Corrosion Management of the Hanford High-Level Nuclear Waste Tanks

The Hanford site is located in southeastern Washington State and stores more than 200,000 m³ (55 million gallons) of high-level radioactive waste resulting from the production and processing of plutonium. The waste is stored in large carbon steel tanks that were constructed between 1943 and 1986. The leak and structurally integrity of the more recently constructed double-shell tanks must be maintained until the waste can be removed from the tanks and encapsulated in glass logs for final disposal in a repository. There are a number of corrosion-related threats to the waste tanks, including stress–corrosion cracking, pitting corrosion, and corrosion at the liquid–air interface and in the vapor space. This article summarizes the corrosion management program at Hanford to mitigate these threats.     

HCl-Induced High Temperature Corrosion of Stainless Steels in Thermal Cycling Conditions and the Effect of Preoxidation

Gaseous HCl released during combustion is one reason for the severe materials degradation often encountered in power generation from waste and biomass. In this study, three stainless steels (the low alloyed EN 1.4982, the standard EN 1.4301 and the higher alloyed EN 1.4845) were tested by repeated thermal cycling in an environment comprising N₂?C10%O₂?C5%H₂O?C0.05%HCl at both 400 and 700 °C. The materials were exposed with ground surfaces and preoxidised at 400 or 700 °C. A positive effect of preoxidation is evident when alloys are exposed at 400 °C. Oxide layers formed during preoxidation effectively suppress chlorine ingress for all three materials, while chlorine accumulation at the metal/oxide interface is detected for surface ground specimens. The positive effect of preoxidation is lost at 700 °C and corrosion resistance is dependent on alloying level. At 700 °C metal chloride evaporation contributes significantly to the material degradation. Based on the results, high temperature corrosion in chlorinating environments is discussed in general terms.     

Hot Corrosion Behavior of Some Superalloys in a Simulated Incinerator Environment at 900 °C

Incinerators are being used to burn solid waste of all types. This burning of waste creates a very aggressive environment at extremely high temperature. This environment attacks the various components of the incinerators. Some studies have been reported regarding behavior of steels in simulated incinerator environment at 550 °C. In present work superalloys Superco 605, Superni 600, and Superni 718 have been subjected to cyclic oxidation in 40 wt.% K₂SO₄ + 40 wt.% Na₂SO₄ + 10 wt.% KCl + 10 wt.% NaCl environment at 900 °C under cyclic condition. Weight change measurements have been done and weight change has been plotted against the numbers of cycles. The oxide scales formed on the surface of the corroded superalloys have been characterize by FESEM, EDS, XRD, cross-sectional analysis, and x-ray mapping. The nickel-based superalloys Superni 600 and Superni 718 indicated better resistance to corrosion in the above environment whereas Superco 605 lead to massive weight gain.     

Mechanical Properties of Carbon Fiber-Reinforced Aluminum Manufactured by High-Pressure Die Casting

Carbon fiber reinforced aluminum was produced by a specially adapted high-pressure die casting process. The MMC has a fiber volume fraction of 27%. Complete infiltration was achieved by preheating the bidirectional, PAN-based carbon fiber body with IR-emitters to temperatures of around 750 °C. The degradation of the fibers, due to attack of atmospheric oxygen at temperatures above 600 °C, was limited by heating them in argon-rich atmosphere. Additionally, the optimization of heating time and temperature prevented fiber degradation. Only the strength of the outer fibers is reduced by 40% at the most. The fibers in core of fiber body are nearly undamaged. In spite of successful manufacturing, the tensile strength of the MMC is below strength of the matrix material. Also unidirectional MMCs with a fiber volume fraction of 8% produced under the same conditions, lack of the reinforcing effect. Two main reasons for the unsatisfactory mechanical properties were identified: First, the fiber-free matrix, which covers the reinforced core, prevents effective load transfer from the matrix to the fibers. And second, the residual stresses in the fiber-free zones are as high as 100 MPa. This causes premature failure in the matrix. From this, it follows that the local reinforcement of an actual part is limited. The stress distribution caused by residual stresses and by loading needs to be known. In this way, the reinforcing phase can be placed and aligned accordingly. Otherwise delamination and premature failure might occur.     

