1. Introduction
Aluminum alloys have been widely applied as both structural and functional materials [1,2,3]. Anodic oxidation is almost a requisite treatment technique to achieve variable excellent performance [4,5] in terms of enhancing the interfacial bonding between top-coating and aluminum substrates [6], improving corrosion-preventing ability [4], building multilayer solid super-capacitors [7], assembling self-confinement nanomaterials [4,5] and growing functional membranes with anodizing preparatory coating [6,8,9,10]. Typically, 1060 aluminum alloys are the series used as industrial pure aluminum, while 2024 and 7075 are important aluminum alloys for aviation. Although these three kinds of aluminum alloys are applied in all kinds of areas, systematic studies related to their anodizing parameters and as-formed pore dimensions are rare. Most studies on aluminum alloys above concentrate on anodic oxidation in sulfuric or chromic acid electrolytes but pay less attention to anodic oxidation in phosphoric acid, which easily brings about superior spatial ordering and size uniformity of the pores [4,5,11,12]. Using phosphoric acid as the electrolytes, larger pore size can be usually prepared, which is a crucial aspect in membrane performance [13]. Yang et al. [14] achieved surface treatment of an aluminum alloy by phosphoric acid anodizing and tested its adhesive properties. The results showed that pre-treatment and anodizing had no effect on the tensile properties of aluminum alloy components, and the lap shear strength of anodized aluminum alloys was 3.38 times that of un-anodized aluminum alloys, which significantly improved the bond strength of aluminum alloys.
Regardless of the application field, pore dimension and surface roughness of anodic film are key contributors to the final performance of aluminum alloys. Despite the mature technology of phosphoric acid anodic oxidation of aluminum alloys, few details of anodizing 1060, 2024 and 7075 aluminum alloys have been revealed. In this study, these three alloys were employed as the matrix, and the effects of various factors on anodic Al2O3 film structure were thoroughly investigated. Additionally, highly uniform and orderly aluminum alloy anodic oxide films were prepared.
2. Experimental Section
2.1. Materials
The materials under study were samples of aluminum alloy of grades 1060, 2024 and 7075, the composition of which is presented in Table 1.
Aluminum samples were represented by metal plates of size (for example, 2 cm × 4 cm × 0.1 cm).
2.2. Preparation of Samples before the Formation of Anodic Oxide Films
Degreasing: 1060, 2024 and 7075 aluminum alloy samples were immersed into absolute ethanol and sonicated in an ultrasonic cleaner for 10 min; after sonication, the specimens were rinsed and dried.
Alkaline washing and pickling: The freshly prepared 1.5 mol/L NaOH solution and 1.5 mol/L HNO3 solution were placed in a water bath at 80 °C for several minutes. The aluminum alloy samples were degreased and then immersed in NaOH solution at a constant temperature for 1 min. After washing, they were immediately immersed in HNO3 for 30 s. The samples were rinsed and dried.
Sealing: One side of the aluminum alloys was sealed by adhesive CW-32, which did not react with the electrolyte solution and dried quickly. It formed a uniform layer which could be torn off without affecting the anodic oxidation reaction.
2.3. Anodizing of Al Samples
In order to avoid the difference in anodizing caused by uneven electrical wires, a phosphoric acid anodic oxidation method under a constant pressure was utilized in this study. The anodic oxidation experiment was set up as shown in Figure 1. The appropriate concentration of H3PO4 solution was used as the electrolytes [6,14], a graphite sheet was used as the cathode and aluminum alloy samples (with one side sealed) were used as the anode. The anodic oxidation time, anodic oxidation temperature, anodic oxidation voltage and electrolyte concentration were tuned by the controlled variable method, while the IT6874A DC voltage stabilizer provided a stable and secure power supply for the entire experimental device.
2.4. Characteristics of Oxidized Samples
2.4.1. Morphological Research
The morphologies of aluminum alloy anodic oxide films were investigated using a field emission scanning electron microscope (FESEM, SU8010 Hitachi). Sheet samples were cut into small pieces (about 1 cm × 1 cm) and stuck onto the copper platform. The prepared samples were finally observed, and findings were carefully recorded though FESEM.
