Incorporation of alloying elements into porous anodic films on aluminium alloys: the

role of cell diameter

J.M. Torrescano-Alvarez,1 M. Curioni,1 H. Habazaki,2 T. Hashimoto,1 P. Skeldon,1* X. Zhou1

Corrosion and Protection Centre, School of Materials, The University of Manchester, Oxford Rd., Manchester

M13 9PL, U.K

Division of Materials Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan

*corresponding author:

e-mail:p.skeldon@manchester.ac.uk

1

Abstract

The presence of alloying elements in aluminium alloys has a significant impact on the

anodizing behaviour and results in the formation of porous anodic films with different

chemical composition and morphology compared with those generated on pure aluminium. In

this work, the effect of alloy element enrichment at the alloy/film interface and of cell

diameter on the incorporation and distribution of alloying element species in porous anodic

films is considered. It is proposed that above a critical cell diameter, Dcrit, the critical alloy

enrichment sufficient for oxidation of the alloying element and its incorporation into the film

can be maintained across the alloy/film interface. Below Dcrit, only a sub-critical enrichment

can be maintained and the alloying element is then incorporated into the film at the cell

boundaries. Dcrit depends on the concentration of the alloying element in solid solution and on

the critical enrichment. The proposed role of Dcrit is supported by alloying element

distributions from literature data for model Al-Au and Al-W alloys and new results for anodic

films on AA 2024-T3 alloy.

Keywords: aluminium alloy; porous anodic film; composition; cell size

1. 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 [1-4]. 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 [7-11].

2

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 higher 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

3

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 [20-22] model alloys,

and also with new data for the distribution of copper in the film on a commercial AA 2024-

T3 alloy.

2. Model for alloying element oxidation during porous film growth

In order to maintain a critical enrichment of the alloying element beneath 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 solution alloy is considered as follows. Cells are assumed

to be in a close-packed, hexagonal arrangement, with a diameter D defined as shown in Fig.

2. D is equal to twice the length of hexagon side (r). The rate of formation of cell boundary

area per unit area of the alloy surface, dA/dt, owing to retreat of the alloy/film interface by

oxidation of the alloy at a rate dx/dt is therefore given by

dAdt

=3 rn dxdt

=1.5 Dn dxdt (1)

in which x is the thickness of the alloy consumed and n is the number of cells per unit area

given by

n= 1Ah

= 10.65 D2 (2)

4

where Ah is the area of a hexagon..

In Eq. 1 only half of the hexagonal cell sides are considered, since each cell wall is shared

with a neighbouring cell. From Eq. (1) and (2)

dAdt

=2.31D

dxdt (3)

Although the cell boundary area is usually created at a rate greater than indicated by Eq, (3),

due to the volume of anodic oxide exceeding that of the oxidized alloy and to the flow of

oxide from the barrier layer to the cell walls, only the cell boundary area created by the

retreat of the alloy/film interface accounts for the loss of alloying element from the enriched

layer.

The cell diameter is proportional to the anodizing voltage, V, with a ratio of about 2.8 nm V-1

[5, 6], and hence n is proportional to V-2, and dA/dt is proportional to V-1. Accordingly, the

rate of cell boundary area creation, (Eq. (3)), during anodizing at a given film growth rate is

higher at a lower anodizing voltage. Therefore, as the film thickens, more of the alloying

element must be supplied from the matrix to maintain the critical enrichment of the alloying

element, since a greater proportion of the alloying element is incorporated into the film at the

cell boundaries.

The number of atoms of alloying element per unit area, dna, in an alloy layer of thickness dx

is given by

dna=N A ρ ca

M aldx (4)

where NA is Avogadro’s number, ρ is the density of the alloy, Mal is the mean atomic mass of

the alloy and ca is the atom fraction of the alloying element in the alloy.

Therefore, the alloying element is made available for either oxidation or enrichment at the

alloy/film interface at a rate given by

5

d na

dt=

ρ N A ca

M al

dxdt

(5)

This rate must equal the rate of supply of enriched alloying element to the cell boundaries to

maintain the critical alloy enrichment. Since adjacent cells sides supply alloying element to

the cell boundary, the alloying element accumulates at the cell boundaries at a rate, r,

r=2N critdAdt (6)

where Ncrit is the critical enrichment required for oxidation of the alloying element.

