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Cathodic polarization of aluminium and Al-0.18 wt.%Mg and Al-0.08 wt.% Ti alloys in 0.24 mol dm-3 nitric acid solution at 38 °C has been employed to assist understanding of the roles of alloying elements in electrograining. The findings indicate that additions of magnesium and titanium to aluminium accelerate the corrosion of the substrate under the alkalization caused by the cathodic reactions. The accelerated dissolution and the consequent formation of hydrated alumina result in a decreased net cathodic current density in potentiostatic and potentiodynamic polarization conditions relative to the behaviour of aluminium. [ABSTRACT FROM AUTHOR]

Differences between chromate conversion coatings and sol-gel coatings formed on aluminium and aluminium-copper alloys are highlighted with respect to the formation of cracks, the effect of the substrate composition on coating thickness and the influence of curing in air on coating re-growth. For sol-gel coatings, it is shown that (i) cracking can be eliminated by inclusion of a sufficient amount of organic moieties, (ii) the coating thickness is unaffected by copper and (iii) coating re-growth occurs readily after curing. These attributes are shown to contrast with the significantly different behaviours of chromate conversion coatings. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Self-assembled alkyl phosphate layers have been formed on a flat, anodized aluminum substrate in dilute ethanol solution containing 2 wt% n-tetradecylphosphonic acid (TDP) and examined by low-voltage scanning electron microscopy as well as atomic force microscopy and X-ray photoelectron spectroscopy. Locally, multi-layered alkyl phosphate films have been formed on aluminum, being clearly observed by a low-voltage scanning electron microscope operated at less than 1 kV. Atomic force microscopy observations disclosed that bilayers of tetradecylphosphonic acid are stacked on the substrate to form multilayers. The present study revealed that the uniform self-organized monolayer is not always formed readily on an oxidized aluminum surface. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Anodizing of aluminium alloys is often used to improve appearance, corrosion resistance or adhesion with organic coatings, with properties of the oxides tailored by controlling the anodizing conditions. Some electrolytes can be used in relatively wide ranges of concentration, temperature and potential or current, while others display a narrower operational range. Optimization of the anodizing process is a non-trivial task, involving the control of voltage and electrolyte nature, concentration and temperature. In this work, potentiodynamic anodizing is proposed as a tool to characterize rapidly the behaviour of electrolyte/alloy combinations over a wide range of potential. It is shown that each electrolyte/alloy displays a fingerprint response, carrying information on the potential/current intervals suitable for porous oxide growth, on the oxidation behaviour of the second phase material on the alloy surface and on the maximum applicable potential or current. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

ABSTRACT In the present study, intergranular corrosion (IGC) in AA2024 and AA2099 aluminium alloys was investigated. The IGC propagation path was examined using scanning and transmission electron microscopies. Further, electron backscatter diffraction was employed to investigate the relationship between the grain structure and the IGC susceptibility. It was found that IGC could occur at grain boundaries where second phase precipitates were absent. It was also revealed that IGC occurred at the grain boundaries that surround grains of relatively high-stored energy, and corrosion was not confined within the region immediately adjacent to the grain boundaries but had developed into the grains of relatively high-stored energy, suggesting that IGC propagated within the grains that have relatively high levels of defects. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In order to assess the practicality of plasma electrolytic oxidation of parts consisting of more than one light metal, coupled metals, namely aluminium/AA 7075 aluminium alloy and AA 7075 aluminium alloy/ZE41 magnesium alloy, have been treated under alternating current conditions using constant total currents. Monitoring of the current flow to each component of the couples revealed differences in current densities on each metal, which varied with time. However, for the selected conditions, microdischarges were sustained on both metals and coatings were produced with thicknesses reasonably similar to those of coating on metals treated individually. Further, the transition to 'soft' sparking occurred at about the same time on both components of the couples. [ABSTRACT FROM AUTHOR]

