Coating Types on the Surface of Aluminum Alloy: Comparison
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Please note this is a comparison between Version 2 by Fanny Huang and Version 1 by Xiaoqing Du.

Aluminum alloy is widely used in many fields for its excellent performance. However, in practical application, aluminum alloy is easy to become corroded and be invalidated, owing to the influence of environmental factors. Some pollutants or corrosive media q5 its surface adhesion will further deepen the failure rate of aluminum alloy, resulting in a large amount of workforce waste, waste of material and financial resources, and energy consumption. So the anti-corrosion and anti-fouling treatment of aluminum alloy surfaces to expand their practical application range and reduce energy loss are of great significance.

  • aluminum alloy
  • superhydrophobic coating
  • anti-corrosion
  • anti-fouling

1. Introduction

Aluminum and its alloys are widely used in aviation, military, transportation, decoration, and other fields owing to their light weight, good thermal conductivity, electrical conductivity, flexibility, and recyclability [1,2][1][2]. At the same time, aluminum is rich in crustal reserves, ranking third in annual production and second only to steel [3,4][3][4]. However, a large number of existing studies and applications have found that, due to the influence of external environmental factors (such as high temperature, high humidity, or a marine environment containing high content of corrosive particles), aluminum and its alloys will undergo local corrosion phenomena such as pitting corrosion, stress corrosion cracking, intergranular corrosion, and layered corrosion during their service time [5], resulting in a shortened service life and limited application range. Furthermore, the adhesion of some corrosive media or pollutants to the surface of the aluminum and its alloys will also lead to their accelerated corrosion and affect their service performance, such as increasing the running resistance and weight of vehicles, increasing energy consumption, etc. [6]. Therefore, carrying out surface anti-corrosion and anti-fouling treatment of aluminum and its alloys is enormously significant.
In recent years, superhydrophobic coatings have attracted widespread attention because of their unique low surface energy characteristics and non-wettability to aqueous media. Surface wettability is one of the characteristics of solid surfaces, and the static water contact angle (WCA) is defined as the included angle between the liquid–air and the solid surface [7]. When the WCA > 90°, the solid surface exhibits hydrophobicity. At the same time, the water droplets are repelled by the solid surface and shrink into a spherical shape. Apart from WCA, the sliding angle (SA) is another criterion for describing the behavior of solid surfaces. The SA is defined as the inclination angle of a solid surface when water droplets first roll. Generally, the characteristics of superhydrophobic surfaces are WCA > 150° and SA < 10°. In addition, the wetting behavior of solid surfaces has been deeply studied. Three wetting models exist: Young’s, Wenzel, and Cassis–Baxter [8]. Young’s model describes droplets on an ideal smooth, solid surface. Wenzel’s model, rather, describes droplets on a rough solid surface and assumes that the droplets fully penetrate the groove. However, both of the above assumptions are uniform infiltration states. The droplet on the solid surface, explained by the Cassis–Baxter model, is a heterogeneous wetting state, because the air is trapped in a rough groove below the droplet, preventing it from penetrating the groove and forming an air cushion. The preparation of superhydrophobic coatings on the surface of aluminum alloys can not only effectively prevent direct contact between the corrosive medium and the matrix but also significantly shorten the residence time of pollutants or corrosive media on its surface, and inhibit the formation of continuous electrolyte film on the surface of the coating, thereby playing the dual role of anti-corrosion and anti-fouling (self-cleaning) on the aluminum alloy substrate [9,10,11][9][10][11].

