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

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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...

Section: Resultsmentioning

confidence: 74%

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

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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: 74%

Characterization of 2024 T3 Aluminum Alloy after Rare Earth Desmutting

Gordovskaya

Curioni

Hashimoto

et al. 2016

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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...

“…The mechanism for the formation of the surface texture during alkaline etching has been reported elsewhere. 18 Anodizing of the alloys of different tempers.-Previous work 8,19 has indicated that intermetallics of different compositions in aluminum alloys are oxidized at specific voltages or voltage ranges. The characteristic voltages/voltage ranges could be determined from the current density-voltage responses of the alloys which were linearly polarized from the OCP to higher voltages.…”

Section: Resultsmentioning

confidence: 99%

Discoloration of Anodized AA6063 Aluminum Alloy

Zhou

Wang

et al. 2014

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The effect of the anodizing process parameters and the influence of β (Mg 2 Si) phase on the appearance of the anodized AA6063 aluminum alloy have been investigated. It was found that β (Mg 2 Si) phase was preferentially oxidized at low anodizing voltages and the selective oxidation of β (Mg 2 Si) phase resulted in the development of a rough alloy/film interface and the incorporation of silicon into the porous anodic film, which led to the discoloration of the anodized alloy. © The Author Discoloration often appears on extruded AA6063 aluminum alloy after anodizing in sulfuric acid solutions and, to date, achieving reproducible and well-controlled surface appearance of anodized AA6063 alloy profiles is still an intractable issue. Discoloration refers to undesirable changes in the appearance of anodized aluminum alloy profiles (usually darker than the normal appearance), either in specific regions or over the macroscopic alloy surface. The effects of precipitation, surface pre-treatments and anodizing process on discoloration of anodized profiles have been explored by previous investigators.1-6 Nakagawa et al.1 suggested that discoloration was caused by rapid cooling of the extrusions on the run-out-table and reheating the extrusions upon removal from the carbon plates. It was believed that such thermal process accelerated the precipitation of β (Mg 2 Si) phase, 2 which subsequently resulted in discoloration of the anodized extrusions. Further, Nakagawa et al. 3 found that the discoloration became more marked with increase of the time and temperature of the desmutting treatment in sulfuric or nitric acid solutions. Additionally, it was found that the discoloration of the anodized alloys was strongly dependent on the pH of the rinsing water after the desmutting procedure. It was further suggested that the discoloration of anodized alloys originated from reactions between β (Mg 2 Si) precipitates and the acid solution during the desmutting and subsequent water-rinsing treatment. Hayashi et al. 5 anodized Al-Mg-Si alloys that had been heat treated under various conditions initially at 15 V and then at 4 V in sulfuric acid solution. The anodized alloys appeared in colors from gray to black and the degree of coloring increased with the amount of β (Mg 2 Si) precipitates formed during heat-treatment. Based on the roughening of the alloy/oxide interfaces, it was suggested that the coloring might result from the formation of a branched-pore morphology due to the local concentration of the anodizing current on the β (Mg 2 Si) precipitates with consequent uneven dissolution of the barrier layer.Although the literature [1][2][3]5 indicates that the discoloration of the anodic films is related to β (Mg 2 Si) precipitates in the alloy matrix, whether the discoloration is introduced in the surface pre-treatment stage or the anodizing stage remains the subject of discussion. Recent work of the authors on an AA6063 aluminum alloy profile with discolored anodic films 7 suggested that the anodizing process parameters and the statu...