The effects of the anodising potential and of the combined addition of tartaric acid and cerium nitrate to a sulphuric acid anodising bath on the corrosion behaviour of high purity aluminium and on AA2024T3 aerospace aluminium alloy have been systematically investigated. It is found that the anodising potential is critical in determining the anticorrosion performance; lower potential generates finer pores that provide enhanced corrosion protection compared with the larger pores generated at higher potentials. At both anodising potentials, the addition of cerium nitrate alone to the sulphuric acid bath does not increase significantly the anticorrosion performance. Conversely, the addition of tartaric acid alone is generally beneficial. Finally, when cerium nitrate is added in combination with tartaric acid a further improvement of the corrosion resistance is observed. [ABSTRACT FROM AUTHOR]

A comparison is made between plasma profiling time-of-flight mass spectrometry (PP-TOFMS) and nuclear reaction analysis (NRA) for depth profiling of 18O tracer in porous anodic oxide films on aluminum. The films were formed galvanostatically, for a range of times, using phosphoric acid electrolytes that were either enriched in 18O or of the natural isotopic concentration. The morphologies of the films were determined by electron microscopy. The findings from PP-TOFMS and NRA reveal a partitioning of the tracer between the surface regions and buried layers of the films. However, a relatively high background of 16O in PP-TOFMS prevents a reliable quantification of the concentration of 18O. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

18O tracer is used to investigate the development of porous anodic films at constant current in phosphoric acid on electropolished aluminum. A barrier layer and porous region form initially with the pore size related to the surface texture of the substrate. Subsequently, major pores emerge, with their sizes related to the anodizing voltage. The evolution of the film is accompanied by increases in growth rate and formation efficiency. The 18O ions of a preformed oxide are retained in the film during anodization in a nonenriched electrolyte, with 18O being partitioned among (i) the surface region of texture-dependent porosity, (ii) the walls of major pores, and, in diminishing amounts, (iii) the inner region of the barrier layer. [ABSTRACT FROM AUTHOR]

Composite ceramic coatings were formed on aluminium by AC plasma electrolytic oxidation (PEO) using Na6P6O18 or Na2SiO3 · 5H2O/KOH electrolytes with monoclinic zirconia nanoparticles in suspension. The coatings grown in Na2SiO3 · 5H2O/KOH electrolyte revealed γ-Al2O3 and amorphous phase; α-Al2O3 and AlPO4 were additionally produced with the Na6P6O18 electrolyte. Higher temperature zirconia phases, possibly tetragonal and orthorhombic, in addition to the monoclinic phase, were indicative of elevated temperatures at sites of microdischarges. Further, local melting resulted in zirconium-rich dendrites in the coating formed in silicate electrolyte. Zirconium was mainly located in the relatively compact, outer layer of the coating, constituting ∼70–90% of the coating thickness. Nanoparticles appeared to be incorporated at the coating surface and following transport to the interface regions between the inner and outer layers along short-circuit paths through the outer coating. [ABSTRACT FROM AUTHOR]

Abstract: Using a tungsten-containing layer, incorporated into sputtering-deposited aluminium, as a tracer, the growth of porous anodic films in malonic and oxalic acid electrolytes has been investigated using transmission electron microscopy, Rutherford backscattering spectroscopy and nuclear reaction analysis. Comparisons were also made with films formed in phosphoric acid electrolyte, which have been studied previously. The findings reveal a distortion of the tracer layer within the barrier region of the porous films, evident as a lagging of the tracer beneath the pores relative to that in the adjacent cell wall region. Further, the films are significantly thicker than the layer of metal consumed during anodizing and display smooth-sided pores. The anodizing behaviours are consistent with a major role for field-assisted flow of film material within the barrier layer in the development of the pores. [Copyright &y& Elsevier]

Spatially resolved electron energy loss spectroscopy (EELS) has been used to investigate the oxidation state of tungsten species in a barrier anodic oxide film formed on an Al–6·5 at.-%W alloy in 0·1M sodium tungstate electrolyte at 293 K. Tungsten 4f core level excitation indicates the presence mainly of W6+ species in the anodic film. Insight into the bonding arrangements within the anodic oxide is also provided by valence band excitation and revelation of the oxygen edge and its associated pre-edge structure. [ABSTRACT FROM AUTHOR]