2. Coating Types on the Surface of Aluminum Alloy for Anti-Corrosion and Anti-Fouling

2.1. Chemical Conversion Film Coatings

Preparing a protective film or coating on the surface of aluminum alloy can effectively improve its durability. The inorganic conversion coating is one of the protective coatings which has been studied for decades. Its preparation process is mature and has been widely used. The chemical conversion coating of aluminum alloy is a stable and sound adhesion of a thin layer formed by the chemical or electrochemical reaction between anions in solution and the aluminum or alumina substrate surface [12]. The following reaction equation can represent the typical reaction:
mAl + nAz− → AlmAn + nze−     
In the above reaction equation, Az− represents the anions with a valence state of -z in the medium. During recent decades, Chromate conversion coatings (CCC) have been widely used in aerospace due to their advantages, such as good corrosion resistance, simple preparation process, and low cost. When the conversion film is damaged, Cr (VI) in the film layer will transfer to the broken place to passivate the substrate, thereby effectively protecting the substrate material [13]. However, Cr (VI) is carcinogenic and environmentally unfriendly and has been restricted by many countries, so finding an alternative coating for CCC is urgent [14,15,16][14][15][16]. The trivalent chromium process (TCP) is one of the replacements. Kim et al. [17] put 3003 aluminum alloy in hexafluoro-zirconate solution containing trivalent chromium for 18 min to obtain a conversion coating. Then, immerse the above conversion coating in 3.5 wt.% NaCl solution for 24 h, or perform a 24 h simulated acid rain test, expressing good corrosion resistance. As Peltier et al. [14] mentioned, the conversion baths generally contain Cr3+ salts, fluoride (ZrF62−), and H2SO4 or NaOH. Therefore, Cr (III) may be oxidized to Cr (VI) during production.
Except for CCC, there is also a traditional conversion coating called phosphate conversion coating (PCC). The formation of PCC is mainly divided into two parts: the dissolution of Al substrate and surface alumina and the deposition of insoluble phosphate [18]. Huang et al. [19] prepared PCC on the surface of 2A12 Al alloy by chemical conversion. The corrosion current density of PCC is three orders of magnitude lower than that of the 2A12Al matrix after immersing the PCC in 3.5 wt.% NaCl solution for 30 min, and the corrosion inhibition efficiency of PCC can be as high as 99.91%. The highest oxidation state of transition metals can form cations in aqueous solutions, which can be reduced by electrochemical reactions to form insoluble oxides, similar to the formation of chromate conversion coatings [18]. Therefore, permanganate (MnO4−), molybdate (MoO42−), and vanadate (VO43−) conversion coatings are also trends in chemical conversion coatings. Janqour et al. [20] prepared a vanadium-based conversion coating by immersing 2024 aluminum alloy in sodium vanadate salt. They found that the conversion coating markedly enhanced the corrosion resistance of 2024 aluminum alloy. Even after soaking the vanadium-based conversion coating in 3.5 wt% NaCl solution for 15 min, the corrosion current density (Icorr) of vanadium-treated 2024 aluminum alloy was still lower by one order of magnitude than that of untreated, while the corrosion potential (Vcorr) and the charge transfer resistance (Rct) of the vanadium-treated were still more positive and larger than the untreated, respectively. Dmitry et al. [18] indicated that alumina’s catalytic and poor conductivity inhibits the reduction of V5+ adsorbed on the surface of aluminum alloys, playing a corrosion-inhibiting role.
In addition to the alternative mentioned above to hexavalent chromate conversion coatings, cerium ions show an inhibitory effect on the corrosion of aluminum alloys, so rare earth conversion coatings have also become an effective surface anti-corrosion coating for aluminum alloys. It is reported that rare earth metal ions, such as Ce, La, Pr, Nd, and Y, have a specific resistance to the local corrosion of aluminum alloy by forming an insoluble hydroxide/oxide layer [21]. Valdez et al. [22] immersed 6061-T6 aluminum alloy in cerium chloride solution to obtain a cerium-based conversion coating. Then, soak the coating in 3 wt.% NaCl solution for one hour; the coatings’ Icorr, being 1 × 10−7 A·cm−2, was almost two orders of magnitude lower than that of the bare aluminum alloy (5.1 × 10−6 A·cm−2). At the same time, the Rct of the cerium-based was much higher than that of the untreated. Although conversion coatings have the advantages of simple preparation, low cost, and large-scale application in industrial production, there is currently no aluminum alloy chromium-free conversion coating comparable to chromate conversion coatings since Cr (VI) was banned.
In addition, the film layer, which is composed of layered double hydroxides (LDH) on the metal substrate surface, exhibits an excellent physical barrier effect and interlayer anion exchange on the metal substrate (such as aluminum and its alloys) due to its unique layer structure and ionic composition [23], thereby blocking or trapping aggressive anions (such as Cl−), so that the aggressive ions cannot be in direct contact with the substrate metal to achieve an anti-corrosion effect. Moreover, due to the weak interaction between the anions and the interlaminates of LDH films, interlayer anions can also be exchanged by corrosion inhibitor ions to prepare LDH films containing corrosion inhibitor ions. Then, the modified LDH can release corrosion inhibitor ions when the surface film layer erodes, slowing the corrosion process of substrate metals [24]. He et al. [25] prepared a Zn-Al LDH-VOx film with vanadium ion intercalation on the surface of AA2024 aluminum alloy by combining electrodeposition and ion intercalation. The experiment found that the exposed AA2024 aluminum alloy suffered severe corrosion after being immersed in 3.5 wt.% NaCl solution for two hours. However, the aluminum alloy covered with Zn-Al LDH-VOx film did not have noticeable corrosion pits even after one month of immersion, which indicated that, compared with the traditional hydrothermal method, the LDH film prepared by electrodeposition is thicker, but it has better corrosion resistance. Besides, there are also studies [11] using a hydrothermal method and ion exchange method to obtain methionine-modified Al-Li-Met LDH film on the surface of 6063 aluminum alloy, which found that this film layer reduces the corrosion current density of aluminum alloy in 3.5 wt.% NaCl by one order of magnitude compared with blank aluminum alloy. However, it can also automatically restore its original morphology and structure after external mechanical scratches, showing particular self-healing performance and improving its corrosion protection durability.

