M. Curioni

Macroscopic and Local Filming Behavior of AA2024 T3 Aluminum Alloy during Anodizing in Sulfuric Acid Electrolyte

The anodic film generated in acid electrolytes on high-purity aluminum shows a porous morphology, formed by the packing of approximately hexagonal cells of alumina, containing a centrally located cylindrical pore. Conversely, during anodizing of an AA2024 T3 alloy, the presence of alloying elements, both in solid solution and as second-phase material, influences the filming behavior, with a less regular film morphology developed above the aluminum alloy matrix, and characteristic morphologies generated above and in the zone of influence of second-phase particles. Further, the anodizing behavior of the alloying elements is determined by the applied potential. In this work, the anodizing behavior of AA2024 T3 commercial alloy in sulfuric acid electrolyte has been characterized. Specifically, phenomena related to the oxidation of the second-phase particles have been separated from those associated with the oxidation of the copper-containing aluminum matrix. This has used electrochemical data obtained under potentiostatic and potentiodynamic conditions for the AA2024 T3 alloy, high-purity aluminum, and model Al-Cu alloys, which have been correlated with detailed examination of plan and sectional views of the anodized substrates.

Volume Expansion Factor and Growth Efficiency of Anodic Alumina Formed in Sulphuric Acid

The growth of anodic alumina in sulphuric acid is investigated at constant current on bulk and sputtering-deposited aluminium. The ratio of the thickness of the film to the thickness of oxidized aluminium is shown to increase with increase of the current density (from 0.5 to 50 mA cm-2) and with decrease of the electrolyte temperature (from 20 to 0°C). In addition, the sulphur content of the films and the efficiency of film formation increase. It is suggested that pores are generated primarily by dissolution at current densities below �?�2 mA cm-2, with flow of film material dominating at higher current densities.

Role of Tartaric Acid on the Anodizing and Corrosion Behavior of AA 2024 T3 Aluminum Alloy

Tartaric acid is added to sulfuric acid anodizing baths to generate porous anodic film that provides corrosion resistance to practical aerospace alloys and reduces the environmental impact of the traditional chromic acid anodizing process. Here, a fundamental study on the effects of the addition of tartaric acid to the sulfuric acid anodizing electrolyte has been undertaken. During anodizing, it was evident that tartaric acid does not significantly affect the mechanism of porous film growth, but it reduces the growth rate of the porous anodic film. After anodizing, in acidic environments, it may reduce the dissolution rate of a previously formed oxide. Furthermore, it was found that in a nearly neutral, chloride-rich environment, tartaric acid limits the anodic reaction of aluminum dissolution at concentrations in the hundreds of ppm range. The previous suggests that the good anticorrosion performance of alloys anodized in the presence of tartaric acid is due to residues of tartaric acid in the pore solution.

Anodic Film Formation on AA 2099-T8 Aluminum Alloy in Tartaric–Sulfuric Acid

The anodizing behavior of a lithium-containing aluminum alloy (AA 2099-T8) in an environmentally friendly electrolyte, namely tartaric-sulfuric acid (TSA), has been examined under potentiodynamic and potentiostatic conditions. Specifically, the dependence of the anodic film morphology and composition on the anodizing voltage was investigated. It is revealed that porous anodic films with well-defined cells were formed at relatively low voltages while porous anodic films with pores of increased dimensions and lateral porosity were formed at increased voltages. In addition, it is indicated that copper in the alloy matrix can be occluded in the anodic film material as copper-rich nanoparticles or it can be oxidized and incorporated into the film material as copper ions, depending on the anodizing voltage. In the latter case, the process is accompanied by oxygen gas generation within the film material, resulting in the lateral porosity in the anodic film. Further, the structures of the copper-rich nanoparticles have been determined and the mechanism of the formation of such nanoparticles has been discussed.

Anodizing of Aluminum under Nonsteady Conditions

Porous anodic alumina is used in nanotechnology, electronics, and corrosion protection with the film morphology tuned by appropriate selection of anodizing electrolyte and anodizing voltage or current. Specifically, tailored potential-time or current-time regimes involving an initial voltage ramp may be used to modify the porous oxide morphology for improved corrosion resistance or nanotechnology applications. In this work, a fundamental study was performed on superpure aluminum to understand the processes of initiation and growth of the porous anodic oxide during anodizing under potentiodynamic conditions. The current-potential response comprises an initial current plateau followed by a region of quasi-exponential dependence of current on applied potential. The first region was associated with initial thickening of the air-formed oxide, and the subsequent quasi-exponential region was related to the establishment of porous anodic film growth. Phenomena related to cell reorganization associated with the continuous variation in the anodizing potential during the growth were examined by direct transmission electron microscopy of ultramicrotomed sections and stripped porous anodic films and compared with anodic oxides generated under steady potential conditions. ?? 2009 The Electrochemical Society.

Incorporation of Gold into Porous Anodic Alumina Formed on an Al – Au Alloy

An Al-1 atom % Au gold alloy is employed to investigate the incorporation of a nonoxidized alloying element into porous anodic alumina films formed in phosphoric acid and sulfuric acid electrolytes. Gold is shown to enrich in the alloy in the initial stages of film growth, when a gold-free film is formed. On sufficient enrichment of the alloy, gold nanoparticles are incorporated into the film above an enriched alloy layer containing nanocrystals of Al2 Au. The formation of the nanoparticles is accompanied by a reduced rate of film growth due to generation of oxygen. The gold is incorporated into the films preferentially at the cell boundary regions of the scalloped alloy/film interface. Elongated and approximately circular nanoparticles are observed in film sections, the former type being particular to films formed in sulfuric acid electrolyte. The incorporation of gold into the film commences earlier than predicted for a planar alloy/film interface. The behavior suggests that transport of gold occurs within the enriched alloy layer, possibly associated with the stresses at the alloy/film interface due to film growth processes.