2.4.2. Structural Investigations
The structures of aluminum alloy anodic oxide films were investigated using an X-ray diffractor (XRD, DX2700BH, Haoyuan, China; excitation light source = Cu Ka, wavelength = 0.154056 nm, tube voltage = 30 kV, tube current = 25 mA).
3. Results
3.1. Effects of Anodic Oxidation Time on Morphology of Anodic Al2O3 Film
Figure 2 shows the anodized structure of the 1060 aluminum alloy prepared at 23 °C, 30 V and with 4 wt% of phosphoric acid as the electrolyte.
As observed, pores in the 1060 aluminum alloy anodic oxide film became larger and larger with anodizing time until partial cracking occurred. For the 1060 aluminum alloy, when the anodic oxidation time was 15~45 min, as time increased, the number of pores in the anodic oxide film increased, the pore size increased from ca. 15 nm to ca. 90 nm (average size) and the porous structure arrangement became more regular and orderly. The anodic oxide film surface was relatively flat, with only localized non-porous areas. At 45 min, the pores were most regular. When the anodic oxidation time was 70~120 min, the pores became slightly larger as the time increased. At 70 min, the surface of the anodic oxide film had a porous structure; however, perforations and deep craters occurred. When the time reached 120 min, the anodic oxide film was composed of porous structures and fibrous materials. The porous structure became irregular and was replaced by fibrous material. The pore wall structure was extremely thin. It could be inferred that the dissolution of anodized aluminum in phosphoric acid became more obvious with an increase in the anodizing time. The ordered porous structure of the pore wall was corroded.
Figure 3 and Figure 4 depict the 2024 and 7075 aluminum alloys prepared at 21 °C, 30 V and with 4 wt% of phosphoric acid as the electrolyte for different anodizing times, respectively.
As shown in Figure 3, unlike the 1060 aluminum alloy, the pores on the 2024 aluminum alloy anodic oxide film became larger, until partial cracking occurred. There were numerous particles and fibers on the anodic oxidation surface. For the 2024 aluminum alloy, when the anodic oxidation time was 10–20 min, the number of pores in the anodic oxide film increased and pore size increased from ca. 20 nm to ca. 50 nm (average size). The anodic oxide film was relatively flat. When the anodic oxidation time was 20 min, the pores were the most regular. When the anodic oxidation time was 30 to 100 min, the pores of the anodic oxide film were more regular. As time increased, the pore size increased and the anodic oxide film became uneven. When the anodic oxidation time increased to 100 min, the holes disappeared and the oxide film was severely fractured. The stacked-fibers-like structure was observed and there were irregular particles on the surface. This might be caused by the Cu element in the 2024 aluminum alloy.
As shown in Figure 4, the 7075 aluminum alloy anodic oxide film became larger until partial cracking occurred. There were numerous particles and fibers on the surface of the substrate. For the 7075 aluminum alloy, when anodic oxidation time was 10~30 min, the number of holes in the anodic oxide film increased and the pore size increased from ca. 15 nm to ca. 60 nm (average size). The anodic oxide film was relatively flat, especially when the anodic oxidation time was 30 min, as pores were more regular and orderly. When the anodic oxidation time was 30~100 min, the pores of the anodic oxide film were further enlarged, but irregularly. When the anodic oxidation time was 100 min, the anodic oxide film had fractured edges with white particles or fibrous materials above the pores. Compared to the 2024 aluminum alloy, there were fewer particles or fibrous materials above the pores of the 7075 aluminum alloy.
3.2. Effects of Anodic Oxidation Temperature on Morphology of Porous Anodic Al2O3 Film
Figure 5 and Figure 6 show the anodic oxidation structures of 1060 and 2024 aluminum alloys prepared at different temperatures at 30 V, anodic oxidation time of 60 min and with 4 wt% of phosphoric acid as the electrolyte, respectively.
