A method is proposed to allow a more accurate evaluation of thermogravimetric data to identify diffusion or partial diffusion control of scaling kinetics. This method is based on the ® tting of mass-gain data to a parabola over a short time interval. The translation of the time interval over the entire test time period provides an actual instantaneous parabolic rate constant independently of any transient stage or simultaneous reaction steps. The usefulness and limitations of this procedure are illustrated from oxidation tests performed on several metallic materials (pure nickel, single-crystal superalloys, and Nb± Ti± Al alloy).

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A B S T R A C TThe oxidation behaviour of IN 718 alloys produced by laser beam melting and electron beam melting was compared to that of the wrought alloy at 850°C in laboratory air. Oxide scales of all alloys were similar in nature and morphology with small differences due to powder particles sintered on the surface of additive manufacturing parts. Nevertheless, major differences in surface topography were noticed, these could affect surface area estimations and consequentlymass gain estimations. A quantitative correlation was determined between apparent parabolic rate constant and surface area.

International audienceThis paper presents a study on the Ti-6Al-4V behaviour in presence of NaCl deposit under dry and moistair environments at 560◦C. The results evidence a detrimental effect of the NaCl deposit with a synergisticeffect in presence of moist air environment. Treatments under dry and moist air with NaCl deposit for600 h, lead respectively to weight gains per unit area 5 and 15 times higher than observed under classicoxidation in dry air. Enhancement of the corrosion phenomenon is attributed to the presence of gaseousmetal chlorides, leading to the establishment of an active corrosion process

Model Fe-Cr and Fe-Cr-Ni alloys were exposed to Ar-CO 2 and Ar-CO 2-H 2 O gas mixtures at 650 • C. While all alloys initially formed protective Cr 2 O 3 scales, nucleation and growth of iron-rich oxide nodules resulted in some cases in breakaway oxidation. The conditions leading to departure from the protective stage are discussed in terms of Cr 2 O 3 thermodynamic and kinetic stability. The morphological and compositional evolutions accompanying nodule development were examined. The influence of carbide precipitation on alloy chromium diffusion and the ability of the alloy to form and maintain Cr-rich oxide layers was investigated.

The oxidation kinetics of high-purity nickel were studied between 500 and 1200°C, in pure oxygen at atmospheric pressure, for aûerage oxide-scale thicknesses of 1, 5, 10, and 30 µm. In the oûerall temperature range studied, a decrease in the parabolic rate constant k p with increasing scale thickness was obserûed. Depending on temperature and oxide-scale thickness, growth kinetics can be interpreted as a mixture of parabolic-and cubic-growth kinetics. Possible correlations between growth kinetics and microstructures of the oxide scales were inûestigated. From this set of experimental data, oxidation-kinetics models were tested. In particular, the effect of grain-boundary diffusion on NiO-growth kinetics was discussed. The correlations between growth kinetics and oxide microstructures appear to be more complex than usually reported.

The oxidation of iron has been studied at low temperatures (between 260 and 500°C) in dry air or air with 2 vol% H 2 O, in the framework of research on dry corrosion of nuclear waste containers during long-term interim storage. Pure iron is regarded as a model material for low-alloyed steel. Oxidation tests were performed in a thermobalance (up to 250 h) or in a laboratory furnace (up to 1000 h). The oxide scales formed were characterized using SEM-EDX, TEM, XRD, SIMS and EBSD techniques. The parabolic rate constants deduced from microbalance experiments were found to be in good agreement with the few existing values of the literature. The presence of water vapor in air was found to strongly influence the transitory stages of the kinetics. The entire structure of the oxide scale was composed of an internal duplex magnetite scale made of columnar grains and an external hematite scale made of equiaxed grains. 18 O tracer experiments performed at 400°C allowed to propose a growth mechanism of the scale.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 18112 To link to this article:slowing down the oxide scale growth but also by limiting the oxygen dissolution in the alloy.