Oxidation of a chromia-forming nickel base alloy at high temperature in mixed diluted CO/H2O atmospheres

International audienceCorrosion of a chromia-forming nickel base alloy, Haynes 230_, has been investigated under impure helium containing a few Pa of CO and H2O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H2O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H2O is the major oxidant of chromia-forming nickel base alloys in impure helium

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Digitalisat der Ausgabe von 1919, erschienen 201

Mechanism of destruction of the protective oxide layer on Alloy 230 in the impure helium atmosphere of Very High Temperature Reactors.

International audienceAlloy 230 which contains 22wt.% chromium could be a promising candidate material for structures and heat exchangers (maximum operating temperature: 850°-950°C) in Very High Temperature Reactors (VHTR). The feasibility demonstration involves to valid its corrosion resistance in the reactor specific environment namely impure helium. The alloys surface reactivity was investigated at temperatures between 850 and 1000°C. Two main behaviours have been revealed: the formation of a protective Cr/Mn rich oxide layer at 900°C and its following destruction at higher temperatures. Actually, above a critical temperature called TA, oxide is reduced at the oxide/metal interface by carbon in solution in the alloy. To ascribe the scale destruction, a model is proposed based on thermodynamic interfacial data for the alloy (chromium and carbon activity), oxide layer morphology and carbon monoxide partial pressure in helium. The proposed mechanism is then validated regarding experimental results and observations on alloy 230 and model alloys

Use of electrochemical techniques to study the corrosion of metals in model fluoride melts

Molten fluorides are appealing coolants for innovative nuclear systems but structural alloys may undergo corrosion at high temperature. Because corrosion primarily occurs via electrochemical reactions, electrochemical techniques are ideal for the study of corrosion thermochemistry and kinetics. Examples are given. An electrochemical series was established using voltammetry in LiF-NaF at 1173 K. Stability increases in the following order: Na, Cr, Fe, Ni, Mo/W, Ag, Au. Various alloys were also classified according to their oxidation resistance. A cathodic protection method was developed to curb the intergranular attack of some nickel alloys in molten LiF-CaF₂-MgF₂-ZrF₄ containing tellurium vapor at 953 K. Voltammetry and polarization resistance measurement were used to estimate the rate of chromium selective dissolution for nickel base alloys immersed in LiF-NaF at 1073 K and 1173 K

Severe hereditary spherocytosis and distal renal tubular acidosis associated with the total absence of band 3

Absence of band 3, associated with the mutation Coimbra (V488M) in the homozygous state, caused severe hereditary spherocytosis in a young child. Although prenatal testing was made available to the parents, it was declined. Because the fetus stopped moving near term, an emergency cesarean section was performed and a severely anemic, hydropic female baby was delivered. She was resuscitated and initially kept alive with respiratory assistance and hypertransfusion therapy. Cord blood smears revealed erythroblastosis, poikilocytosis, and red cells with stalk-like elongations. Band 3 and protein 4.2 were absent; spectrin, ankyrin, and glycophorin A were significantly reduced. Renal tubular acidosis was detected by the age of 3 months. Nephrocalcinosis appeared soon thereafter. After 3 years of follow-up the child is doing reasonably well on a regimen that includes regular blood transfusions and daily bicarbonate supplements. The long-term prognosis remains uncertain given the potential for hematologic and renal complications

Stress-strain curves and derived mechanical parameters of P91 steel from spherical nanoindentation at a range of temperatures

Nanoindentation allows extracting local mechanical properties out of small regions of interest, such as welds, coatings or ion-irradiated layers. Probing the surface with spherical tips combined with several data analysis procedures allows deriving the complete elastic-plastic behaviour of the material under test, from the initial elastic response during loading, to the onset of plasticity and the post-yield behaviour. This works aims at comparing different measurement and analysis protocols to spherical nanoindentation tests performed at different temperatures on a ferritic/martensitic P91 grade steel, in order to derive meaningful indentation stress−strain curves (ISSC) and estimate parameters such as indentation modulus, yield strength, work hardening exponent and ultimate tensile strength. Indentations using spherical indenters have been carried out from room temperature to 600°C in vacuum in a set-up where thermal drift has been minimised by an active surface referencing system and an accurate temperature stabilisation in the contact area. To evaluate indentation tensile properties from nanoindentation results, the determination of the contact area, the definition of representative stress and strain, and the fitting to constitutive equations are the important steps, the most adequate choice of which is still matter of discussion and may depend on the instrument, material analysed and testing procedure. In the present work it is shown that the methodology used to determine the radius of the contact is critical to achieve consistent results. The geometrical definition of the contact radius provides a consistent shape of the ISSC; however it requires a good calibration of the true indenter radius as a function of depth. On the other hand, the Hertz model for the contact radius is very sensitive to the measurement of stiffness and presumes that the elastic modulus of the material is known or derived form the initial loading. The application of the different combinations of contact radius and strain definitions to nanoindentation data obtained by multi-cycle and continuous stiffness measurements revealed that Tabor’s approach combined with geometrically determined radius best represented the ISSC relationship for the P91 characterized. This method was then extended to predict the high temperature tensile properties of the steel. The results of the nanoindentation characterization will be presented and discussed thereby comparing the performance of different measurement and analysis protocols. Please click Additional Files below to see the full abstract

Investigation of crack propagation resistance of 304L, 316L and 316L(N) austenitic steels in liquid sodium

In order to assess the susceptibility of candidate structural materials to Liquid Metal Embrittlement (LME), the fracture behavior of three grades of austenitic steels was investigated in oxygenated (200 wppm) liquid sodium in the temperature range [473–673 K] on notched axisymmetric tensile specimens. The tests were carried out in an inert glove box at very low concentrations of dioxygen and humidity (<1 ppm) to prevent further contamination after pre-exposure in oxygenated liquid sodium. A decrease in crack propagation resistance of the three austenitic steels (304 L, 316 L(N), 316 L) is observed in oxygenated liquid sodium (200 wppm) from 573, 623 and 673 K respectively after pre-wetting in oxygenated sodium. This reduction is correlated with a ductile to brittle change of the fracture surface. This effect observed with the three austenitic steels is attributed to the onset of LME after significant plastic deformation