Abstract: The generation of pores is investigated in anodic films formed at 5mAcm−2 on aluminium in 0.4M sulphuric acid electrolyte at 293K. The study follows the behaviour of a fine tungsten tracer layer, initially located in the aluminium, during anodizing. Significantly, the tungsten is incorporated into the anodic film with negligible loss of the tracer to the electrolyte. The findings indicate that pores develop primarily due to flow of film material in the barrier layer under the influences of the stresses of film growth. The flow of material from beneath pores toward the cell walls is accommodated by the increased thickness of the anodic film relative to that of the oxidized metal by a factor of about 1.35. [Copyright &y& Elsevier]

Abstract: The development of pores in a classical porous anodic film formed on aluminium in phosphoric acid solution is investigated. The study employs a tungsten tracer layer that is incorporated into the anodic film from the aluminium substrate, followed by detection of the tracer by transmission electron microscopy and Rutherford backscattering spectroscopy. Distortions of the tungsten layer on entry into the film and retention of tungsten species in the film are compatible with porosity arising mainly from flow of anodic oxide beneath the pore bases towards the cell walls. The behaviour is contrary to expectations of a dissolution model of pore formation. [Copyright &y& Elsevier]

The unusual occurrence of anodic films of unlimited thickness has been recently reported for anodizing of tantalum in certain dehydrated, high-temperature electrolytes with organic solvents. The precise nature of these films is still uncertain. In the present work, non-thickness-limited (NTL) anodic films were formed at 0.1 mA cm-2 on aluminium and tantalum in glycerol/phosphate electrolyte at 453 K and then examined by Rutherford backscattering spectroscopy (RBS). The results disclosed films composed of alumina and tantala, free of phosphorus species at the resolution of the measurements. Most notably, the densities of the NTL alumina and tantala were about 2.4 and 3.6 g cm-3, respectively. These values are less than those of compact anodic films of the type usually grown at high efficiency in aqueous electrolytes by respective factors of about 1.3 and 2.2. This difference in density is attributed primarily to the morphology and structure of NTL film materials, which incorporate significant porosity. [Copyright &y& Elsevier]

The growth kinetics of chromate/fluoride conversion coatings are examined for 99.99% aluminium, and Al–2.3at.%Cu, Al–1.9at.%Au and Al–20at.%Au alloys. The thickening of coatings and thinning of substrates, the latter deposited by magnetron sputtering, are determined by transmission electron microscopy. The results reveal consumption of substrates during the coating process, to the maximum immersion time of 24 min. Initial relatively rapid thinning in the period to 6 min is followed by slower, approximately constant thinning at about 9.3, 6.2, 6.0 and 5.4 nm min−1 for the 99.99%Al, Al–2.3at.%Cu, Al–1.9at.%Au and Al–20at.%Au alloys respectively. The ratio of the number of chromium atoms in the conversion coating, determined by Rutherford backscattering spectroscopy, to the number of oxidized substrate atoms is about 3, indicating a low efficiency of coating growth. Alloying decreases the coating thickness, as well as metal consumption. Alloying elements initially enrich beneath the coating, in alloy layers of a few nanometres thickness. Later, copper may be oxidized, and then enter the coating, whereas gold is occluded as nanoparticles. The gold nanoparticles act as markers to indicate formation of coating material at the metal/coating interface. Sudden loss of coating on the alloys, after about 6–9 min, occurs soon after entry of alloying element species into the coating. The loss may be related to the presence of nanoparticles of gold and copper near the alloy/coating interface, the latter possibly forming by reduction of copper species, that disrupt the bonding of the coating either directly though their presence or through their action as local cathodes. [Copyright &y& Elsevier]

The present study examines the behaviour of hydrogen impurity in an Al–6.5 at.% W alloy during anodizing, using elastic recoil detection and nuclear reaction analyses. Increased concentrations of hydrogen are found near the alloy/anodic film interface, amounting to ∼2×1015 H atoms cm−2 for the particular alloy, containing 0.1–0.3 at.% hydrogen in the bulk regions, and conditions of anodizing. The enrichment arises from hydrogen in the alloy (i) diffusing to the interface, which acts as a trap, or (ii) accumulating at the interface, due to the growth of the anodic film, or a combination of both processes. Diffusion is consistent with the known mobility of hydrogen in aluminium near ambient temperature. Further, accumulation, and subsequent oxidation, of hydrogen are expected based on the general behaviour of alloying elements in anodized aluminium. The anodic films contained ∼0.1–0.3 at.% hydrogen, originating from either the electrolyte or the alloy. [Copyright &y& Elsevier]

