Corrosion protection of clad 2024 aluminum alloy anodized in tartaric-sulfuric acid bath and protected with hybrid sol–gel coating

The behaviour of second phase particles during anodizing of aluminium alloys

Anodic growth of titanium oxide: Electrochemical behaviour and morphological evolution

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. © 2008 The Electrochemical Society.

Macroscopic and local filming behavior of AA2024 T3 aluminum alloy during anodizing in sulfuric acid electrolyte

The present study investigates the effect of tartaric acid on anodic film morphology and on corrosion resistance of hydrothermally sealed anodized AA2024. Anodizing treatment was performed in dilute sulfuric acid electrolyte with or without addition of tartaric acid. Hydrothermal sealing was carried out in boiling water for each anodized specimen. The morphology of the sealed anodic films was examined using field-emission scanning electron microscopy (FESEM). Electrochemical impedance spectroscopy (EIS) was performed to assess sealing quality and corrosion resistance of the anodic films. It was shown that the sealing was more efficient in the case of anodic films formed in tartaric acid electrolyte mainly due to their lower porosity. It was also observed that the properties of the barrier layer were higher when sealing was performed on specimens anodized in the presence of tartaric acid. This suggests a specific role of the species on the barrier layer, which contributes to the enhancement of the performance in terms of corrosion resistance of the sealed anodic films. The present study clearly validates the beneficial role of tartaric acid in anodizing baths for the corrosion protection of AA2024.

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FESEM and EIS Study of Sealed AA2024 T3 Anodized in Sulfuric Acid Electrolytes : Influence of Tartaric Acid

Cyclic oxidation processes during anodizing of Al–Cu alloys

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. © 2009 The Electrochemical Society.

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

Porous anodic oxides generated on copper-containing aluminium alloys are less regular than anodic oxides generated on pure aluminium. Specifically, a porous oxide morphology comprising layers of embryo pores, generated by a cyclic process of oxide film growth and oxygen evolution, is generally observed. In this work, the relation between the oxidation behaviour of copper during anodising and the specific porous oxide film morphology was investigated by electrochemical techniques, transmission electron microscopy and Rutherford backscattering spectroscopy (RBS). It was found that the anodising potential determines the oxidation behaviour of copper, and the latter determines the porous oxide morphology. At low voltage, relatively straight pores with continuous cell walls were obtained on Al-Cu alloys, but selective oxidation of aluminium atoms resulted in the occlusion of copper-containing metallic nanoparticles in the anodic film. At higher potentials, copper oxidation promoted oxygen evolution within the barrier layer, and generation of a less regular film morphology. RBS, performed on Al-Cu alloy specimens, revealed a high volume fraction of copper atoms in the anodic films generated at low potentials and a reduced amount of copper atoms in the anodic oxide films generated at high potentials. Copyright ©2010 John Wiley & Sons, Ltd.

Enrichment, incorporation and oxidation of copper during anodising of aluminium - copper alloys

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 .

Graded anodic film morphologies for sustainable exploitation of aluminium alloys in aerospace.

The presence of alloying elements such as copper, magnesium and zinc in the 2xxx and 7xxx aluminium alloys increases mechanical properties but requires specific anticorrosion measures such as anodizing followed by painting. In this work, the performance of the anodic film was initially examined showing that, for a fixed anodizing electrolyte, a finer porous film morphology, generated at reduced potentials, provides better corrosion resistance than a coarser morphology generated at increased potentials. Complementing the previous, the effects of addition of tartaric acid to the anodizing electrolyte were examined, showing that tartaric acid does not affect significantly the porous film growth, but it reduces the dissolution rate of a previously formed oxide. Further, it was demonstrated that tartaric acid acts as an anodic inhibitor at concentrations in the ppm range, suggesting that the contributions to the anticorrosion performance are due to residues of tartaric acid in the pore solution. ??The Electrochemical Society.

J. Ferguson

Papers

Cyclic oxidation processes during anodizing of Al–Cu alloys

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

Enrichment, incorporation and oxidation of copper during anodising of aluminium - copper alloys

Graded anodic film morphologies for sustainable exploitation of aluminium alloys in aerospace.

Graded Anodic Films for Corrosion Protection of Aerospace Alloys: Anodizing Procedure and Corrosion Behaviour