Electrochemical Corrosion Behavior of SiO₂ Superhydrophobic Inhibitor in Al7075

Abstract

The automotive industry has been employing Al alloys to reduce the weight of chassis; however, this can present some corrosion problems. In this research, we study the electrochemical behavior of SiO₂ superhydrophobic on Al 7075. The electrochemical techniques employed were cyclic potentiodynamic polarization (CPP), performed at a scan rate of 60 mV/s from −800 to 800 mV vs. OCP, and electrochemical impedance spectroscopy (EIS) at ±10 mV with frequencies ranging from 10 mHz to 100 kHz, as per ASTM G61 and ASTM G106. The electrolytes employed were NaCl and H₂SO₄ at 3.5 wt.% simulating marine and industrial atmospheres. The results showed that the coating presented an efficiency of 81% when exposed to NaCl, but the corrosion in this medium is localized. In H₂SO₄, the corrosion type is uniform.

1. Introduction

Currently, the automotive industry is focused on reducing vehicle weight by utilizing materials such as aluminum alloys, advanced high-strength steels, magnesium, and composites. The use of Al alloys in the automotive industry has increased in recent years; however, those materials are susceptible to present corrosion problems [1].

Al alloys employed in the automotive industry are the series 7XXX, 6XXX, and 5XXX. These alloys are available in sheets or by extrusion. Nissan has significantly reduced its use of Al alloys and AHSS in the BIW of the vehicle. However, those alloys are exposed to corrosion problems due to the environment in which they are employed [1,2,3].

The cause of corrosion can be related to environmental factors, such as the presence of oxygen in water and salt, which can promote the process. Salts often serve as electron carriers, enabling water to transport them through redox reactions. In humid environments, metals corrode significantly faster. This happens because moisture-laden air interacts with oxygen and electrons on the metal’s surface. Prolonged exposure to humid air accelerates the corrosion of metal components. Vehicles encounter diverse environmental conditions, ranging from extreme heat and cold to operation in coastal regions or near chemical plants. These factors all contribute to atmospheric corrosion. Even a small scratch during use can create a corrosion cell within the surface’s moisture film, while the application of de-icing salts in cold climates can accelerate the process [4].

One option to prevent corrosion problems in the automotive industry is the application of coatings that reduce the interaction between metal and the environment. There are various methods to protect materials from corrosion, including anodization, vapor deposition, chemical conversion, plasma spraying, and organic coatings. Inhibitors, which can be part of both organic and inorganic coatings, primarily serve to slow down the corrosion process. Their effectiveness is closely linked to their adsorption capacity; however, factors such as inhibitor properties, electrolyte concentration, and changes in surface charge can influence the adsorption performance of the inhibitor [5,6,7,8,9,10,11].

Silicon dioxide (SiO₂), an inorganic nanoparticle, is commonly used to improve the performance of various organic-based resins. Its strong mechanical and thermal properties make it an effective barrier material. Additionally, numerous studies have demonstrated that SiO₂ exhibits hydrophobic characteristics, which help block corrosive agents from interacting with the surface and prevent the penetration of corrosive ions [12]. Superhydrophobic surfaces, known for their water-repellent behavior, are considered highly effective in corrosion protection. They are defined by a static water contact angle greater than 150° and a tilt angle below 10° [12,13,14,15,16,17]. The hierarchical ordering of superhydrophobicity is important to determine the behavior of the coating; usually, this ordering structure gives self-cleaning properties, helping to remove the electrolyte from the surface. For that reason, the use of superhydrophobic coatings is an excellent option for coating materials [18].

