G.M. Brown

The morphology, structure and mechanism of growth of chemical conversion coatings on aluminium

Abstract The morphology and structure of the chemical conversion coating developed on aluminium in an acid chromate/fluoride solution have been examined by transmission electron microscopy of stripped films and ultramicrotomed sections. The use of annealed, high purity aluminium specimens which had been electropolished and subsequently etched prior to the conversion coating treatment allowed a clear identification of the anodic and cathodic sites for the coating growth process. Thus, for the first time, it has been found that the morphology of the coating is strongly dependent on the crystallographic orientation of the substrate which, in turn, appears to control the preferential deposition of the hydrated chromium oxide at grain boundaries or cellular boundaries. Such boundaries predominantly contain the presumed flaw sites due to impurity segregation in the substrate. Based on these observations, the mechanism of conversion coating growth has been developed. Deposition of hydrated chromium oxide is observed at cathodic sites, associated with metal ridges developed on the aluminium surface as a result of the initial pretreatment. The anodic sites lie between the metal ridges, where the initial air-formed film is chemically dissolved in the fluoride-containing electrolyte. Dissolution of aluminium proceeds through the chemically thinned film, revealing scalloped regions between the regions of cathodically developed chromium oxide.

The growth of chromate conversion coatings on high purity aluminium

The manner of chromate conversion coating development on very high purity (99.9996%) aluminium is considerably different from that observed previously on specimens of reduced purity (99.99% Al). This results from the absence of preferential cathodic sites associated with reduced impurity segregation within the high purity aluminium substrate. Consequently, a relatively uniform hydrated chromium oxide is observed on high purity aluminium, with the occasional presence of holes of various sizes. No preferential coating growth is evident, unlike that for 99.99% Al. Furthermore, elemental analysis of ultramicrotomed sections revealed the presence of only chromium and oxygen within the conversion coating. Neither aluminium nor fluorine was detected, suggesting that Al3+ and F− transport through the developed coating is unlikely during coating growth. The features of conversion coating growth on high purity aluminium can be explained readily on the basis that the deposition of the coating occurs by electron tunnelling through a thin, alumina passive film which is always present on the aluminium surface.

Impurity distributions in barrier anodic films on aluminium : a GDOES depth profiling study

The impurity distributions in the barrier anodic films formed on aluminium in a wide variety of electrolytes have been investigated by glow discharge optical emission spectroscopy (GDOES) depth profiling. The depth profiles obtained were compared, wherever available, with those obtained by other techniques. It was found that GDOES profiling is an extremely powerful and reliable technique for depth profiling analysis of thin, non-conducting alumina films. Surface charging is insignificant and the sputtering rate of the film is kept constant throughout the analysis, giving rise to excellent depth resolution which is comparable to, or better than, secondary ion mass spectrometry (SIMS) depth profiling analysis. Further sensitivity is also high, given the amount of impurity species being detected successfully. Thus, GDOES is expected to play a great role in depth profiling analysis of non-conducting anodic alumina films or other films where SIMS depth profiling is only of limited use due to ion beam bombardment-induced sample surface charging which has significant influence on the sputtering rate of the films.

Ultramicrotomy—a route towards the enhanced understanding of the corrosion and filming behaviour of aluminium and its alloys

Abstract Through an ultramicrotomy approach, it is now possible to prepare thin cross-sectional specimens, less than 10 nm thick, of the aluminium substrate and its surface film with negligible damage to the film material, allowing direct examination of the real film material at atomic-scale resolution with simultaneous analysis of a local area of a few nanometres in diameter. Here, selected examples, some new and some previously reported, are collected together and reviewed briefly to show that the understanding of the nature of various surface films on aluminium and aluminium alloys is increased greatly by cross-sectional transmission electron microscopy. Examples included are: (a) porous anodic oxide growth over an Al-0.5 wt% Fe alloy containing finely dispersed Al 3 Fe particles, (b) porous anodic oxide growth over an Al-1.4 wt% Fe alloy containing finely dispersed Al 6 Fe particles where the complex interplay between the local film formation over the particles and general film formation over the surrounding matrix regions is now revealed precisely, (c) chromate conversion coating growth over high purity aluminium where the surface inhomogeneities associated with the grain boundaries or cellular boundaries of impurity segregation within the metal play a crucial role in the coating development, (d) corrosion of aluminium covered with organic coatings where the highly localized nature of corrosion and its association with impurity segregation within the metal is highlighted, (e) adhesion between aluminium and organic coatings where surface films on the aluminium substrate play a critical role, and (f) crystalline barrier oxide growth over etched aluminium surfaces which is of great practical importance in improving the performance of aluminium electrolytic capacitors. Further, it is shown that ultramicrotomy is an extremely powerful technique for the preparation of clean and microscopically flat surfaces both for high purity aluminium and aluminium alloys, allowing AFM to be utilized to its full potential in revealing highly localized processes, such as pit initiation, during corrosion of aluminium and aluminium alloys.

Glow discharge optical emission spectrometry (GDOES) depth profiling analysis of anodic alumina films—a depth resolution study

Anodic alumina films with precisely known distributions of incorporated species have been used as standards for glow discharge optical emission spectrometry (GDOES) depth profiling analysis to quantify depth resolution. It is evident that the depth resolution of GDOES is excellent and is comparable with, or better than, secondary ion mass spectrometry depth profiling of similar films. Further, the sensitivity for detection of elements is also high, given the amounts of impurity species detected successfully. Thus, GDOES, with its further ability of routine and rapid analysis of films (organic, inorganic or metallic) of thicknesses up to several hundreds of microns, has significant potential in studies of the corrosion and filming behaviour of materials.