At the critical cell diameter, Dcrit, the rate of supply of alloying element made available by

oxidation of the alloy, dna/dt, equals the rate of loss of the alloying element from the

scalloped alloy/film interface, r. Hence, from Eq. (3), (5) and (6)

Dcrit=4.62 N crit M al

ρN A ca (7)

The schematic diagrams of Fig. 2 show the distributions of alloying element species predicted

by the model in porous anodic films with D < Dcrit (Figs. 2(a) and 2(b)) and D > Dcrit (Fig.

2(c)). In Figs. 2(a) and 2(b), the film is shown at time t1 and a later time t2, respectively. In

this interval an alloy layer of thickness x has been oxidized and a thickness of oxide, l, has

been added to the film. Since D < Dcrit, the alloy enrichment beneath the cell bases is < Ncrit

and the alloying element is incorporated into the film at the cell boundaries, while the

remainder of the anodic alumina within each cell is free of alloying element species. As noted

previously, l may exceed x if oxide flow occurs. However, incorporation of enriched alloying

element at the ridges of the alloy/film interface occurs in proportion to distance of retreat of

the alloy/film interface, x. It has been assumed that the alloying element species are

incorporated uniformly around the cell boundaries, although it is possible that the

incorporation is enhanced at preferred regions, for instance triple points [19], since enriched

alloying element may also be transported along the curved cell boundary ridges. In Fig. 2(c),

since D > Dcrit, the critical enrichment of the alloying element can be sustained beneath the

cell bases and hence the alloying element species are incorporated into the film both at cell

6

boundaries and within the cell volume. The alloying element species in Fig. 2(c) are shown to

be distributed throughout the cell volume, which would occur if alloying element ions

migrate outward at a similar or faster rate than Al3+ ions. If the alloying element ions

migrated outward slower than Al3+ ions, the alloying element species would be confined to an

inner region of the cell, with an alloying element free region adjacent to the pore, as observed

in anodic films formed in phosphoric acid on an Al-W alloy [23].

If D > Dcrit, a portion of the alloying element atoms accumulated in the enriched layer by the

retreat of the alloy/film interface is used to maintain the critical enrichment, since alloying

element is still being transported to the cell boundaries. Therefore, relatively fewer alloying

element atoms may be available for oxidation and incorporation into the film at cell bases,

when D is only slightly greater than Dcrit compared with D » Dcrit. If D < Dcrit, all of the

alloying element is incorporated at the cell boundaries.

3. 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 [20-22] 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 concentration of alloying element in the alloy owing to

its lower nobility [12]. Table 1 summarizes the findings. It also includes data for AA 2024-T3

alloy, which are considered in Section 4. The results indicate a correlation between the

calculated Dcrit (Eq. (7)) and the alloying element distributions in the films. For the

calculations, the alloys were assumed to have an atomic density and atomic mass similar to

that of aluminium. Dcrit values of 491 and 675 nm were calculated for anodizing the Al-1 at.%

Au alloy in sulphuric and phosphoric acid, respectively. Formation of such cell diameters

would require anodizing voltages of 175 and 241 V, respectively, as estimated using a ratio

of cell diameter to forming voltage of 2.8 nm V-1 [5, 6]. The actual anodizing voltages were

much smaller than these values, indicating that the resultant cell diameters were below Dcrit,

and gold was incorporated into the films primarily at the cell boundaries. Similarly, tungsten

species were observed to be located at the cell boundaries of a film formed on an Al-3.5 at.

7

%W alloy in sulphuric acid when the cell diameter was below Dcrit. In contrast, anodizing the

alloy in phosphoric acid and borax solution led to incorporation of tungsten species at both

the cell boundaries and within the cells when the cell diameter was greater than Dcrit. Thus,

the distributions of gold and tungsten were consistent with expectations from the model.