Research has shown that SiO₂ coatings can lower the corrosion rate by altering surface porosity, and corrosion resistance is further enhanced when SiO₂ is combined with epoxy resins. Various corrosion inhibitors have also been developed based on the hydrophobic properties of coatings. A reduction in corrosion kinetics occurs with a hydrophobic coating in NaCl at 3.5 wt.% [19]. Superhydrophobic surfaces are particularly useful in anodized materials, as they help reduce Cl⁻ ion penetration due to anodized porosity. These surfaces offer solutions to corrosion issues caused by environmental pollutants. The role of hierarchical structures in superhydrophobic surfaces is based on the contact angle of 160° and a sliding angle of 1°, enabling a self-cleaning effect [20,21,22]. Localized corrosion, such as intergranular corrosion, can significantly reduce the service life of materials like the AA7075 aluminum alloy. The alloy’s microstructure—determined by the heat treatments applied—plays a key role in influencing its susceptibility to localized corrosion. However, the relationship between microstructure and localized corrosion behavior remains not fully understood, with varying interpretations in the literature. For instance, the T6 aging process has been linked to an increased risk of intergranular corrosion (IGC) [23,24,25]. Xiong et al. [26] found that applying different heat treatments to Al-7075 resulted in distinct localized corrosion patterns, influenced by the anodic nature of the affected areas. Their research also suggests that the GL test, which is faster, avoids the need for complex test-time selection, and yields cleaner specimen surfaces, may be a more efficient method for predicting localized corrosion behavior under natural conditions—without the need for an applied current—compared to the OCP test.

Some researchers showed that when superhydrophobic is applied on the surface, the Ecorr value increases 0.1 V vs. SCE, meaning that the potential is more noble. The current values are decreasing by three orders of magnitude, indicating that corrosion kinetics are lower when the coating is applied, which is characterized by potentiodynamic polarization [22,27,28,29].

When EIS was conducted, the results showed that the n values are close to 1, indicating the presence of some porosity, but with a trend to be homogenous. However, the inhibition efficiency is nearly 100%, indicating that the corrosion resistance is higher and the material is protected against corrosion [30,31].

This work aims to evaluate the electrochemical corrosion behavior of Al7075 coating with a superhydrophobic coating of SiO₂ nanoparticles. The use of a superinhibitor helps reduce the corrosion rate in Al alloys, employing an organic coating as an alternative to conventional coatings such as anodizing or other coatings that require more preparation and budget. The electrochemical techniques employed to realize this study are cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy in the electrolytes of NaCl and H₂SO₄ at 3.5wt.% to simulate marine and industrial environments based on ASTM G106 and G61.

2. Materials and Methods

2.1. Materials

Silica nanoparticles (SiO₂ NPs) were prepared using tetraethyl orthosilicate (TEOS, 98%, Sigma-Aldrich, Burlington, VT, USA) as the precursor, with ammonium hydroxide (28–30% NH₃, Sigma-Aldrich, Burlington, USA) serving as the catalyst. Deionized water (18 MΩ·cm) and isopropyl alcohol (C₃H₈O, Baker, Philadelphia, PA, USA) were used as solvents during hydrolysis. For surface modification and formation of a superhydrophobic layer, hexane (95%, J.T. Baker, Philadelphia, USA) and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES, 97%, Matrix Scientific, Columbia, IN, USA) were employed. Glass slides for coating experiments were obtained from Fisher Scientific, Hampton, VA, USA.

2.2. Equipment

A water purification unit (Water-Pro PS, Labconco^®, Kansas City, MO, USA) provided the deionized water. Solution mixing was performed using a magnetic stirrer (PC 410, Corning^®, Somerville, MA, USA), the coating was deposited onto glass substrates using an airbrush (Master Performance G233 system, Tokyo, Japon), and the resulting coatings were examined with a scanning electron microscope (JSM-7401, JEOL, Tokyo, Japan).

2.3. Synthesis and Coating Procedure

SiO₂ nanoparticles were synthesized following the Stöber method [32]. A mixture of 95 mL of isopropyl alcohol was stirred at 300 rpm, and then 350 μL ammonium hydroxide, 1 mL TEOS, and 3.65 mL deionized water were added. The reaction proceeded at 40 °C for 24 h under continuous stirring. Subsequently, 500 μL PFDTES and 25 mL hexane were incorporated, and the temperature was raised to 60 °C. Stirring continued for 48 h, yielding 125 mL of the superhydrophobic solution [33].