Reinterpretation of Type II Hot Corrosion of Co-Base Alloys Incorporating Synergistic Fluxing

The components of gas-turbine engines operating in marine environments are highly susceptible to hot corrosion, which is typically classified as Type II (650–750 °C) and Type I (900–950 °C) hot-corrosion attack. Even though hot-corrosion has been widely investigated in the last 50 years, several critical questions remain unanswered and new ones have emerged based on recent observations that, in part, are associated with the increasing complexity of the alloy systems and the sulfate-deposit chemistries. The present work is focused on the Type II hot-corrosion mechanism for Co-base alloys. Observations for a CoCrAlY model alloy (isothermally exposed at 700 and 800 °C under different atmospheres, including: air and O₂ with 100 and 1000 ppm SO₂) suggest the rapid dissolution of Co (as Co-oxide) is not the controlling factor in the degradation mechanism, as was proposed by Luthra, since the γ-phase which is richer in Co, is not attacked as significantly as the Al-rich β-phase. To the contrary, it is suggested that Al (and Cr) is (are) the element(s) which is (are) removed first. A modified interpretation of the Type II hot-corrosion mechanism is proposed, which is based on the synergistic fluxing model developed by Hwang and Rapp.     

Quality Considerations for the Evaluation of Thermal Spray Coatings

Revealing the true structural and mechanical properties is of utmost importance for the optimized use of thermal sprayed coatings. Only the true properties can be expected to correlate to the spray parameters. During the recent decade, the gas turbine industry has experienced a focus on the laboratory procedures being the weakest link in a frozen and robust process. This article will show several results indicating that the laboratory procedures are more essential to the evaluation results than the spray parameters themselves. With new and robust laboratory techniques, the true properties of thermal spray coatings are revealed, causing a major problem with respect to the quality standards developed 30-40 years ago. In many cases, these old specifications need updates, which is a difficult task from a cost, time, and quality perspective for OEM’s. Coatings that have been successfully used for almost half a century no longer conform to the specification they were optimized to, because of these new appropriate laboratory techniques and procedures. What is actually meant when stating the following? (1) The coating has 5% porosity; (2) No cracks are allowed; (3) Tensile bond is 50 Mpa; (4) Hardness is 1000 HV; and (5) Coating thickness is 100 μm. This article also initiates a discussion on the measurement inaccuracies, for testing of thermally sprayed coatings, with respect to the commonly used general international standards (such as QS9000, ISO17025, AS9003, and ISO10012), as well as with respect to recommendations from the Six Sigma methodology.     

Influence of Surface Finish on the Cyclic Oxidation Lifetime of an EB-PVD TBC, Deposited on PtAl and Pt-diffused Bondcoats

Thermal barrier coatings (TBCs) are well established as protective systems for gas turbine hot path components, due to their ability, with substrate cooling, to reduce the maximum surface temperature experienced by the metal component. However, when subject to high temperature oxidation, cyclic heating and cooling during service, TBCs degrade in both thermal protection capability and mechanical stability as a result of a combined thickening of the alumina-thermally grown oxide and sintering of the ceramic top coat. Eventually the ceramic top coat spalls from the metallic substrates. The detailed failure mechanisms for the TBC often are complicated, reflecting a balance between defects introduced into the TBC during manufacture and service and the stored energy generated in the TBC as a result of cyclic thermal exposure. It has been shown that the surface finish influences the residual stress in the thermally grown oxide and thus the stored energy. In this study, the influence of substrate surface finish, prior to bondcoat manufacture, on the cyclic oxidation lifetime is examined. Two EB-PVD TBC systems, a zirconia 8 wt% yttria topcoat on a platinum aluminide bondcoat and a zirconia 8 wt% yttria topcoat on a platinum diffused γ+γ′ bondcoat have been studied. For these two systems, various substrate surface finishes have been investigated, including ground, grit blasted and polished and grit blasted surfaces. The lifetime data for these cyclic oxidation tests of EB-PVD TBCs on these two diffusion bondcoats, platinum aluminide and platinum diffused, on CMSX4, have been analysed statistically for the various surface finishes. It is shown that the variability in measured lifetime can be modelled using Weibull statistics. The role of surface finish on the Weibull model parameters, characteristic life (η) and Weibull modulus (β), are discussed in this paper and hence the role surface finish plays on the likelihood of early, short life, TBC failure. Based on this analysis a more optimised surface finish is recommended to extend TBC lifetimes with diffusion based bondcoats. Further, the platinum diffusion bondcoat is shown to outperform the platinum aluminide system once the substrate surface finish has been optimised.     