Using medium energy ion scattering, combined with Rutherford backscattering spectroscopy, transmission electron microscopy and atomic force microscopy, the development of the copper-enriched alloy layer during anodizing of a sputtering-deposited Al–0.4 at.% Cu alloy has been examined. The enriched layer is located just beneath the amorphous alumina film that is produced on the anodized alloy. Importantly for understanding the mechanism of formation of the enriched layer, the layer is revealed to be of thickness ∼2.1 nm from the start of the anodizing, when enrichment of copper is very low, with no significant increase in the thickness of the layer as the copper enriches to ∼3×1015 Cu atoms cm−2, the maximum measured in the present experiments. The findings are consistent with a model of the layer in which the copper is present mainly in copper-rich clusters. [Copyright &y& Elsevier]

Barrier-type anodic oxide films, with relatively uniform distributions of Al[sup 3+] and Zr[sup 4+] ions, have been formed on sputter-deposited, non-equilibrium Zr-Al alloys, containing 5, 15 and 43 at.% Al, in 0.01M ammonium pentaborate electrolyte at 293K. The films formed on the last two alloys are primarily amorphous, with transport numbers of cations of about 0.18 and 0.25 respectively, i.e., increasing with increase in aluminium content. Thus, as for other amorphous anodic oxides, both cation and anion transport contribute significantly to film growth. However, nanocrystals, suggested to be based on cubic ZrO[sub 2], are formed near the alloy-film interface in the amorphous oxide matrix, in association with bubbles due to generation of oxygen gas in the vicinity of the alloy-film interface. Increase in the zirconium content to 95 at.% leads to the formation of a relatively uniform, crystalline anodic film with a monoclinic ZrO[sub 2] structure; film growth now proceeds predominantly due to migration of anions, similar to films formed on high purity zirconium. [ABSTRACT FROM AUTHOR]

MgB 2 particles have been incorporated into a coating formed by AC plasma electrolytic oxidation (PEO) on aluminium under a constant current density. The incorporation of MgB 2 into the coating was characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The additions of particles to the electrolyte promoted a voltage decrease, possibly a result of the initiation of a "soft" sparking regime. The transition occurred at earlier times with increase of the MgB 2 concentration and enhanced MgB 2 incorporation into the coating. The coating containing particles was thicker than one in which MgB 2 was absent and had a nodular surface, with MgB 2 accumulations in the valleys between the nodules. A superconducting quantum interference device revealed superconductivity below about 39 K, the critical temperature of MgB 2 , and hysteresis in magnetic moment-field curves that typifies a type II superconductor. • PEO coatings containing MgB 2 were formed on aluminium in silicate electrolyte. • Particles were mainly distributed in the outer 40% of the coating thickness. • Coatings exhibited superconductivity below the critical temperature of MgB 2 (39 K). • MgB 2 was incorporated primarily during "soft" sparking period of PEO. • The transition to "soft" sparking was promoted by increase of MgB 2 in electrolyte. [ABSTRACT FROM AUTHOR]

Abstract: Electrograining of a binary Al–Si alloy has been undertaken in nitric acid based electrolytes, with the resultant surfaces examined by scanning and transmission electron microscopies. Depending on electrograining conditions, the pit appearance varies from hemispherical to large lateral pits, with the latter favoured in relatively acidic electrolytes. The conditions prevailing in the pit have been explored through use of aluminium ion additions to the nitric acid electrolyte as well as additions of species which influence the precipitation and dissolution of aluminium hydroxide. These confirm that control of the pit solution pH, through hydroxide generation, as a result of the selected electrograining conditions and consequent anodic and cathodic polarisation, enables tailoring of the resultant electrograined surface appearance. [Copyright &y& Elsevier]