In the case of the 1060 aluminum alloy, when the temperature was maintained at 3 °C, the electrochemical reaction was slow, the surface of the aluminum alloy had unevenly distributed micropores and there were large non-porous areas. When temperature was maintained at 23 °C, the number of pores increased and the pore size varied, while the porous structure was disorderly. There were only localized porous areas and pits. The interior of the pits was porous. At 26 °C, the porous structure became more orderly and regular and the pores were larger (ca. 100 nm). This might be due to the increase in temperature and current density, which accelerated the dissolution of the oxide film in the electrolyte [15] and the anodic oxidation process; the non-porous areas on the surface were also dissolved.
For the 2024 aluminum alloy, when the anodic oxidation temperature was maintained at 3 ℃, the surface of the aluminum alloy was non-porous. When the anodic oxidation temperature was maintained at 21 ℃, truncated pore channels were generated with an average size ca. 70 nm. As temperature increased, sufficient Al3+ was ejected and pores became noticeable. The heat generated assisted the dissolution of the pore wall oxides and played an important role in pore expansion, resulting in the truncation of the pore channels [16]. In addition, the increase in temperature and current density accelerated the dissolution of the oxide film in the electrolyte under an electric field. At a temperature of 25 ℃, there were almost no holes, but there were cracks in the oxide film. There were fibrous or granular products on the anodic oxidation surface and there might be other alloying elements on the surface of the oxide film.
Figure 7 shows anodic oxidation structures prepared by the 7075 aluminum alloy at 30 V, anodic oxidation for 30 min and phosphoric acid concentration of 4 wt% in the electrolyte at different temperatures.
For the 7075 aluminum alloy, when the anodic oxidation temperature was 3 ℃, the aluminum alloy had micropores. At 21 ℃, the pores were evenly distributed with an average size of ca. 50 nm. At a temperature of 26 ℃, there were almost no holes, but there were cracks in the oxide film. There were fibrous or granular products on the anodic oxidation surface and there might have been other alloying elements on the surface of the oxide film. It can be concluded that, as the anodic oxidation temperature increases, the anodic oxidation process accelerates when the reaction temperature is sufficiently high. Although the dissolution of oxides does not occur [17], the kinetics of anodic oxide film growth are accelerated, the breakdown potential is lowered and the ordered porous anodic Al2O3 structure is destroyed with the cracking of the oxide film and the gradual disappearance of pores.
3.3. Effects of Electrolyte Concentration on Morphology of Porous Anodic Al2O3 Film
Figure 8 shows the anodic oxidation structures prepared by the 1060 aluminum alloy in an electrolytic cell at 10 V, anodic oxidation for 60 min and with different electrolyte concentrations at 23 ℃.
For the 1060 aluminum alloy, when the concentration of phosphoric acid was 4 wt%, the porous structure was destroyed, leading to a granular anodic oxide film; pits were observed between the particles. When the concentration of phosphoric acid was 10 wt%, the porous structure disappeared. Perforated holes and cracks appeared in some areas. The non-porous region consisted of a large number of rods stacked on top of each other and dumped on the substrate. Ravines started to form between the porous and non-porous areas. Two forms of aluminum alloy anodic oxide film existed, which might be caused by the increase in electrolyte concentration, the intense electrochemical reaction and the uneven anodic oxidation.
Figure 9 shows the anodic oxidation structures prepared by the 2024 aluminum alloy in an electrolytic cell at 21 ℃, 30 V, anodic oxidation for 20 min and with different electrolyte concentrations.
For the 2024 aluminum alloy, when the concentration of phosphoric acid was 4 wt%, the anodic oxide film was relatively flat and there were porous areas and pits. There were white particles or fibrous materials on the surface of the anodic oxide film. When the concentration of phosphoric acid was 10 wt%, the aluminum alloy anodic oxide film was slightly uneven, with an increasing number of particles or fibrous materials.
Figure 10 shows the anodic oxidation structures prepared by the 7075 aluminum alloy in an electrolytic cell under the following conditions: 21 ℃, 30 V, anodic oxidation for 30 min and with different concentrations of electrolyte.