Influence of Applied Potential on Titanium Oxide Nanotube Growth

The effect of applied potential on the morphology, growth efficiency, and composition of self-organized titanium oxide nanotubes, generated in 0.2 M N H4 F /glycerol electrolyte, has been examined by electron microscopy, Rutherford backscattering spectroscopy, and nuclear reaction analysis. A linear increase in nanotube diameter, length, and wall thickness, with increasing applied potential is evident up to 40 V, whereas the growth efficiency decreases with increase of the applied potential. Due to the semiconductive properties of the titanium oxide, at potentials above 40 V some of the oxygen anions migrating across the barrier layer lose electrons and generate oxygen gas within the oxide. This results in a morphological transition above 40 V, characterized by the degeneration of nanotube geometry and uniformity, and the appearance of oxygen-filled voids within the barrier layer and nanotubes walls.

Graded Anodic Film Morphologies for Sustainable Exploitation of Aluminium Alloys in Aerospace

High strength aluminium alloys are widely used in the civil and military aerospace industry due to their low weight and high mechanical properties, achieved by selected alloying elements and heat treatments. The resulting multiphase alloy system, a solid solution of alloying elements in the aluminium matrix and a variety of second phase material, requires specific anticorrosion measures in order to prevent localized corrosion, which is promoted by microgalvanic coupling between the different metallographic phases. Traditionally, the anticorrosion performances are achieved by chromic acid anodizing (CAA), followed by painting. However, environmental issues and associated costs for the disposal of chromate wastes, require the development of new approaches for anodizing of aluminium alloys. In this work, the potential for tailoring the porous anodic film morphology through the film thickness by controlled variations of the anodizing potential is inspected. The procedure developed is, in principle, applicable to any aluminium alloy in any anodizing electrolyte and results in the generation of innovative graded porous anodic film morphologies which promise improvement of anticorrosion properties and replacement of CAA .

The contribution of hydrogen evolution processes during corrosion of aluminium and aluminium alloys investigated by potentiodynamic polarisation coupled with real-time hydrogen measurement

Water reduction, which leads to the evolution of hydrogen, is a key cathodic process for corrosion of many metals of technological interest such as magnesium, aluminium, and zinc; and its understanding is critical for design of new alloys or protective treatments. In this work, real-time hydrogen evolution measurement was coupled with potentiodynamic measurements on high-purity aluminium and AA2024-T3 aluminium alloy. The results show that both materials exhibit superfluous hydrogen evolution during anodic polarisation and that the presence of cathodically active alloying elements enhances hydrogen evolution. Furthermore, it was observed for the first time that superfluous hydrogen evolution also occurs during cathodic polarisation. Both the anodic and cathodic behaviours can be rationalised by a model assuming that superfluous hydrogen evolution occurs locally where the naturally formed oxide is disrupted. Specifically, during anodic polarisation, oxide disruption is due to the combined presence of chloride ions and acidification, whereas during cathodic polarisation, such disruption is due to alkalinisation. Furthermore, the presence of cathodically active alloying elements enhances superfluous hydrogen evolution in response to either anodic or cathodic polarisation, and results in ‘cathodic activation’ of the dissolved regions.Corrosion of aluminium: the contribution of hydrogen evolutionAqueous corrosion is an electrochemical reaction resulting in materials degradation, involving simultaneous oxidation of metal and reduction of species in a wet environment. The overall corrosion rate depends on how fast the two processes proceed. Although the reduction of oxygen is the most important cathodic reaction, for more reactive materials such as Al and Mg the reduction of hydrogen is thermodynamically allowed and could contribute to the overall rate. Now a team led by Michele Curioni at University of Manchester look at the hydrogen evolution behaviour of high purity aluminium and AA2024T3 alloy utilising coupled real-time hydrogen evolution and potentiodynamic measurements. Superfluous hydrogen evolution was observed during both anodic and cathodic polarisation and associated in both cases to local oxide disruption. The derived understanding of corrosion could enable us to develop new protection treatments.

Study of the Linear Friction Welding Process of Dissimilar Ti-6Al-4V–Stainless Steel Joints

Linear friction welding (LFW) is an innovative joining method that can be used to obtain high-strength joints between dissimilar materials. A key factor that influences the joints performances are the intermetallic compounds that could be formed during the welding process. These intermetallics are brittle and could compromise the mechanical performances of the joint. This article deals with the analysis of the LFW process of dissimilar titanium–stainless steel joints. Two different types of joints were studied: AISI 304–Ti6Al4V and AISI 316–Ti6Al4V. Particular attention was paid to characterizing the intermetallic compounds using scanning electron microscopy, Electron probe microanalysis and X-ray diffractometry. Zones with different microstructure were observed. Due to the diffusive phenomena occurring during the welding, Kirkendall effect and occurrence of several intermetallics were observed. Moreover, it was found that the joint with AISI 316 formed brittle intermetallic compounds, which led to crack formation close to the weld line.