The effects of permanent mold cast microstructure on the growth of anodic oxide layers on three different aluminum substrates (i.e. Al99.8, AlSi10, and AlSi10Cu3, wt.%) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and laser scanning confocal microscopy (LSCM). The anodic oxidation was performed galvanostatically in 2.25 M H2SO4, at 0 °C. The oxide layers developed a microscale topography mainly determined by the morphology of aluminum grains and cells. A low amount of insoluble impurities, uniformly distributed, would contribute to the growth of oxide layers with minimum defects and uniform thickness on the pure aluminum substrate whereas for the binary and ternary systems, a fine cell structure and a modified morphology of Si particles would be favorable. The Al–Fe and Al–Fe–Si particles were occluded in the oxide layers next to Si particles, blocking locally the oxide growth whereas Al2Cu particles were preferentially oxidized. In addition, the presence of Si particles in the layer influenced pore morphology by development of deflected pores around the particles. [Copyright &y& Elsevier]

The ionic transport numbers, relative migration rates of cation species and formation ratios are reported for barrier anodic films formed on metastable, solid solution Al–Ta alloys, with compositions extending from the aluminium-rich to the tantalum-rich ends of the system. The data were obtained by marker experiments, using ion implanted xenon, transmission electron microscopy, Rutherford backscattering spectroscopy and medium energy ion scattering. The films are amorphous and form by migration of metal and oxygen species. The ionic transport numbers and the formation ratios depend approximately linearly upon the composition of the alloy, between the values for anodic alumina and anodic tantala. The migration rate of Al3+ ions exceeds that of Ta5+ ions, but reduces in relative magnitude as the tantalum content of film increases. The faster migration of Al3+ ions is consistent with the higher energy of the Ta5+–O bond compared with that of the Al3+–O bond. Due to the difference in mobilities of the cation species, the films comprise an outer layer of alumina and an inner layer containing units of both alumina and tantala. The two-layered films can develop fingers of inner layer oxide that penetrate the outer alumina layer due to the higher ionic resistivity of the alumina. Such channelling of current can lead to mixing of inner layer oxide and alumina and thereby hinder formation of an alumina layer, particularly in films on the more tantalum-rich alloys. [Copyright &y& Elsevier]

The present study demonstrates that the phenomenon of non-thickness limited (NTL) growth of anodic films in a phosphate/glycerol electrolyte at 453 K on tantalum and aluminium is due to the formation of porous films. For both tantala and alumina films, the resultant morphology comprises cells of material orientated approximately normal to the metal/film interface, with each cell containing a central pore of typical diameter in the range 3–15 nm and a barrier layer of film material at the base of the pore. The total porosity is about 5%. The hardness of NTL tantala films, determined by nanoindentation, was 5.1 GPa, reasonably similar to that of conventional anodic tantala. [Copyright &y& Elsevier]

The generation of oxygen gas within an amorphous anodic alumina film is reported. The film was formed by anodizing aluminum, which was first electropolished and then chemically polished in CrO3–H3PO4 solution, in sodium tungstate electrolyte. The procedure results in incorporation of mobile Cr3+ species, from the chemical polishing film, and mobile W6+ species, from the electrolyte, into the amorphous structure. The tungsten species are present in the outer 27% of the film thickness, while Cr6+ species occupy a thin layer within the tungsten-containing region. Above the Cr3+ containing layer, a band develops that contains oxygen bubbles of a few nanometres size. The oxygen is generated by oxidation of O2− ions of the alumina. A mechanism of oxygen generation within the alumina is proposed based on the electronic band structure of the oxide, modified by the Cr3+ and W6+ species, and on the ionic transport processes during oxide growth. [Copyright &y& Elsevier]

The study examined the influence of fluorozirconic acid additions to the electrolyte on the growth rate and composition of porous anodic films formed on AA 2024-T3 alloy in sulphuric acid and tartaric-sulphuric acid electrolytes. A range of anodizing voltages, electrolyte temperatures, fluorozirconic acid concentrations and anodizing times was employed. High purity aluminium was employed as a reference material. Fluorozirconic acid increased the film growth rate, with a reducing influence as the temperature increased from 0 to 37 °C. The growth rate was enhanced by increased additions of fluorozirconic acid to the electrolyte, although chemical dissolution of the film was also accelerated. The films contained fluoride and sulphate ions from the electrolyte, with the fluoride concentrations decreasing with increasing temperature whereas the sulphate concentration was relatively unaffected. [ABSTRACT FROM AUTHOR]

