Study of oxidation and corrosion at high temperature for nickel based polycristalines alloys

Le développement de nouvelles générations de turboréacteurs se base sur un rendement énergétique amélioré, ce qui conduit notamment à une diminution de la consommation de carburant. Un des moyens pour parvenir à une meilleure efficacité moteur consiste à augmenter la température en service du turboréacteur. Cette élévation de température, estimée à 50°C soulève des problématiques nouvelles et surtout pour les disques de turbines basse pression qui voient désormais localement des températures pouvant atteindre 700°C. En effet, l'alliage utilisé jusqu'alors - l'Inconel 718 - n'est plus stable microstructuralement à des températures supérieures à 650°C. Des nouveaux alliages sont donc développés pour répondre à cette problématique. Néanmoins, les sous - produits de combustion (SO2/SO3) associés aux particules ingérées en vol (sels marins, cendres, sable...) peuvent mener à la formation de dépôt alcalins (Na2SO4) et conduire alors à l'établissement d'un environnement particulièrement agressif. Les phénomènes mis en jeu dans ce phénomène de dégradation connu sous le nom de corrosion de type - II (ou également L.T.H.C) sont mal connus et varient fortement en fonction des paramètres expérimentaux choisis. Les travaux de cette thèse se divisent en trois parties distinctes. Dans les deux premières, trois alliages commerciaux sont comparés tour à tour à 650/700°C sous air du laboratoire, puis sous conditions de corrosion de type - II (650/700°C sous Air +400-1000 ppmSO2). L'inconel 718 montre dans tous les cas le meilleur comportement suivi de l'AD730 pour ensuite finir par le René 65. Afin de compléter l'analyse, des alliages modèles ont également été élaboré en ciblant certains éléments d'alliage pour préciser leur rôle dans le phénomène de corrosion chaude basse température.The development of next generation turbojet is based on a better energetic performance which leads to the reduction of fuel consumption (- 15%) and fine particle emission. A way to achieve such engine - efficiency consists in increasing the service temperature of the engine. This increase of temperature, estimated to be of 50°C, raises some new issues, and specifically for low pressure turbine discs that are submitted locally to temperatures between 650 and 700°C. Indeed, the alloy hitherto used for those engine parts - Inconel 718 - is not stable above 650°C. New alloys have been design to face this issue. Nevertheless, some combustion by-products (SO2/SO3) associated with ingested particles (salts, ashes, dust) can lead to the formation of alkaly deposits (Na2SO4) and thus yielding to an aggresive environment. The phenomena involved in this kind a rapid degradation known as type - II hot corrosion (or LHTC) are not very well known and can vary from one study to another according to the chosen set of experimental parameters. The work proposed here is made of three main parts. For the two firsts, three commercial nickel based alloys are compared at 650°C and 700°C both under long - term (10 000h) oxidation and short - term (100h) hot corrosion (Air + 400 - 1000 ppm SO2) conditions. Inconel 718 shows the best behavior in every case scenari , followed by AD730 and René 65. It shows indeed the lowest kinetic constant under oxydation and the lowest amount of degradation under hot corrosion. To complete the study, a set of model alloys have been manufactured by targeting specific alloying element to better understand their role in type - II hot corrosion

Corrosion in molten chlorides - CEA developments and research program

International audienceA French R&D program (CEA, CNRS, Orano) has been launched on molten salts cooled reactors which aims at assessing the feasibility of fast MSRs and confirm their potential assets. Chloride salts have been selected and in the French context (closed cycle, transmutation, Pu multirecycling), we focus on chlorides fast MSR with a focus on actinides conversion (reducing ultimate wastes). One of the major constraints on the use of molten chlorides is the corrosion behaviour of structural materials. To overcome this constraint, different approaches are performed with different time scales:- Evaluation of already available and /or qualified materials such as nickel base alloys but also high entropy alloys or ceramics.- Evaluation of different corrosion protection strategies: protective coatings, electrochemical potential control, chemistry control…- Evaluation of more innovative solutions: advanced manufacturing processes, smart materials, architectured materials…In the same time a large French research program has been launched together with CEA and CNRS to work on the global acceleration of materials discovery, the DIADEM (DIscovery Acceleration for the Deployment of Emerging Materials) project. In the frame of this large project which will be launched in 2022 for 8 years, shorter targeted projects have been already selected (duration of 3 years), as the A-DREAM project which is dedicated to the acceleration of the development of corrosion resistant materials in molten chlorides. For this, the A-DREAM project proposes an integrated approach implementing: (i) the digital design of materials/coatings, (ii) the high throughput synthesis of these materials and (iii) the implementation of an accelerated corrosion methodology. In this presentation, the French corrosion strategy for MSR will be presented together with results obtained