Preferential anodic oxidation of second‐phase constituents during anodising of AA2024‐T3 and AA7075‐T6 alloys

Miera, M. Saenz de; Curioni, Michele; Skeldon, P.; Thompson, G.E.

doi:10.1002/sia.3191

Cited by 54 publications

(38 citation statements)

References 10 publications

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“…The current response displayed two peaks during the initial voltage ramp, associated with the oxidation of Al-Mg-Cu and Al-Cu-Fe containing second phase particles, respectively. [23][24][25] During the initial voltage ramp, the current density increased to about 8 mA/cm 2 . As the voltage ramp was interrupted and the constant potential applied, the current decreased slightly, to attain a steady value in the region of 7 mA/cm 2 .…”

Section: Resultsmentioning

confidence: 99%

Application of EIS to In Situ Characterization of Hydrothermal Sealing of Anodized Aluminum Alloys: Comparison between Hexavalent Chromium-Based Sealing, Hot Water Sealing and Cerium-Based Sealing

Carangelo

Curioni

Acquesta

et al. 2016

J. Electrochem. Soc.

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Chromic acid anodizing has been used for almost a century to enhance corrosion protection of aerospace alloys. For some applications, hydrothermal sealing in hexavalent chromium-containing solution is required to enhance further the corrosion resistance but, due to environmental concerns, the use of hexavalent chromium must be discontinued. Good progress has been made to replace chromates during anodizing but comparatively less effort has focused on the sealing process. In this work, for the first time, electrochemical impedance spectroscopy (EIS) has been used to characterize in-situ the sealing processes occurring during hot water sealing, sodium chromate sealing and cerium sealing. The results suggest that the processes occurring during sodium chromate sealing are significantly different compared to hot water and cerium sealing. In particular, during chromate sealing, the porous skeleton is significantly attacked, suggesting that the anticorrosion performance is likely to arise from the residuals of chromate rather than from the improvement of the barrier properties. In contrast, during hot water sealing, little attack occurs on the porous skeleton, and the improved corrosion protection is due to the enhanced barrier effect. During cerium sealing, precipitation of cerium products occurs, providing an inhibitor reservoir, and little, if any, attack occurs on the pre-existing oxide. Chromic acid anodizing (CAA) is widely used in the aeronautic industry to improve corrosion resistance of aluminum alloys.1 Since the beginning of the 1990s, however, the high toxicity associated with Cr (VI) has imposed restrictions on their use in industrial applications. As a consequence, numerous attempts have been made to find less toxic alternatives. 2,3Anodizing with dilute sulfuric acid (DSA) has been used to obtain thin anodic films (1-5 μm) that provide some protection without excessive deterioration of the fatigue life for specific aerospace alloys. Although the fatigue performance of DSA is acceptable, the corrosion resistance is lower than that of parts anodized in chromic acid (CAA). More recently, a new anodizing procedure, involving the addition of tartaric acid in dilute sulfuric acid electrolyte and called tartaric-sulfuric acid anodizing (TSA), was introduced.4-6 The addition of tartaric acid to sulfuric acid baths improves significantly the anticorrosive properties of the anodic layers compared to those obtained by sulfuric acid anodizing.7 Recent work, 7 however, indicates that the mechanism of porous film growth is not significantly affected by tartaric acid additions and that tartaric acid is not incorporated in significant amounts into the oxide material. Thus the corrosion resistance provided by TSA is likely to be associated with residuals of tartaric acid adsorbed on the porous skeleton. Tartaric acid concentration in the order of ppm, has been proved to be effective in reducing both the oxide dissolution rate in acidic environments and the anodic reaction rate. The effect of tartaric acid on the anodic fi...

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Section: Resultsmentioning

confidence: 99%

Application of EIS to In Situ Characterization of Hydrothermal Sealing of Anodized Aluminum Alloys: Comparison between Hexavalent Chromium-Based Sealing, Hot Water Sealing and Cerium-Based Sealing

Carangelo

Curioni

Acquesta

et al. 2016

J. Electrochem. Soc.

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“…Intermetallic particles in the alloy oxidize at higher or lower potentials than the aluminum matrix during potentiodynamic anodizing in sulfuric acid electrolyte. 5,31,32 The absence of the peak at 0 and 4.8 V confirms the removal of intermetallic particles in the cerium-containing solutions. …”