The ratio Ncrit:ca has been previously designated an enrichment factor that expresses the

dependence of the critical enrichment on the concentration of the alloying element available

to enrich in the alloy [14]. In the case of Al-W and Al-Cu alloys, the enrichment factor has

been reported to increase significantly for alloy concentrations below ≈ 1 at.% [13, 14]. Thus,

the critical cell diameter should increase with decrease of the concentration of the alloying

element.

In addition to transport of alloying elements, impurity elements may be transported to cell

boundaries during anodizing of nominally pure aluminium and aluminium alloys. The

accumulation of such species at cell boundaries may affect the fracture of films. Previous

work has reported that films on aluminium (which typically contain copper, iron and

hydrogen impurity that can enrich beneath oxide films) may fracture around cell boundaries

or through cells depending on the anodizing conditions [24-28]. Alloying element and

impurity transport may also occur during alkaline etching or electropolishing that proceeds in

the presence of an oxide film on a scalloped substrate/film interface. It may also be

anticipated in porous anodic films on magnesium alloys [29] that exhibit enriched alloy

layers [30].

4. Comparison of the model with new experimental data for AA 2024-T3 alloy

4.1. 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 [31]. The films were examined by TEM using a FEI Titan G80-200 ChemiSTEM

instrument, operated at 200 kV, with four energy dispersive x-ray (EDX) spectroscopy

8

detectors. Electron-transparent cross-sections were cut on a Leica EMUC6 ultramicrotome

and collected on lacey carbon films on gold grids. Scanning electron microscopy (SEM) of

film cross-sections, prepared by cutting using a diamond knife, used a Zeiss Ultra 55

microscope, operated at 1.5 kV; EDX data were obtained at 5 keV.

4.2 Results

SEM revealed mainly intercell fracture of the film (Fig. 3(a)), with backscattered electron

imaging (Fig. 3(b)) disclosing regions of relatively heavy species at the cell boundaries. The

enriched species is later shown to be copper. From examination of film cross-sections, the

cell diameter varied over a relatively wide range at a particular film depth and increased from

the top region (50 to 100 nm) to the bottom region of the film (60 to 140 nm). A barrier layer

about 40 nm thick was present at the film base (Fig. 3(c)). The discontinuity in the copper

distribution at individual cell boundaries might be due to loss of copper-enriched material

during cross-sectioning, non-uniform incorporation of copper species around cell boundaries,

or misalignment of the electron beam with the thin copper-rich layer. Notably, the barrier

layer appeared to be free of cavities (Fig. 3(c)). This suggests that little or no copper was

oxidized and incorporated into the film at the cell bases, since the presence of Cu2+ ions in the

alumina leads to formation of bubbles of high pressure oxygen gas [33]. SEM/EDX analysis

of the whole film thickness made at 7 different regions of the cross-section, (in rectangular

areas of size about 16 x 6 μm and avoiding regions of intermetallic particles), revealed an

average Cu:Al atomic ratio of 2.5 ± 0.7 x 10-2, which was 1.08 ± 0.36 times the average ratio

in the alloy immediately beneath the film (2.3 ± 0.4 x 10-2).

The loss of Al3+ ions from the film by ejection to the electrolyte at the pore bases was

determined by estimating the anodizing efficiency from the ratio of the initial slope of the

voltage-time curve, when a barrier film is growing, with that for anodizing in ammonium

pentaborate solution during which Al3+ loss is negligible [34]. An efficiency of 0.90 was

calculated, which is in agreement with the tendency toward a high efficiency of porous film

growth with increase of current density and decrease of temperature [35]. The efficiency may

increase by a small amount following the transition barrier to porous film growth, as reported

for anodizing aluminium in sulphuric acid [36]. Since the present film grew at a relatively

high efficiency, the similarity of the Cu:Al ratio in the film and in the alloy indicates that

most of the copper oxidized at the alloy/film interface was retained in the film.