The Al7075 substrates were coated using a spray deposition method. The coating parameters were established through preliminary optimization tests to ensure a uniform and defect-free superhydrophobic surface [18]. A spray gun equipped with a 0.5 mm diameter nozzle was positioned 15 cm from the substrate. An air pressure of 30 psi was applied to achieve consistent atomization of the solution and prevent droplet agglomeration on the surface. The final coating consisted of 7 layers, applied in multiple passes to guarantee homogeneous coverage. After coating, the samples were allowed to cure at ambient temperature. After coating, the samples were exposed to outdoor conditions for 35 days to evaluate the stability of their superhydrophobic properties.

2.4. Scanning Electron Microscopy

Samples were introduced in the following ways: before coating application, after coating application, and after corrosion characterization. This is to see the morphology of the coating and material by secondary electrons. The energy applied was 2 kV at a working distance between 7.5 and 8 mm.

2.5. Electrochemical Characterization

A three-electrode setup (Figure 1) was employed to study corrosion, where the test material acted as the working electrode, a saturated calomel electrode (SCE) served as the reference, and a platinum wire functioned as the counter electrode in a steady state. All measurements were repeated twice at ambient temperature using a Princeton Applied Research Model 263A (UK). The experiments were conducted in a 3.5 wt.% NaCl and H₂SO₄ aqueous solution. Cyclic potentiodynamic polarization was performed based on ASTM G61-86 [34]. One cycle was used in a polarization range of −0.8 to +0.8 V relative to E_corr with a 60 mV/min sweep rate [34]. The electrochemical impedance spectroscopy (EIS) was performed based on ASTM G106, with a frequency range of 0.01 Hz to 100 kHz, an applied signal amplitude of 10 mV RMS, and 35 measurement points per decade. EIS data were analyzed through ZView-4 software (Scribner Associates, Inc., Southern Pines, NC, USA), using equivalent circuit modeling to interpret the spectra.

3. Results

3.1. SEM Before Corrosion

Figure 2 shows the morphology of Al 7075 at SEM before and after the application of SiO₂ hydrophobic. Figure 2a,b shows the Al 7075, which presents some porosity and the lines due to the grind. In addition, Figure 2c,d shows the coating; the inhibitor does not present a homogeneous distribution, presenting zones with cracks that resemble dry earth.

3.2. Wettability Test

Figure 3 presents the wettability results, showing a water contact angle (WCA) of 157.5 ± 1.18° and a water sliding angle (WSA) of 3.2 ± 0.29°. These values confirm that the coating retains its superhydrophobic behavior.

3.3. Electrochemical Test

3.3.1. Cyclic Potentiodynamic Polarization

Figure 4 shows the CPP of samples when they are exposed to NaCl and H₂SO₄. Figure 4a shows the behavior of samples when they are exposed to NaCl. The uncoated Al 7075 sample presents a lower Ecorr, −750 mV, and the coated sample presents a value of −721 mV (See

Table 1. CPP parameters obtained by experimentation.

Sample	Ecorr (mV)	icorr (A/cm2)	Hysteresis
NaCl
Al 7075	−750 ± 5	1.54 × 10−4–750 ± 5 × 10−5	Positive
Al 7075 SiO2	−721 ± 4	5.91 × 10−5 ± 4 × 10−5	Positive
H2SO4
Al 7075	−589 ± 7	1.45 × 10−3 ± 2 × 10−4	Negative
Al 7075 SiO2	−420 ± 6	2.28 × 10−4 ± 5 × 10−5	Negative

). The increase in E_corr indicates that the coated sample has a higher corrosion potential, and the corrosion probability decreases. The i_corr is lower for Al 7075 SiO₂ (5.91 × 10⁻⁵ A/cm²), indicating a lower corrosion rate compared to the 1.54 × 10⁻⁴ A/cm² of Al 7075, which presents a higher corrosion rate. The samples presented positive hysteresis, indicating a localized corrosion in both samples.

Figure 4b shows the results of samples when they are exposed to H₂SO₄. The sample Al 7075 SiO₂ obtained an E_corr of −420 mV and Al 7075 −589 mV, indicating that the coated sample requires more energy to initiate an anodic process. The i_corr of Al 7075 SiO₂ is 2.28 × 10⁻⁴ A/cm², while Al 7075 presented 1.45 × 10⁻³ A/cm², indicating that the corrosion rate increases when the sample is uncoated. The samples presented uniform corrosion due to the positive hysteresis.

