*2.1. Material*

Commercial aluminum alloy 7075 was employed as the material for the AAO surface because it is widely used in the 7000 series aluminum. Components of aluminum 7075 are aluminum (87.1–91.4 wt%), zinc (5.1–6.1 wt%), magnesium (2.1–2.9 wt%), copper (1.2–2 wt%), chromium (0.18–0.28 wt%), iron (Max 0.5 wt%), silicon (Max 0.4 wt%), manganese (Max 0.3 wt%), titanium (max 0.2 wt%), and other total (max 0.15 wt%). Its density is approximately 2.81 g/cc. The ultimate tensile and the tensile yield strengths are 572 MPa and 503 MPa, respectively.

#### *2.2. Preparation of the AAO Surface*

Not only a plate shape but also a cylindrical shape were prepared for the experiment in this study because the cylindrical shape is frequently used in the subsea industry as housing for an electronic device. The AAO devices for the plate and cylindrical shapes are shown in Figures 1 and 2, respectively. The following describes the details of the AAO preparation for both shapes.

**Figure 2.** Anodic aluminum oxide device for the cylindrical shape.

## 2.2.1. Plate shape

All the samples were first rinsed with ethanol and isopropyl alcohol, and then they were cleaned using deionized water in a sonicator for 5 min. The anode was aluminum 7075, and the cathode was aluminum 6061. The distance between the anode and the cathode was fixed at approximately 5 cm as shown in Figure 1. A DC power supply (DRP-92305DU power supply) provided a constant voltage of 40 V, and the temperature was maintained by a circulator (Lab. Companion. RW-0525G) at 25 ◦C. The solution was 0.3 M oxalic acid solution, and it was stirred by a magnetic bar at a constant speed. The anodization was conducted for 3 h to generate 15 μm pore depth. After the anodizing, widening was conducted for 1 h under the same condition with anodizing to enlarge the pore diameter. The next step was oil impregnation. Oil (rust preventive oil, NP-7) was deposited on the nanoporous structure. The oil used in this study was a lubricant, which mostly consists of paraffin.

## 2.2.2. Cylindrical shape

The cylindrical shape's anode was installed as shown in Figure 2 and the cylindrical shape's cathode was also built for homogeneous anodizing. First, the samples were rinsed with ethanol and isopropyl alcohol. The anode was commercial aluminum 7075, and the cathode was aluminum 6061. The diameter of the cylindrical anode was 25 cm, and that of the cylindrical cathode was 35 cm. The distance between the anode and the cathode was adjusted to be about 5 cm. A DC power supply (PNCYS. EP-10010) provided a constant voltage of 40 V, and the temperature was maintained by a circulator (Lab. Companion. RW-0525G) at 25 ◦C. The anodizing time was 3 h, and the solution was 0.3 M oxalic acid solution—the same condition as the anodizing condition of the plate shape. Finally, the oil was impregnated on the surface of the AAO.

#### *2.3. Salt Spray Test*

The salt spray test was performed to investigate the corrosion e ffect on the surface directly. The (bare) aluminum and the oil impregnated AAO surfaces of aluminum 7075 were analyzed for comparison. This salt spray test complies with 'Standard: KSA–KS D 9502. This standard contains a salt spray test for the anodic oxidation of metallic materials and various coatings. It conforms with ISO 9227:2006 (Corrosion tests in artificial atmospheres—salt spray tests). A 5% concentration of brine was utilized for the salt spray test, and the temperature of the test was maintained at 35 ◦C. The test was performed for 720 h for the oil impregnated AAO surface of aluminum 7075.

## *2.4. Pressure Test*

This study carried out a pressure test to investigate the surface of the oil impregnated AAO of aluminum 7075 under high-pressure conditions. As the subsea equipment is installed in the seabed, it is exposed to the high-pressure environment—when going down 10 m below sea level, the pressure increases by approximately 1 bar. Since the surface of the oil impregnated AAO can be damaged under the high-pressure condition, a pressure test was required. Figure 3a,b shows the facility for the high-pressure test, which complies with 'MIL-STD-810G' (Department of defense test method standard, part two-laboratory test methods, 512.5 Immersion) and 'ABS NOTICE' (Rules for building and classing underwater vehicles, systems and hyperbaric facilities, Section 3 general requirement and safeguards, 3 proof testing, 3.1 hydrostatic test). Two cylindrical samples, which had 80 mm (diameter) × 150 mm (length), were prepared (Figure 3c). The pressure of the facility was gradually increased to 600 bar for 6000 s as shown in Figure 4. The samples were examined at 600 bar for 1800 s.

**Figure 3.** Facility for pressure test and samples.

**Figure 4.** Pressure with time in pressure test facility.
