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Poster De Conférence Année : 2015

In situ high temperature oxidation analysis of metallic alloys using acoustic emission coupled with thermogravimetry

Résumé

High temperature corrosion of metallic alloys causes significant damage on industrial pipes, tanks or reactors in the field of refining and petrochemical processes. Oxidation or carburization can create stresses in the corroded outer layer of the alloys; the relaxation of these stresses produces cracks followed by transient elastic waves. Different methods allow to record and analyze these transient elastic waves. In this study an innovative In situ equipment has been developed. Thermogravimetric analysis (TGA) has been coupled with an acoustic emission (EA) device to measure acoustic signals emitted during the corrosion process at high temperature. The sample mass variation was recorded simultaneously with the acoustic signals in order to understand the corrosion mechanisms. Piezoelectric sensors were fixed on the cold part of a symmetric thermogravimetric analyzer. An alumina waveguide used to transmit waves from the metal sample to sensors was specially designed based on several factors: internal diameter of the furnace, maximum acceptable weight for the balance, experimental atmosphere conditions, temperature. Moreover alumina best preserves the waveforms. The Hsu Nielsen test was applied to check the waveguide at room temperature. To check it at high temperature, a test based on the oxidation of a zirconium alloy which produces cracks in the oxide layer was developed and presented here. Zircaloy-4 oxidation behavior at high temperature (900°C), which can be reached in case of severe accidental situations in nuclear pressurized water reactor, was studied using acoustic emission analysis coupled with thermogravimetry. Two different atmospheres were used to study the oxidation of Zircaloy-4: a-helium and pure oxygen, b- helium and oxygen combined with slight addition of air. The mass gain rate curve under 20% of oxygen confirms the parabolic law due to oxygen diffusion in the inward oxide scale. Under a mixture of oxygen and air in helium, an acceleration of the corrosion was observed due to the detrimental effect of nitrogen. The kinetic rate increased significantly after a kinetic transition (breakaway). This acceleration was accompanied by an acoustic emission activity. Most of the acoustic emission bursts were recorded after the kinetic transition (post-transition) or during the cooling of the sample. The characteristic features of the acoustic emission signals clearly appear to be strongly correlated with the different populations of cracks and their occurrence in the ZrO2 layer or in the α - Zr(O) layer. Acoustic events were recorded during the isothermal dwell time at high temperature under air. They were associated with large cracks in the zirconia porous layer. Acoustic events were also recorded during cooling after oxidation tests both under air or oxygen. For the latter, cracks were observed in the oxygen enriched zirconium metal phase and not in the dense zirconia layer after 5 hours of oxidation. Thermogravimetric experiments coupled with acoustic emission analysis are of interest in improving understanding of metallic material corrosion at high temperature. AE analysis is complementary for post-mortem oxidized sample characterizations. The AE technique has enabled detailed study of Zircaloy-4 oxidation behavior at high temperatures.
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Dates et versions

hal-01252720 , version 1 (08-01-2016)

Identifiants

  • HAL Id : hal-01252720 , version 1

Citer

Véronique Peres, Omar Al Haj, Eric Serris. In situ high temperature oxidation analysis of metallic alloys using acoustic emission coupled with thermogravimetry. Gordon Research Conference High Temperature Corrosion, Jul 2015, New London, United States. . ⟨hal-01252720⟩
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