The development of chemical conversion coatings on aluminium

Abstract The growth of the chemical conversion coatings on aluminium is strongly dependent upon the purity of the metal. For specimens up to 99.99% purity, the aluminium surface is not homogeneous. Flaws are always present in the thin oxide layer covering the aluminium surface; such flaws are sited above the grain boundaries and cellular boundaries associated with impurity segregation in the aluminium substrate. These flaws are of the residual type and provide easy paths for electronic conduction in an otherwise insulating oxide layer. Consequently, coating growth (i.e. the reduction of the dichromate species to hydrated chromium oxide) occurs preferentially along the grain boundaries or cellular boundaries of the aluminium substrate. However, for aluminium specimens of 99.9996% purity or higher, where the cellular structure is absent, the population density of the residual-type flaws in the oxide layer is considerably reduced. Consequently, coating growth proceeds by the tunnelling of electrons through the thin, insulating, passive oxide layer to produce a coating of more uniform appearance than associated with substrates of reduced purity.

The early stages of high temperature oxidation of an Al-0.5wt% Mg alloy

The high temperature oxidation of an Al-0.5 wt% Mg alloy in air at 723 and 823 K is investigated using atomic force microscopy, secondary ion mass spectrometry and transmission electron microscopy, with attention directed to the degradation and breakdown of the thin, protective amorphous Al2O3 layer covering the initial alloy surface. Following the commencement of oxidation, the amorphous Al2O3 layer thickens slightly. However, migration of Mg2+ ions through the amorphous layer leads subsequently to formation of an outer layer of fine MgO crystals, accompanied by thinning of the Al2O3 layer and eventual complete transformation to less protective spinel MgAl2O4. The degradation of the Al2O3 layer is caused by either reaction between the layer and Mg in the alloy, with the formation of randomly oriented MgAl2O4 crystals of several nanometres size, or solid-state reaction between the inner and outer layers. Extensive local attack occurs upon breakdown of the oxide at sites where the cellular boundaries of faster Mg diffusion in the alloy intersect the surface, resulting in the formation of V-shaped cavities, filled with fine, cubic crystals of MgO, extending deep into the alloy along the cellular boundaries.

The growth of a porous oxide film of a unique morphology by anodic oxidation of an Al-0.5 wt% Ni alloy

Porous anodic oxide film growth on an Al-0.5 wt% Ni alloy, containing finely dispersed Al3Ni intermetallic particles, has been investigated by transmission electron microscopy of ultramicrotomed sections. It was found that film growth is influenced greatly by the localised processes proceeding at the intermetallic Al3Ni particles, with consequent development of a unique film morphology, which has not been observed previously. At the initial stage of anodizing, the majority of the applied current is consumed by dissolution of the Al3Ni particles at the alloy surface, accompanied by oxygen gas evolution, which leads to the suppression of the forming voltage, at 10 to 15 V, for a sufficiently long period until the particles are removed from the alloy surface. During this period, a thin porous film of fine dimensions, corresponding to the voltage of 10 to 15 V, is formed over the matrix surface generally. After the particles have been removed from the surface, the applied current is used for the growth of the porous oxide over the matrix surface as well as over the walls of cavities formed by dissolution of the particles. As the forming voltage increases, some of the pores grow further, at the expense of other pores in the immediate surrounding regions, with their dimensions controlled precisely by the increasing forming voltage, resulting in the formation of large pores of a ‘‘tear-drop’’ shape. Implications of these findings for forming porous anodic oxide films of controlled morphologies on aluminium and its alloys are discussed.

Selective oxidation of aluminium and interfacial enrichment of iron during anodic oxide growth on an Al6Fe phase

Abstract Porous anodic oxide formation on an Al-1.4wt%Fe alloy containing finely dispersed Al 6 Fe particles has been examined by transmission electron microscopy of ultramicrotomed sections, with particular attention directed towards the local film formation over the Al 6 Fe particles. It was found that anodic oxide formation over the Al 6 Fe phases involves initial oxidation of aluminium and interfacial enrichment of iron, with the enrichment confined to a thin layer, about 1–2 nm thick, in the Al 6 Fe phase immediately beneath the oxide. With EDX analysis, using an electron probe of 0.8 nm diameter, the composition of the iron-enriched layer has been determined to be about Al 80 Fe 20 . Additionally, close examination of the iron-enriched layer, using an ultra-high resolution transmission electron microscope, has revealed that the layer has a heavily distorted structure with the occasional presence of ordered domains, only 1 nm diameter, with the Al 6 Fe structure. No evidence was obtained to suggest the presence of nano-sized iron clusters in the enriched layer. These observations are discussed in terms of the Gibbs free energy criteria for alloy oxidation proposed recently by several of the authors.

Direct observation of anodic films formed on tantalum in concentrated phosphoric and sulphuric acid solutions

The anodic films formed at 1 mA cm−2 on tantalum in concentrated H3PO4 (85%) and H2SO4 (95%) solutions at 25°C have been examined directly in the transmission electron microscope employing ultramicrotomed sections. In the former electrolyte, duplex films comprising an outer PO4[3]−-doped layer, constituting about 70% of the film thickness, and an inner relatively pure Ta2O5 layer are developed, contrasting with the uniformly SO4[2]−-doped films developed during anodizing in concentrated H2SO4. The doped layers contain high concentrations of PO4[3]− and SO4[2]− ions, with compositions represented by Ta2O5(1−x)(PO4)10×\3 and Ta2O5(1−x)(SO4)5x, where x = 0.27 and 0.30 respectively. The increased incorporation of electrolyte species relative to films formed in dilute electrolytes significantly enhances the resistivity of the film material with an associated reduction in the dielectric constant.