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The present work investigates the corrosion behaviour of magnesium with different Fe contents. It is found that the micro-galvanic coupling between Fe-rich particles and magnesium matrix results in surface film formation and decrease in anodic reaction rate, consequently, increase of corrosion potential. Depending on the population density of Fe-rich particle, if the corrosion potential increases to the breakdown potential of surface film, localized corrosion initiates at the sites of breakdown. It is observed that Mg with different Fe contents exhibit the same critical potential value for corrosion initiation. Thus, corrosion potential is a critical factor for the corrosion tolerance limit of Fe in Mg.

The effect of Ag on cathodic activation and corrosion behaviour of Mg-Mn-Ag alloys are investigated in the present work. During corrosion, Mg-Ag intermetallic phase undergoes dealloying to form porous Ag-rich remnant whereas Ag in the alloy matrix is dissolved to form Ag+ ions that subsequently redeposit on the alloy surface via displacement reaction with Mg to form Ag nanoparticles, which lead to significant cathodic activation. The cathodic activation is a self-catalysed process since enhanced cathodic activity promotes anodic dissolution which, consequently, results in further enhancement of cathodic activity. Thus, the corrosion rate of Mg-2 Mn-4Ag alloy increases significantly with immersion time.

The behaviour of Fe-containing particles during alkaline etching and desmutting of AA5052 aluminium alloy is investigated by quasi in situ monitoring of the compositional and morphological evolution of the particles. In alkaline solution, multi-layered oxide/hydroxide films are rapidly formed on the particles, rendering them electrochemically inactive. The particles are inactive in nitric acid due to formation of thin alumina layer. The treatment doesn’t significantly change the total population of Fe-containing particles with the population of Al3Fe being reduced and that of Al(Fe,Mn,Cr)Si remaining almost unchanged. Further, relevance of the observed behaviour to localized corrosion of aluminium alloys is also discussed.

The surface nanotexture developed on the lithium-containing aluminium alloy AA2099-T8 during alkaline etching has been investigated. The formation of the surface nanotexture is associated with copper-rich nanoparticles in the copper-enriched layer beneath the residual alumina film. Specifically, the copper-rich nanoparticles support local cathodic reactions, which support the anodic dissolution of the adjacent aluminium matrix, thereby resulting in the surface nanotexture. The initial development of the surface nanotexture is grain orientation dependent. Further, the presence of smut (etching products), which is porous in nature and rich in O, Mg, Mn, Cu and Zn species, also affects the surface nanotexture evolution.

Aluminium alloy 2024-T351 was examined using a range of electron microscope techniques at the early stages of attack after immersion in 0.1 M NaCl aqueous solution. At attack sites, a significant portion of the intergranular network was attacked, even though only 15% of the boundaries were decorated by second phase particles (θ-phase). Structure of the grains and the grain boundaries, including grain misorientation angle and grain stored energy, based on dislocation density, were probed further. A strong correlation of intergranular attack around grains with higher grain stored energy and therefore high dislocation density was revealed.

SEM based tomography of localized corrosion has been achieved using selective detection of backscattered electrons. The high resolution tomography provides direct evidence that links the surface appearance of corroded alloy, the alloy microstructure and the corrosion propagation path. Stable localized corrosion of AA2024-T351 aluminium alloy was initiated at locations where large clusters of S phase particles were buried beneath the surface. Propagating away from the initiation sites, corrosion developed preferentially along the grain boundary network. The grain boundary attack started beneath the alloy surface, proceeded along preferred grain boundaries and may emerge at the alloy surface.

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.

Polished specimens of AA2024-T3 alloy were immersed for up to 120 min in 0.1 M NaCl. The development of corrosion was monitored using scanning electron microscopy with energy dispersive X-ray spectroscopy (EDXS) and particle induced X-ray emission spectroscopy (PIXE). Both techniques revealed the intermetallic (IM) particle distributions and attack sites as distinguished by detection of chloride species. The earliest stages of attack involved localized attack around isolated IM particles as reported in Part I. Additionally attack occurred on a larger scale developing rapidly with rings of corrosion product surrounding clusters of IM particles. There were significantly higher numbers of IM particles within the corrosion rings, indicating that local clustering played an important role in co-operative corrosion.

