Oxidation Mechanism of Ni—20Cr Foils and Its Relation to the Oxide-Scale Microstructure

The oxidation behavior of Ni—20Cr foils of 100- and 200-m thickness wasstudied in air between 500 and 900°C. Simultaneously, the morphology,microstructure, and composition of the oxide layers were determined byscanning and transmission electron microscopies. Depending on thetemperature, the oxide layer differed significantly. The scale formedat all temperatures was complex, with an outer NiO layer having columnargrains, and an inner layer of equiaxedNiCr₂O₄+NiO+Cr₂O₃ grains. At low temperatures (500 and 600°C),the chromium content was insufficient to form a continuousCr₂O₃ layer, while such a continuous layer formed at theinner interface at oxidation temperatures of 700 to 900°C. At 600°C,internal oxidation of chromium occurred in the substrate. The oxidationmechanism is described taking into account these morphologies and theoxidation kinetics. The observation of no significant differences betweenthe oxidation behavior of thin strips and thick materials is related to thelimited exposure times of the study.     

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. II. Influence of sulphur

The oxidation of presulphidized Ni-Cr alloys has been studied by taking into account the influence of the two distinct oxidation mechanisms described in part I of this article. Sulphur enters the Cr₂O₃ scale (in Ni-34Cr alloys) mainly as S^2– species, which at high temperatures increases the V_Cr content, and hence the oxidation kinetics. Sulphur is randomly distributed in the scale, except at the inner oxide-alloy interface, where intergranular microsulphides are analyzed in the oxide-scale zone. In the case of NiO, NiCr₂O₄, Cr₂O₃ oxide multilayers (in a Ni-20Cr alloy), sulphur in the S^2– state is distributed in the oxide layers or at Si-precipitate interfaces. Such a distribution leads to crack formation, especially during cooling.     

Application of high resolution autoradiography to study the microsegregation of carbon in thin foil microstructures of a 12% Cr steel using 14C as a radioactive tracer

The technique of high resolution microautoradiography has already been applied to study the microsegregation of carbon and tritium in iron and its alloys, but autoradiographs have so far been obtained using liquid nuclear emulsions and carbon replicas on massive specimens. This technique has now been extended to study the distribution of carbon in thin foil microstructures of a 12% chromium steel in the quenched and tempered condition. An experimental procedure has been developed for obtaining autoradiographs on thin foils, and is described in detail. The limitations of the technique with respect to efficiency and resolution have been considered. The results obtained show that high resolution microautoradiography on thin foils permits a more precise localization of the radioactive tracer atoms with respect to the microstructural features and gives a better resolution, even with a radioisotope emitting high energy particles, in comparison with the replica method on massive specimens.     

Theoretical analysis of the deflection test used in single-surface oxidation of metallic samples

On the basis of the experimental results on deflection obtained by Delaunay during single-surface oxidation of metallic samples, a theoretical analysis of this deflection test was carried out. Two important problems were established concerning the physical signification of the oxide stress calculated from the measured deflection and the relation between the oxide adherence and the oxide stress. In particular, it appears that the oxide stress level determined by the deflection test is only representative of the stresses relieved by the sample curvature. Secondly, spalling of the oxide layer is directly related to the product of the residual stress value by the curvature of the sample. Some experimental results, recently obtained, confirm this theoretical analysis.     

Influence of impurities,such as carbon and sulphur,on the high temperature oxidation behaviour of Fe72Cr23Al5 alloys

Kinetic and analytical study of oxidation behaviour of Fe₇₂Cr₂₃Al₅ alloy in relation to the microstructure, particularly carbon and sulphur localization and chemical state, indicates that these elements have a strong influence on alumina growth and adherence. Competition between oxidation and a carbide precipitation slightly increases the oxidation rate during isothermal treatments but has a beneficial influence on the oxide scale adherence. Sulphide precipitation in the metallic substrate has a disastrous effect on oxidation: large weight gains are observed, due to internal oxidation at sulphides and presence of sulphur in the alumina. Moreover, it was confirmed that alumina growth is controlled by preferential oxygen diffusion along short-circuit paths.     

Effect of the Application of a Mechanical Load on the Oxide-Layer Microstructure and on the Oxidation Mechanism of Ni–20Cr Foils

The effect of a tensile load on the oxidation kinetics and mechanism ofNi–20Cr was studied by comparison of the oxidation behavior ofNi–20Cr thin strips in air under classical conditions, i.e., withoutany applied mechanical load and under tensile creep, at temperatures between500 and 900°C. The study was performed mainly by comparisons of crosssections of oxidized samples observed by SEM. The results obtained clearlyindicate that applying a tensile load induces an increase in the oxidationrate, does not modify the oxide-film morphology, but promotes the formationof internal oxidation at low temperatures, 500–600°C, and notablyincreases the thickness of the intermediate NiCr₂O₄layer at 900°C. This is related to the acceleration of anionic diffusionwhen a tensile load is applied, due to the formation of fast-diffusion byshort-circuit paths.     

Diffusion of18O in massive Cr2O3 and in Cr2O3 scales at 900°C and its relation to the oxidation kinetics of chromia forming alloys

The lattice and grain-boundary diffusion coefficients of¹⁸O atP _{O 2}=0.1 atm and at 900°C were determined in massive Cr₂O₃ and in Cr₂O₃ scales which were grown on a Ni–30Cr alloy. The diffusion profiles were established by SIMS and analyzed considering two domains in the case of polycrystalline Cr₂O₃ (massive or scales), the first one relative to apparent diffusion and the second to grain-boundary diffusion. A ridge model is proposed for Cr₂O₃ scales to modify thef value, fraction of sites associated with the grain boundary. With such a model,f is equal to 0.0006 and 0.0005 for the scales formed during 15 hr and 165 hr, respectively. The oxygen-lattice diffusion coefficients determined in Cr₂O₃ scales are in very good agreement with those in massive Cr₂O₃. With some assumptions, our diffusion data lead to a calculated parabolic oxidation constant equal to the experimental one. Scale growth occurs by countercurrent diffusion of oxygen and chromium, mainly by grain-boundary diffusion.     

Improvement of the application of an electrochemical method for the determination of transport properties of anα-alumina scale. Part II: Influence of yttrium and palladium on alumina scales developed on aβ-NiAl alloy

An electrochemical method based on analysis of potential-current curves at various appliedP _{O 2} for an alumina scale developed at 1100°C on either undoped or yttrium-palladium doped -NiAl alloys was used to determine transport parameters in the scale. In the case of scales formed on undoped or Y-doped NiAl alloys, thet _i variations, at least in theP _{o 2} range experimentally studied, are very close: the scale consists of an outer domain characterized byt _i0.1, thent _i increases with decreasingP _{o 2}. The ionic conductivity in both cases shows a V-shape and decreases in the presence of Y. The presence of palladium notably modifies the shape of thet _i variation and induces an inversion of the shape of ₁ variation withP _{o 2}. The analysis of these results suggests that at lowP _{O 2} yttrium, localized in the inner part of the scale, decreasest _i, creates defect complexes and intergranular yttrogarnet precipitates, and prevents the outer part of the scale from contamination by nickel, which is thought also to give defect complexes when present. The variation of the oxygen chemical potential inside the scale shows a progressive variation of _o. The calculated oxidation constants are close to the experimental ones, which indicates that the alumina scale growth is ensured by grain-boundary diffusion of ionic species.