For the 7075 aluminum alloy, when the concentration of phosphoric acid was 4%, the anodic oxide film was relatively flat. There were porous areas and pits. When the concentration of phosphoric acid was 10 wt%, the aluminum alloy anodic oxide film was slightly uneven, with an increasing number of particles or fibrous materials on the surface. Compared with the 2024 aluminum alloy, the 7075 aluminum alloy anodic oxide film had fewer particles or fibrous substance above the pores.
3.4. Effects of Anodic Oxidation Voltage on Morphology of Porous Anodic Al2O3 Film
Figure 11 shows the anodic oxidation structures prepared by the 1060 aluminum alloy under different voltages with the following conditions: 23 °C, anodic oxidation for 60 min and with 4 wt% of phosphoric acid as the electrolyte.
For the 1060 aluminum alloy, when the anodic oxidation time was 60 min with the voltage at 10 V, it can be seen that there were a large number of particles on the surface of the anodic oxide film and the film was porous, but the pore structure was destroyed and the pore size was not uniform. This might be due to the long anodic oxidation time and partially dissolved porous structure. At 30 V, the pore size was uniform, the pore walls were visible, the porous structure was perforated or cracked in some areas and the interior of the cracks was still porous. Compared to 30 V, when the voltage was 50 V, the pore size increased, the porous structure became more orderly and the presence of pore-encapsulating materials in certain areas was observed; however, the non-porous areas were still present, which indicated that the anodic oxidation was still continuing and the anodic oxidation time was insufficient.
Figure 12 shows the anodic oxidation structures prepared from the 2024 and 7075 aluminum alloys under the following conditions: 23 ℃, anodic oxidation for 60 min and with 4 wt% of phosphoric acid as the electrolyte. Herein, Figure 12a–c depicts the 2024 anodic Al2O3 film, while Figure 13a–c depicts the 7075 anodic Al2O3 film.
Figure 13 shows the anodic oxidation structures prepared from the 7075 aluminum alloy under the following conditions: 23 ℃, anodic oxidation for 60 min and with 4 wt% of phosphoric acid as the electrolyte. Herein, a, b and c depict the 7075 anodic Al2O3 film.
When anodic oxidation voltage was 30 V, the 2024 aluminum alloy oxide film was relatively loose, while the 7075 aluminum alloy exhibited uneven pores and cracks in certain areas. At 120 V, blistering of both the 2024 and 7075 aluminum alloy oxide films was observed and pores were unevenly distributed, which indicated that the breakdown potential had been reached. When the voltage was 160 V, there were only a few pores formed in the 2024 and 7075 aluminum alloys, mostly bumps.
3.5. X-ray Diffraction (XRD) Characterization of Anodic Oxidation Structures
Figure 14 shows X-ray diffraction patterns of the 1060, 2024 and 7075 aluminum alloys and their anodic oxide films. As observed, the 2024 and 7075 aluminum alloys and their anodic oxide films showed diffraction peaks at 2θ ≈ 38.45°, but the intensities of the diffraction peaks were different. These diffraction peaks were corresponding to an aluminum alloy, indicating that the anodic oxide film obtained was amorphous Al2O3. This is consistent with the results of the amorphous 2024 and 7075 aluminum alloy anodic oxide films in the previous section. The 1060 aluminum alloy showed a diffraction peak at 2θ ≈ 38.45°, which was corresponding to an aluminum alloy. However, no diffraction peak at this diffraction angle was detected for the 1060 aluminum alloy anodic oxide film. This might be due to the thick barrier layer of the amorphous anodic Al2O3 film, which could not be penetrated by X-rays, and the aluminum peak information could not be obtained. This also indicated that the 1060 aluminum alloy anodic film was thicker than that of the 2024 and 7075 aluminum alloys. In addition, the absence of a diffraction peak of aluminum alloy in the 1060 aluminum alloy anodic oxide film was consistent with its morphological characterization.
4. Discussion
4.1. Optimized Anodization Parameters of 1060, 2024 and 7075 Aluminiun Alloys
All three kinds of aluminum alloys can be anodized to form uniform and porous Al2O3 membranes. However, the optimized anodization parameters are series dependent. We can see the details in Table 2.