3.3.2. Electrochemical Impedance Spectroscopy

Figure 5 shows the results of EIS for samples exposed to NaCl. The value of CPE is higher for Al 7075 SiO₂ with a value of 6.6 × 10⁻⁵ F/cm² (see

Table 2. Parameters obtained by EIS.

Sample	Rs (Ω·cm2)	CPE1-T (F·sn−1/cm2)	n	Rct (Ω·cm2)	CPE2-T (F·sn−1/cm2)	n	RC (Ω·cm2)	IE (%)
NaCl
Al 7075	21 ± 0.4	3.6 × 10−5 ± 2 × 10−6	0.7 ± 0.09	874 ± 9.1	3.42 × 10−4 ± 3.3 × 10−4	−0.7 ± 0.09	1.04 × 103 ± 94	-
Al 7075 SiO2	10 ± 1.2	6.6 × 10−5 ± 1.5 × 10−6	0.9 ± 0.07	177 ± 7.2	2.09 × 10−3 ± 4.1 × 10−4	−0.6 ± 0.09	1.01 × 104 ± 86	81.37
H2SO4
Al 7075	1.9 ± 0.08	1.1 × 10−4 ± 1.7 × 10−5	0.9 ± 0.04	99 ± 8.4	0.066 ± 4.9 × 10−4	−0.8 ± 0.08	52 ± 4.6	-
Al 7075 SiO2	1.9 ± 0.09	1.5 × 10−4 ± 2.2 × 10−5	0.9 ± 0.05	52 ± 6.1	0.11 ± 1.9 × 10−3	−0.7 ± 0.09	22 ± 1.8	−102

), indicating a high ionic energy. The n value of Al 7075 is 0.7, suggesting a non-homogeneous current distribution on the surface, which may be due to a localized process. On the other hand, the value of Al 7075 SiO₂ is 0.9, indicating that the surface is nearly homogeneous, but is not perfect. It is most important that corrosion is attacked in preferred zones. The second layer presented negative values of n, indicating that the CPE has an inductor behavior. This means that an adsorption process is occurring on the surface. Figure 5b shows the absolute impedance vs. frequency. This graphic indicates that Al 7075 SiO₂ exhibits higher corrosion resistance; therefore, the efficiency of inhibitors (IE) is higher, with a value of 81.37%. The IE is calculated with the following equation [36]:

$$IE={\displaystyle \frac{\left(R_T-R_{TC}\right)}{R_T}}\times 100$$

(1)

where

$$R_T=R_{ct}+R_C$$

(2)

The inhibitor increased corrosion resistance; therefore, the efficiency is higher. The Bode phase angle diagram shows how the adsorption behavior begins to occur at low frequencies, around 1 × 10⁻¹ Hz, and that behavior occurs in the second layer, when the electrolyte penetrates the material. For that reason, although the inhibitor protects Al 7075 from corrosion, it cannot change the mechanism of corrosion of aluminum.

Figure 6 shows the behavior when samples are exposed to H₂SO₄. Figure 5a shows the Nyquist diagram, supported by Figure 6b of absolute impedance. In both graphics, it is notable how the behavior of Al 7075 and Al 7075 SiO₂ presented the same characteristics at high frequencies; however, at middle and low frequencies, the corrosion resistance of Al 7075 is higher than that of the sample coated. It is because the H₂SO₄ generates an oxide layer in aluminum that protects the material. Additionally, electrolytes can be very aggressive for superhydrophobic material. The IE is negative due to the lower resistance presented by the material. N values of the first layer are 0.9 and 0.9; therefore, the behavior is capacitive, and the distribution of charge is nearly homogeneous. On the second layer, the values are negative, indicating an adsorption process that occurs on the surface. The adsorption process is more homogeneous for the uncoated sample; therefore, the n value is −0.87.

Figure 7 shows the equivalent circuits for the sample uncoated (a) and the sample with inhibitor (b). The behavior is the same; the difference is that the first layer is related to the oxide generated by the interaction of the electrolyte with the metal for Al 7075. In Figure 7b, the first layer is related to the resistance of the inhibitor. The behavior of CPE from the second layer is associated with an inductor by the negative values of n.