The formation of a trivalent conversion coating on aluminium has been investigated using analytical electron microscopy, atomic force microscopy, ion beam analysis, glow discharge optical emission spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The coating is shown to comprise a chromium- and zirconium-rich outer layer and an aluminium-rich inner layer. Zirconium and chromium are present in chemical states consistent with ZrO 2 , ZrF 4 , Cr(OH) 3 , Cr 2 (SO 4 ) 3 , CrF 3 and CrO 3 or CrO 4 2 − . However, negligible amounts of Cr(VI) species occurred in coatings formed in de-aerated solution. Electrochemical impedance spectroscopy revealed that the inner layer provides the main corrosion protection during short-term tests in 0.1 M sodium sulphate solution at room temperature. [ABSTRACT FROM AUTHOR]

The combination of ultramicrotomy and low-voltage scanning electron microscopy, with selective detection of backscattered electrons, significantly improved depth and lateral resolutions and atomic number contrast (Z-contrast) sensitivity, enabling high-resolution tomography of localized corrosion. This approach has advanced the understanding of the relationship between the distribution of S-phase particles and the initiation and development of localized corrosion, and the transition to intergranular corrosion in an AA2024-T3 aluminium alloy. [ABSTRACT FROM AUTHOR]

Abstract: The deterioration of AA2024, AA6061 and AA7475 anodised in an environmentally-compliant tartaric acid/sulphuric acid electrolyte has been examined as a function of the immersion time in the electrolyte after termination of anodising. By transmission electron microscopy and scanning electron microscopy, degradation of the porous oxide film was qualitatively observed on AA2024. Electrochemical impedance spectroscopy revealed that AA2024 and AA7075 were more sensitive to prolonged immersion in the anodising electrolyte compared with AA6061, due to increased barrier layer thinning rates and increased susceptibility to localized corrosion. Salt spray tests confirmed the previous, indicating decay of anticorrosion performance for AA2024 and AA7075. [Copyright &y& Elsevier]

Abstract: Surface modification by excimer laser surface melting (LSM) has been performed with the aim to improve the corrosion resistance of the AA2050-T8 alloy. LSM produced melted surfaces, largely free of precipitates, with both microstructure and corrosion behaviour depending upon the number of laser pulses employed. Increased number of laser pulses resulted in thicker melted layers, but also in greater trapped porosity and formation of micro-cracks at the overlapping area. Nevertheless, the LSM-treated specimens exhibited enhanced corrosion resistance compared to the untreated alloy, which was associated with the formation of a relatively uniform melted layer and a diminished presence of precipitates. [Copyright &y& Elsevier]

Abstract: The formation of organic-modified sol–gel coatings on 1050 aluminium alloy has been examined. The coatings were prepared by the combination of a completely hydrolysable precursor of tetra-n-propoxyzirconium (TPOZ), with a partly hydrolysable precursor of glycidoxypropyltrimethoxysilane (GPTMS). Influences of GPTMS/TPOZ ratio, withdrawal speed and curing temperature on the barrier-type corrosion protection of the alloy were examined. By control of process parameters and ratios of organic and inorganic moieties, crack-free sol–gel coatings with improved corrosion protection were produced on the alloy surface. Optimum protection was achieved by a coating formed with a GPTMS/TPOZ ratio of 2.7, a withdrawal speed of 180mm/min and curing at 110°C. The coating formed under the optimum conditions revealed good adhesion to the etched and desmutted alloy substrate. The pitting potential of the coated alloy increased significantly, indicative of an improved corrosion protection of the alloy. [Copyright &y& Elsevier]

Abstract: The influence of an anodizing pre-treatment in sulphuric acid is investigated on plasma electrolytic oxidation (PEO) of aluminium in silicate electrolyte under constant rms current. The presence of the anodic film is shown to promote the establishment of a micro-arc regime that is favourable for growth of the PEO coating. The incorporation of the pre-formed film into the coating appears to proceed by thermal transformation of the anodic alumina, accompanied by formation of oxide beneath the pre-formed layer. The final coatings contain α- and γ-Al2O3, with increased concentrations of silicon, sodium and potassium in an outer region of the coating. [Copyright &y& Elsevier]