Among all three kinds of aluminum alloys, pore size and regularity are related to their Al content. The higher the Al content, the larger and more regular the pores that can be formed during anodic oxidation. Combining the results from the last section, the higher the non-aluminum content, the rougher the anodizing surface. This can be attributed to the fact that non-valve-metals cannot be anodized, only reserved on an Al2O3 surface with particles. Temperature, time and phosphoric acid concentration all affect the surface morphography of anodic membranes. Increasing these parameters can promote pore formation, as well as the pores’ cracks, by accelerating the dissolving of Al2O3. An appropriate voltage is necessary to provide a driving force of Al→Al2O3 and to influence the formation rate of Al2O3. According to the mechanism of direct injection of cations, the direct injection of Al3+ into the electrolyte does not promote the growth of oxides at the oxide/electrolyte interface, resulting in the transformation of aluminum alloys from a compact oxide film into a loose or even porous honeycomb structure [4,16,17,18]. At constant pressure, the thickness of aluminum alloy anodic oxide films increases linearly with time. When the anodic oxidation voltage reaches the breakdown potential, it not only prevents the uniform growth of the porous anodic film on the macroscopic metal surface, but also the aluminum alloy anodic oxide films will crack [19].
4.2. Evolutionary Process of the 1060, 2024 and 7075 Aluminum Alloys during Anodization
By studying the surface changes of alloys with time, four distinct stages can be found during anodic oxidation of 1060, 2024 and 7075 aluminum alloys in a phosphoric acid electrolyte, namely the non-porous stage, the mixed stage, the ordered porous stage, the disordered porous stage or the non-porous stage. As Figure 15 reveals, the surface of un-anodized aluminum alloys is flat, with some pits and decorating granules, and features no pores. With anodization proceeding, namely through the first stage, non-porous alloys become non-porous oxide films. With the extension of the anodic oxidation time, pores start to form, and the non-porous area decreases until the anodic oxide film becomes a porous area with regularly shaped pores of uniform size. Then, the porous structure is perforated and cracked, and the anodic oxide film is composed of a disordered porous structure or fibrous particles.
With the extension of the anodic oxidation time; the increase in the anodic oxidation temperature, electrolyte concentration and anodic oxidation voltage; and the speeding of the anodic oxidation process, the non-porous areas on the surface are dissolved and the pores become more regular and uniform. By exceeding the optimized anodic oxidation parameters, the porous anodic Al2O3 structure is destroyed and the pores gradually disappear, along with cracked pore walls.
5. Conclusions
It is concluded that uniform and porous anodic membranes are prepared on industrial pure aluminum 1060 and two aviation aluminum alloys (2024 and 7075). Pores on the surface of the 1060 aluminum alloy are the most regular, while 2024 and 7075 aluminum alloys display poor porous anodic Al2O3. Optimized anodization parameters were investigated systematically through experiments. The details can be expressed as follows: 1060 (23 ℃, 30 V, 45 min, 4 wt% phosphoric acid); 2024 (21 ℃, 30 V, 20 min, 4 wt% phosphoric acid); 7075 (21 ℃, 30 V, 30 min, 4 wt% phosphoric acid). With the conditions above, the pore structure of anodic membranes afforded on the substrates was more regular and orderly.
As the experiment data revealed, the anodic oxide films have gone through four stages, namely the non-porous stage, the mixed stage, thee ordered porous stage, the disordered porous stage or the non-porous stage. With the extension of the anodic oxidation time, pores started to form, and the non-porous area decreased until the anodic oxide film became a porous area with regularly shaped pores of uniform size. Then, the porous structure was perforated and cracked, and the anodic oxide film was composed of a disordered porous structure, fibrous particles or ravines. Our experiment findings provide a reference for the efficient assembly of coatings on 1060, 2024 and 7075 anodic oxide films and may be helpful for modifying and constructing all kinds of functional composite coatings on aluminum alloys.
Conceptualization, X.T.; data curation, C.J.; formal analysis, C.J.; writing—original draft, C.J. and X.T.; writing—review and editing, R.W., F.Z., L.Z., Q.H. and X.T. All authors have read and agreed to the published version of the manuscript.
We acknowledge the financial support from the National Natural Science Foundation of China (No. 21501196) and Fundamental Research Funds for the Central Universities (No. 3122019147).