3.4. SEM After Corrosion Test

Figure 8 shows the samples after the corrosion test in NaCl. Figure 8a,b show the uncoated Al 7075, where the distribution of oxide and corrosion residuals is not homogeneous; therefore, the values of n are not 1 in EIS. Additionally, this figure supports the results obtained by CPP, where the type of corrosion was localized due to the preferential localization of corrosion in specific zones. Something similar occurs with Figure 8c,d, where the superhydrophobic presented degradation in the cracked zones. The corrosion preferentially attacks the cracking zones, generating a localized attack.

Figure 9 shows the samples after the corrosion test on H₂SO₄. Figure 9a,b show that the distribution of attack is more homogeneous; however, the corrosion residuals are more concentrated in some zones. Figure 9c,d shows how dissolution begins in the cracking zones as a localized process, resulting in a uniform dissolution with the generation of an oxide layer. The formed oxide layer shows areas of cracking, suggesting that corrosion becomes uniform over time.

4. Discussion

Water droplets are retained due to the hierarchical structure of the coating. Designing such hierarchical features and modifying surface roughness are essential steps in developing an effective hydrophobic layer. In the case of superhydrophobic coatings, the presence of multiscale roughness enhances water repellency, durability, and self-cleaning properties. Morphological and structural analyses confirmed that the coating’s hierarchical architecture, formed by nano- and microscale particles, contributes to its roughness and superhydrophobic behavior [37,38].

The anodic reaction in aluminum involves the transformation of Al³⁺ into ions, as represented in the following equation.

$${Al}_{\left(s\right)}\to {Al}_{\left(aq\right)}^{3+}+{3e}^{-}$$

(3)

In NaCl composites, the chloride ion (Cl⁻) plays a key role in the electrochemical corrosion of the material. The following equation illustrates the reaction between Cl⁻ and aluminum. These reactions promote active oxidation; as oxygen diffuses through the surface, the resulting corrosion products become porous and fail to maintain their passivating properties. Consequently, localized attacks occur, and pitting develops in the ferrite regions, as Cl⁻ ions preferentially penetrate those zones [39,40]. Chloride ions hinder the formation of a stable oxide film on the metal surface by destabilizing the protective layer and creating weak spots that expose the substrate to localized corrosion, entering as interstitial ions.

$${{Al}^{3+}}_{\left(aq\right)}+{{3Cl}^{-}}_{\left(aq\right)}\to {AlCl}_3$$

(4)

Although aluminum spontaneously generates a passive layer, it often shows inhomogeneities. The presence of Cl⁻ further disrupts its stability [41]. According to different research, certain ions pose greater risks to specific alloys, with Cl⁻ being particularly aggressive. While H₂SO₄ may induce higher dissolution, chloride ions cause more long-term damage by penetrating and dissolving the passive film through diffusion. As a result, the oxide layer formed by NaCl in Al-7075 exhibits pitting corrosion due to the instability generated by Cl⁻ at the surface.

This susceptibility of aluminum to chloride ions reinforces the earlier statement that Cl⁻ governs the behavior of the passive film in NaCl electrolytes, preventing its uniform formation [41,42,43]. Additionally, hydroxide reactions further intensify degradation in various materials, as aluminum hydroxide is produced according to the following equation.

$${{Al}^{3+}}_{\left(aq\right)}+{{3OH}^{-}}_{\left(aq\right)}\to {Al\left(OH\right)}_{3\left(s\right)}$$

(5)

In contrast, the reaction of H₂SO₄ can lead to the formation of a stable oxide layer; however, due to its aggressiveness, the oxide may sometimes become unstable and dissolve in the acidic medium. Under such conditions, aluminum undergoes dissolution driven by the electrolyte’s aggressiveness, producing hydrogen gas and causing a shift in the electrolyte’s pH [44,45,46,47]. The corrosion behavior of Al-7075 with inhibitors in H₂SO₄ can present a uniform corrosion mechanism within the acidic environment. The surface of the specimen was almost entirely covered with corrosion, and the presence of consistent cracks and depressions confirmed the uniform nature of the process. Furthermore, immersion in H₂SO₄ leads to greater material degradation, reflected in lower hardness values compared to specimens exposed to NaCl solution [48].