Abstract: Hybrid organic–inorganic sol–gel-matrices, with up to 20wt.% incorporated ceria nanoparticles, have been employed as coatings for an AA2024-T3 aluminium alloy. The morphology of the coatings and associated nanoparticles has been examined by conventional and high-resolution transmission electron microscopy, revealing a relatively uniform distribution of 5nm size nanoparticles across the coating thickness. Electrochemical studies indicate a general beneficial effect of incorporation of ceria nanoparticles, although the performance of the coated alloy depends on the nanoparticle content. Electrochemical polarisation behaviour revealed that the coating decreased the anodic current density by about seven orders of magnitude compared with the uncoated alloy, with high breakdown potentials in chloride-containing solution. Accelerated salt spray testing showed that corrosion in an artificial scratch is blocked most efficiently by high ceria contents, whereas general corrosion is inhibited effectively with comparatively low ceria contents. Electrochemical impedance spectroscopy indicated degradation of the barrier properties of coatings with increased amounts of incorporated nanoparticles. Assessment of the abrasion and scratch resistance, and hydrophobicity also revealed additional beneficial functional properties of the coatings containing nanoparticles. [Copyright &y& Elsevier]

Abstract: A study has been carried out to investigate the incorporation of zirconia into alumina-based coatings formed on aluminium using DC plasma electrolytic oxidation in phosphate and silicate electrolytes containing zirconia nanoparticles in suspension. The nanoparticles were incorporated mainly near the coating surface and within cavities inside the coatings. Additionally, a silicon-rich, surface deposit appeared to entrain the nanoparticles of the silicate electrolyte. Zirconia was also present in cellular microstructures of the main alumina-rich coating layers, suggesting melting at >1710°C and subsequent solidification. The coatings frequently contained tetragonal zirconia in addition to monoclinic zirconia, with the presence of zirconia also affecting the proportions of γ- and α-alumina in favour of the former phase. [Copyright &y& Elsevier]

Abstract: The role of pre-treatment in the formation of a cerium conversion coating is investigated for the protection of AA2024-T3 and 7075-T6 alloys. The alloys were alkaline-etched and de-smutted in nitric acid, prior to cerium treatment in Ce(NO3)3 at 85 °C with H2O2 accelerator. Potentiodynamic polarization studies in 3.5% NaCl solution revealed a large shift of ∼300 mV of the corrosion potential below the pitting potential for the 7075-T6 alloy, which correlated with the development of a finely-textured, uniform coating. However, the formation of a uniform coating and protection was dependent upon the time of de-smutting, with non-uniform coatings resulting from extended times of de-smutting. In contrast, non-uniform coatings developed on the 2024-T3 alloy, with pitting potential at the corrosion potential, irrespective of the time of de-smutting. Findings for the 2024-T3 alloy indicate that extended de-smutting affects the enrichment of alloying elements. [Copyright &y& Elsevier]

Abstract: Spark anodizing of aluminium at 5Adm−2 in sodium metasilicate/potassium hydroxide electrolytes is studied, with particular emphasis on the mechanism of coating growth, using transmission electron microscopy and surface analytical techniques, with coatings typically 10μm, or more, thick. Two-layered coatings develop by deposition of an outer layer based on amorphous silica, associated with low levels of alkali-metal species, at the coating surface and growth of an inner, mainly alumina-based, layer, with an amorphous region next to the metal/coating interface. Formation of crystalline phases in the inner layer, mainly γ-Al2O3, with some α-Al2O3 and occasional δ-Al2O, is assisted by local heating, and possibly also by ionic migration processes, arising from the rapid coating growth at sites of breakdown. Due to local access of electrolyte species in channels created by breakdown events, the silicon content in the inner coating regions varies widely, ranging from negligible levels to about 10at.%. Silica deposition at the coating surface and formation of Al2SiO5 and Al6Si2O13 phases is promoted by increased time of anodizing and concentration of metasilicate in the electrolyte. However, at sufficiently high concentration of metasilicate and pH, when more extreme voltage fluctuations accompany breakdown, the two-layered nature of coatings is replaced by a mixture of aluminium-rich and silicon-rich regions throughout the coating thickness. [Copyright &y& Elsevier]