2.2. Anodizing Film Coatings

In the natural environment, aluminum is easy to passivate with oxygen to form a raw alumina film due to its chemical activity, which can prevent further contact oxidation between the aluminum substrate and the atmosphere, thereby reducing the corrosion rate of the substrate. The stability of the naturally formed alumina oxide film is very high in the service environment, with a pH of 5–8. Still, it is not ideal in extreme environments (such as strong acids, alkali, and ultraviolet irradiation) [26]. Therefore, to improve the anti-corrosion stability of the alumina film in most environments, an anodic oxidation method has been adopted to fabricate the oxide film on the surface of aluminum and its alloys. Anodizing is an electrochemical process in which aluminum oxide grows on the aluminum anode plate through current action, continuously consuming the aluminum anode. Still, the part of the hole bottom connecting the substrate is closed [27]. Since most of the surface of the anodized alumina film is porous, the corrosive medium can readily enter the film through these pores, thus affecting the protective performance of the entire film layer. Given the problem of the film’s poor corrosion resistance due to the porous structure of the alumina film layer, there are two leading solutions. One is to change the surface structure of the film layer by changing the anodic oxidation parameters during the preparation process [28] to improve the film layer’s compactness. The other is to seal the resulting film layer after the end of anodizing [29] to reduce the penetration of the corrosive medium into the film layer and ultimately provide the corrosion resistance of the film layer. However, the current mainstream research direction is more inclined to the latter because adding sealing materials gives aluminum or aluminum alloys more diverse functions.
Khan et al. [30] anodized 2024 aluminum alloy, followed by pore sealing treatment using boiling distilled water. The results showed that the corrosion current density of the anodic oxide film on the surface of 2024 aluminum alloy is reduced by four orders of magnitude compared with the blank 2024 aluminum alloy in 3.5 wt.% NaCl solution, indicating an excellent anti-corrosion property of the anodic oxide film. Yu et al. [31] anodized 6061 aluminum alloy in a citric acid–sulfuric acid system and then sealed the hole with a potassium dichromate solution. It is found that, after soaking in 3.5 wt.% NaCl solution for nine days, the corrosion current density of the aluminum alloy samples treated by anodization and hole sealing decreased by two orders of magnitude compared to blank aluminum alloys. Also, it fell by one order of magnitude compared with the samples only undergoing anodization, indicating that post-sealing treatment can further improve the corrosion resistance of the film layer.