Figure 10 shows the corrosion diagram when Al 7075 SiO₂ is exposed to the different electrolytes. Figure 10a shows the behavior when it is exposed to NaCl. The Cl⁻ ions attack in the cracking zones, with the reaction of the oxygen generating the corrosion residual presented in Equation (4). The Cl⁻ ions attack on the cracking zones, propitiating the dissolution of the superhydrophobic. The zone where the ions penetrate an oxide layer is generated; therefore, the corrosion type that is presented in the sample is localized. However, it is important to check that the results show a decrease in corrosion rate, with a magnitude of ×10⁻⁵ A/cm² for the coated sample and ×10⁻⁴ A/cm² for the uncoated sample, indicating that the inhibitor achieves its function in NaCl.

When a sample is exposed to H₂SO₄, the corrosion mechanism is similar. The corrosive ions attack the cracking areas. The difference is that the H₂SO₄ is more aggressive and dissolves SiO₂. After that, the oxide layer is generated. Therefore, Al7075 presented a higher corrosion resistance in H₂SO₄ when evaluated by EIS. The oxide layer created by exposure to this medium is more resistive. However, the results obtained by CPP of the Al 7075 SiO₂ showed a lower corrosion rate, due to the corrosion inhibitor delaying the corrosion. In EIS, the results are different because this technique evaluates the oxide layer generated, and the perturbation generated by EIS is lower than that generated by CPP. Therefore, the oxide layer generated by CPP can be dissolved more easily. The oxide layer generated in Al 7075 by H₂SO₄ is not unstable. Figure 10 shows how the oxide layer is non-homogeneous.

The results obtained by CPP in H₂SO₄ did not present a passivation zone. An anodic breach is occurring, indicating a current demand. However, the current demand decreases in comparison with the CPP when samples are exposed to NaCl, particularly when the activation zone is pure. Consequently, the anodic breach of samples in H₂SO₄ exhibits a behavior similar to pseudopassivation, resulting in the generation of an oxide layer on the surface.

The mechanism of attack in H₂SO₄ is based on the dissolution of the inhibitor. After coating dissolution, an oxide layer is created on the Al 7075 surface, as shown in Equation (5). The reaction in acid, which is natural in sulfur compounds, plays a significant role in the atmospheric corrosion of aluminum. Sulfate ions tend to incorporate into the corroded surface layers, and amorphous aluminum sulfate hydrate is commonly identified as the main corrosion product when aluminum is exposed to marine or industrial environments. Because this product is noncrystalline and metastable, equilibrium diagrams are not effective for determining the chemical mechanisms behind aluminum corrosion. The next equation shows the corrosion mechanism that occurs on the surface:

$${SO}_{2\left(g\right)}\to {SO}_{2\left(aq\right)}+H_{2}O\to H^{+}+{HSO}_3^{-}$$

(6)

$${HSO}_3^{-}+H_2{O}_2\to {HSO}_4^{2-}+H_{2}O$$

(7)

$${HSO}_3^{-}+O_3\to {HSO}_4^{-}+O_2$$

(8)

An additional oxidation is the result of metal ions supplied to the solution on the metal surface, as Equation (9) shows. After that, a production of sulfur VI is show in Equation (10).

$${HSO}_3^{-}+O_3\to {HSO}_4^{-}+O_2$$

(9)

$${HSO}_4^{-}\to H^{+}+{SO}_4^{2-}$$

(10)

Fundamental aluminum sulfates are produced through a sequence of stepwise reactions, which can be collectively represented as

$${x(Al}^{3+})+y\left({SO}_4^{2-}\right)+z\left({OH}^{-}\right)\to {Al}_x{\left({SO}_4\right)}_y{\left(OH\right)}_z$$

(11)

The composition of the resulting product can vary, depending in part on the amount of water present. Typically, the aluminum-to-sulfur ratio is approximately 2 [49].

