Incorporation of alloying elements into porous anodic films on aluminium alloys: The role of cell diameter
Introduction
The growth of barrier-type and porous anodic films on aluminium involves outward migration of Al3+ and inward migration of O2− ions through amorphous alumina under a high electric field [[1], [2], [3], [4]]. During porous film growth, ion migration is confined to a thin compact barrier layer, next to the substrate, located beneath a usually much thicker porous layer, classically comprising hexagonal cells each with a central cylindrical pore [5,6]. The cell and pore diameter, typically in the range 10–200 nm, depend mainly on the anodizing voltage. Such films are usually formed in sulphuric, phosphoric or oxalic acid electrolytes. Under galvanostatic anodizing, the voltage depends on the applied current density, and the composition, concentration and temperature of the electrolyte. In films formed in sulphuric and phosphoric acid electrolytes, the pores have been shown to arise from flow of oxide from the barrier layer to the cell walls under typical conditions of film growth [[7], [8], [9], [10], [11]].
The introduction of alloying elements into the film during anodizing of aluminium alloys modifies the film growth and film composition depending upon the types of alloy element species present, their nobility with respect to aluminium, their influences on the solubility and electronic conductivity of the alumina, and their ionic migration rates in the film, which may be faster than, slower than, or similar to Al3+ ions [12]. The oxidation of alloying elements during formation of barrier-type films on binary solid-solution aluminium alloys has been shown to depend on the Gibbs free energy per equivalent for formation of the alloying element oxide (ΔGO n−1) relative to that for formation of alumina [12]. A more negative value relative to alumina, indicative of an alloying element of lower nobility than aluminium, results in immediate oxidation of the alloying element and aluminium. A less negative value compared with alumina results in an enrichment of the alloying element in the alloy by a prior period of oxidation of aluminium only. The enrichment is confined to a layer a few nanometres thick immediately beneath the anodic film.
At a critical enrichment of the alloying element, measured as the number of enriched atoms per unit area of the alloy/film interface, the oxidation of the alloying element commences and both aluminium ions and alloying element ions are incorporated into the film [12]. The amount of enriched alloying element in the enriched alloy layer then remains relatively constant with further alloy oxidation. The critical enrichment in binary alloys containing about 1 at.% of alloying element increases approximately linearly with ΔGO n−1 [12] and reduces with decreasing alloy concentration [13,14] and in the presence of a second enriching alloying element [15]. Gold exhibits the highest enrichment [16,17]. However, unlike the usual behaviour of other elements, gold is incorporated into the film as nanoparticles of gold metal [18]. In the case of porous anodic films, it has been proposed that the scalloped morphology of the alloy/film interface leads to transport of the enriched element from beneath the cell bases to the cell boundaries, as shown in Fig. 1 for enriched copper during the growth of a porous anodic film on an Al-Cu alloy [19]. The alloy/film interface moves inward parallel to direction of the local electric field, thereby carrying the enriched copper toward the cell boundary.
The present paper considers the consequence of such transport to the distribution of enriched alloying element species in the film, and reveals a dependence of the alloying element distribution on the cell diameter. A model is proposed that predicts a critical cell diameter for a particular alloying element and alloy composition: below the critical cell diameter, the alloying element is incorporated into the film at the ridges in the alloy/film interface that coincide with the cell boundaries; above the critical diameter, the alloying element is incorporated into the film at all regions of the alloy/film interface, including both the ridges and the cell bases. Hence, in the former case, the alloying element species are confined to the cell boundary regions of the film. In the latter case, the alloying element species may be found within the cells in addition to the cell boundary regions. The critical cell diameter is dependent on the alloy composition and the critical enrichment for the particular alloying element. The model predictions are compared with literature data for distributions and critical enrichments for gold and tungsten in anodized Al-Au [17] and Al-W [[20], [21], [22]] model alloys, and also with new data for the distribution of copper in the film on a commercial AA 2024-T3 alloy.
Section snippets
Model for alloying element oxidation during porous film growth
In order to maintain a critical enrichment of the alloying element at all locations beneath the cell bases of a porous anodic alumina film, sufficient alloying element must be supplied to the alloy/film interface to replenish the loss to the cell boundaries that occurs due to the lateral transport of the alloying element to the ridges of the scalloped alloy/film interface. The requirement on the cell diameter for maintaining the critical enrichment of the alloying element in a binary solid
Comparison of the model with experimental data from the literature for model binary aluminium alloys
Previous work, using Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM), has measured the critical enrichments and distributions of gold and tungsten in films on Al-1 at.% Au [17] and Al-3.5 at.% W [[20], [21], [22]] alloys produced by magnetron sputtering. Similarly to gold, tungsten is more noble than aluminium and hence enriches in the alloy beneath the oxide film. However, the critical enrichment of tungsten is lower than that for gold for the same
Experimental details
Specimens of etched AA 2024-T3 alloy (4.19 Cu, 1.36 Mg, 0.06 Si, 0.07 Fe, 0.42 Mn, 0.002 Cr, 0.03 Zn, 0.01 Ti, bal. Al (wt.%)) were hard anodized for 600 s at 50 mA cm−2 in 10 vol % H2SO4 (Fisher Scientific, 96 vol %) at −2 ± 1 °C, resulting in a film thickness of 18 μm. Copper, the principal alloying element, enriches in the alloy owing to its higher nobility with respect to aluminium [12]. Details of the specimen preparation and voltage response can be found in Ref. [31]. The films were
Conclusions
- 1.
A model has been proposed to explain the distribution of alloying element species in porous anodic films on aluminium alloys. For an alloying element that is enriched in the alloy during anodizing, the model predicts a dependence of the distribution on the cell diameter.
- 2.
According to the model, a critical cell diameter exists for a particular alloy composition. The critical diameter is determined by the magnitude of the critical enrichment of the alloying element in the alloy and the
Acknowledgments
The authors thank the Engineering and Physical Sciences Research Council (LightForm - EP/R001715/1 Programme Grant) for support of this work. J.M. Torrescano-Alvarez acknowledges receipt of a scholarship from Consejo Nacional de Ciencia y Tecnología (CONACYT) and a fellowship from the Roberto Rocca Education Program to undertake her Ph.D. studies.
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