Effect of thermal residual stresses on the strength for both alumina/Ni/alumina and alumina/Ni/nickel alloy bimaterials

International audienceThis paper describes some technical limitations encountered in joining ceramics-ceramics or ceramics-metals, and how, to some extent, they have been practically overcome. The effect of the residual stresses on the strength of joints fabricated between alumina-alumina or alumina and the nickel base alloy HAYNES 214TM using a solid-state bonding technique with Ni interlayer was studied. Finite element analyses (FEA) for the elastic-plastic and elastic-plastic-creep behavior have also been used to better design the joints and to predict their performance. It was found that the residual stresses caused by the thermal expansion mismatch between alumina (Al2O3) and the Ni-based superalloy (HAYNES 214TM) have severely deteriorated the joints compared to Al2O3-Al2O3 joint fabricated with the same solid-state bonding parameters. The high residual stresses zones obtained through the FEA simulation fitted well with the fractographic observations of the Al2O3/Ni/HAYNES 214TM joints. Also, in order to use the joint material as a structural material, the study about the effect of geometrical parameters has been performed. Optimal geometries have been determined

FEM CALCULATIONS AND EXPERIMENTAL DETERMINATION OF RESIDUAL STRESSES IN ALUMINA/NICKEL ALLOY JOINTS. OPTIMIZATION OF FABRICATION PARAMETERS

International audienceThe study relates to joints fabricated by solid state bonding between alumina and nickel alloy HAYNESTM214®, using an intermediate nickel metallic foil. Experimentally, damages and cracks often are observed close to the metal/ceramics interface. Consequently, the residual stresses distributions in the specimen were characterized experimentally using X-ray diffraction (XRD) and indentation techniques and predicted by Finite Element Analysis (FEA) calculations using an elastic-plastic-creep model. We demonstrate that a good correlation between FEA calculations and experimental results is obtained. Then, the effect of elaboration and geometrical parameters has been studied in order to minimize the residual stresses in alumina close to the metal-ceramics interface. However, the Al2O3/Ni/HAYNESTM214® system always leads to high residual stresses. To solve this problem, we show that the use of a multi-layer Cu/Ni/Cu joint, associated with the Direct Copper Bonding method (DCB), by pre-oxidation of copper, allows reducing significantly the tensile residual stresses in ceramics

Corrosion in amine solvents used for the removal of acid gases

International audienceProcesses using amine solvents for the removal of CO 2 have long been used for the treatment of natural gas. More recently, developments in the field of post-combustion CO 2 capture have gained considerable interest for greenhouse gas mitigation. Even though the nature of the amine is different for both applications, the process flow diagrams are very similar. The raw gas is contacted with the amine solvent in an absorber column, and the CO 2 is absorbed by the solvent. The rich solvent containing the CO 2 then circulates to a regenerator, where the temperature is increased to strip the CO 2. The lean solvent then returns to the absorber for a new cycle. For these processes, corrosion is known to be a major issue. Usually, amines are not corrosive but during operation of amine units, degradation of the solvent may occur due to the reaction with contaminants like oxygen. As a result, corrosive species are formed including other amines and acids products like oxalates, formates and acetates also known as Heat Stable Salts (HSS). Corrosion problems may also depend on other parameters for instance type and concentration of the alkanolamine, temperature of the solvent or CO 2 loading. In order to extend the limits of gas treating processes, but also to develop the new processes for CO 2 capture, a better understanding of corrosion in amine solvents is required. This paper presents the first results of a study where the impact of several parameters will be evaluated by electrochemical methods. The corrosivity of Monoethanolamine (MEA), Diethanolamine (DEA) and methyl-diethanolamine (MDEA) was compared. Other parameters were studied, among which the impact of CO 2 loading and the concentration in HSS. Back-

Influence of the Oxygen Partial Pressure on the High Temperature Corrosion of A 38Ni-34Fe-25Cr Steel in Presence of NaCl Salt

