G. C. Wood

The influence of yttrium additions on the oxide-scale adhesion to an iron-chromium-aluminum alloy

The oxidation behavior of an Fe-27%Cr-4%Al alloy and similar alloys containing 0.023% and 0.82% Y in 1 atm oxygen at 1200°C has been examined. The oxide formed on the yttrium-free alloy develops a highly convoluted configuration, apparently resulting from lateral growth of the oxide. The latter leads to oxide detachment from the alloy at temperature and extensive spalling during cooling. It is postulated that lateral growth results from the formation of oxide within the existing oxide layer by reaction between oxygen diffusing inward down the oxide grain boundaries and aluminum diffusing outward through the bulk oxide. Additions of yttrium to the alloy apparently prevent the formation of oxide within the oxide layer, the oxide-forming reaction occurring as the alloy-oxide interface. Thus lateral growth is prevented and spalling during cooling does not occur. Secondary advantages conferred by the addition of 0.82% Y to the alloy are the prevention of void formation at the alloy-oxide interface, the avoidance of alloy grain growth during oxidation, and the creation of an oxide “keying” or “pegging” effect.

The influence of alloying elements on the development and maintenance of protective scales

Some of the important principles that determine the establishment, growth and long-term maintenance of protective Cr2O3, Al2O3 and SiO2 scales on hightemperature iron-, nickel- and cobalt-base alloys are reviewed and discussed. Emphasis is placed on the effects of alloying elements and other additions, such as third elements and reactive elements or oxide dispersions, on each of these processes. Particular attention is paid to transport processes in the scales and the importance of short-circuit paths. Some of the important parameters that influence the long-term mechanical stability of such scales are considered and evaluated.

The application of ultramicrotomy to the electronoptical examination of surface films on aluminium

Abstract Ultramicrotomy has been used to produce thin sections of surface films on aluminium after different treatments and these have been examined by transmission electron microscopy. Films studied are those produced by electropolishing, outdoor exposure, a hydrothermal treatment, immersion in sodium hydroxide, a chemical conversion process, an a.c. treatment and cathodic polarization. Anodic films including porous and barrier-type films and the thick coating on a superplastic ZnAl alloy have been examined. It has been found that the application of ultramicrotomy enables observation of the thickness and morphology of both very thin and friable surface films on aluminium, and it is useful in the study of certain phenomena concerning thicker porous-type anodic films. Various novel features are visible that have not been observed previously, and it is apparent that ultramicrotomy may be used advantageously to examine surface films on aluminium.

High-temperature oxidation of alloys

Some recent developments in the understanding of the oxidation of alloys at elevated temperatures are reviewed, with special reference to binary and ternary alloys upon which many commercial materials are based. Following an initial classification of alloy systems, certain basic principles and their limitations are considered, including factors determining whether an alloy displays surface scaling only, internal oxidation only, or both phenomena; the mode of distribution of cations in an oxide solid solution growing on an alloy; and doping and the Wagner-Hauffe rules. Previous alloy oxidation theory has concerned itself principally with steady-state oxidation but, in practice, behavior of the oxidizing sample in approaching the steady state, both upon initial exposure to the environment and after scale spalling induced isothermally or during thermal cycling, is often as important. Such behavior is illustrated in terms of scale establishment and with respect to compositional changes in the subjacent alloy, the importance of certain alloy and oxide properties being emphasized. After further brief consideration of steady-state scaling, the causes and consequences of breakaway oxidation are presented. The role of major and minor concentrations of third elements on scaling behavior is discussed. The paper concludes with brief reference to situations not considered in detail and with aspects requiring further study.

A study of the pitting corrosion of Al byscanning electron microscopy

Abstract The essentials of a theory are proposed for the pitting of Al, supporting air-formed or barrier-type anodic oxide films, in Cl− solutions. Pits propagate immediately at flaws in the otherwise essentially inert oxide films and proceed to undermine these surface films. Under natural immersion conditions, the morphology and distribution of attack as pitting proceeds depend upon the changing relative cathode/anode area ratio on the electrode, as well as the degree of polarization of the anodic and cathodic processes. During applied polarization, the morphology and distribution of attack are dependent upon the applied potential. Certain concepts developed are applicable to other anodic processes on Al. Crucial information, for testing the theory, is provided by examination of pitted surfaces by scanning electron microscopy. The technique is particularly useful in elucidating the relation between the distribution and morphology of pits and in establishing the role of the surface oxide film throughout the pitting process. The complicated distribution of corrosion products above and around pits on naturally corroded surfaces becomes apparent.

Effects of Alloying Elements in Anodizing of Aluminium

SummaryThe anodic oxidation of aluminium alloys is reviewed and discussed with reference to recent results of the authors on the anodizing of model binary aluminium alloys. Attention is given primarily to the oxidation of alloying elements at the alloy/film interface during the formation of barrier-type anodic films. However, the findings are also considered to be applicable to the formation of porous anodic films. The enrichments of alloy layers of about 1–5 nm thickness is revealed to be a relatively common occurrence following anodizing of aluminium alloys. The enrichments, present in the alloy just beneath the anodic film, are a direct consequence of the formation of the anodic film on the alloy. The influence of alloy composition on the enrichments of the alloying element within the alloy is highlighted, and correlated with the Gibbs free energy per equivalent for formation of the alloying element oxide. The development of enriched alloy layers is not confined to anodizing and is found following othe...