9

A high angle annular dark field (HAADF) TEM image of two adjacent cells revealed cell and

pore diameters of ≈ 60 nm and 10 nm, respectively (Fig. 4(a)). EDX maps showed copper

was enriched and aluminium was depleted at the boundary between the cells (Fig. 4(b) and

4(c)). Other potentially enriching alloying elements, such as manganese, iron, chromium and

zinc [12], were not detectable due to their low concentrations in the alloy and their

incorporation into second phases. Magnesium does not enrich owing to its lower nobility

relative to aluminium [12]. Oxygen was present at all cell regions (Fig. 4(d)). As expected,

sulphur was also present in the film (map not shown). Table 2 lists the results of EDX

analyses of the Cu:Al atomic ratio in the boxed regions of Fig. 4(a). The highest ratio of 2.8 x

10-2 was obtained in the central box along the copper-rich boundary between the two cells.

Lower ratios of about 1.0 x 10-2 were determined at locations in the middle of cells. Table 2

also lists similar results that were obtained from the analyses of the middle and boundaries of

other cells. The highest Cu:Al atomic ratio at a cell boundary was 3.9 x 10-2. The lowest

Cu:Al atomic ratio within a cell was 0.7 x 10-2.

An EDX linescan analysis (Fig. 5) along the line LS (Fig. 4(a)) showed that the majority of

the copper in the film was located at the cell boundaries. The width of the copper peaks and

the detection of copper in the cell interiors arise from the significant thickness of the section

relative to the cell diameter, the orientation of the cell boundaries with respect to the electron

beam (diameter = 0.2 nm) and irradiation damage. Using the enrichment of the etched alloy

determined by RBS (5.0 x 1015 Cu atoms cm-2), ca of 1.8 x 10-2 (from ICP-OES analysis of

the alloy) and an alloy atomic density equal to that of aluminium, a Dcrit of 213 nm (Table 1)

was calculated, which would be obtained at a Vcrit of 76 V. In contrast, the maximum

anodizing voltage was 55 V. Thus, the location of copper primarily at the cell boundaries is

consistent with the model of Section 2. Copper enrichment at the cell boundaries has also

been reported for porous films on AA 2099-T8 alloy formed at potentials of 3 and 12 V [37,

38]. Assuming a critical enrichment of copper similar to that in the AA 2024-T3 alloy, a Dcrit

of about 330 nm and a critical voltage of 119 V may be estimated (Eq. 7) for the films, which

is consistent with copper incorporation at cell boundaries. θ’, θ’’ and θ phases were detected

in the copper-enriched regions of the film, and similar phases may be present at the cell

boundaries of the present films. However, the factors that determine whether the alloying

element is oxidized at ridges in the alloy/film interface or incorporated into the film as

metallic nanoparticles still require elucidation.

10

5. 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 concentration of the alloying element in solid solution in the bulk

alloy. Below the critical cell diameter, the alloying element is incorporated into the film at the

ridges in the alloy/film interface coincident with the cell boundaries. Above the critical cell,

the alloying element is incorporated both at the ridges next to cell boundaries and also

beneath the cell bases.

3. The model prediction is compatible with the observed distributions of gold and tungsten in

porous anodic films formed on Al-Au and Al-W alloys, and of copper in AA 2024-T3 alloy.

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.

References

[1] Yasuo Oka, Toshiro Takahashi, Kiyoshi Okada, Shin-ichi Iwai, Structural analysis of

anodic alumina films, J. Non-Cryst. Solids 30 (1979) 349-357.

[2] R. Dupree, I. Farnan, A.J. Forty, S. El-Mashri, L. Bottyan, A MAS NMR study of the

structure of amorphous alumina films, J. de Phys. Colloques 46 (1985) C8-113-C8-116.

11

[3] F. Brown, W.D. Mackintosh, The use of Rutherford backscattering to study the behavior

of ion-implanted atoms during anodic oxidation of aluminum: Ar, Kr, Xe, K, Rb, Cs, Cl, Br,

and I, J. Electrochem. Soc. 120 (1973) 1096-1102.

[4] C. Cherki, J. Siejka, Study by nuclear microanalysis and O18 tracer techniques of the

oxygen transport processes and the growth laws for porous anodic oxide layers on aluminum,

J. Electrochem.Soc.120 (1973) 784-791.

[5] G.C. Wood, J.P. O’Sullivan, The anodizing of aluminium in sulphate solutions,

Electrochim. Acta 15 (1970) 1865-1870.

