A study on the self-sealing process of anodic films on aluminum by EIS

The electrochemical behaviors of anodized aluminum in neutral NaCl and Na₂SO₄ solutions were studied using electrochemical impedance spectroscopy (EIS). The results reveal that there is a self-sealing process for unsealed anodic film in neutral NaCl and Na₂SO₄ solutions. The resistance of the porous layer (R_p) and the capacitance of the barrier layer (CPE_b) increase and the capacitance of the porous layer (CPE_p) decreases with immersion time in the initial stage. Corrosion resistance provided by the anodic film is improved by the self-sealing process of the porous layer. However, chloride ions have an opposite effect. The improving effect of the self-sealing process on film resistance is decreased with the increase of chloride concentration of the solution.     

AZ91D镁合金外加电场下自封孔微弧氧化膜层微观形貌及耐蚀性的研究

微弧氧化技术可以实现对金属表面的高耐蚀、耐磨等改性,传统微弧氧化所得陶瓷膜具有多孔结构,影响了其耐蚀性能及高温氧化性能。本文针对氧化膜多孔结构与腐蚀性能之间的关系开展基础研究。采用外加电场微弧氧化技术实现自封闭孔结构,并研究了不同孔结构膜层的耐蚀性能;讨论了封孔过程中胶体运动-电位-孔结构表征之间的规律性关系,评价了自封孔后膜层腐蚀性能。主要研究结果表明：膜层中的多孔结构是腐蚀介质的通道,自封孔后耐蚀性能提高,此外,耐蚀性与孔隙率及封孔填充物的成分和形态具有极大的相关性,通过调整外加电场强度和时间可以实现对自封孔的调控,从而改善耐蚀性能。     

Self-sealing of fractures in indurated claystones measured by water and gas flow

Self-sealing of fractures in the indurated Callovo-Oxfordian (COX) and Opalinus (OPA) claystones, which are considered as host rocks for disposal of radioactive waste, was investigated on artificially fractured samples. The samples were extracted from four lithological facies relatively rich in clay mineral, carbonate and quartz, respectively. The self-sealing of fractures was measured by fracture closure, water permeability variation, gas penetration, and recovery of gas-induced pathways. Most of the fractured samples exhibited a dramatic reduction in water permeability to low levels that is close to that of intact rock, depending on their mineralogical composition, fracture intensity, confining stress, and load duration. The self-sealing capacity of the clay-rich samples is higher than that of the carbonate-rich and sandy ones. Significant effects of sample size and fracture intensity were identified. The sealed fractures become gas-tight for certain injection pressures. However, the measured gas breakthrough pressures are still lower than the confining stresses. The gas-induced pathways can recover when contacting water. These important findings imply that fractures in such indurated claystones can effectively recover to hinder water transport but allow gas release under relatively low pressures without compromising the rock integrity.     

氧化石墨烯调控GO/TiO2陶瓷膜层自封孔效应研究

自封孔是提高微弧氧化膜层耐磨性和耐蚀性等综合性能的重要技术之一。针对物理封孔因封孔剂稳定性和膨胀作用限制膜层服役安全性等问题,本论文利用氧化石墨烯电性调控微弧氧化陶瓷膜层孔结构,制备具有减摩效应的GO/TiO₂微弧氧化自封孔陶瓷膜层,研究了氧化石墨烯浓度对微弧氧化陶瓷膜层孔结构和减摩性的影响。研究发现,通过氧化石墨烯改变电解液导电性等参数影响反应过程,从而实现对GO/TiO₂陶瓷膜层孔结的调控。氧化石墨烯浓度为5g/L时制备出了孔隙率、孔径和平均摩擦系数分别为3.6%、2.5μm和0.1的自封孔陶瓷膜层,相较于传统微弧氧化膜层分别下降了83.2%、78.4%和87.5%。研究认为通过控制氧化石墨烯浓度可以实现对微弧氧化陶瓷膜层表面孔结构的调控,为制备具有减摩效应的自封孔微弧氧化膜层提供了新思路。     

A facile strategy for fabricating self-sealing dense coating grown in-situ on porous silicon nitride ceramics

This present work explores initially the feasibility of producing in-situ surface oxidized coating on porous silicon nitride (Si₃N₄) ceramics. Theoretical prediction identifies the applied conditions of self-sealing strategy and oxidation time required to form dense coating. Experimentally, the porous Si₃N₄ ceramics with different pore structures were selected to fabricate in-situ oxidized coatings. The phase compositions, microstructures and mechanical properties of the porous Si₃N₄ ceramics were investigated before and after oxidation. The results show that flat and dense coatings are prevailed in all samples, which consist of amorphous SiO₂ and its precipitates besides dominant Si₃N₄ phase. The strengthened substrate and strengthening effect of coating are the essential mechanisms associated with the improved mechanical properties. Self-sealing method seems to offer an inexpensive and efficient route to prepare coating on porous Si₃N₄ ceramics.     