Section: Resultsmentioning

confidence: 76%

Characterization of 2024 T3 Aluminum Alloy after Rare Earth Desmutting

Gordovskaya

Curioni

Hashimoto

et al. 2016

J. Electrochem. Soc.

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The effect of CeCl 3 and CeCl 3 + H 2 O 2 additions to a nitric acid solution for desmutting of 2024 T3 aluminum alloy has been investigated using scanning electron microscopy and energy dispersive X-ray analysis, potentiodynamic polarization and X-ray photoelectron spectroscopy. No S-, θ-phase and Al-Cu-Fe-Mn-(Si) particles were detected on the alloy surface after desmutting with added CeCl 3 . A similar result was attained with addition of H 2 O 2 , although with slower removal of the particles. In contrast, θ-phase, Al-Cu-Fe-Mn-(Si) and dealloyed S-phase particles remained following desmutting in nitric acid alone. The absence of the particles on the alloy surfaces following desmutting in the presence of CeCl 3 was also confirmed by potentiodynamic anodizing of the alloy in H 2 SO 4 solution. Surface pre-treatment of aluminum alloys is important for improvement in the properties and the anticorrosion performance of anodic films and conversion coatings used in the aerospace industry. 1The main purpose of the pre-treatment is to prepare a reproducible, chemically active surface for application of the subsequent treatment. The pre-treatment of the alloys involves several steps, including degreasing, acid or alkaline etching and desmutting.Enhanced mechanical properties of the alloys are achieved by addition of alloying elements that form second phase particles.2,3 The presence of cathodically active intermetallics on the alloy surface sustains cathodic reactions that cause susceptibility to corrosion due to microgalvanic coupling between the intermetallics and the aluminum matrix and the associated alkiline corrosion. Porous anodic films formed on Al-Cu alloys are less regular than films produced on high purity aluminum due to the incorporation of copper into the film and oxygen evolution during anodizing. 4 Furthermore, the anodic layer formed on the intermetallic particles has an altered morphology and contains various defects, thus providing reduced corrosion protection compared with the aluminum matrix. [5][6][7] For the last two decades, rare earth metals (cerium, neodymium and lanthanum) have been investigated in surface treatments for enhancing the corrosion resistance of anodized aluminum alloys. [8][9][10] The treatments can reduce the number of residual cathodic intermetallic particles on the alloy surface and hence enable formation of an anodic film that contains fewer defects. In this regard, relatively little information about cerium-containing desmutting solutions has been published. 8,[11][12][13] Hughes et al. Subsequently, Hughes et al. 12 and Kimpton et al. 13 investigated a lower etch rate cerium-containing desmutter (0.5 M H 2 SO 4 + 0.05 M (NH) 4 Ce(SO 4 ) 4 · 2 H 2 O) for pre-treatment of 2024 T3 and 7075 T6 aluminum alloys, which was shown to improve the performance of subsequently applied cerium conversion coating.In the present work, alkaline etching followed by desmutting in nitric acid with CeCl 3 and CeCl 3 / H 2 O 2 additions are investigated for the removal of intermetallics from th...

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“…The corrosion enhancement of aluminium alloys is usually achieved by anodizing in sulfuric acid media [5]. However, the large size of the intermetallic precipitates also disturbs the anodizing layer growth [6,7]; consequently, the anodizing process must be followed by a sealing process to increase the corrosion resistance. Since the new ecological regulations have prohibited the use of hexavalent chromium, many alternative treatments are currently studied.…”

Section: Introductionmentioning

confidence: 99%

Initial stages of multi-phased aluminium alloys anodizing by MAO: micro-arc conditions and electrochemical behaviour

Veys‐Renaux

Rocca

2015

J Solid State Electrochem

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International audienceThe electrochemical behaviour of AA1050 and AA2214 alloys was studied in a KOH/silicate in order to investigate the first stages of the coating formation during MAO process (by SEM and electrochemical measurements). Before the sparking initiation, occurring around 300 V for both alloys, a thin inner layer of aluminium oxide grows by a classical anodizing mechanism, and induces a great enhancement of the electrochemical resistance. Then, the micro-arc regime beyond 300 V leads to the formation of a dense, thicker but cracked external aluminium oxide layer. Afterwards, from 400 V, a sharp increase of current density named ``current wall'' and due to an increase of oxide conductivity, leads to a significant oxygen release and the occurrence of large and energetic sparks. Microscopic analyses reveal that the presence of copper-rich intermetallic phases in the multi-phased alloy (AA2214) induces the incorporation of copper nanoparticles in the gamma-alumina layer formed by the sparking phenomenon. These copper particles increase the reaction of water oxidation during the classical anodizing stage, and result in a delay of the sparking initiation during the galvanostatic anodizing and in a lower electrochemical resistance of the anodized layers

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Preferential anodic oxidation of second‐phase constituents during anodising of AA2024‐T3 and AA7075‐T6 alloys

Cited by 54 publications

References 10 publications

Application of EIS to In Situ Characterization of Hydrothermal Sealing of Anodized Aluminum Alloys: Comparison between Hexavalent Chromium-Based Sealing, Hot Water Sealing and Cerium-Based Sealing

Application of EIS to In Situ Characterization of Hydrothermal Sealing of Anodized Aluminum Alloys: Comparison between Hexavalent Chromium-Based Sealing, Hot Water Sealing and Cerium-Based Sealing

Characterization of 2024 T3 Aluminum Alloy after Rare Earth Desmutting

Initial stages of multi-phased aluminium alloys anodizing by MAO: micro-arc conditions and electrochemical behaviour

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