Growth of anodic oxide films on AC2A alloy in sulphuric acid solution

Growth behaviour of anodic oxide films on AC2A Al cast alloy was investigated in sulphuric acid solution using SEM, optical microscope (OM) and confocal scanning laser microscope (CSLM) and energy dispersive spectroscopy (EDS). The AC2A alloy contains three different types of second-phase particles: Al–Cu, Al–Cu–Fe–Si and Al–Si particles. The growth of anodic oxide films was critically retarded by the presence of non-reactive particles of Al–Si, while little effect was observed by the presence of active particles of Al–Cu and Al–Cu–Fe–Si. The most severe retardation effect on the growth of anodic films on AC2A alloy resulted from agglomerated Al–Si particles.     

Dielectric breakdown and healing of anodic oxide films on aluminium under single pulse anodizing

Single pulse anodizing of aluminium micro-electrode has been employed to study the behaviour of dielectric breakdown and subsequent oxide formation on aluminium in alkaline silicate and pentaborate electrolytes. Current transients during applying pulse voltage have been measured, and surface has been observed by scanning electron microscopy. Two types of current transients are observed, depending on the electrolyte and applied voltage. There is a good correlation between the current transient behaviour and the shape of discharge channels. In alkaline silicate electrolyte, circular open pores are healed by increasing the pulse width, but such healing is not obvious in pentaborate electrolyte.     

EIS characterisation of anodic films formed on 2024 aluminium alloy, in sulphuric acid containing molybdate or permanganate species

Electrolytes composed of sulphuric acid and corrosion inhibitors (molybdate or permanganate species) were proposed in order to replace chromic acid for the anodising of 2024 aluminium alloy. The electrochemical impedance spectroscopy (EIS) method was used to visualise the correlation between the corrosion performance in NaCl and the morphology of these new anodic layers. From an appropriate equivalent circuit, EIS parameters concerning the porous and barrier layers were detected. Their evolution during corrosion tests was discussed. The results indicate that the morphology and the corrosion resistance of anodic films formed in acid sulphuric with molybdate species remain unchanged. On the contrary, morphological properties of anodic films formed in presence of permanganate species are modified, favouring their corrosion performance. EIS analyses were completed with SEM technique.     

Corrosion behaviour of sintered NdFeB deposited with an aluminium coating

A protective, pure Al coating was deposited by direct current (DC) magnetron sputtering onto sintered NdFeB magnets. Separated, single phases of sintered NdFeB (the Nd-rich phase, the B-rich phase and the matrix phase) were prepared by arc melting for open circuit potential (OCP) tests. The corrosion process of the sintered NdFeB magnets coated with Al (Al/NdFeB) was studied experimentally. It was found that the corrosion process can be divided into three different stages. The Al coating cannot provide complete sacrificial protection for the sintered NdFeB magnets.     

Degradation of the corrosion resistance of anodic oxide films through immersion in the anodising electrolyte

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.     

Influence of molybdate species on the tartaric acid/sulphuric acid anodic films grown on AA2024 T3 aerospace alloy

AA2024 T3 alloy specimens have been anodised in tartaric acid/sulphuric media and tartaric acid/sulphuric media containing sodium molybdate; molybdate species were added to the anodising bath to enhance further the protection provided by the porous anodic film developed over the macroscopic alloy surface. Morphological characterisation of the anodic films formed in both electrolytes was undertaken using scanning electron and transmission electron microscopies; the chemical compositions of the films were determined by Rutherford backscattering spectroscopy that was complemented by elemental depth profiling using rf-glow discharge optical emission spectrometry. The electrochemical behaviour was evaluated using potentiodynamic polarisations and electrochemical impedance spectroscopy; the corrosion performance was examined after salt spray testing. The porous anodic film morphology was little influenced by the addition of molybdate salt, although thinner films were generated in its presence. Chemical composition of the anodic film was roughly similar; however, addition of sodium molybdate in the anodizing bath resulted in residues of molybdate species in the porous skeleton and improved corrosion resistance measured by electrochemical techniques that was confirmed by salt spray testing.     

Observations of anomalies in the anodic behaviour of microcrystalline aluminium

The anodic behaviour of sputtered microcrystalline Al (mc-Al) was investigated in neutral Na₂SO₄ electrolyte under varied conditions. Our results revealed that Cl⁻ addition led to a reduction in the anodic current density, which we considered unusual. Mott-Schottky analysis showed that Cl⁻ introduction altered the semiconducting property of the passive film from n-type to p-type, implying that the p-type film can possessed a relative higher stability. Immersion of mc-Al in other electrolytes yielded films with n-type, p-type and positive p-n junction structure. The results also indicate that the p-type film was most stable and the positive p-n junction film least stable.     

Development of anodic coatings on aluminium under sparking conditions in silicate electrolyte

Spark anodizing of aluminium at 5 A dm⁻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 10 μm, 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 10 at.%. 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.     