2.3. Organic Painting Coatings

At present, the anti-corrosion organic coatings for aluminum alloy mainly include alkyd resin coatings, phenolic resin coatings, epoxy resin coatings, polyurethane coatings, polyvinyl alcohol coatings, perchloroethylene resin coatings, furan resin coatings, organo-silane coatings, etc. [32,33,34,35][32][33][34][35]. There are five main anti-corrosive paths for organic coatings on aluminum alloys: [36,37][36][37]: (1) the organic coatings’ simple shielding effect on the substrate; (2) the coatings’ slow-release impact on the passivation of the substrate; (3) the enhanced wet adhesion between the organic film and the substrate; (4) the increased circuit resistance between the coating and the substrate; (5) adding metal powder (such as zinc [37]) to the paint plays a cathodic protection role at the expense of the anode. Ge et al. [38] sprayed epoxy resin primer containing aluminum tripolyphosphate (ATP) and polyurethane topcoat sequentially on the surface of 5083 aluminum alloy to form a double-layer organic coating. Its charge transfer resistance increased ten times compared with blank aluminum alloy after soaking in 3.5 wt.% NaCl solution for 15 days. It was found that there were no bubbles and evident rust around the scratches after exposing the scratched coating to a neutral salt spray environment for ten days, indicating that the organic coating could act as an effective barrier against the corrosion of aluminum alloys. Gad et al. [39] pointed out that, when the surface of 2198-T851 aluminum alloy coated with epoxy resin was sprayed with a layer of vinyl chloride (PVC) coating containing cerium ion further, after five weeks of exposure to a salt spray environment, the modified coatings’ charge transfer resistance increased by two orders of magnitude. The increased charge transfer resistance means that adding cerium ions provides an idea for improving the durability of the coating.
However, aluminum and its alloy products will not only be corroded by environmental media during actual use but also become fouled due to the adhesion of environmental pollutants, such as microorganisms, dust, or other contaminants. For example, ships serving in the ocean often slow their operation and increase energy consumption due to the attachment of marine organisms. As microorganisms grow, they will destroy the hull’s anti-corrosion coating, accelerating its corrosion. Thus, improving the anti-pollutant adhesion ability of aluminum and its alloy surface is also essential to enhance its protection ability and expand its application range. Currently, widely used self-polishing coatings can release toxic compounds (such as TBT [40] or copper oxide [41]) through the hydrolysis reaction of the resin to achieve the effect of killing microorganisms. But TBT has been restricted worldwide as early as 2008. At the same time, the long-term use of low-toxicity copper oxide also has potential environmental hazards, and the galvanic reaction of copper ions with aluminum or aluminum alloy substrates will affect the anti-corrosion performance [42]. If non-toxic compounds can replace the toxic compounds in self-polishing coatings, the problem of environmental hazards would disappear. Sha et al. [43] prepared a self-polishing anti-fouling coating polymer based on eugenol methacrylate, which solved the problem of microbial attachment by interpreting eugenol with antibacterial and anti-algae properties through phenolic ester-based water. In addition, except for the self-polishing coatings that achieve anti-fouling through chemical reactions, the stain-release anti-fouling films with low surface energy can also make the pollutants challenging to adhere to solid surfaces [44,45][44][45]. Zhao et al. [46] prepared a silicone anti-fouling coating containing a fluorinated side chain of natural ice chips, which was able to resist 95.6% of marine bacteria while reducing the coverage area of the biofilm by 86.9%, effectively preventing the adhesion of diatoms.
Finding environmentally friendly organic painting coatings has always been one of the research directions for organic painting coatings on aluminum alloy surfaces, and it is also necessary to address the mechanical durability and thermal stability of organic painting coatings in corrosive environments. Due to the complexity of organic painting coating failure, it is only discovered after the complete failure of the coating. Therefore, a new and effective failure criterion is necessary, which can detect and guide maintenance work in the early stages of coating failure. Based on the low surface energy characteristics of the fouling release coating, the bionic superhydrophobic coating is also an anti-fouling coating that has been studied more often and has a good application prospect. Moreover, the non-wettability and the unique micro-nano structure of the superhydrophobic layer also provide it with two simultaneous functions: anti-corrosion and anti-fouling.

