**1. Introduction**

Aluminum alloy is one of the most widely used materials in the subsea industry because of its low cost, excellent thermal conductivity, good strength, acceptability for short-term development, and low density. The main reason for use in the subsea industry is the excellent thermal conductivity. The generated heat in subsea electronic equipment should be removed to prevent thermal deformation. The produced heat can be eliminated by high thermal conductivity materials by exchanging the heat with the surroundings.

There are eight series of aluminum alloys depending on its composition elements. The broadly used aluminum alloys in the subsea industry are 6000 series. The 6000 series has moderate strength, and is resistant to corrosion. The 7000 series has the highest strength among all the series, but it requires some treatment for the corrosion issue. Although the 7000 series has higher strength than the 6000 series, the 6000 series is more frequently used in the subsea industry.

Several methods have been suggested to prevent aluminum corrosion such as electrodeposition of cerium or silane films, cathodic protection using an Mg-rich coating, organic coating, conversion coating, and polymer coating [1–6]. Anodizing (Anodic Aluminum Oxide, AAO) is considered one of the most practical methods for corrosion protection. Anodizing forms a nanoporous oxide layer on the top of the aluminum surface. The well-established nanoporous structure increases the corrosion resistance. The dimensions of the nanoporous structure like pore diameter, interpore distance, and thickness

of the nanoporous oxide layer can be controlled by changing the type and concentration of the acid electrolyte solution, anodic voltage, anodizing time, and temperature [7–13]. The anodizing time affects the pore depth, and the anodic voltage and the temperature of the electrolyte solution influence the pore diameter and the interpore distance [14]. S.-K. Hwang et al. [13] experimentally showed that the anodizing time determines the pore depth. The pore diameter is an important characteristic of the nanoporous oxide layer [15,16]. The pore diameter can be enlarged through an etching process [12,17].

A corrosive medium can permeate the pores in the produced nanoporous structure [18–20]. Several solutions have been suggested to prevent absorbing the corrosive medium. One method is to fill the pores with solid-state oxide materials such as hydrothermal, dichromate, and nickel-salt [18,21,22]. Another solution is the coating because the hydrophobic coatings on the hydrophilic metallic surface make the surface superhydrophobic [23–27]. A further solution is oil impregnation.

Oil impregnation into the nanoporous oxide layer is suggested for corrosion protection and self-healing to withstand external damages or local defects using high purity aluminum [28]. Several studies have shown that the retention of oil on the surface resisted ingress of water or organic liquid [29,30]. The marine industry has employed oil impregnation into the porous structure to prevent corrosion of the material in a marine environment using high purity aluminum, 5000 series aluminum and low alloy steel [31–34]. Some studies have indicated that the oil impregnated nanoporous oxide layer is likely to lose the oil under a dynamic environment [35].

Although several studies have investigated oil impregnation into the nanoporous oxide layer using pure aluminum or 6000 series aluminum for corrosion protection, no studies have analyzed the oil impregnated nanoporous oxide layer using 7000 series aluminum, to our best knowledge. There are no studies on conducting a pressure test for the oil-impregnated nanoporous oxide to investigate the influence of high-pressure conditions. Therefore, this study investigated the corrosion resistance of aluminum 7075 surfaces with an oil impregnated nanoporous oxide layer to enhance its corrosion resistance for subsea application. The structure of this study is as follows. The experiment for the oil impregnated nanoporous oxide layer is described including information on the materials, preparation of the AAO surface, oil impregnation on the AAO surface, a salt spray test, a pressure test, and characterization in Section 2. The results and discussion are indicated in Section 3. Finally, the conclusion is presented.