[6] J.P. O’Sullivan, G.C. Wood, The morphology and mechanism of formation of porous

anodic films on aluminium, Proc. R. Soc. London, Ser. A 317 (1970) 511-543.

[7] P. Skeldon, G.E. Thompson, S.J. Garcia-Vergara, L. Iglesias-Rubianes, C.E. Blanco-

Pinzon, A tracer study of porous anodic alumina, Electrochem. Solid-State Lett. 9 (2006)

B47-B51.

[8] J.E. Houser, K.R. Hebert, The role of viscous flow of oxide in the growth of self-ordered

porous anodic alumina films, Nat. Mater. 8 (2009) 415-420.

[9] K.R. Hebert, S.P. Albu, I. Paramasivam, P. Schmuki, Morphological instability leading to

formation of porous anodic oxide films, Nat. Mater. 11 (2012) 162-166.

[10] D. Barkey, J. McHugh, Pattern formation in anodic aluminium oxide growth by flow

instability and dynamic recrystallization, J. Electrochem. Soc. 157 (2010) C388-C391.

[11] J. Oh, C.V. Thompson, The role of electric field in pore formation during aluminium

anodization, Electrochim. Acta 56 (2011) 4044-4051.

[12] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, X. Zhou, Effects of

alloying elements in anodizing of aluminium alloys, Trans. Inst. Met. Finishing 75 (1997) 18-

23.

12

[13] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, The composition of

the alloy/film interface during anodic oxidation of Al-W alloys, J. Electrochem. Soc. 143

(1996) 2465-2470.

[14] Y. Liu, F. Colin, P. Skeldon, G.E. Thompson, X. Zhou, H. Habazaki, K. Shimizu,

Enrichment factors for copper in aluminium following chemical and electrochemical surface

treatments, Corros. Sci. 45 (2003) 1539-1544.

[15] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, The co-enrichment

of alloying elements in the substrate by anodic oxidation of Al-W-Cu alloys, Corros. Sci. 39

(1997) 339-354.

[16] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, X. Zhou, Anodic

film formation on Al-Au alloys: enrichment of gold in the alloy and subsequent evolution of

oxygen, J. Phys. D 30 (1997) 1833-1841.

[17] S.J. Garcia-Vergara, M. Curioni, F. Roeth, T. Hashimoto, P. Skeldon, G.E. Thompson,

H. Habazaki, Incorporation of gold into porous anodic alumina films, J. Electrochem. Soc.

155 (2008) C333-C339.

[18] X. Zhou, H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, Formation

and accommodation of gold atom clusters and oxygen bubbles during amorphous anodic

alumina growth, Proc. R. Soc. A 455 (1999) 385-399.

[19] I.S. Molchan, T.V. Molchan, N.V. Gaponeneko. P. Skeldon, G.E. Thompson, Impurity-

driven defect generation in porous anodic alumina, Electrochem. Commun. 12 (2010) 693-

696.

[20] S.J. Garcia-Vergara, P. Skeldon, G.E. Thompson, H. Habakaki. Pore development in

anodic alumina in sulphuric acid and borax electrolytes, Corros. Sci. 49 (2007) 3696-3704.

[21] L. Iglesias-Rubianes, P. Skeldon, G.E. Thompson, H. Habazaki and K. Shimizu,

Influence of current density in anodizing of an Al-W alloy, Corros. Sci. 43 (2001) 2217-2227.

13

[22] S.J. Garcia-Vergara, P. Skeldon, G.E. Thompson, H. Habakaki, Pore development during

anodizing of Al-3.5at.%W alloy in phosphoric acid, Surf. Coat. Technol. 201 (2007) 9506-

9511.

[23] S.J. Garcia-Vergara, D.J. Le Clere, T. Hashimoto, P. Skeldon, G.E. Thompson, H.

Habazaki, Optimized observation of tungsten tracers for investigation of formation of porous

anodic alumina, Electrochim. Acta 54 (2009) 6403-6411.

