Defect and Diffusion Forum Vols. 323-325

Abstract: The presented original work examines how sorption and diffusion can be combined at molecular scale in nanocomposite materials to improve the resistance to diffusion of biosourced or biodegradable polymers. The concept is applied to apolar penetrants and discussed on polycaprolactone (PCL) containing organomodified montmorillionites acting as nanoadsorbents.

Abstract: Today, recycling industrial waste plays a major role in our society. Waste is used as additive in the production of a certain number of products. It enables to reduce the costing price per piece and to enhance some of the properties of the material, and to reduce pollution problems. To meet the various criteria, and manufacturing requirements of contemporary ceramic tile and bring a solution to our environment, an addition of waste glass from lamps public lighting and neon (cullet) is incorporated in various ceramic masse for substituted a feldspar. A comparative study is made between the properties obtained with each type of waste on the one hand and the other with the properties of tile produced in the industry. It was found that the tiles made with waste glass lighting lamps offer higher mechanical strength than that obtained with the feldspar and a good agreement between the shard - enamel. Although the value of strength obtained with the waste of neon is lower than that obtained with the first type of glass. The latter always remains higher than that of industrial value.

Abstract: When an alloy component is selectively oxidised but cannot reach the surface quickly enough to form a scale, then internal oxidation results. In this process, a gas phase oxidant dissolves in an alloy and diffuses inwards, reacting with a dilute solute metal to precipitate metal oxide or carbide, etc. Penetration kinetics are parabolic, the rate being controlled by oxidant diffusion and the concentration of reacting metal. Rates are predicted from classical oxidation theory on the basis that the reaction product is exceedingly stable, no solute metal remains in the reacted alloy, and oxidant diffusion is via a solvent metal matrix. This paper is concerned with situations where these approximations fail: the development of low stability precipitates and the growth of elongated precipitates which allow interfacial diffusion of the oxidant. Effects on the rates of internal oxidation are discussed.

Abstract: Thermogravimetry measurements associated to concentration profiles allow determining a diffusion coefficient at high temperature of the most oxidable one among the metallic elements belonging to the chemical composition of an alloy. In this work the employed method is described and applied to chromia-forming alloys essentially based on nickel but also to selected cobalt-based and iron-based alloys. More precisely D_Cr values were estimated for chromium diffusing through the carbide-free zones developed during high temperature oxidation. The effects of the base element, of the chromium carbides density and of the dendritic orientations on the chromium diffusion were evidenced.

Abstract: The papers focus is to establish the criterion for the transition from internal to external oxidation. This criterion is a simple value of oxide volume fraction where the coalescence of the nodular oxide in the continuous layer is considered as inevitable. It is obtained by the Wagners analytical solution and by the data of Giggins et al. [1], which give the experimental transition from internal to external oxidation between 10 and 11 weight percent of chromium in the initial alloy. This paper also enables to obtain the oxygen diffusion coefficient in pure nickel thanks to the experimental results of oxidation at 950 °C during 10 hours on Ni-0.2Cr, Ni-1Cr and Ni-5Cr model alloys.

Abstract: γ-TiAl intermetallics are attractive materials for high-temperature structural applications in the aerospace and automobile industries. However, they show environmental embrittlement at elevated temperatures that is mainly related to their low high-temperature corrosion resistance. One way how to improve the high-temperature corrosion resistance is the deposition of protective coatings on the surface of the base material. In this study, samples of a Ti-Al alloy with the chemical composition Ti-48Al-2Cr-2Nb (at.%) were covered by physically vapour deposited (PVD), by metalorganic chemically vapour deposited (MOCVD) and by high-velocity oxy-fuel (HVOF) sprayed coatings. All coatings were based on the Ti-Al alloys and contained different amounts of alloying elements. The corrosion experiments were performed in molten salts containing 75 wt.% Na₂SO₄ and 25 wt.% NaCl at 850°C up to 336 h. Both, PVD and CVD protected coatings reduced the changes in the mass of the samples over the corrosion time. Still, the formation of TiO₂ could not be avoided, as it was confirmed by glancing-angle X-ray diffraction experiments.

Abstract: Water vapour has little effect on the oxidation rate and scale composition of a nickel-based SY 625 alloy oxidized between 900 and 1100°C. At 900 and 1000°C, the outer scale is composed of Cr₂O₃ and a continuous NbNi₄ - Ni₃Mo subscale is found at the oxide/alloy interface. At 1100°C the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. The oxide scale morphology differs between dry and wet conditions. Under dry conditions the oxide scale appears to be compact and chromia pegs are observed at the internal interface. Under wet conditions, porosities are observed inside the scale. At 1100°C some scale spallation is observed under dry and wet conditions.

Abstract: Because of considerable experimental difficulties, the defect structure of NiS₂ has not been elucidated so far. The first step in these investigations was to explain which sublattice of this compound is predominantly disordered. In order to solve this problem, the mechanism of sulphidation of NiS to NiS₂ has been studied using marker technique. These experiments have been carried out at temperatures 823-923 K in sulphur vapors under pressure 10³ 10⁵ Pa. It has been found that the predominant defects in NiS₂ occur in cation sublattice. The problem then arised whether these defects are cation vacancies or interstitial cations. This phenomenon could have been explained in studying the kinetics of NiS sulphidation as a function of sulphur activity. It has been found that the parabolic rate constant of this reaction increases with sulphur activity, strongly suggesting that cation vacancies, and not cation interstitials, are the prevailing defects. If, namely, interstitial cations would prevail, the sulphidation rate would be virtually pressure independent.

Abstract: Ni-14wt%W model-alloy has been used to study the kinetics and the mechanism of high temperature internal oxidation in presence of an external oxide scale. Oxidations have been done in air at 1000°C for different oxidation times and have allowed the evaluation of the oxidation kinetic from mass gain and from oxide thickness measurements. The oxidation kinetic which follows a parabolic oxidation rate indicates a diffusive behaviour of species during the oxide formation. Cross-section observations by Scanning Electron Microscopy (SEM) and analysis by Grazing Incidence X-Rays Diffraction (GIXRD) show a multi-layered structure of the oxide. Starting from the surface towards the bulk alloy, the scale is layered as follows : (i) a pure nickel oxide (NiO) in the outer part (ii) a porous NiO matrix containing NiWO₄ second phase and (iii) an internal oxidation zone of tungsten, first forming WO₃ sub-micrometer oxides that progressively transform into NiWO₄ oxide precipitates in the alloy.

Abstract: The corrosion behavior of four valve steels (X33CrNiMn23-8, X50CrMnNiNbN21-9, X53CrMnNiN20-8 and X55CrMnNiN20-8) in combustion gases of fuel oil, containing different bio-component concentrations of (5 and 10 wt. %) has been studied under thermal shock conditions. It has been found that the addition of bio-components to the fuel oil decreases the corrosion resistance of all steels under investigation, this effect being the stronger, the higher was the concentration of bio-component addition. It has been shown, that in spite of bio-component addition, the X33CrNiMn23-8 steel containing the highest chromium concentration, behaved much better than three remaining steels due to the formation of the highly protective chromia scale.