The early Stages of the Scale Growth on FeCrAl(+RE)-Type Alumina Formers

The early stages of the scale growth process were studied of two FeCrAl alloys: one synthetic (Fe23Cr5Al) and one commercial (Kanthal APM alloy). In addition, Yttrium was implanted to the Fe23Cr5Al alloy. Oxidation exposures were carried out at 1000°C using two-stage-oxidation exposures in atmospheres containing significantly different amounts of18O2-tracer. The scales were analyzed in terms of SIMS, PLS and SEM. The distribution of oxygen isotopes which corresponded to the location of new oxide formation, the scale phase composition, scale morphology and microstructure were determined which enabled description of the scale evolution on all studied alloys. Similar evolution stages were observed, but minor differences were related to the rate of disappearing of the transient aluminium oxides.

Growth Mechanism vs Matter Transport in Thermally Growing Oxides on High Temperature Materials: A Brief Survey Based on the Case Study of Alumina Formers

This paper reviews briefly the relationship between the growth mechanism and matter transport using as an example the best currently applied metallic materials being alumina formers. The attention is paid to the experimental approach as well as to the interpretation procedure of experimental results. The scale structure, microstructure, morphology and phase composition are indicated as factors strongly affecting its growth mechanism. The attempt is made to elucidate the possible relationships between the obtained experimental results and actual scale growth mechanisms operating during oxidation exposures.

High Temperature Oxidation and Oxidation-Induced Degradation of Alumina Formers

This paper focuses on a systematic description of the alumina formers oxidation behaviour by referring to a variety of observables, features and/or processes and their evolution during the exposure. An attempt is also made to identify the key effects relevant to degradation process. An output of this approach consists of the ‘Oxidation Routes’ and the ‘Degradation Paths’.

Why the Growth Rates of Alumina and Chromia Scales on Thin Specimens Differ from those on Thick Specimens

For a number of chromia and alumina forming high temperature alloys and coatings, recent studies revealed, that in some cases the specimen/component or coating thickness may substantially affect the growth rates of the surface oxides. For the alumina formers the thickness dependence is mainly governed by depletion of oxygen active elements such as Y, Zr, Hf, Mg which are either intentionally added alloying elements or manufacturing related alloy impurities. In the case of the chromia forming materials, which tend to exhibit a more substantial dependence of oxidation rate on specimen/component thickness, depletion of minor alloying additions is also an important factor to be considered. However, for these alloys relaxation of oxide growth stresses by plastic deformation of the metallic substrates seems to be the dominant parameter which governs the observed behaviour.

Metal Dusting Reaction Mechanisms

Iron and nickel, model alloys of Ni-Cu and Fe-Cr, and commercial heat resisting alloys were exposed at 650-680oC to flowing CO-H2-H2O gases which were supersaturated with respect to carbon. All ferritic materials, including chromia and alumina formers, developed a coke deposit of carbon nanotubes, the growth of which was catalysed by nanoparticles of Fe3C. Austenitic materials formed graphite filaments and clusters in association with nanoparticles of austenite. Graphite cluster formation was suppressed by alloying copper with nickel. The sensitivity of coking kinetics to alloy copper content was consistent with a mechanism involving graphite nucleation within the subsurface metal. Chromia forming alloys resisted dusting until damage to the scale could no longer be repaired by Cr2O3 regrowth, and carbon gained access to chromium – depleted metal.

Defect-Diffusion-Stress Relationships in Modeling the Oxidation and Degradation Processes of Alumina Formers: A Brief Survey

In the modeling of the oxidation and degradation processes of the superior metallic materials in terms of the high temperature oxidation resistance, being the so-called alumina formers, the matter transport only via grain boundaries of α-Al2O3 is usually taken into account. This paper indicates that such an approach needs to be re-visited. It is pointed out that the following factors should be taken into account (i) diffusion via point defects in the unstable alumina polymorphs (γ, δ, θ) which grow during the early oxidation stages; (ii) dislocation-related processes; (iii) formation of the oxide in three-dimensional defects, being the cracks in the oxide layer; (iv) outward oxide growth resulting from the formation of the new oxide within the existing layer. The mentioned above effects as well as the other ones, related to the interfacial and stress-induced processes are illustrated and discussed. It is pointed out that alumina formers constitute a wide group of materials and several categories should be distinguished among them with respect to their composition and production route on one hand, and the oxidation and degradation mechanisms, on the other hand. It is shown that the comprehensive approach enables better and self-consistent modeling of the oxidation and degradation mechanisms of alumina formers.