*3.1. Plate Shape*

#### 3.1.1. FE-SEM Image of Plate Shape

Figure 5 shows the FE-SEM images of AAO and AAO with 1-h widening. Before widening, the pore diameter was about 35 nm, and the pore depth approximately 15 μm. The uniform surface is shown in the microscale image (Figure 5a), and the nanoporous structure is indicated in the nanoscale image (Figure 5b). After 1 h widening, a crack was generated in the microscale image (Figure 5c) and the nanoporous structure was collapsed in the nanoscale image (Figure 5d). The widening process was not considered in this study due to the collapse of the nanoporous structure.

**Figure 5.** FE-SEM images of anodic aluminum oxide with widening of aluminum 7075.

#### 3.1.2. Salt Spray Test

Figure 6 shows the results of the salt spray test for bare aluminum and AAO-O (oil impregnation on anodic aluminum oxide) surfaces of aluminum 7075. Corrosion was indicated after 5 h in the case of bare aluminum, whereas the corrosion was not generated in the case of the AAO-O after 720 h. The salt spray test was stopped after 100 h in the bare aluminum case as the corrosion was intensively examined after 100 h. In the case of AAO-O, the test was continuously conducted up to 720 h. The results of the salt spray test showed that the oil impregnation on the AAO surface of aluminum 7075 could enhance the corrosion resistance.

**Figure 6.** Results of salt spray test for bare aluminum and AAO-O (oil impregnation on anodic aluminum oxide) surfaces.

#### 3.1.3. Contact Angle of Plate Shape

Figure 7 indicates the contact angles of AAO, AAO-O (oil impregnation), and AAO-O-SST (salt spray test). The contact angles of bare aluminum and bare aluminum-SST (salt spray test) were also investigated for comparison. The contact angle of a water droplet on the bare aluminum was 74.4◦ ± 5.8◦, and it was significantly decreased to 24.8◦ ± 5.6◦ after the salt spray test. The bare aluminum is susceptible to corrosion. After anodizing, the contact angle was dropped to 21.0◦ ± 1.7◦. It showed that the nanoporous structure enhanced the wettability of water by increasing surface roughness. After oil impregnation on the AAO surface, the contact angle was increased. The contact angle of a water droplet on AAO-O-SST was 81.1◦ ± 1.9◦. Although the salt spray test was conducted on the oil-impregnated AAO surface, the contact angle of the water droplet on AAO-O-SST was significantly increased.

**Figure 7.** Contact angles of bare aluminum, bare aluminum-SST (salt spray test), AAO (anodic aluminum oxide), AAO-O (oil impregnation), and AAO-O-SST (salt spray test) surfaces of aluminum 7075.

Figure 8 presents the schematic cross-section diagram of bare aluminum, bare aluminum-SST (salt spray test), AAO, AAO-O (oil impregnation), AAO-O-SST (salt spray test) surfaces. Bare aluminum

indicated a relatively flat surface, whereas the roughness was considerably increased after the salt spray test as shown in Figure 6. The results of the contact angle test showed that the contact angle of bare Al was decreased after the salt spray test. A surface of high roughness generally presents a low contact angle. Oxidized aluminum by the salt spray test increased the roughness of the surface, and the increased roughness reduced the contact angle. After anodizing, a nanoporous structure was generated. The produced nanoporous structure by anodizing decreased the contact angle. After oil impregnation on the AAO surface, the contact angle was increased. We assumed that the oil entirely covered the surface of the AAO. The contact angle on the surface of the AAO-O-SST was significantly increased after the salt spray test. It could be imagined that some oil remained on the nanoporous structure after the salt spray test as shown in Figure 8e. If there was no oil remaining on the surface of the AAO-O-SST, the contact angle would be similar to the contact angle of the AAO. When the oil layer was completely covered on the top of the surface of the AAO-O-SST, the contact angle would be similar to the contact angle of the AAO-O. The contact angle of the AAO-O-SST had a similar contact angle to the bare aluminum because the exposed nanoporous structure held the droplet.

This study analyzed the interfacial tension between the proposed surfaces and water. The interfacial tension shows the adhesive force between the liquid phase of one substance and the liquid, solid or gas state of another element. When there is high interfacial tension, the water spreads on the surface. The high interfacial tension indicates a hydrophilic surface, whereas little interfacial tension shows a hydrophobic surface. Therefore, the high interfacial tension poses a low contact angle, and the low interfacial tension has a high contact angle. The interfacial tension was calculated by proposed equations from previous studies.