International audienceIn biomass gasification process, some molten salts of the feed can generate high temperature corrosions. In this study the chromia-forming austenitic alloy Haynes ® HR-120 was oxidized with a deposit of sodium chloride during 96 hours at 825°C and 900°C. Two different atmospheres were selected; one with a high oxygen partial pressure (Ar/O 2 90/10 %vol.) and one, named syngas, with a low oxygen partial pressure (CO/H 2 /CO 2 45/45/10 %vol.). While at 900°C the behaviour of the alloy in presence of sodium chloride was catastrophic in high oxidizing conditions, the impact of sodium chloride was insignificant in the other atmosphere. Under Ar/O 2 mixture, the catastrophic oxidation was attributed to the setting up of an active oxidation. At 900°C under syngas atmosphere, the protective behaviour of the alloy seems linked to the association of a faster evaporation of the salt and a very low oxygen partial pressure. At 825°C a catastrophic behaviour is observed under syngas atmosphere as the NaCl evaporation rate is much slower

Study of SiC-nickel alloy bonding for high temperature applications

International audienceWe have studied the formation of metal/ceramic joints by solid state bonding technique for applications at temperatures >600 ◦C. The bonding is obtained between silicon carbide (SiC) and Ni-based super-alloy (HAYNES® 214TM) via metallic foils (Ni, Ag). In some cases a thin coating on the ceramic or the alloy by the electroless JetMétalTM process has been used. Often used in brazing, nickel, when added to silicon carbide, usually give silicides. These reactions yield the "Pest Effect" ("pesting") that induces a catastrophic brittleness of this type of assembling. To minimize the reaction of these metals with silicon carbide, addition of elements limiting the "Pest Effect" on the one hand and, diffusion barriers on the other hand, have been performed. Indeed, the choice of the thin Ni0.93 B0.07 coating is based on the ability of boron of improving the mechanical properties of silicides, thus avoiding the "Pest Effect". However, we demonstrate that boron does not allow one to suppress the joint brittleness. Another new joining method employing a thin Ag coating or a Ag foil was tested. This process revealed the absence of chemical reaction at the Ag/SiC interface, thus proving the beneficial role of silver, which acts as an effective diffusion barrier for nickel beyond a certain thickness. This method has led to fabrication of joints presenting high shear resistance (>40MPa)

Evaluation of hydrogen embrittlement induced damages in steels using acoustic emission

International audienceIn the oil and gas industry, it is known that equipments which operate in hydrogen sulfide (H 2 S) media can be subjected to damages like Hydrogen Induced Cracking (HIC) and Sulfide Stress Cracking (SSC). At the present time, material selection for sour environments is generally obtained using standard tests (NACE TM0177 and NACE TM0284) which give information about the resistance of steels in sour service. These tests are suitable for a quick assessment of candidate steels by operators or steel suppliers. However they cannot provide detailed information to better understand and quantify the damage occurring during the service life of steel components. Acoustic emission (AE) is an efficient technique to monitor degradation of materials. In the present work it is used for the early detection, characterization and time progress description of cracking phenomena caused by hydrogen embrittlement of steel in sour media. The methodology used for the identification of AE sources related to HIC (H 2 bubbles, FeS corrosion layer, cracking) is described. AE results are discussed and correlated with crack length ratio and AE performance for quantifying HIC damage is evaluated

Hydrogen evolution in aqueous solutions containing dissolved CO2: Quantitative contribution of the buffering effect

International audienceThe hydrogen evolution reaction (HER) occurring on steel in an oxygen-free aqueous solution containing dissolved CO2 was investigated. This reaction was modelled by taking into account the two dissociation reactions of dissolved CO2. The mathematical problem was solved numerically using a finite element method (FEM). A fair agreement between the measurements performed on a steel rotating disc electrode and the theoretical calculations was obtained. Thus, the cathodic behaviour of steel in CO2-containing solutions can be fully explained by the buffering effect induced by the presence of CO2. Some interfacial pH measurements performed on a gold electrode also support this conclusion

The effect of chemical species on the electrochemical reactions and corrosion product layer of carbon steel in CO 2 aqueous environment: A review

