure feedwater heater working in a nuclear power plant. The samples cut from tubes have been analyzed by different methods: visual examination, optical (metallographic) microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Following analyses, on the surface of tubes was highlighted the presence of many pits in which cracks have started. Inside the pits, impurities of the type S, Cl, K, Ca were detected too. The branched cracks most likely have occurred as a result of precipitation of small amounts of chlorides deposited and concentrated on the surface of the tubes over a long period of operation. The stresses that favored this type of corrosion cracking were both residual stresses and stresses occurring at the torsion and bending of the tubes, while the high water temperature from feed heater was also a favorable factor for corrosion. The final conclusion was that the tubes failed by a chloride induced stress corrosion cracking mechanism.

Factors Involved in Stress Corrosion Cracking of Tubes from a Nuclear Power Plant Feedwater Heater

This study has been carried out to identify the operating factors involved in the failure of two tubes made of 304 stainless steel removed from a high pressure feedwater heater working in a nuclear power plant. The samples cut from tubes have been analyzed by different methods: visual examination, optical (metallographic) microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Following analyses, on the surface of tubes was highlighted the presence of many pits in which cracks have started. Inside the pits, impurities of the type S, Cl, K, Ca were detected too. The branched cracks most likely have occurred as a result of precipitation of small amounts of chlorides deposited and concentrated on the surface of the tubes over a long period of operation. The stresses that favored this type of corrosion cracking were both residual stresses and stresses occurring at the torsion and bending of the tubes, while the high water temperature from feed heater was also a favorable factor for corrosion. The final conclusion was that the tubes failed by a chloride induced stress corrosion cracking mechanism.

The effect of salt loading on chloride-induced stress corrosion cracking of 304L austenitic stainless steel under atmospheric conditions

The effect of salt loading on chloride-induced stress corrosion cracking in 304 L was studied at atmospheric pressure. Stress relieved samples were uniaxially pre-strained to 5% and were loaded with nine levels of MgCl2, investigating Cl− deposition levels from 1.7 × 10−4 to 3.1 x 10−2 g cm−2. Samples were subject to 60 MPa stress, 90 °C at 70% relative humidity, for 480 h. A direct correlation between chloride deposition and the density of cracking and corrosion was observed between 5.7 × 10−4 and 1.96 × 10−2 g cm−2. Crack propagation rates were constant between salt loadings of 5.7 × 10−4 and 2.4 × 10−2 g cm−2 at 1–2 µm h−1.

Electrochemical Polarization Behavior of 304L Stainless Steel in a Thin Layer of Chloride Solution

Spent nuclear fuels are stored in stainless steel canisters. The containers are cooled naturally by a convective airflow through the annulus between the overpack and the canister. This air flow draws dust into the overpacks, and the surfaces of containers are covered with dust. Salts carried by the dust will deliquesce as heat generated by radioactive decay declines over time. The deliquescence of salt deposit could induce various forms of corrosion attack such as pitting and chloride-induced stress corrosion cracking (SCC). In this study, the corrosion behavior of 304L SS - a typical dry storage container material, is evaluated by simulating the deliquescence of salt deposit using a thin electrolyte test condition. Electrochemical corrosion test was performed on 304L Stainless Steel in 3.5 wt% NaCl solution at 20 OC. The tests were carried out in a newly fabricated electrochemical cell with exposed surface area of test specimen being 1 cm2. The cell has a movable PTFE wall which houses a Pt foil as a counter electrode. The adjustable wall enables the thickness of electrolyte to be varied between the SS specimen and the counter electrode. The thickness of the electrolyte was maintained at 1 mm in all the tests. This resulted in the volume of the electrolyte to be 10 ml. Open circuit potential, cyclic polarization and potentiostatic tests at different passivation potential were carried out. The potentiostatic passivation tests were followed by electrochemical impedance spectroscopy and Mott-Schotky analysis in order to understand the initiation of localized corrosion. For comparison, the electrochemical tests are being performed by using bulk electrolytes. The electrochemical polarization behavior in the thin film electrolyte was more or less similar to that in the bulk electrolyte. However, the pitting protection was shifted in the negative direction by about 80 mV in the thin electrolyte as compared to that of bulk electrolyte. Potentiostatic tests conducted at potentials below the pitting protection potential did not show pit initiation. Relatively a quick pit initiation event was observed when the applied potential was 60 mV anodic to the pitting protection potential. Bulk electrolytes showed no passivity breakdown at this potential. The results will be analyzed based on the EIS data, semiconducting properties of the passive film, and UV-Vis absorption of the test electrolytes.

C(T) 시편을 이용한 오스테나이트계 스테인리스 강의 염화물에 의한 응력부식균열 실험 기법 개발

본 논문에서는 해안가에 설치되는 사용후핵연료 보관용 캐니스터의 염화물에 의한 응력부식균열 평가를 위해 새로운 실험 방법을 제시한다. 개발된 실험기는 표준 C(T) 시편을 사용하여 분석이 용이하고, 볼트 조임으로 정 변위 하중을 가해준다. 또한 침수 방식을 이용한 가속 실험이 가능하고 염화 환경과 로드셀의 분리를 통해 하중 신호의 안정성을 높인다. 오스테나이트계 304 스테인리스 강(STS304)으로 제작한 2개의 시편으로 28일간 예비 실험을 수행하였고, 그 결과 수렴된 하중 값은 각각 4,382.2, 3,980.8N, 균열 진전량은 1.41E-4m, 1.77E-4m로 계측되었다. 계측 값을 바탕으로 수렴된 하중을 이용해 계산한 응력확대계수는 각각 40.0, 41.8 MPa· m 이다. 결론적으로, 실험 결과를 바탕으로 개발된 실험기를 이용한 새로운 CISCC에 의한 결함 평가 방법을 제시하였다.

Stress Corrosion Cracking of Heat Exchanger Tubes in District Heating System

The purpose of this paper is to present failure analysis, of the heat exchanger tube in a district heating system. SS304 stainless steel is used, as material for the heat exchanger tube. The heat exchanger operates in a soft water environment containing a small amount of chloride ions, and regularly repeats operation and standstill period. This causes concentration of chloride ions on the outer surface of the tube, as well as repeat of thermal expansion, and shrinkage of the tube. As a result of microscopic examination, cracks showed transgranular as well as branched propagation, and many pits were present, at the initiation point of each crack. Energy disperstive spectroscopy analysis showed Fe and O peak, as well as Cl peak, meaning that cracks were affected by Cl ion. Failure of the tube was caused by chloride-induced stress corrosion cracking by thermal stress, high temperature, and localized enrichment of chloride ions.