Not applicable.
Not applicable.
Some or all data that support the findings of this study are available from the corresponding author upon reasonable request.
The authors declare no conflict of interest.
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Enlarge this image.Figure 2. SEM images of 1060 anodic oxide films at different anodic oxidation times: (a) 15 min; (b) 30 min; (c) 45 min; (d) 70 min; (e) 90 min; (f) 120 min.
Enlarge this image.Figure 3. SEM images of 2024 anodic oxide films at different anodic oxidation times: (a) 10 min; (b) 20 min; (c) 30 min; (d) 100 min.
Enlarge this image.Figure 4. SEM images of 7075 anodic oxide films at different anodic oxidation times: (a) 10 min; (b) 20 min; (c) 30 min; (d) 100 min.
Enlarge this image.Figure 5. SEM images of 1060 anodic oxide films at different anodic oxidation temperatures: (a) 3 ℃; (b) 21 ℃; (c) 26 ℃.
Enlarge this image.Figure 6. SEM images of 2024 anodic oxide films at different anodic oxidation temperatures: (a) 3 ℃; (b) 21 ℃; (c) 25 ℃.
Enlarge this image.Figure 7. SEM images of 7075 anodic oxide films at different anodic oxidation temperatures: (a) 3 ℃; (b) 21 ℃; (c) 26 ℃.
Enlarge this image.Figure 8. SEM images of 1060 anodic oxide films at different electrolyte concentrations: (a) 4 wt%; (b) 10 wt%.
Enlarge this image.Figure 9. SEM images of 2024 anodic oxide films at different electrolyte concentrations: (a) 4 wt%; (b) 10 wt%.
Enlarge this image.Figure 10. SEM images of 7075 anodic oxide films at different electrolyte concentrations: (a) 4 wt%; (b) 10 wt%.
Enlarge this image.Figure 11. SEM images of 1060 anodic oxide films at different anodic oxidation voltages: (a) 10 V; (b) 30 V; (c) 50 V.
Enlarge this image.Figure 12. SEM images of 2024 anodic oxide films at different anodic oxidation voltages: (a) 30 V, (b) 120 V and (c) 160 V.
Enlarge this image.Figure 13. SEM images of 7075 anodic oxide films at different anodic oxidation voltages: (a) 30 V; (b) 120 V; (c) 160 V.
Enlarge this image.Figure 14. XRD patterns of the 1060, 2024 and 7075 aluminum alloys (a,c,e) and their anodic oxide films (b,d,f). * represents the aluminum peak.
Enlarge this image.Figure 15. SEM images of un-anodized aluminum alloys: (a) 1060, (b) 2024 and (c) 7075.
Elemental composition of aluminum alloys used.
| Aluminum Alloy | Al (%) | Si (%) | Cu (%) | Mg (%) | Zn (%) | Mn (%) | Ti (%) | V (%) | Fe (%) | Cr (%) |
|---|---|---|---|---|---|---|---|---|---|---|
| 1060 | Bal. | 0.25 | 0.05 | 0.03 | 0.05 | 0.03 | 0.03 | 0.05 | 0.35 | - |
| 2024 | Bal. | 0.5 | 3.8–4.9 | 1.2–1.8 | 0.25 | 0.3–0.9 | 0.15 | - | 0.5 | 0.10 |
| 7075 | Bal. | 0.4 | 1.2–2.0 | 0.3 | 5.1–6.1 | 0.3 | 0.2 | - | 00.5 | 0.2–0.3 |
Optimized anodization parameters of the 1060, 2024 and 7075 aluminum alloys.
| Aluminum Alloys | Temperature (℃) | Time (min.) | Electrolyte Concentration | Voltage | Pore | |
|---|---|---|---|---|---|---|
| Regularity | Size (nm) | |||||
| 1060 | 23 | 45 | 4 wt% | 30 | good | ca. 90 |
| 2024 | 21 | 20 | 4 wt% | 30 | bad | ca. 50 |
| 7075 | 21 | 30 | 4 wt% | 30 | bad | ca. 60 |
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