We employed the formulas in the Smith et al. study [30] to calculate the interfacial tension between the water and the proposed surfaces. The formula for the interfacial tension between the aluminum surfaces and the water is indicated in Equation (1). The formula for the interfacial tension between the AAO surfaces and the water is shown in Equation (2). Equation (3) shows the formula for the interfacial tension between the AAO-O-SST surfaces and the water. Since we assumed that the oil on the surface of the AAO-O-SST was thoroughly infused, the interfacial tension between the aluminum and the oil was not taken into account.

$$
\gamma\_{sw} = \sigma\_s + \sigma\_w - 2\sqrt{\sigma\_s \cdot \sigma\_w} \tag{1}
$$

$$
\gamma\_{Aw} = r \gamma\_{sw} \tag{2}
$$

$$
\gamma\_{ASw} = f\gamma\_{sw} + (1 - f)\gamma\_{ow} \tag{3}
$$

where γsw is interfacial tension between the solid (aluminum) and the liquid (water), γow is that between the oil and the water, γAw is that between the AAO surfaces and the water, and γASw is that between the AAO-O-SST surfaces and the water. σ*s* is the surface free energy of the solid, and σ*w* is the surface tension of the liquid. *r* is the roughness factor which is the ratio of the total surface area to the projected area of the solid in contact with liquid, and *f* is the fraction of the projected area of the surface that is occupied by the solid.

The roughness factor (r) was calculated by Equation (4).

$$r = 1 + \frac{2\pi \cdot \frac{a}{2} \cdot h}{\left(a + b\right)^2} \tag{4}$$

where *a*, *b*, and *h* are the pore diameter, edge-to-edge spacing, and the height, respectively. *a* and *b* were 30 and 70 nm, and *h* was 15 μm. *f* is predicted to be 0.65 using the FE-SEM image of the AAO surface.

There are various models to predict the interfacial tension between different substances and phases: Owens–Wendt–Rabel–Kaelble (OWRK), Wu, Oss and Good (acid-base), Fowkes, and Extended Fowkes [36,37]. This study employed the Fowkes model. Although the Extended Fowkes model furthermore consideres polar interactions and a hydrogen bonding fraction as shown in Equation (5), those two fractions are insignificant in these cases. The surface tension of water, aluminum, and oil are 72.8, 45.0, and 25.0 mN/m, respectively. The dispersion contribution of water is 21.8 mN/m.

$$\gamma\_{a\emptyset} = \sigma\_a + \sigma\_\beta - 2\left(\sqrt{\sigma\_a^D \cdot \sigma\_\beta^D} + \sqrt{\sigma\_a^P \cdot \sigma\_\beta^P} + \sqrt{\sigma\_a^H \cdot \sigma\_\beta^H}\right) \tag{5}$$

where α is aluminum or oil, and β is water. σ*<sup>D</sup>* is the dispersion force contribution, and σ*<sup>P</sup>* is the polar interaction. σ*<sup>H</sup>* indicates the hydrogen bonding fraction.

The diagrams of the proposed surfaces and the interfacial tensions are tabulated in Table 1. The interfacial tension of bare aluminum and AAO-O-SST showed relatively low values, whereas that for AAO noted a considerably high value. As indicated before, the surfaces of aluminum and AAO-O-SST had a relatively high contact angle. Since the high contact angle means low interfacial tension, the calculated results of interfacial tension agreed with the results of the contact angle. The interfacial tension of the AAO-O-SST was slightly less in value than that of bare aluminum, and the contact angle of the AAO-O-SST was slightly higher than that of bare aluminum. The interfacial tensions at AAO had a significantly high value due to the high roughness factor. The deep pore depth caused the high roughness factors.

**Table 1.** Interfacial tensions between water and bare aluminum, AAO (anodic aluminum oxide), and AAO-O-SST (oil impregnation—salt spray test).

## *3.2. Cylindrical Shape*

Contact Angle of Cylindrical Shape

Figure 9 indicates the contact angles of AAO, AAO-O, and AAO-O-PT (pressure test) for the cylindrical shape of aluminum 7075. After anodizing, the contact angle was decreased. After oil impregnation, the contact angle was increased to 50.0◦ ± 4.4◦ like that in the plate shape. The contact angle of the AAO-O-PT was approximately 89.1◦ ± 13.6◦. Although the pressure test was conducted on AAO-O, the contact angle was increased by 39.1◦. This result showed that some oil remained in the nanoporous structure, even though the oil on the top of the nanoporous structure was removed. The combination of the nanoporous structure and the oil in the pores caught the water droplet. Therefore, the high contact angle was investigated in the case of oil impregnation.

**Figure 9.** Contact angles of AAO (anodic aluminum oxide), AAO-O (oil impregnation), and AAO-O-PT (pressure test) surfaces for cylindrical shape of aluminum 7075.

#### *3.3. Comparison of Plate with Cylindrical Shapes*

The cylindrical shape indicated a similar tendency to the plate shape. Although the contact angle of AAO was low in both shapes, they were increased by the oil impregnation. The salt spray test and the pressure test eliminated the covered oil on the top of the nanoporous structure, but could not remove the oil in the pores. The combination of the exposed nanoporous structures and the oil filled in the pores increased the contact angle significantly. These results showed that the oil impregnation on the AAO surface of aluminum 7075 in both shapes increased the corrosion resistance.