Optical and electron microscopies and EBSD were used to study the early stages of corrosion propagation during stable pit formation on AA2024-T3. Polished AA2024-T3 developed large scale rings of corrosion product, typically a few hundred microns in diameter, within 2 h of exposure to 0.1 M NaCl at room temperature. These features were sectioned using diamond ultramicrotomy and substantial subsurface attack, in the form of intergranular corrosion was observed beneath these sites with virtually no grain etchout. A model is proposed for the mechanism of stable pit progression which involves extensive grain boundary attack, followed by grain etchout leading to open pit formation.

The corrosion of aluminium alloy 2024-T3 (AA2024-T3) was studied as a function of immersion time from 2.5 to 120 min in 0.1 M aqueous NaCl solution. At immersion times as short as 5 min, rings of corrosion product of 100 to 200 μm diameter, containing smaller domes of corrosion product, were observed using SEM. The domes of corrosion product had greater chloride concentrations than elsewhere on the surface and represented sites of anodic attack. As the immersion time was increased, significant grain boundary attack was observed within the rings of corrosion product. Analyses of Particle Induced X-ray Emission (PIXE) maps of the corroded surfaces showed a significantly higher number of IM particles around the chloride attack sites than the average particle density for the maps, indicating clustering of IM particles. These results suggest a co-operative corrosion effect as a result of clustering of the IM particles. A mechanism for the generation of the corrosion rings is discussed.

The corrosion behaviour of Al–5Zn–3Mg–0.6Cu–0.8Zr–0.25Cr–0.15Ni–0.15Ti alloys, produced by traditional and powder technologies, with similar thermo-mechanical treatments, in 3% sodium chloride solution, has been examined by electrochemical methods, scanning electron microscopy, transmission electron microscopy and X-ray microanalysis. The alloys reveal similar precipitation but of different shape, size and distribution; further, both alloys experience localized corrosion. Copper-rich precipitates initiate the dissolution of surrounding particles, enriched in Zn and Mg. As a result, the surface is enriched with other alloying elements after a full polarisation run. Cast material has lower corrosion properties because of the higher heterogeneity of the structure. The structure heterogeneity of the cast material involves a more non-uniform distribution of the precipitates, larger Zn- and Mg-rich particles, and depletion of the matrix and areas around the precipitates by alloying elements compared with the powder material.

The corrosion behaviour of a friction stir welded AA7108 T79 aluminium alloy has been investigated using accelerated testing (ASTM G34 EXCO) and electrochemical measurements. The welded alloy showed the expected zones associated with friction stir welding, namely nugget, thermomechanically affected zone and heat affected zone. Corrosion testing revealed that the edge regions of the thermomechanically affected zone were most susceptible to corrosion. The localized corrosion occurs intergranularly due to the non-uniform distribution of η/η′ (MgZn2) precipitates within the thermomechanically affected zone.

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.

Filiform corrosion of AA 5005 H14 aluminium alloy sheet has been investigated. Painted and scribed panels, with different surface treatments, were inoculated in HCl and exposed in a constant humidity cabinet maintained at 40 °C and 75–85% RH for 1000 h. After exposure, the panels were examined by optical and electron microscopy. It is evident that filiform corrosion susceptibility is determined largely by the near-surface microstructure. Heavily deformed layers, comprising oxide-decorated fine grains and dispersoids on as-rolled and mechanically ground alloy surfaces, are readily susceptible to filiform corrosion. Removal of these deformed layers, by caustic etching and acid cleaning, results in a high filiform corrosion resistance. On such surfaces, underfilm corrosion progresses by localized corrosion of the substrate; with comparatively slowly growing filaments propagating by repeated blistering of the overlying lacquer.

Surface treatments, including chemical polishing, alkaline etching, acid pickling, and electropolishing, of aluminium and copper-containing aluminium alloys lead to enrichment of solid solution copper in the metal just beneath the residual oxide films of the treatment processes. The paper presents the enrichment factor for copper as a function of the copper content of the bulk matrix material. The factor is defined as the ratio of the copper enrichment, measured in units of 1015 atoms cm−2, to the copper content of the matrix in at.%. Although absolute levels of enrichment increase with increase in copper content of the alloy, the enrichment factor increases in the opposite sense.

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 � 10 15 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. 2002 Elsevier Science Ltd. All rights reserved.

Previous work of the authors has revealed detachment of anodic films during anodizing of electropolished, solid-solution Al–3 at.%Mg alloy at constant current density, with the possibility of the electropolishing pre-treatment having a significant influence on the loss of the film material. Here, anodic films were formed on a similar alloy, with pre-treatment by electropolishing, alkaline etching or mechanical polishing. The results disclose the detachment of the anodic film for all pre-treatments, thus demonstrating that prior electropolishing is not essential for initiation of detachment. However, the occurrence of detachment was sensitive to the current density, with lower current densities promoting detachment at earlier stages of growth of the anodic film for a particular pre-treatment.