Corrosion evaluation of microarc oxidation coatings formed on 2024 aluminium alloy

Dense alumina ceramic coatings of 7 μm thickness were fabricated on 2024 aluminium alloy by microarc oxidation (MAO). The corrosion behaviour of the MAO coated alloys was evaluated using potentiodynamic polarisation and EIS measurements. The results show that the corrosion process of the coated alloy can be divided into three stages: (1) the initial stage (the first 2-6 h of immersion): penetration of corrosion medium into the aluminium alloy was inhibited by coating; (2) the second stage (after 24 h of immersion), corrosion medium penetrated to attack the interface between the substrate and the coating; (3) the final stage (after about 96 h): corrosion process was controlled by the diffusion of corrosion products.     

Corrosion of hot extrusion AZ91 magnesium alloy. Part II: Effect of rare earth element neodymium (Nd) on the corrosion behavior of extruded alloy

The corrosion behavior of extruded Nd-free AZ91 and extruded AZ91 + 1.5Nd alloy was investigated by weight loss and electrochemical measurements. The results showed that the extruded AZ91 + 1.5Nd alloy had higher corrosion resistance compared to the extruded Nd-free AZ91 alloy, which could been explained from point of view of microstructure changes: (1) the significant decrease of twins and dislocation decreased the anodic dissolution rate; (2) the micro-galvanic corrosion was inhibited by the formation of Al₃Nd phase; and (3) Nd not only increased the percent of Non-Faraday process, but also led to anisotropic feature on the corrosion mechanism.     

Development of porous anodic films on 2014-T4 aluminium alloy in tetraborate electrolyte

Anodic film growth on 2014-T4 aluminium alloy at 60 V in 50 g l⁻¹ di-sodium tetraborate at 60 °C has been examined by transmission electron microscopy and Rutherford backscattering spectroscopy. Initial film growth proceeds at relatively high efficiency on the initially etched and desmutted alloy. During the subsequent period of current decline, the reactive electrolyte species penetrate the outer film at preferred regions, establishing conditions for pore development by field-assisted dissolution. In the alkaline electrolyte, such field-assisted dissolution also appears to proceed locally, probably through mechanical disruption of the film, giving rise to a feathered film morphology. The oxidation of copper from the alloy, in the presence of an enriched layer of copper, developed largely by initial etching, also influences film morphology through parallel oxygen gas generation, creating oxygen-filled voids. Such gas-filled voids may rupture or be removed from the alumina film material through field-assisted dissolution at the pore base. In the former case, cracking allows access of the anodizing electrolyte to the enriched alloy/film interface, with subsequent dissolution of the enriched layer and local film growth; these give rise to lateral porosity in addition to that from pores passing perpendicularly to the alloy surface. The efficiency of anodizing is about 12%, with losses from Al³⁺ ion ejection, field-assisted dissolution, oxygen gas generation, film rupture, interface dissolution and local film repair.     

Formation of barrier-type anodic films on sputtering-deposited Al-Ti alloys

The formation of amorphous anodic films at constant current is investigated for sputtering-deposited Al-Ti alloys containing from 3-30 at.% Ti. The films were grown at high efficiency in a borate electrolyte and comprised a main region containing units of Al₂O₃ and TiO₂, with a thin surface region enriched in titanium species. The formation ratios of the films increased with increase of titanium content of the alloys. The presence of the outer region is explained by the faster migration of Ti⁴⁺ ions relative to that of Al³⁺ ions through the films.     

Stability of aluminium in 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid and ethylene glycol mixtures

The stability of electrochemically passivity aluminium has been investigated in mixtures of ethylene glycol, 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, Na₂B₄O₇.10H₂O and NaH₂PO₄. The effects of (i) borax and sodium dihydrogen phosphate salts presence, (ii) water absorption and (iii) ethylene glycol content are studied by electrochemical impedance spectroscopy (EIS). The results show a decrease in the polarization resistance and an increase in the capacitance associated with the passive oxide dielectric properties with an increase in the water and/or ethylene glycol content in the mixtures. The presence of Na₂B₄O₇.10H₂O and NaH₂PO₄ salts stabilize the oxide layer.     

Anodic dissolution of Al sacrificial anodes in NaCl solution containing Ce

The corrosion behaviour of Al–Zn–In sacrificial anodes has been investigated in a sodium chloride solution containing CeCl₃. Scanning electron microscopy, energy-dispersive X-ray analysis, and inductively coupled plasma mass spectrometry have been employed to gain knowledge of the micro-morphology and corrosion process of the Al alloy. Cerium, both as the alloy element and as the additive in the NaCl solution, improves the electrochemical properties of the Al–Zn–In alloy. The activation of Ce in the Al–Zn–In alloy in the NaCl solution has been studied.     