[24] S.-Z. Chu, K. Wada, S. Inoue, M. Isogai, A. Yasumori, Fabrication of ideally ordered

nanoporous alumina films and integrated alumina nanotube arrays by high-field anodization,

Adv. Mater. 17 (2005) 2115-2119.

[25] S.-Z. Chu, K. Wada, S. Inoue, M. Isogai, Y. Katsuta, A. Yasumori, Large-scale

fabrication of ordered nanoporous alumina films with arbitrary pore intervals by critical-

potential anodizing, J. Electrochem. Soc. 153 (2006) B384-B391.

[26] W. Lee, R. Ji, U. Gösele, K. Nielsch. Fabrication of long-range ordered porous alumina

membranes by hard anodization, Nat. Mater. 5 (2006) 741-747.

[27] K. Schwirn, W. Lee, R. Hillebrand, M. Steinhart, K. Nielsch, and U. Gösele, Self-

ordered anodic aluminium oxide formed by hard anodization, 2 (2008) 302-310.

[28] W. Lee, K. Schwirn, M. Steinhart, E. Pippel, R. Scholz, U. Gösele, Structural

engineering of nanoporous anodic aluminium oxide by pulse anodization of aluminium, Nat.

Mater. 3 (2008) 234-239

[29] S. Ono, K. Asami, T. Osaka, N. Masuko, Structure of anodic films formed on

magnesium, J. Electrochem. Soc. 1996 143(3): L62-L63.32.

[30] A. Němcová, O. Galal, P. Skeldon, I. Kuběna, M. Šmíd, E. Briand, I.Vickridge, J-J.

Ganem, H. Habazaki, Film growth and alloy enrichment during anodizing AZ31 magnesium

alloy in fluoride/glycerol electrolytes of a range of water contents, Electrochim. Acta 219

(2016) 28-37.

14

[31] J.M. Torrescano-Alvarez, M. Curioni, P. Skeldon, Effects of oxygen evolution on the

voltage and film morphology during galvanostatic anodizing of AA 2024-T3 aluminium alloy

in sulphuric acid at -2 and 24 °C, Electrochim. Acta 275 (2018) 172-181.

[32] L.R. Doolittle, Algorithms for the rapid simulation of Rutherford backscattering spectra,

Nucl. Instrum. Meth. B 9 (1985) 344-351.

[33] P. Skeldon, G.E. Thompson, G.C. Wood, X. Zhou, H. Habazaki, K. Shimizu, Evidence

of oxygen bubbles formed within anodic films on aluminium-copper alloys. Phil. Mag. A 76

(1997) 729-741.

[34] A.C. Harkness, L. Young, High resistance anodic oxide films on aluminium, Can. J.

Chem. 44 (1966) 2409-2413.

[35] G. Patermarakis, K. Moussoutzanis, Development and application of a holistic model for

the steady state growth of porous anodic alumina films, Electrochim Acta 54 (2009) 2434-

2443.

[36] D. Mercier, Q. Van Overmeere, R. Santoro, J. Proost, In-situ optical emission

spectrometry during galvanostatic aluminum anodising, Electrochim. Acta 56 (2011) 1329-

1336.

[37] Y. Ma, X. Zhou, G.E. Thompson, M. Curioni, P. Skeldon, X. Zhang, Z. Sun, Anodic

film growth on Al–Li–Cu alloy AA2099-T8, Electrochim. Acta 80 (2012) 148-159. AA

2099-T8

[38] Y. Ma, X. Zhou, G.E. Thompson, M. Curioni, T. Hashimoto, P. Skeldon, E. Koroleva,

Single-step fabrication of metal nanoparticle loaded mesoporous alumina through anodizing

of a commercial aluminum alloy. Electrochem. Solid-State Lett. 15 (2012) E4-E6.

15

Figure captions

Figure 1. Schematic diagram showing transport of an enriched element (copper) to the cell

boundaries during the growth of a porous anodic film [19]. The transport of copper results in

formation of oxygen bubbles at the cell boundaries. A close-packed arrangement of

hexagonal cells is assumed, with cells of diameter D and side length r as shown in (d).

