Anodizing Aluminum and Its Alloys in Etidronic Acid to Enhance Their Corrosion Resistance in a Sodium Chloride Solution

Four types of aluminum specimens were pretreated by chemical etching in NaOH and HNO₃ solutions and were then anodized in etidronic acid. Figure 1 summarizes the voltage-time curves during galvanostatic anodizing in a 0.2 M etidronic acid solution (293 K) at current densities of 10–250 Am⁻² for 60 min. In the case of 5N-Al (Fig. 1a), the voltage-time curves consisting of the initial increase, slight decrease, and plateau voltage were measured at low current densities of 10 Am⁻² and 30 Am⁻², and the shapes of these curves are typical of the formation of the porous oxide film. The initial slope and the subsequent plateau voltage increased with the current density, and steady-state high-voltage anodizing of greater than 200 V could be achieved at 30 Am⁻². However, unstable voltage oscillations were measured at a higher current density of 40 Am⁻² after the peak value appeared (burning), and a nonuniform film with a locally thick oxide was formed on the surface under this burning condition. The voltage-time curves during the anodizing of the A1050 and A5052 alloys are shown in Figs. 1b and 1c. Similar voltage-time curves were measured during galvanostatic anodizing of each aluminum alloy, whereas the voltage peak disappeared or became unclear compared to that of 5N-Al. Although a higher current density of 200 Am⁻² led to the formation of a nonuniform film on these aluminum alloys due to oxide burning, uniform oxide formation without burning could be achieved up to 150 Am⁻². As a result of the high current density conditions, the plateau voltages measured after the initial transients increased to greater than 250 V.

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Figure 1d shows the voltage-time curves during the anodizing of the A7075 alloy under the same operating conditions. At higher current densities of more than 50 Am⁻², the voltage increased gradually in the initial period and then slightly increased with the anodizing time for up to 60 min, and there is no clear voltage peak on the voltage-time curves. Relatively higher voltages of greater than 300 V were measured at higher current densities, whereas a uniform oxide film without burning was formed on the surfaces of whole specimens anodized at up to 250 Am⁻². Because numerous visible sparking behaviors were observed on the aluminum surface as the voltage increased to greater than 300 V during anodizing, the formation behavior of the anodic oxide is expected to be different from that of the previous three aluminum specimens. Therefore, morphological characterization of the specimens anodized at the highest current densities without oxide burning was performed via SEM and TEM observations.

Hereafter, the aluminum specimens were anodized under the highest current density condition without oxide burning: 5N-Al at 30 Am⁻², A1050 at 150 Am⁻², A5052 at 150 Am⁻², and A7075 at 250 Am⁻². Figure 2 shows SEM images of the surface (upper) and fracture cross-section (lower) of the (a) 5N-Al, (b) A1050, and (c) A5052 specimens anodized for 60 min. There is no visible sparking behavior on the aluminum surface during these anodizing processes. The whole SEM images show a slightly uneven surface with micrometer-scale concavities due to chemical etching for pretreatment before anodizing. Several white deposits are observed on the surface of the A1050 and A5052 alloys, which are the intermetallic compounds exposed to the surface by the chemical pretreatment. Nanoscale pores of the porous oxide film were observed on the whole anodized surface. Although the fracture shape of the oxide films is slightly different, thick porous oxide films with vertically grown pores to the oxide/substrate interface were observed in the cross-sectional SEM images. These SEM observations suggest that typical porous oxide films can be formed on the A1050 and A5052 alloys by anodizing in etidronic acid, similar to the high-purity aluminum specimen.

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Figure 3a shows low- and high-magnification SEM images of the surface of the A7075 alloy anodized at 250 Am⁻² for 60 min. The anodic oxide formed on the A7075 alloy possessed numerous submicrometer- to micrometer-scale pores, which were irregularly distributed over the entire surface, and the morphology of this oxide film is very different from that of the typical porous oxide film. A cross-sectional SEM image of the A7075 alloy anodized at the same current density for 120 min is shown in Fig. 3b. Different from the vertical nanopores formed in the porous oxide film, a thick outer porous oxide with disorderly distributed submicron-scale spherical pores and a thin inner dense barrier oxide are observed in the cross-sectional image. Considering the continuous plasma generation during the anodizing of A7075 at a high voltage of greater than 300 V, such a characteristic oxide is considered to be a PEO film. In summary, the porous oxide film is formed on the high-purity aluminum, the A1050 alloy, and the A5052 alloy by galvanostatic anodizing in etidronic acid, whereas the PEO film is formed on the A7075 alloy.

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Figure 4 shows bright-field (BF) STEM and TEM images of the PEO film formed on the A7075 alloy by anodizing at 250 Am⁻² for 7 min, which is 3 min after the start of visible sparking. Because the interface between the aluminum substrate and the PEO film is not smooth due to the chemical pretreatment and plasma generation, it is represented by a white dotted line. The porous oxide film formed by anodizing in etidronic acid typically consists of an amorphous aluminum oxide.³¹ However, this oxide possessed a two-layer structure consisting of a nonuniform outer layer with varying contrasts and a relatively uniform inner layer (Fig. 4a). The inner layer consisted of a uniform amorphous structure with a hollow feature (Fig. 4b), whereas the outer layer consisted of many fine crystalline oxides with a spot diffraction pattern and nanoscale voids (Figs. 4c and 4d).