Conversion coatings have been formed in two stages on sputtering-deposited aluminium using a chromate/fluoride bath. The first stage, common to all specimens, was conversion treatment for 1 min to produce a coating of thickness of about 70 nm, with associated thinning of the aluminium substrate. Further treatment was then carried out for 13 min, either immediately or with intervening ageing in humid air, water or laboratory air. Notably, ageing in laboratory air for 1 h was sufficient to prevent significant growth of new coating material upon re-immersion in the coating bath. In contrast, ageing in humid air or water allowed additional thickening of the coating, although with a reduced growth rate. The behaviour appears to be related to loss of free or weakly bound water from the coatings in laboratory air, with the composition and structure of the coating being modified such that transport of reactant and product species of the coating process is impeded. [Copyright &y& Elsevier]

The potential of low-voltage, high-resolution scanning and scanning transmission electron microscopy (SEM/STEM) for morphological characterization of various surface insulating films on aluminium and its alloys has been assessed by examination of porous anodic films, barrier anodic films and corrosion product layers. The characterization shows clearly the value of the approaches, particularly the ability to image directly fine details of appropriately-prepared aluminium surfaces that have usually required examination by transmission electron microscopy (TEM). Such ready characterization assists mechanistic understanding of the contributions of the macroscopic surface and flaws or second phase to the filming and corrosion processes. Further, the approaches are applicable to other materials where such understanding was limited by the sample preparation routes available for TEM. [Copyright &y& Elsevier]

Transmission electron microscopy (TEM) has been employed to examine anodic oxide film formation on 99.8 wt.% aluminium, Al–10 wt.%Si and Al–10 wt.%Si–3 wt.%Cu alloys under conditions relevant to hard anodizing. In particular, anodic oxidation of silicon particles proceeded at a significantly reduced rate compared with that of the adjacent aluminium matrix. This gave rise to alumina film encroachment beneath the particles with development of tortuous porosity and, eventually, occlusion of partially anodized particle in the anodic film. Additional effects included the presence of gas-filled cavities above the silicon particles, associated with oxygen generation above the anodizing particle. The presence of such particles and the corresponding gas-filled voids across the anodic film thickness and at the alloy/film interface is considered responsible for the continuous voltage rise during anodizing of the Al–10 wt.%Si alloy, effectively blocking electrolyte access to the pore base and providing local region of high resistance at the alloy/film interface. A direct consequence of the voltage rise was a thickening of the barrier layer at the base of the porous anodic film. For the ternary alloy, with the additional presence of copper and the CuAl2 particles, the latter appear to have undergone complete oxidation, with copper detected in local film regions. [Copyright &y& Elsevier]

Anodizing of solid-solution Al–1at.%Cu alloy in ammonium pentaborate electrolyte is shown to develop two distinct types of amorphous film. On alloy grains of {1 0 0} orientation, the alumina film is of uniform thickness and relatively featureless. For other grains, the film is of non-uniform thickness and contains oxygen bubbles. In both cases, copper species are distributed throughout the film. Copper is enriched in the alloy to ∼5.8×1015 Cu atoms cm−2 for bubble-free grains, with similar or slightly lower levels for other grains. Evidently, copper enrichment alone does not lead to generation of oxygen. Other factors suggested to be involved, each dependent upon grain orientation, are the structure of the enriched alloy layer, the cyclic nature of the oxidation of copper, and the generation of modulated film compositions. [Copyright &y& Elsevier]

The origin of grain orientation dependent textures on a surface treated aluminium Al–0.2% Mg alloy has been probed by atomic force microscopy, transmission electron microscopy and elemental depth profiling by radio frequency glow discharge optical emission spectroscopy. Magnesium cations are observed to act as a tracer species, assisting definition of the residual alumina film thickness, developed by pretreatment. Copper impurities, enriching in the alloy immediately below the alumina film, act as a marker, assisting definition of the alloy/film interface. Conversely, the absence of iron and silicon in the depth profiles, present in the alloy as impurities at similar levels to copper, suggests their role in the development of the texture. Such elements are considered to be present in the alloy as fine segregates; their location at alloy ridges defines the positions of cathodic sites, with the adjacent matrix undergoing dissolution through the residual alumina film during pretreatment. [ABSTRACT FROM AUTHOR]