On the Degradation Modes and Oxidation Behavior of Platinum Aluminide Bond Coats in Thermal Barrier Coating Used as Surface Protection System for a Turbine Blade Superalloy

Adhesion of thermally grown oxide (TGO) to the bond coat is known to limit the useful life of thermal barrier coatings used in gas turbine blade applications. This is determined by the structure and composition of the bond coat as well as its thermal stability and in turn, its ability to develop and maintain a protective oxide. In this study, the degradation modes of platinum aluminides of the β-(Ni,Pt)Al- and PtAl₂ + β-(Ni,Pt)Al-types used as bond coats in thermal barrier coatings deposited on Ni-base superalloy and utilizing zirconia-7 wt% yttria and as top coat have been examined. Thermal exposure tests have been carried out at 1150 °C with cycling to room temperature every 24 h. Various electron-optical techniques have been used to characterize the microstructures of the bond coats and TGO. Particular emphasis has been placed upon the susceptibility of the bond coat to degradation by interdiffusion, oxidation, rumpling and formation of internal cavities. It is shown that the oxidation behavior and thermal stability characteristics are functions of the exact distribution of Pt in the bond coats. The β-(Ni,Pt)Al-type bond coat is found to have higher thermal stability and oxidize at a slower rate in comparison with the PtAl₂ + β-(Ni,Pt)Al₂-type. However, both bond coats are observed to exhibit a similar behavior in that the Al-rich and Pt-modified β-phase is progressively transformed into the Al-depleted γ′- and γ-phases with continued thermal exposure but at a slower rate in the β-(Ni,Pt)Al bond coat. Under the test conditions used in the study, there has been no evidence for rumpling, however, internal cavities are observed near the surface of each bond coat during the later stages of thermal exposure showing that rumpling is not necessarily a prerequisite. Failure of the respective thermal barrier coating systems is found to occur by loss of adhesion between the TGO and bond coat whose composition has approached that of the superalloy substrate by interdiffusion.     

Recovery of waste rare earth fluorescent powders by two steps acid leaching

The effects of the acid leaching and alkali fusion on the leaching efficiency of Y,Eu,Ce,and Tb from the waste rare earth fluorescent powders were investigated in this paper.The results show that hydrochloric acid is better than sulfuric acid in the first acid leaching,and NaOH is better than Na2CO3in the alkali fusion.In the first acid leaching,the Wloss is 20.94%when the waste rare earth fluorescent powders are acid leached in H?concentration 3 mol L-1and S/L ratio 1:3 for 4 h due to red powders dissolved.The better results of the alkali fusion can be got at 800℃ for 2 h when the NaOH is used.The blue powders and the green powders can be dissolved into NaAlO2and oxides such as rare earth oxide(REO).The REO can be dissolved in H?concentration 5 mol L-1,S/L1:10 for 3 h in the second acid leaching.The leaching rates of the Y,Eu,Ce,and Tb are 99.06%,97.38%,98.22%,and 98.15%,respectively.The leaching rate of the total rare earth is 98.60%.     

Predicting the Lifetimes of Nuclear Waste Containers

As for many aspects of the disposal of nuclear waste, the greatest challenge we have in the study of container materials is the prediction of the long-term performance over periods of tens to hundreds of thousands of years. Various methods have been used for predicting the lifetime of containers for the disposal of high-level waste or spent fuel in deep geological repositories. Both mechanical and corrosion-related failure mechanisms need to be considered, although until recently the interactions of mechanical and corrosion degradation modes have not been considered in detail. Failure from mechanical degradation modes has tended to be treated through suitable container design. In comparison, the inevitable loss of container integrity due to corrosion has been treated by developing specific corrosion models. The most important aspect, however, is to be able to justify the long-term predictions by demonstrating a mechanistic understanding of the various degradation modes.     

