Most high-temperature-resistant alloys oxidize to form an external alumina layer, or scale, whose slow growth protects the underlying alloy from continued aggressive oxidation. The growth of the Al 2 O 3 scale is controlled by the transport of oxygen inward and aluminum outward through it, with the rate dominated by the fastest diffusing species down the fastest path. Components in the alloy can be incorporated into the growing Al 2 O 3 layer, hence affect the transport rates of oxygen and/or aluminum. This paper summarizes existing experimental data to assess the possible effect of these incorporated impurities on the growth rate and transport properties of Al 2 O 3 scales formed on Fe-, Ni-, and Pt-based alloys. The amount and distribution of the alloy base metal, sulfur impurity, and reactive elements, such as Hf, Y, Zr, and Ce, in the alumina scale are evaluated. Their effect on the oxidation and transport rates through the scale are discussed and compared with Al and O diffusion rates deduced from creep studies.

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Impurity Effects on Alumina Scale Growth

The protective character of oxide scales developed at high temperature on metallic alloys strongly depends on the adherence of the scales which must be related to the stress level in the scales. Thus, after a review of the origins of the stresses in such scales, the methods actually used for determining the stresses either at room temperature or during the oxidation sequences are briefly mentioned. Results concerning NiO, Cr 2 O 3 and Al 2 O 3 scales are presented. These results are discussed in relation with the stress origin and they are compared with results obtained by theoretical modelling. It is suggested that growth stresses are particularly related to the scale growth mechanism, and that they are of minor importance compared with thermal stresses so that, for the systems considered, the adherence loss mainly results from the thermal stresses and/or from the poor quality of the scale-metallic substrate interface due to the defect concentration at this interface.

Stresses in NiO, Cr2O3 and Al2O3 oxide scales

The effect of Y additions on the oxidation mechanism of NiAl at 1270 K has been investigated. Mass transport in the alumina scale was examined with18O tracers. Proton activation analysis (18O(p, α)15N) and SIMS were used to measure the18O distribution in the scale. On pure NiAl and low-doped (0.07% Y) NiAl mainly outward scale growth was observed. Addition of 0.5% Y induces oxide formation at both interfaces of the scale. Larger quantities of Y added to the alloy (>0.5%) do not dissolve completely in the NiAl, but form Y-rich segregates. Internal oxidation of these intergranular segregates was observed. The relation between the modification of the scale-growth mechanism and the improved adherence of the scale to the alloy due to the addition of Y is discussed.

An18O tracer study on the growth mechanism of alumina scales on NiAl and NiAlY alloys

Simple mechanical models which could be used to calculate the stresses in an oxide scale on a flat metal substrate are presented. The sources for these stresses and the experimental techniques for measuring stresses and oxide failure are also briefly summarized. The options for stress relief in oxide scales are listed. The importance of lateral oxide growth and oxide plasticity is emphasized. Both processes result in stress relief without scale failure. This is followed by a detailed survey of the proposed mechanisms and models for oxide scale failure under tensile or compressive forces. Experimental evidence in support of these mechanisms is examined and the critical factors for predicting oxide failure are given.

The mechanical failure of oxide scales under tensile or compressive load

An attempt was made to determine the strain and the stresses generated by the growth of an oxide film using several approaches: an experimental one by means of deflection tests and modeling using either a recently developed creep analysis or a finite-element simulation A new deflection apparatus was developed and NiO growth studied during the early stages of oxidation of a Ni80Cr20 alloy at 900°C, since many microstructural, kinetics and mechanical data are available for this system The comparison of experiments and modeling indicate that the oxide layers are mostly subjected to compressive stresses when NiO is growing and the stress level and evolution clearly show that viscoplastic strain occurs in both the substrate and the oxide during oxidation The comparison between the two modeling approaches with experiment leads to good agreement and suggests that the compressive-growth stresses derive from the lateral expansion of the fraction of new oxide that is formed within the oxide layer

Comparison of oxidation-growth stresses in NiO film measured by deflection and calculated using creep analysis or finite-element modeling

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

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

Impurities influence on oxidation kinetics of Fe-Ni-Cr-Al alloys

The influence of structural parameters on the oxidation of a FeNi45Cr20Al5Y0.02 alloy (composition by weight) is pointed out. The substrate grain size has no influence on the oxidation kinetics when β-phase (NiAl) has precipitated in the matrix. This second phase promotes Al2O3 formation and, consequently, increases the oxidation resistance. The thinner the samples, the greater the aluminium depletion in the substrate and the worse the cyclic oxidation resistance.

Influence of structural parameters on oxidation of austenitic Fe-Ni-Cr-Al alloys

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 β particles, in comparison with the replica method on massive specimens.

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

A. M. Huntz

Papers

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

Impurities influence on oxidation kinetics of Fe-Ni-Cr-Al alloys

Influence of structural parameters on oxidation of austenitic Fe-Ni-Cr-Al alloys

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

High temperature corrosion of FeNiCr, FeCrAl and FeNiCrAl alloys in controlled H2/H2O/CH4 atmospheres