The behaviour of second phase particles during anodizing of aluminium alloys

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...

Section: Resultsmentioning

confidence: 71%

“…10a). 5 With addition of CeCl 3 and CeCl 3 / H 2 O 2 to the desmutting solution, these two peaks were absent (Figs. 10a and 10b respectively).…”

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

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Characterization of 2024 T3 Aluminum Alloy after Rare Earth Desmutting

Gordovskaya

Hashimoto

et al. 2016

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“…For the low-current density condition (5 mA cm −2 ), the initial peak observed at the lower current density coincided with an initial low voltage plateau, which is well known to be associated with oxidation of copper-rich intermetallic particles. 24,25 The data indicate that approximately 14% of the applied current was consumed by oxygen evolution. The voltage-time curves revealed steady voltages, of 8 and 24 V at 5 and 50 mA cm −2 respectively, during the growth of the porous films.…”

Section: Methodsmentioning

confidence: 99%

Gravimetric Measurement of Oxygen Evolution during Anodizing of Aluminum Alloys

Torrescano-Alvarez

Skeldon

2017

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A gravimetric method based on changes in buoyancy force of a submerged gas-collecting container has been optimized to measure the evolution of oxygen during anodizing of an aluminum alloy. Previously, the gravimetric method has been used to measure hydrogen evolution from magnesium and aluminum during corrosion processes, either at the free corrosion potential or under relatively low polarization. However, during anodizing, the comparatively higher values of current applied and the heating effects associated with power dissipation might introduce artefacts in the gas measurement. Optimization of the experimental setup enabled reduction or elimination of such artefacts, so that a reliable measurement could be obtained. The results show that typically about 15 to 20% of the current applied to an AA 2024-T3 aluminum alloy during anodizing in sulfuric acid under the present conditions was used in generating oxygen. Anodic treatments are commonly used to produce protective oxide films and coatings on metals such as aluminum, magnesium, titanium and their alloys. During such processes, in addition to the electrochemical oxidation of the substrate that produces the desired protective layer, oxygen may be generated electrochemically as a side reaction.1-5 For aluminum, oxygen generation during anodizing can be significantly increased by the addition of certain alloying elements or by the presence of impurities in the substrate, such as, for example, copper and iron.6,7 The oxygen may be formed at locations of intermetallic particles, where the high concentration of alloying elements locally modifies the composition, morphology and electronic properties of the oxide.2 Oxygen can also form above the matrix regions, when elements such as iron or copper in solid solution are oxidized and their ions incorporated into the oxide. The resultant modifications of the oxide properties enable the reactionto proceed within the oxide resulting in the development of nanobubbles of oxygen gas within the anodic film. 7Owing to the plasticity of the amorphous alumina during the growth of the film, 8,9 and the high pressure of the contained oxygen gas, 7 the bubbles are able to grow within the film until the film eventually ruptures and the gas is released to the electrolyte.7,10 The liberation of oxygen has been proposed to account for changes in the morphology of the porous oxide structure.11 In particular, the process is believed to be responsible for the degeneration of the well-ordered porous morphology, typically observed on high purity aluminum, 12 into a globular or sponge-like morphology typically observed on copper containing alloys, 13,14 such as those of the 2xxx series 11,12 and 7xxx series.14 The morphology of the porous films on these alloys is of great technological interest, since anodizing is one of the most commonly applied corrosion-protection measures, 15 and anticorrosion performance and oxide morphology are closely related.16 As a consequence, the understanding of the interaction between the anodizing conditions, th...

“…2,[16][17][18][19][20][21][22] In particular, oxidation of copper at the metal-oxide interface results in injection of copper ions into the oxide. 1,18,[23][24][25][26][27] The presence of copper ions increases significantly the electronic conductivity within the barrier layer, triggering oxygen evolution within the oxide.…”

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confidence: 99%

Cerium-Based Sealing of Anodic Films on AA2024T3: Effect of Pore Morphology on Anticorrosion Performance

Carangelo

Acquesta

et al. 2016

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In this work, porous anodic oxides were produced by traditional and modified tartaric sulfuric anodizing (TSA) processes and sealed in hot water, chromate and cerium based solutions. The sealing behavior of a film with relatively coarse porosity, generated at high voltage (traditional TSA), was compared to the sealing behavior of a film with finer porosity and generated at reduced potential (modified TSA). After sodium chromate sealing, the two anodizing cycles produced film with similar anticorrosion performance. Conversely, after hot water or cerium sealing, the finer oxides generated at low voltage (modified TSA) provided much better corrosion resistance. EIS performed in-situ during sealing revealed that chromate sealing is very aggressive to the porous skeleton compared to the other sealing treatments. Therefore, the original morphology has little effect on the final performance, since both fine and coarse oxides are substantially attacked. In contrast, the oxide morphology has a substantial effect when sealing is performed in hot water or cerium-based solution. Overall, it is possible to obtain films with anticorrosion performance equivalent or improved compared to that obtained by traditional TSA anodizing cycle sealed with chromate by combining the low voltage anodizing cycle with the cerium-based sealing. Aerospace aluminum alloys display outstanding mechanical properties but require specific protection measures in order to meet the requirements of durability and corrosion resistance. Anodizing in acidic electrolytes is one of the methods that are most widely employed for this purpose, since it produces porous oxides that improve corrosion resistance and adhesion with organic coatings.The porous anodic oxide morphology generated on high copper alloys is significantly different from that generated on high purity aluminium.1-3 Specifically, on aluminum, anodizing in phosphoric, sulfuric, or oxalic acid, results in the generation of a well-ordered oxide morphology, comprising closely-packed hexagonal cells with a central cylindrical pore, and having a diameter that is proportional to the applied potential.4-10 Under these conditions, the pore walls are generally straight and uniform (provided that anodizing is conducted under steady applied potential or current). At the bottom of the pores, close to the metal, a barrier layer is observed with thickness proportional to the applied potential. On the other hand, the thickness of the porous oxide is proportional to the charge passed. Due to the dependence of barrier layer thickness and pore diameter on the applied potential, and to the dependence of the film thickness on charge, porous morphologies can be tailored by controlling the electrical regime (potential-time or current-time), and complex morphologies can be achieved to enhance specific properties.11-14 It has been shown that fine pores and thick films are beneficial for corrosion protection.13 However fatigue life can be an issue for aerospace alloys 15 and therefore film thickness should be limited to...