The valence state of copper in anodic films formed on Al-1at.% Cu alloy

During anodising of Al-Cu alloys, copper species are incorporated into the anodic alumina film, where they migrate outward faster than Al³⁺ ions. In the present study of an Al-1at.% Cu alloy, the valence state of the incorporated copper species was investigated by X-ray photoelectron spectroscopy, revealing the presence of Cu²⁺ ions within the amorphous alumina film. However, extended X-ray irradiation led to reduction of units of CuO to Cu₂O, probably due mainly to interactions with electrons from the X-ray window of the instrument and photoelectrons from the specimen. The XPS analysis employed films formed on thin sputtering-deposited alloy/electropolished aluminium specimens. Such an approach enables sufficient concentrations of copper species to be developed in the anodic film for their ready detection.     

Influence of incorporated Mo and Nb on the Mott-Schottky behaviour of anodic films formed on AISI 304L

The effect of incorporated Mo and Nb on the electronic properties of oxide films formed on AISI 304L was investigated by Mott-Schottky analysis. The films show a bi-layer structure and behave as n-type and p-type semiconductors at potentials above and below the flat band potential. The inner p-type layer is Cr-enriched, while the outer n-type layer shows a slight increase in Fe-content close to the outer surface, where NbO³⁺-oxalate or MoO₄²⁻ incorporation occurred. The observed enhancement of pitting corrosion resistance of anodized steels is most probably related to compositional changes and thickness increase of the film after the surface treatments.     

Modelling the anodizing behaviour of aluminium alloys in sulphuric acid through alloy analogues

Anodic film morphologies on aluminium aerospace alloys are strongly influenced by alloying elements. The present study uses model alloys to interpret the early stages of anodizing of AA2024-T3 and AA7075-T6 aluminium alloys in 0.4 M sulphuric acid electrolyte. Further, coupled model alloys, representative of matrix and second phase regions, are employed as alloy analogues. The findings enable assignment of transient anodic currents during potentiodynamic polarization of the commercial alloys to oxidation of Al₂CuMg phase at 0 V SCE and of Al₂Cu, Al₇Cu₂Fe and Al–Cu–Fe phases at 5–6 V SCE. The phases that oxidize at the latter potential also cause voltage arrests during galvanostatic anodizing.     

Discontinuities in the porous anodic film formed on AA2099-T8 aluminium alloy

The anodizing behaviour of constituent particles (Al–Fe–Mn–Cu) and dispersoids (Al–Cu–Mn–Li and β′(Al₃Zr)) in AA2099-T8 has been investigated. Low-copper-containing Al–Fe–Mn–Cu particles anodized more slowly than the alloy matrix, forming a highly porous anodic oxide film. Medium- and high-copper-containing Al–Fe–Mn–Cu particles were rapidly dissolved, resulting in defects in the anodic film. The anodizing of Al–Cu–Mn–Li dispersoids is slightly slower than the alloy matrix, forming a less regular anodic oxide film. β′(Al₃Zr) dispersoids anodized at a similar rate to the alloy matrix. Further, the potential impact of the discontinuities in the resultant anodic films on the performance of the filmed alloy is discussed.     

Influences of ion migration and electric field on the layered anodic films on Al–Mg alloys

Anodizing of sputtering-deposited Al–Mg alloys containing 27 and 32 at.% magnesium in sodium hydroxide electrolyte is shown to develop two-layered anodic oxide films. The outer layer contains aluminium and magnesium species, and is enriched in the latter species relative to the alloy, particularly towards the film surface. The inner layer also contains the two alloy species but is depleted in magnesium, due to Mg²⁺ ions migrating to the outer layer faster than Al³⁺ ions. The ratio of the thickness of the outer layer to that of the film increases with increase of magnesium content of the alloy. The presence of aluminium species in the outer layer is attributed to the penetration of the outer layer by oxide of the inner layer with lower ionic resistance. This mechanism of film growth appears to be sustainable to alloy concentrations to 40 at.%Mg, when the inner layer may no longer form. Enrichment of alloying elements can accompany film growth on Al–Mg alloys, as shown by enrichment of tungsten to 2–3 × 10¹⁵ atoms cm⁻² in an Al–26 at.%Mg–1 at.%W alloy.     

Kinetics of H2O2 reaction with oxide films on carbon steel

The nature of carbon steel surfaces in 0.01 M borate solutions (pH 10.6) have been characterized using a range of electrochemical techniques and ex situ analyses such as Raman and Auger spectroscopy. Their subsequent behaviour on exposure to 10⁻³ M H₂O₂-containing solutions has also been studied. The anodically oxidized carbon steel surfaces have been characterized according to three regions: (I) the potential range <−0.5 V (vs SCE), when the surface is active and covered by Fe^II/Fe^III oxide/hydroxide; (II) the potential range −0.5 V to 0.0 V when the surface is passivated by an outer layer of Fe^III oxide/hydroxide over the inner layer of Fe^II/Fe^III oxide/hydroxide; and (III) potentials >0 V when further growth of the underlying layer appears to lead to minor film breakdown/restructuring. The addition of H₂O₂ to films grown in the passive region or above (II and III) leads initially to a degradation of the outer layer allowing increased growth of the inner layer. Subsequently, the outer passivating layer is repaired and passivity re-established. These changes appear to be confirmed by Raman spectroscopy.