The n values obtained by EIS are 0.68 for Al 7075, indicating that the surface is not homogeneous, and −0.70 for the second layer, indicating that the absorption process is occurring in the surface. Adsorption continues to be heterogeneous, and the same behavior is observed for Al 7075 SiO₂ in the second layer, where a heterogeneous adsorption process occurs on the surface due to an n value of 0.68. The first layer obtained an n value of 0.90, indicating that the superhydrophobic coating is well distributed; however, it is not perfect, and the cracking zones from Figure 1 caused the 0.90 value [50,51,52,53,54].

The value of R_ct is the value associated with the inhibitor resistance. The resistance of the coating in NaCl is 177 Ω·cm², and in H₂SO₄, it is 52 Ω·cm². This indicates a higher efficiency of coating in NaCl than in H₂SO₄. It is recommended to use this coating in NaCl. In H₂SO₄, the coating does not protect the material in the same way as in NaCl.

Various coatings can be applied to enhance the corrosion resistance of aluminum. Methods such as thermal oxidation, sol-gel processing, sputtering, electrodeposition, passivation, and anodizing are used to produce a protective oxide layer. However, passivation has the drawback of forming a thin oxide film with structural irregularities, making it vulnerable to localized corrosion. The sol-gel technique generates an oxide coating enriched with Al–OH. Plasma electrolytic oxidation (PEO) is another highly effective approach, though the need for specialized equipment and the challenges of treating large components increase production costs. Among these methods, anodization is a particularly suitable option for developing oxide layers. It produces more uniform coatings than passivation and is also more cost-effective than PEO since it can be applied to large parts [55,56,57,58,59]. Furthermore, spray application is a method that helps reduce costs and is easier than other methods.

It is important to mention that the effect of structural defects can generate problems in the adhesion of coatings and the generation of oxide layers. The heterogeneous surface can be associated with surface defects. Those defects generate a poor adhesion of the coating. In the case of oxide layer generation, the oxide created on the surface is not homogeneous or can be from contaminated imperfections [60,61]. This imperfection reduces the corrosion resistance of the material and coatings, making the coating susceptible to localized attacks on cracking zones; after this, the coating is dissolved.

The efficiency of the coating was 81% in NaCl. Rasitha et al. [62] obtained an efficiency of 99% when exposed to 0.1 M NaCl, aided by a self-cleaning process. Also, superhydrophobic coatings applied to anodized aluminum substrates exhibit a two-time constant response when analyzed through electrochemical impedance spectroscopy (EIS). R_ct denotes the charge transfer resistance at the aluminum–coating interface, Cdl represents the double-layer capacitance related to substrate/coating interactions, and Rs corresponds to the solution resistance. R_c is associated with the resistance of electrolyte penetration through pores in the coating or along ion-conductive paths formed within it, while Cc accounts for the capacitance of the surface coating. The R_c of the superhydrophobic film increases significantly when the anodized aluminum is further treated with an organic coating. Elevated R_ct values indicate an enhancement in the corrosion protection of the aluminum substrate [62,63,64].

5. Conclusions

• The coating of SiO₂ presented a hierarchical structure with a heterogeneous distribution, yet it was nearly homogeneous; however, it exhibited crack zones that served as corrosion concentrators.
• Samples coated with superhydrophobic coatings showed a reduction in corrosion rate obtained by CPP, indicating that the coating protects the material.
• The Al 7075 coated with SiO₂ presented localized corrosion when exposed to NaCl. Cl⁻ ions attacked the cracking zone. On the other hand, the H₂SO₄ coating presented a uniform corrosion due to the dissolution of the coating in this medium.
• The inhibitor efficiency in NaCl is 81%, indicating higher corrosion resistance. Also, the R_ct value of Al 7075 coated by SiO₂ exposed to H₂SO₄ is 70% lower than the R_ct in NaCl; therefore, this coating can be employed in marine atmospheres.
• The behavior of Al 7075 coated with SiO₂ presented an inductive behavior, indicating that adsorption phenomena are occurring on the surface. This is the natural behavior of aluminum, and the coating did not alter it.
• The imperfections generated by cracking zones help form corrosion concentrators.
• Localized corrosion is occurring on the coating, which initiates the dissolution process of the coating.