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The XRD diffraction patterns of the anodic oxide formed on the four aluminum specimens are summarized in Fig. 5. The shapes of the diffraction patterns obtained with 5N-Al, A1050, and A5052 are very similar, i.e., there are three strong peaks corresponding to the aluminum substrate and a broad peak at a wide range of 15°–35° corresponding to the amorphous Al₂O₃. In the case of the A7075 alloy, in addition to these four peaks, there are many additional peaks caused by the intermetallic compounds contained in the alloy matrix, such as Mg₃₂(Al, Zn)₄₉, Al₆Mn, Mg₂Si, CuMg₂, and Fe₃Si, as well as crystalline γ-Al₂O₃. Therefore, the STEM observations and XRD measurements suggest that the PEO film formed on the A7075 alloy consisted of a two-layer structure of the outer crystalline γ-Al₂O₃ and the inner amorphous Al₂O₃. Such an outer crystalline oxide film may be formed by the rapid cooling phenomenon that occurs after the dielectric breakdown and the spark discharge of amorphous Al₂O₃ during anodizing at greater than 300 V.

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Figure 6 shows SEM images and aluminum and oxygen elemental distribution maps of the vertical cross-section of the (a) A1050, (b) A5052, and (c) A7075 alloys anodized in etidronic acid at 150 Am⁻² for 1 h (a and b) and 250 Am⁻² for 2 h (c), respectively. Although several defects caused by the intermetallic compounds were observed in the porous oxide film, relatively uniform films were successfully formed on the A1050 and A5052 alloys, and there is almost no difference in film thickness from location to location. Alternately, a nonuniform microporous PEO film measuring approximately 10 μm in average thickness was formed on the A7075 alloy.

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To increase the corrosion resistance of the anodized aluminum specimens, hydration sealing of the formed nanopores in boiling water is a simple and useful technique. Anodic amorphous Al₂O₃ easily reacts with boiling water, and hydroxides are formed by the following chemical reaction⁴⁷:

$$\begin{eqnarray}&&{{\rm{Al}}}_{2}{{\rm{O}}}_{3}+{{\rm{H}}}_{2}{\rm{O}}\,\to \,2{\rm{AlO}}\left({\rm{OH}}\right)\end{eqnarray} \tag{ 1 }$$

In a previous study, we found that plate-like hydroxides grew from the bottom of nanopores in the porous oxide film formed in etidronic acid and that long-term immersion caused complete pore-sealing.⁴⁵ Figure 7 shows SEM images of the surfaces of the anodized aluminum alloys after immersion in boiling water for 240 min. Hereafter, the thickness of the anodic oxide formed on entire aluminum specimens was adjusted to approximately 10 μm by controlling the anodizing time. Numerous thin, plate-like hydroxides were completely covered on the surface of the anodized A1050 and A5052 alloys (Figs. 7a and 7b), and the pores were completely filled by hydroxide deposits from the bottom to top (Fig. 7c). This pore-sealing behavior was in a good agreement with the previous results obtained using a high purity aluminum specimen.⁴⁵ Conversely, no plate-like hydroxide was observed on the surface and the vertical cross-section of A7075 alloy after immersion in boiling water, and many micrometer-scale pores still remained on the surface (Figs. 7d and 7e), possibly due to the presence of the crystalline γ-Al₂O₃ formed on the outer oxide by PEO and because it is difficult to seal the micropores in the PEO film.

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To examine the corrosion-resistant property of the anodic oxides formed in etidronic acid, potentiodynamic polarization measurements of the anodized specimens were conducted in a 3.5 wt% NaCl solution at 298 K. Here, in addition to etidronic acid, the aluminum specimens were covered with a typical porous oxide film that had a thickness of 10 μm and was formed by anodizing in sulfuric acid, and these polarization measurements were also carried out for the comparison of corrosion behaviors. Figure 8a shows polarization curves using the 5N-Al specimens covered with various oxide films, and the calculated corrosion current densities, i_corrs, are summarized in Table II (Here, the i_corr values for the pure aluminum, the A1050 alloy, and the A5052 alloy were measured by using two different specimens). The polarization curve of the chemically polished 5N-Al (CP, black curve) exhibited a large value overall due to the presence of only a thin native oxide film with many defects, and a larger corrosion current of i_corr = 2.9 × 10⁻³–9.6 × 10⁻³ Am⁻² was obtained. When this 5N-Al specimen was covered with the porous oxide film formed in sulfuric acid (sulfuric, orange curve), the corrosion current decreased to i_corr = 2.3 × 10⁻⁶–3.4 × 10⁻⁵ Am⁻². In this case, the aluminum substrate is separated by only a narrow barrier layer at the bottom of the porous oxide film from the NaCl solution because the nanopores formed in the porous oxide film are still open. The thickness of the bottom barrier layer, δ, is calculated with the following equation⁹:

$$\begin{eqnarray}&&\delta \,=\,{\rm{k}}U\end{eqnarray} \tag{ 2 }$$

where k is a proportional constant obtained by the experimental value (approximately 1.0 nmV⁻¹) and U is the anodizing voltage (15 V for anodizing in sulfuric acid); thus, the calculated thickness of the barrier layer is 15 nm. As a result, the corrosion current was reduced by this thin barrier layer. When this anodized specimen was immersed in boiling water for 10 min, the nanopores were completely sealed by the hydroxides.⁴⁵ Therefore, the corrosion current of the pore-sealed specimen decreased to i_corr = 1.9 × 10⁻⁶–9.4 × 10⁻⁵ Am⁻² (sulfuric; pore-sealing, red curve), and a corrosion-resistant porous oxide film with a three-orders-of-magnitude-smaller corrosion current could be obtained on the 5N-Al surface by anodizing in sulfuric acid.

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Notably, the corrosion-resistant property of the open-pore porous oxide film formed in etidronic acid is greatly improved over that of the pore-sealed oxide formed in sulfuric acid. The corrosion current of the 5N-Al specimen covered with the porous oxide film with a thickness of 10 μm (etidronic, light blue curve) was calculated to be i_corr = 6.7 × 10⁻⁸–3.3 × 10⁻⁷ Am⁻², which is much smaller than the typical pore-sealed porous oxide film formed with sulfuric acid. This smaller corrosion current is due to the presence of a more-than-10-times-thicker barrier layer formed in etidronic acid at a higher voltage (219 nm at 219 V). Moreover, the corrosion current further decreased to i_corr = 2.4 × 10⁻⁹–3.2 × 10⁻⁹ Am⁻² by the subsequent pore-sealing process (etidronic; pore-sealing, dark blue curve). It is noteworthy that this value is smaller than that obtained with sulfuric acid by three orders of magnitude, and a highly corrosion-resistant coating was successfully achieved by anodizing in etidronic acid. Recently, the fabrication of corrosion-resistant pure aluminum surface has been reported via PEO coatings,⁴⁸ plasma electrolytic carbonitridings,⁴⁹ and superhydrophobic layer formations.^50–52 However, the corrosion current densities obtained via these surface finishing processes were reported to be i_corr = 10⁻³−10⁻⁷ Am⁻², thus our anodic coatings via anodizing in etidronic acid has significant advantages over conventional methods for the corrosion protection of aluminum.

A similar corrosion resistance improvement could be achieved on the A1050 alloy (Fig. 8b). Although the corrosion current of the A1050 alloy was overall larger than that of the 5N-Al specimen due to the presence of the alloying elements in the matrix, it decreased to i_corr = 4.2 × 10⁻⁵–1.8 × 10⁻⁴ Am⁻² by etidronic acid anodizing compared to 2.2 × 10⁻⁴–4.0 × 10⁻⁴ Am⁻² by sulfuric acid anodizing. After the subsequent pore-sealing process, the corrosion current further decreased to i_corr = 2.7 × 10⁻⁶–1.5 × 10⁻⁵ Am⁻² by etidronic acid anodizing compared to 3.2 × 10⁻⁶–4.1 × 10⁻⁵ Am⁻² by sulfuric acid anodizing. As a result, the corrosion current decreased by two or three orders of magnitude compared to that of the bare metal substrate. For the A5052 alloy (Fig. 8c), the corrosion current similarly decreased to i_corr = 2.1 × 10⁻⁶–4.1 × 10⁻⁶ Am⁻² by etidronic acid anodizing compared to 3.2 × 10⁻⁵–6.0 × 10⁻⁵ Am⁻² by sulfuric acid anodizing. Alternately, there was no significant difference in the corrosion current after pore-sealing: i_corr = 2.2 × 10⁻⁶–4.2 × 10⁻⁵ Am⁻² by etidronic acid anodizing and i_corr = 1.9 × 10⁻⁶–9.0 × 10⁻⁵ Am⁻² by sulfuric acid anodizing. Based on these experimental results, the coating of the porous oxide film formed by anodizing in etidronic acid is a useful technique for corrosion protection of aluminum in a NaCl solution, particularly for pure aluminum and aluminum alloys with small amounts of alloying elements.

Figure 8d shows the polarization curves of the A7075 alloy covered with the PEO film. The corrosion current after the PEO film coating significantly decreased to i_corr = 8.23 × 10⁻⁵ Am⁻², compared to 4.54 × 10⁻¹ Am⁻² obtained on the bare A7075 substrate, because there was a thin barrier layer under the microporous PEO film (Fig. 3b). Alternately, there was no significant difference in the corrosion current densities after immersion in boiling water (i_corr = 1.58 × 10⁻⁴ Am⁻²) due to the absence of the hydration behavior of crystalline γ-Al₂O₃ during immersion, as shown in Fig. 7c. Although the corrosion resistance of this PEO film is lower than that of pore-sealed porous oxide films, it is expected to be applied to hard, corrosion-resistant coatings with crystalline aluminum oxides for the A7075 alloy.