A Model for the Incorporation of Electrolyte Species into Anodic Alumina

A semiquantitative model is presented for the incorporation of species derived from electrolyte anions into amorphous barrier-type films formed on aluminum in aqueous electrolytes at ambient temperature. The model relates the film compositions to the concentrations of adsorbed electrolyte anions which form the double-layer charge at the film surface. During film growth, the adsorbed anions are incorporated into the film, either directly or following transformation to a new form, as so-called electrolyte species. The incorporated electrolyte species present in the film can have positive, negative, or effectively no charge, and hence electrolyte species can be immobile, migrate inward, or migrate outward within the film under the electric field. The concentration of electrolyte species in the film depends upon the type and concentration of the adsorbed anions, the direction of migration of the electrolyte species in the film, and the faradaic efficiency of film growth. The validity of the model has been assessed by comparing the predicted and experimental compositions of films formed in a wide range of electrolytes. For reasonable selection of the type of adsorbed anion, the measured concentrations of electrolyte species in films, determined by Rutherford backscattering spectroscopy and nuclear reaction analysis, are typically about 70% of the predicted values, which is a satisfactory level of agreement given uncertainties in precise values of model parameters

Anodic oxidation of aluminium

Abstract Key aspects of anodic film growth on aluminium at ambient temperatures in aqueous electrolytes, encompassing barrier- and porous-type films, are described. By direct observation of film sections, incorporating inert marker layers and tracers, in the transmission electron microscope and appropriate analysis, the locations of solid-film growth and electrolyte anion effects can be determined precisely. Thus, during barrier-film formation, at high current efficiency, Al3+ ion egress and O2-/OH− ingress proceed across the pre-existing air-formed film to develop solid material at the film/electrolyte and metal/film interfaces respectively. With decrease in current efficiency, the former contribution declines through a mechanism of direct ejection of Al3+ ions at the film/electrolyte interface. At a critical current density, all outwardly mobile Al3+ ions are lost to the electrolyte. Concerning anions of the forming electrolyte, such species (or more strictly their transformation products) may be mobile...

The structure and mechanism of formation of the ‘glaze’ oxide layers produced on nickel-based alloys during wear at high temperatures

Abstract The structure and composition of the worn surfaces, and in particular of the tribologically important ‘glaze’ region, formed on Nimonic 75, Nimonic C263, Nimonic 108 and Incoloy 901 after sliding in air at elevated temperatures (150–800°C) have been determined. A typical wear scar is comprised of a number of areas, some having a thin, physically homogeneous surface ‘glaze’ layer, the rest having no ‘glaze’ surface. The ‘glaze’ layer lies on top of a region either of highly compacted oxide particles above a growing, steady-state oxide layer, or of alloy, deformed to varying degrees, depending on time of sliding, ambient temperature and the relative strength of the alloy. Electron diffraction shows the surface ‘glazes’ to consist of simple oxides after sliding at temperatures up to 400°C, viz. NiO in that on all four alloys, CoO on N108 and C263, and FeO on Incoloy 901. At the higher temperatures, NiCr 2 O 4 is observed in the ‘glaze’ on all the alloys, with NiO and Cr 2 O 3 on N75, Cr 2 O 3 and probably CoO on C263 and N108 and α-Fe 2 O 3 , FeO and Fe 3 O 4 on Incoloy 901. The ‘glaze’ and underlying, wear-affected, oxidized regions are shown by ion microprobe mass spectrometry, electron spectroscopy and electron probe microanalysis to consist of oxides, containing all the alloying elements, approximately in the same average proportions as in the alloys. Three possible mechanisms for the formation of the observed structure of the ‘glaze’ regions are proposed. It is concluded that high-strength properties and relatively rapid transient oxidation rates at elevated temperatures are desirable qualities in alloys employed under high temperature sliding conditions.

The transient oxidation of alloys

The pre-steady-state oxidation of a wide range of binary alloys of practical importance, in 1 atm oxygen at 600° C, is discussed in terms of the main determining parameters, namely the free energies of formation and growth rates of the component and complex oxides, the bulk alloy composition, the alloy interdiffusion coefficient, the oxygen solubility and diffusivity in the alloy, and effects such as epitaxy. Schematic diagrams are used to illustrate the morphology and structure of the films, as revealed by electron microscopy and diffraction. After tabulation of the sparse, and often apparently unreliable, fundamental parameters, a comparison is made within each of the following groups of alloys: (1) Fe-Cr, Ni-Cr, and Co-Cr alloys, in which the less noble metal is the same and the noble metal is varied, there being composition ranges in which noble metal oxide and less noble metal oxide respectively predominate but in which the oxides are partially miscible or react. (2) Ni-Al, Ni-Cr, Ni-Si, Ni-Mn, and Ni-Co alloys, in which the noble metal is the same, the less noble metals have a wide range of affinities for oxygen and oxidation rates, and the oxide phases produced include solid solutions, largely immiscible simple oxides, and complex oxides. (3) Cu-Ni, Cu-Zn, and Cu-Al alloys, in which the noble metal is the same and the less noble metal is varied, the oxides being largely immiscible.