International audienceThis paper summarizes the chemical effects that can occur during the corrosion process of carbon steel in a CO 2 saturated aqueous environment. Particularly, it focuses more on the results that small chemical contaminations in the environment have on the corrosion process. Underground waters present complex chemistry with several different dissolved ions (chlorides, carbonates) even in high concentrations that impact substantially on the corrosion rates of these materials. Moreover, gas impurities present in the gas mixture, such as oxygen in carbon capture and storage applications, constitute a supplementary form of significant contamination in the CO 2 saturated aqueous environment. In particular, the effect on both electrochemical reactions and corrosion product layer is examined for several chemical species that are commonly present either in the gas mixture or in underground waters

Near surface pH measurements in aqueous CO2 corrosion.

International audienceCorrosion of carbon steels in a carbon dioxide (CO2) corrosive environment is an important issue and in some cases the steel surface can be covered by a protective corrosion product film. Research has shown that under certain conditions, once a truly protective film has precipitated on the steel surface, the corrosion rate can decrease by an order of magnitude. Over many years, both quantitative and qualitative research has been carried out to further understand the initiation and growth of the protective film. However, a main limitation is in the correlation of film properties with bulk solution conditions. Research has shown that serious errors in predicting/reasoning can be made by operating with bulk instead of surface water chemistry conditions. This paper uses a pH sensor design to be used for real time surface pH measurement. The study shows a simultaneous electrochemical and surface pH analysis at two varying conditions of pH (pH 6 and pH 6.6) where a characteristic difference in the morphology and hence protectiveness of a corrosion product film is anticipate

L'apport de l’émission acoustique couplée à l’analyse thermogravimétrique pour la compréhension de la corrosion du fer par metal dusting

National audienceLa corrosion à haute température d'alliages métalliques (fer, nickel, alliages de cobalt) représente un enjeu majeur dans de nombreux domaines industriels. Grâce à sa sensibilité et à son caractère non destructif, l'émission acoustique (AE) apparait comme une méthode intéressante pour suivre en service les dégradations des équipements soumis à des formes sévères de corrosion comme le metal dusting en pétrochimie. Le but de notre étude est d'établir une base de données qui assigne les phénomènes de corrosion à haute température aux signaux acoustiques. L'analyse thermogravimétrique (TGA) a été couplée avec l'émission acoustique. Un guide d'ondes spécifique en alumine compatible avec une atmosphère réductrice riche en carbone comme avec une atmosphère oxydante a été développé. Son fonctionnement a été validé grâce à l’étude de l’oxydation d’un alliage de zirconium dont la couche de zircone superficielle se fissure après la transition cinétique et au refroidissement. Des échantillons de fer pur préalablement oxydés, la couche superficielle d’oxydes de fer favorisant le processus de carburation, ont été corrodés à 650°C sous un mélange gazeux contenant 5% d’isobutane et 5% d’hydrogène dans l’hélium. Les étapes préliminaires d’oxydation et de réduction (avant l’injection de l’atmosphère carburante) sont non émissives en accord avec l’absence de fissures dans les échantillons. Des signaux acoustiques sont en revanche détectés après une augmentation significative de la masse de l’échantillon pendant le palier isotherme. Ils cessent au refroidissement. La prise de masse est due à plus de 90% au dépôt de graphite pulvérulent qui contient des particules de fer et à la diffusion du carbone dans l’échantillon. La caractérisation par microscopie électronique de coupes d’échantillons prélevés à différents stades de l’essai nous permet d’attribuer les signaux d’émission acoustique d’amplitudes et énergies modérées à l’étape de carburation. Les évènements émissifs semblent provoqués par l’insertion du carbone dans la couche externe qui est poreuse après réduction et par la formation de la cémentite. Le couplage innovant TGA/EA in situ est un dispositif performant qui permet d’affiner nos connaissances sur la corrosion des métaux et alliages à haute température en complément des analyses post mortem. Les phénomènes diffusifs comme la diffusion du fer pendant l’oxydation ne sont pas émissifs lorsqu’ils n’entraînent pas de fissures. Les mécanismes de dégradation irréversibles, comme l'initiation et la propagation des fissures sont en revanche audibles et décelables dès leur apparition