Flaws in anodic alumina films are localized sites where the film material is modified morphologically, structurally, or chemically compared with adjacent regions of uniform amorphous alumina. The flaws are preferred sites of pitting and dielectric breakdown, which are of importance to the corrosion resistance and electronic applications of aluminium respectively. Although flaws have been categorized in general terms as either mechanical or residual, there is no detailed understanding of the precise origin and nature of flaws and their mechanisms of influence in pitting and dielectric breakdown. Consideration is given here to the formation of oxygen bubbles within anodic alumina, by oxidation of O2− ions in the vicinity of the metal/film interface, which are suggested to be one type of residual flaw. The compositions of impurities or second phase particles, which may lead to generation of oxygen bubbles, are examined in the light of systematic investigations of anodizing of binary aluminium alloys.

Anodic oxidation of binary Al–3 wt.% Mg and Al–5 wt.% Mg alloys has been undertaken in ammonium pentaborate electrolyte to develop barrier-type anodic films at relatively high current efficiency. During anodizing, aluminium and magnesium are oxidized at the alloy/film interface, entering the film as Al3 and Mg2 ions. Such behaviour is anticipated for an alloying element with a Gibbs free energy of oxide formation per equivalent less than that of alumina; further, the greater outward migration rate of Mg2 ions relative to that of Al3 ions in anodic alumina is expected from consideration of the respective single metal–oxygen bond energies. However, unexpectedly, with continued anodizing, the essentially alumina film detaches from the alloy surface, which is followed by growth of new film on the exposed substrate. The film detachment from the alloy is associated with void formation, considered to result from the significantly reduced Pilling–Bedworth ratio for formation of anodic MgO compared with that for anodic alumina.

Anodic film growth on an electropolished Al–2 wt% Cu alloy is associated with copper enrichment in the alloy immediately below the alumina film, oxidation of copper and incorporation of outwardly mobile Cu 2 ions within the thickening alumina film. Oxygen gas generation is also evident within the alumina which, through bubble formation, film cracking and healing, results in highly roughened anodized surfaces. Interestingly, gas generation is dependent upon alloy grain orientation, suggesting that the shape, size, arrangement and persistence of the individual copper-rich clusters in the enriched layer influence the anodizing behaviour.

The formation of voids at the alloy/anodic film interface and the enrichments of alloying elements in a thin alloy layer immediately beneath the anodic film as a consequence of anodizing have been examined for Al-0.28 at.% In alloy and Al-0.5 at.% Cu alloys containing ∼0.11 at.% Cd, ∼0.07 at.% In or ∼0.27 at.% Sn. Fine voids are formed for the indium- and tin-containing alloys for the selected conditions of anodizing, which is suggested to be associated with the relatively low Pilling–Bedworth ratios of the alloying element oxides incorporated into the anodic films. Thus, the formation of voids is dependent upon oxidation of the alloying element. Consequently, no voids were resolved for the cadmium-containing alloy for which limited or no oxidation of cadmium occurred. The enrichment of the binary alloy was equivalent to 3.5 at.% In, with enrichments of the ternary alloys to the ranges 11–16 at.% Cu and 2–7 at.% Cd, In or Sn respectively, for an assumed 2 nm thick enriched layer.

Potentiodynamic polarization has been employed to determine the pitting potentials of Al-Mo and Al-Mo-Si metastable alloys. A significant increase in the pitting potential of aluminium is found through addition of molybdenum, to 30 at.%, which is further enhanced, by ~ 600 mV, with addition of 6 at.%Si. Auger electron spectroscopy indicates formation of relatively pure alumina passive films, containing negligible amounts of molybdenum and silicon species, under the selected conditions of polarization. The absence of molybdenum and silicon species in the anodic film is attributed to the enrichment of the alloying elements in the alloy, which is a feature of anodic oxidation of many aluminium alloys. The presence of enriched alloy layers, formed by prior oxidation of aluminium, is revealed by transmission electron microscopy of galvanostatically formed passive films. The increased pitting potentials are suggested to be associated with the enrichments of molybdenum, or molybdenum and silicon, in the alloy, although the precise mechanism is not disclosed.