带缺陷液化气球罐安全运行分析

某炼化企业于上世纪70年代建造的2台液化气球罐,投用3个月就发生开裂泄漏,经修复带缺陷运行,在服役30年后,因应力腐蚀开裂报废。本文结合硫化氢应力腐蚀开裂的机理和影响因素,对这两台液化气球罐的运行条件进行了深入分析,给出了液化气球罐运行安全控制建议。     

Corrosion Behavior of Alloy 600 in Simulated Nuclear High Level Waste Medium

Nickel-based alloys are being considered as candidate materials for the storage of high level waste. In the present investigation, Alloy 600 was assessed by potentiodynamic anodic polarization technique for its corrosion behavior in the as-received, solution annealed, and sensitized condition in 3 M HNO₃ and 3 M HNO₃ containing simulated high level waste. From the results of the investigation, it was found that the solution annealed specimen possesses superior corrosion resistance compared to the as-received and sensitized specimen. Double loop electrochemical potentiokinetic reactivation test was carried out to study the degree of sensitization. The effect of different concentrations of chloride ions in 3 M HNO₃ at 25 °C indicated tendency for pitting as the concentration of chloride ions was increased. Microstructural examination was carried out by optical microscope and scanning electron microscope after electrolytic etching. X-ray photoelectron spectroscopy study was carried out to investigate the passive film formed in 3 M HNO₃ and 3 M HNO₃ simulated high level waste.     

Friction Stir Welding of Low-Carbon AISI 1006 Steel: Room and High-Temperature Mechanical Properties

Friction stir welding (FSW) is an ecologically benign solid-state joining process. In this work, FSW of low-carbon AISI 1006 steel was carried out to study the microstructure and mechanical properties of the resulting joints at both room temperature (RT) and 200 °C. In the parameter space investigated here, a rotational tool speed and translation feed combination of 1200 rpm and 60 mm/min produced a defect-free weld with balanced mechanical properties and a superior Vickers microhardness profile compared to all other conditions and to base metal (BM). At faster translation feeds (100 and 150 mm/min), wormhole defects were observed in the weld microstructure and were attributed to higher strain rate experienced by the weld zone. Under tensile loading, welded material exhibited yield strength that was up to 86 and 91% of the BM at RT and 200 °C, respectively. On the other hand, tensile strength of welded material was nearly similar to that of the base metal at both RT and 200 °C. However, at both temperatures the tensile ductility of the welded joints was observed to be significantly lower than the BM. Annealing of the 1200 rpm and 60 mm/min FSW specimen resulted in tensile strength of 102% compared to base material and 47% increase in the strain at failure compared to the as-welded specimen. The Charpy impact values revealed up to 62 and 53% increase in the specific impact energy for the 1200 rpm and 60 mm/min welded joints as compared with the BM.     

胜利油田立式常压储罐的腐蚀分析及检测技术

立式钢制焊接常压储罐是油田油气储运的重要设备.通过对胜利油田立式常压储罐的使用情况和检测结果,分析小结了常压储罐发生腐蚀的主要部位,分析了腐蚀发生的原因.介绍了胜利油田现用的腐蚀检测方法,提出了常压储罐的腐蚀防护措施.     

Waste incineration corrosion processes: Oxidation mechanisms by electrochemical impedance spectroscopy

Molten chloride mixtures are formed in waste incineration plants during waste firing and energy production. These mixtures are responsible for degradation processes like hot corrosion. In order to evaluate the damage of molten salt mixtures in waste incineration environments, the alloys 625 and 617 were exposed beneath a molten KCl‐ZnCl₂ mixture at 650 °C in air. The corrosion process was monitored by electrochemical impedance spectroscopy (EIS). An extensive microscopy analysis has been done in order to correlate the electrochemical results, and to establish an electrochemical mechanism for such high temperature corrosion processes.     

Solutioning and Aging of MAR-M247 Nickel-Based Superalloy

Despite the existence of previous studies on the heat treatment of the MAR-M247 superalloy, there is a lack of microstructural characterization data that support the heat-treatment conditions that are proposed in this study. Thus, the aim of this study is to investigate the changes in microstructure that occur in this alloy when subjected to different solutioning and aging heat treatments. Thermodynamic calculations and differential thermal analysis guided the experimental design and the analysis of experimental results. The MAR-M247 superalloy was produced via vacuum induction melting and investment casting. The samples were solutioned between 1185 and 1270 °C and aged between 770 and 980 °C. The as-cast and heat-treated samples were characterized using scanning electron microscopy in backscattered electron and secondary electron modes. Thermodynamic calculations have shown that the minimum solutioning temperature is approximately 1220 °C, occurring in a γ + MC + MB₂ three-phase field (M = metal). The samples were solutioned at 1250 °C for 310 min before aging heat treatment. During solutioning the carbide composition is the MC phase shifts to higher hafnium (Hf) and lower tantalum (Ta) content, which is in agreement with the thermodynamics calculations. After solutioning, residual aluminum (Al) segregation leads to the formation of large γ′ particles in certain regions of the material following one-step aging heat treatment at 770 and 870 °C. However, a nearly uniform size distribution of γ′ particles was observed after aging at 980 °C as well as after double aging heat treatment at 980 °C for 300 min + 870 °C for 1200 min.     