The Tianjin geothermal field (north-eastern China): Water chemistry and possible reservoir permeability reduction phenomena

Injection of spent (cooled) thermal fluids began in the Tianjin geothermal district, north-eastern China, at the end of the 1990s. Well injectivities declined after 3–4 years because of self-sealing processes that reduced reservoir permeability. The study focuses on the factors that may have caused the observed decrease in permeability, using chemical and isotopic data on fluids (water and gas) and mineral phases collected from production and injection wells. The results of data processing and interpretation indicate that (1) it is very unlikely that calcite and silica precipitation is taking place in the reservoir; (2) the Fe- and Zn-rich mineral phases (e.g. sulfides, hydroxides and silicates) show positive saturation indexes; (3) SEM and XRD analyses of filtered material reveal that the latter mineral phases are common; (4) visual observation of casings and surface installations, and of corrosion products, suggests that a poor quality steel was used in their manufacture; (5) significant quantities of solids (e.g. quartz and feldspar crystals) are carried by the geothermal fluid; (6) seasonal changes in fluid composition lead to a reduction in casing corrosion during the summer.     

铝阳极氧化膜在去离子水中的自封闭效应

采用交流阻抗法研究了工业纯铝L3阳极氧化膜在去离子水中的自封闭效应,并结合等效电路分析了氧化膜多孔层与阻挡层电化学参数的变化。结果表明,未封闭的阳极氧化膜在去离子水中浸泡初期存在一个明显的自封闭过程,导致阳极氧化膜在浸泡初期的耐蚀性提高。     

Structural and hydrogeological characterisation of the excavation-disturbed zone in the Opalinus Clay (Mont Terri Project, Switzerland)

The fracture network in the excavation-disturbed zone (EDZ) of an argillaceous formation, the Opalinus Clay in the Mont Terri Rock Laboratory in Switzerland, is characterised by applying structural and hydrogeological techniques. Unloading fractures are studied by mapping side walls of newly excavated niches and by analysing resin-injected overcores taken from the EDZ. The result is a structural data set of fracture orientations, frequencies, and extent of the EDZ. Pneumatic and hydrogeological tests are carried out in short boreholes crossing the EDZ to derive hydraulic parameters such as permeability and transmissivity distributions of the fracture network. Hydraulic transmissivities are orders of magnitude higher when compared to those of undisturbed rock, and are in the order between 1E−8 and 1E−7 m²/s. Regular repetition of these hydrotests resulted in decreasing transmissivities in the range of two orders of magnitude over a time period of about 2 years. These observations indicate a hydraulic self-sealing of the initially highly transmissive fracture network with ongoing saturation of the EDZ. The dynamic evolution of EDZ fractures can be derived by interpreting pore water pressure measurements during gallery excavation.Finally, all structural and hydrogeological information is synthesised in a conceptual model of the EDZ in the Mont Terri Rock Laboratory.     

Thermo-hydro-mechanical behavior of clay rock for deep geological disposal of high-level radioactive waste

In the context of deep geological disposal of radioactive waste in clay formations, the thermo-hydromechanical(THM) behavior of the indurated Callovo-Oxfordian and Opalinus clay rocks has been extensively investigated in our laboratory under repository relevant conditions:(1) rock stress covering the range from the lithostatic state to redistributed levels after excavation;(2) variation of the humidity in the openings due to ventilation as well as hydraulic drained and undrained boundary conditions;(3)gas generation from corrosion of metallic components within repositories; and(4) thermal loading from high-level radioactive waste up to the designed maximum temperature of 90℃ and even beyond to150℃. Various important aspects concerning the long-term barrier functions of the clay host rocks have been studied:(1) fundamental concept for effective stress in the porous clay-water system;(2) stressdriven deformation and damage as well as resulting permeability changes;(3) moisture influences on mechanical properties;(4) self-sealing of fractures under mechanical load and swelling/slaking of clay minerals upon water uptake;(5) gas migration in fractured and resealed claystones; and(6) thermal impact on the hydro-mechanical behavior and properties. Major findings from the investigations are summarized in this paper.     

新型高压水压试管机端头自密封装置的研制

为保证钢管质量,相关标准要求对钢管进行水压试验。传统的水压试管机存在生产效率低,使用和维护成本高,管端存在试压盲区等不足。介绍了一种新型高压水压试管机端头密封装置,可用于双线管材水压试验设备。其突出特点是,当钢管内充满高压水后,由于水压的作用使密封圈发生变形从而起到自密封作用。该密封装置适合于钢管超高水压外径密封。