The anodizing behaviour of sputter-deposited Al-4 at.% Au-1 at.% Cu and Al-1.6 at.% W-0.6 at.% Zn alloys has been examined by Rutherford backscattering spectroscopy and transmission electron microscopy. Initially, a uniform, amorphous alumina film, free of copper and gold species, is formed on the Al-Au-Cu alloy; at the same time, a thin alloy layer, beneath the anodic film, is enriched in copper and gold. The uniform film growth is eventually terminated by generation of oxygen at gold-rich particles at, or near, the alloy/film interface, leading to bubble formation, subsequent rupture of the film and release of oxygen. Film growth on the Al-W-Zn alloy causes mainly enrichment of tungsten in the alloy, with the limited enrichment of zinc contrasting with the behaviour of Al-Zn alloys. Uniform film growth proceeds to high voltages with incorporation of mobile tungsten and zinc species into the film.

The anodic oxidation of dilute Al–Cr alloys, containing 0.8 and 1.7 at% Cr, has been investigated in order to understand the oxidation behaviour of the alloying element and its influences on the film composition and morphology. The alloys reveal two stages of oxidation: an initial stage, in which only aluminium atoms are oxidized to form a chromium-free anodic alumina film, and a subsequent stage, in which both aluminium and chromium are oxidized, in their approximate alloy proportions, with generation of a chromium-contaminated anodic alumina film. In the first stage, chromium is enriched in a thin layer of alloy, immediately beneath the anodic film, to an amount corresponding to a layer of average thickness 1.5 nm and of average composition, Al–20 at% Cr. Following the oxidation of chromium, oxygen is produced electrochemically within the film at or near the alloy/film interface, probably associated with the development of chromium-rich clusters in the enriched alloy layer and, subsequently, formation of semiconducting chromium-rich oxide. Thus, the film material formed at the alloy/film interface by inward migration of O 2- ions contains many oxygen-filled bubbles with associated high pressures. The chromium species present in the film migrate outward more slowly than Al 3+ ions. Hence, a layer of chromium-free anodic alumina, which thickens as the film grows, is maintained adjacent to the film/electrolyte interface.

As part of a systematic study of anodic oxidation of aluminium alloys, focused on the importance of initial enrichment of thin layers of alloy to the oxidation of alloying elements at the alloy/film interface, the behaviour of Al/Zn alloys is considered, which is also of relevance to surface treatment of commercial aluminium alloys. The alloys investigated, by a combination of transmission electron microscopy and Rutherford backscattering spectroscopy, are bulk Al-0.90 at% Zn alloy, in the electropolished condition, and Al-0.23 at% Zn alloy, produced by magnetron sputtering. Following anodic oxidation at 50 Am−2, both alloys reveal zinc enrichments immediately beneath the amorphous anodic films, in thin layers of alloy which contain average concentrations of about 1.8 × 1019 and 4.6 × 1019 Zn atoms m−2, respectively. For the Al-0.9 at% Zn alloy, the enrichment is sufficient for zinc atoms to be oxidized at the alloy/film interface and, hence, for zinc ions to be incorporated into the mainly alumina film. The incorporated zinc ions migrate outwards in the anodic film about 2.3 times faster than Al3+ ions. For the Al-0.23 at% Zn alloy, the enrichment is insufficient for oxidation of zinc atoms to proceed and, hence, no significant amount of zinc ions was detected in the film. The oxidation behaviour of zinc in Al-Zn alloys is generally similar to that of copper in dilute Al-Cu alloys, although the incorporation of zinc ions into the anodic films does not create the high population density of flaws found in films on Al-Cu alloys following incorporation of copper ions. Further, anodic film material on Al-Zn alloys becomes detached from the substrate at a particular condition of the alloy/film interface which, in the present work, occurs at an anodizing voltage of about 100 V for the Al-0.90 at% Zn alloy. Following detachment of the film, most of the zinc in the zincenriched alloy layer is lost from the exposed alloy surface and the enrichment reforms during subsequent regrowth of the anodic film.

The anodic oxidation of Al-0.9 at%Cu alloy has been investigated, focusing on the effect of a mechanical polishing pre-treatment on incorporation of copper ions into the anodic film and the accumulation of copper atoms in the alloy just beneath the alloy/film interface. For the particular alloy and anodizing conditions, a relatively pure alumina film is formed to thickness of about 170 nm with associated progressive accumulation of copper atoms in the alloy at the alloy/film interface. The thickness of the copper-enriched layer is about 2 nm and the average composition approaches about 42 at%Cu. Only on achieving this level of enrichment are copper ions incorporated into the anodic film. The copper ions are more mobile than aluminium ions and hence, are able to reach the film/electrolyte interface during subsequent film growth.