压力容器无损检测--大型常压储罐的无损检测技术

常压储罐在原油、化学危险品的储存和输送过程中起着重要作用,目前我国拥有的5 000 m3以上储罐就有2万多台。在使用过程中,罐底腐蚀泄漏是引起储罐失效的主要原因,对储罐进行相关的检验可以了解储罐的使用状况,预防储罐失效。简要介绍无损检测技术在储罐检验中的应用,尤其对声发射和罐底板漏磁扫查技术检测储罐泄漏和腐蚀进行了重点介绍,并提出了相关的检测工艺要求。     

Corrosion Behavior of MIG Brazed and MIG Welded Joints of Automotive DP600-GI Steel Sheet

Galvanized dual-phase steel sheets are extensively used by the auto industry for their corrosion resistance property. Welding by the metal inert gas (MIG) process causes degradation of the steel in the vicinity of the joint due to excessive zinc evaporation. In order to minimize Zn loss, the MIG brazing process has been tried out in lap joint configuration over a heat input range of 136–204 J mm^?1. The amount of zinc loss, intermetallic formation and corrosion properties in the joint area has been evaluated for both MIG brazing and MIG welding. Corrosion rate of 21 mm year^?1 has been reduced to 2 mm year^?1 by adopting MIGB in place MIGW. Impedance study has shown that the corrosion mechanism in base metal, MIG brazed and MIG welded joints is dominated by charge transfer, diffusion and mixed mode control processes, respectively.     

The Effect of Surface Chemistry and Morphology on the Properties of HVAF PEEK Single Splats

Thermal spray of polymers has had limited investigation due to the narrow processing windows that are inherent to polymer powders, especially their low temperatures of thermal degradation. The polymer poly aryl ether ether ketone (PEEK) has a continuous use temperature of 260 °C, does not suffer significant thermal degradation below 500 °C (Lu et al., Polymer, 37(14):2999-3009, 1996), and has high resistance to alkaline and acidic attack. These properties led to PEEK being selected for investigation. To minimize thermal degradation of the particles, the high velocity air fuel technique was used. To investigate the effect of substrate pretreatment on single splat properties, single splats were collected on aluminum 5052 substrates with six different pretreatments. The single splats collected were imaged by scanning electron microscopy and image analysis was performed with ImageJ, an open source scientific graphics package. On substrates held at 323 °C, it was found that substrate pretreatment had a significant effect on the circularity and area of single splats, and also on the number of splats deposited on the substrates. Increases in splat circularity, area, and the number of splats deposited on the surface were linked to the decrease in chemisorbed water on the substrate surface and the decrease of surface roughness. This proved that surface chemistry and roughness are crucial to forming single splats with good properties, which will lead to coatings of good properties.     

Modeling the Gas Nitriding Process of Low Alloy Steels

The effort to simulate the nitriding process has been ongoing for the last 20 years. Most of the work has been done to simulate the nitriding process of pure iron. In the present work a series of experiments have been done to understand the effects of the nitriding process parameters such as the nitriding potential, temperature, and time as well as surface condition on the gas nitriding process for the steels. The compound layer growth model has been developed to simulate the nitriding process of AISI 4140 steel. In this paper the fundamentals of the model are presented and discussed including the kinetics of compound layer growth and the determination of the nitrogen diffusivity in the diffusion zone. The excellent agreements have been achieved for both as-washed and pre-oxided nitrided AISI 4140 between the experimental data and simulation results. The nitrogen diffusivity in the diffusion zone is determined to be constant and only depends on the nitriding temperature, which is ~5 × 10^?9 cm²/s at 548 °C. It proves the concept of utilizing the compound layer growth model in other steels. The nitriding process of various steels can thus be modeled and predicted in the future.