Figure 2. Predicted distributions of an enriching alloying element in porous anodic alumina films with cells of diameter,< Dcrit (a, b) and > Dcrit.(c).

Figure 3. (a) Secondary and (b) backscattered electrons SEM micrograph of a fractured

section of an anodic film on an AA 2024-T3 alloy specimen hard anodized at 50 mA cm2 in

10 vol.% H2SO4 at -2 °C. (c) Secondary electron micrograph of film base.

Figure 4. (a) HAADF TEM micrograph and EDX elemental maps of (b) copper, (c)

aluminium and (d) oxygen, for an AA 2024-T3 specimen hard anodized at 50 mA cm-2 in 10

vol.% H2SO4 at -1 ºC for 600 s.

Figure 5. EDX linescan analyses of aluminium, oxygen and copper along the line shown on

Fig. 4(a).

16

Table 1. Experimental data from the literature for the peak (Vpk) and final (Vf) voltages and measured enrichments of alloying elements (Ncrit) for anodizing Al-1 at.% Au [17] and Al-3.5

at.% W [20-22] alloys in different electrolytes; calculated critical cell diameter (Dcrit), determined from Eq. (7); and critical voltage (Vcrit) estimated using a ratio of critical cell diameter to anodizing voltage of 2.8 nm V-1

[5, 6]. The location of the alloying element observed by TEM either at cell boundaries only or additionally in the cell body is indicated

by b or c, respectively.

Substrate Electrolyte Vpk

(V)Vf

(V)Ncrit

(x1015 atoms cm2)Dcrit

(nm)Vcrit

(V)

alloy element location

Al-1 at. % Au H2SO4 18 9 6.4 491 175 b.

Al-1 at. % Au H3PO4 50 38 8.8 675 241 b.

Al-3.5 at.% W H3PO4 180 160 3.2-5.5 70-120 25-43 c.

Al-3.5 at.% W H2SO4 21 18 2.5 55 20 b.

Al-3.5 at.% W Borax 60 60 2.5 55 20 c.

AA 2024-T3 H2SO4 40 55 5.0 213 76 b.

Table 2. Results of EDX analyses in boxed regions made on three different ultramicrotomed sections (denoted as A, B, C) of the film formed on an AA 2020-T3 alloy specimen at 50 mA cm-2 in 10 vol.% H2SO4 at -1 ºC for 600 s. The locations labelled 1 to 5 in analysis A correspond to the regions in Fig. 4(a). Equivalent locations to 1 to 3 were analysed in ultramicrotomed sections B and C.

Analysis location Cu:Al ratio(x10-2)

A B C1 cell edge 3.9 2.3 1.82 cell centre 1 0.7 1.23 cell boundary 2.8 3.8 3.24 cell centre 0.9 - -5 cell edge 1.8 - -

17

Figure 1. Schematic diagram showing transport of an enriched alloying element (copper) to

the cell boundaries during the growth of a porous anodic film [19]. The transport of copper

results in formation of oxygen bubbles at the cell boundaries.

18

O2

Figure 2. Predicted distributions of an enriching alloying element in porous anodic alumina films with cells of diameter < Dcrit (a, b) and > Dcrit.(c). A close-packed arrangement of hexagonal cells is assumed, with cells of diameter D and side length r as shown in (d).

19

Figure 3. (a) Secondary and (b) backscattered electrons SEM micrograph of a fractured section of an anodic film on an AA 2024-T3 alloy specimen hard anodized at 50 mA cm2 in 10 vol.% H2SO4 at -2 °C. (c) Secondary electron micrograph of film base.

20

200 nm

Figure 4. (a) HAADF TEM micrograph and EDX elemental maps of (b) copper, (c) aluminium and (d) oxygen, for an AA 2024-T3 specimen hard anodized at 50 mA cm-2 in 10 vol.% H2SO4 at -1 ºC for 600 s.

21

0 100 200

Cou

nts/

a.u

Position/nm

Al Cu O

Figure 5. EDX linescan analyses of aluminium, oxygen and copper along the line shown on

Fig. 4(a).

22