G.E. Thompson

Porous anodic alumina: fabrication, characterization and applications

Abstract A review is given of anodic alumina film formation on aluminium, with barrier and porous films developed by anodic polarization in an appropriate electrolyte. Initial consideration of barrier-type films, using marker and tracers, allows definition of the locations of film growth and consideration of the transport processes under the high electric field. From this base, the effects of altering film growth conditions, generally using acid electrolytes, to generate porous anodic films are considered, with be resultant films of defined morphology thickening as a result of the dynamic equilibrium between film growth at the metal–film interface and field assisted dissolution at the pore base–electrolyte interface. Means for characterising the developed films are presented as well as applications of the porous, amorphous film material.

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.

Crystallization of anodic titania on titanium and its alloys

Crystallization of amorphous anodic films grown at constant current density on sputteringdeposited titanium, and Ti–Si and Ti–Al alloys, in ammonium pentaborate electrolyte, has been examined directly by transmission electron microscopy. In the case of titanium, anatase develops at relatively low voltage in the inner film region, formed by inward migration of oxygen species. In contrast, the outer film region, formed at the film/electrolyte interface, is composed of amorphous oxide only. Oxide crystals are particularly found near the plane, separating the two regions, which is located at a depth of 35–38% of the film thickness. Oxide zones, of size � 1 nm, with a relatively ordered structure, developed at the metal/film interface, are considered to lead to transformation of the inner region structure. The incorporation into the film of either aluminium or silicon species suppresses the formation of crystalline oxide to much increased voltages. However, eventually nanocrystals form at � 40% of the film thickness, probably originating from pre-cursor nuclei in the air-formed on the as-deposited alloy. 2003 Elsevier Science Ltd. All rights reserved.

Characterization of the oxide/hydroxide surface of aluminium using x‐ray photoelectron spectroscopy: a procedure for curve fitting the O 1s core level

The performance of coated and bonded aluminium relies heavily upon its surface chemistry and hence characterization of the aluminium surface is important. A method to quantify the hydroxide concentration at aluminium oxide/hydroxide surfaces by curve fitting the O 1s peak is developed and tested in this paper. Pseudoboehmite, AlO(OH), is formed at the surface of aluminium after immersion in boiling water. The surface of this material was used to determine the binding energy of the unresolved O 1s component peaks that were referenced to the binding energy of the Al 2p oxide component. In vacuo heating resulted in changes in the elemental and functional composition that were consistent with dehydration of the pseudoboehmite. It is proposed that the resultant film comprises γ-alumina with residual hydroxide groups. The O 1s curve-fitting method was applied to air-formed films with known atmospheric exposure histories before and after heating in vacuo. The change in both the elemental composition and functional stoichiometry of the films upon heating was consistent with significant but incomplete dehydration. The probable surface phases are determined from the functional and elemental composition. Copyright

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...

XPS studies of MoS2 formation from ammonium tetrathiomolybdate solutions

Abstract Preparation of MoS 2 by acidification of ammonium tetrathiomolybdate solution has been studied by X-ray photoelectron spectroscopy (XPS). The precipitates formed from the solution are mainly composed of MoS 3 , the Mo 3d chemical shift of which is approximately 4.2eV. Heat treatments of the above product at 450 and 850°C, in a vacuum of 1.33 × 10 −4 Pa, lead largely to formation of MoS 2 , with typical chemical shifts of abouth 1.2eV. The MoS 2 formed consists of nano-sized, crystalline particles of hexagonal (2H-type) structure. The validity of MoS 2 formation by this route is confirmed by comparative study, namely the decomposition of ATT solids under the same respective heat treatment conditions. Apart from these main aspects, origins of minor XPS peaks are also considered.

Electron microscopy of ion beam thinned porous anodic films formed on aluminium

Abstract Ion beam thinning of porous anodic films formed on aluminium in the major anodizing acids enables observation of the apparent film material substructure to be made upon subsequent examination of the thinned regions in the electron microscope. In addition to irregular pore section shapes and distorted cellular morphology, cell boundary bands are evident for the films formed in phosphoric and oxalic acids. The presence of the bands seems related to the growth mechanism and composition of the porous anodic film. For films formed in chromic and sulphuric acids no cell boundary bands are evident, possibly due to the low level of chromate incorporation in the former and the similar nature of the particle size distributions of the film material regions in the highly anion-contaminated film in the latter. Remarkable changes in the apparent film material substructure are observed for films exposed further to the electron beam. Whether this phenomenon relates to a drying out process, sintering or agglomeration in the electron microscope or to some type of crystallization phenomenon is not yet known. It is possible that the extent of “opening up” of the cell walls, or the acid anion content of the cell walls being greater next to the pores than in the cells remote from the pores, conditions the morphology observed for the films formed in phosphoric and oxalic acids. The low level of chromate incorporated in films formed in chromic acid and the large level of sulphate incorporated in films formed in sulphuric acid may govern the behaviour in the respective cases.

Initial stages of plasma electrolytic oxidation of titanium

The initial stages of oxide growth on titanium are examined in a recently developed commercial alkaline pyrophosphate/aluminate electrolyte of interest for plasma electrolytic oxidation of light metal alloys. Constant current anodizing was employed, with resultant films examined by scanning and transmission electron microscopies and Rutherford backscattering spectroscopy. The initial film is relatively uniform and composed of TiO2, with low concentrations of aluminium and phosphorus species incorporated from the electrolyte. With increase in voltage the film breaks down locally, and regions of original and modified film develop simultaneously, with the latter occupying more of the surface as the voltage rises. Porous regions due to dielectric breakdown also become increasingly evident. At � 240 V, sparking commences, and the surface reveals extensive, relatively uniform porosity, with the coating now containing much enhanced concentrations of aluminium and phosphorus species compared with the coating at lower voltages. The films develop at low efficiency due to generation of oxygen. The oxygen is produced within the original film material and at sites of dielectric breakdown. The former type of film develops a two-layered morphology, with an outer layer of amorphous TiO2 and an inner layer with numerous fine and course cavities. The cavities are due to the generation of oxygen that may be associated with the formation of anatase in the inner layer. 2003 Elsevier Ltd. All rights reserved.