2.4. Superhydrophobic Coatings

In general, a biomimetic superhydrophobic surface is defined when the static water contact angle (WCA) on the coating surface is greater than 150°, the sliding angle (SA) is less than 10° [7[7][8],8], and the Cassie–Baxter model is mainly used to explain the surface-wetting state of existing superhydrophobic coatings. Superhydrophobic coating surfaces with the above wetting properties are often prepared by constructing a graded rough structure and then modifying it with low surface energy substances. When a superhydrophobic surface with a graded rough structure is just immersed in the electrolyte, the air trapped in the hierarchical structure can temporarily leave it in an unstable Cassie–Baxter wetted state, thereby preventing direct contact between the metal matrix and the corrosive medium, reducing the actual contact area between the corrosive electrolyte and the substrate surface, thereby improving the corrosion resistance of the substrate. At the same time, the low surface energy of the superhydrophobic coating can reduce its adhesion to water droplets. When the coating is in contact with an aqueous solution or pollutants, the interaction force between the pollutants is much higher than the adhesion force between the contaminants and the coating. Therefore, the superhydrophobic coating can make it difficult for the polluting liquid–solid mixture to adhere to the surface. The low adhesion between the liquid–solid mixture and the coating surface can effectively shorten the maintenance time of the liquid–solid mix on the surface of the coating, inhibit the formation of a continuous electrolyte film on the surface of the coating, and finally enhance the anti-corrosion and anti-fouling effect of the coating [47]. Additionally, owing to the superhydrophobic coating having a low SA (less than 10°), the liquid contaminants are easy to roll off from the surface and carry away the other solid pollutants during the rolling process to achieve the effect of anti-fouling enhancement. Therefore, bionic superhydrophobic coatings’ inherent rough structure and low surface energy can give aluminum or aluminum alloy surfaces good anti-corrosion and anti-fouling (self-cleaning) performance.
Cao et al. [48] prepared layered double hydroxide (LDH) on 5052 aluminum alloy by in situ growth and then impregnated it into stearic acid (STA) to obtain STA–LDH coating. The WCA of the coating obtained was 151°, and the SA was less than 5°. After immersing the sample covered with STA–LDH in 3.5 wt.% NaCl solution for 20 min, the charge transfer resistance increased from 5.105 × 104 Ω·cm2 (blank aluminum alloy sample) to 8.46 × 106 Ω·cm2. The corrosion current density also decreased by about two orders of magnitude. The above results indicate that the STA–LDH superhydrophobic coating has an excellent anti-corrosion effect on 5052 aluminum alloy. Li et al. [49] obtained a coating with a WCA of 162° by chemical etching and impregnation modification of 1H, 1H, 2H, 2H-perfluorodialkyltriethoxysilane. The obtained coating’s corrosion current and potential were approximately the same as the coating without immersion after immersing the coating in 3.5 wt.% NaCl solution for 72 h. It is not difficult to see that the above studies all adopted a “two-step” method to prepare superhydrophobic coatings. Therefore, to simplify the preparation process of superhydrophobic coatings, the study began to explore the use of a “one-step” method to prepare superhydrophobic coatings. Zhang et al. [50] used perfluorooctanoic acid (PFOA) as raw material to obtain a superhydrophobic coating on the surface of 5083 aluminum alloy by a one-step hydrothermal method, and its WCA could reach 167.2 ± 2°. The superhydrophobic modified 5083 aluminum alloy was soaked in a simulated seawater solution, and it was found that the corrosion inhibition efficiency reached 94.45%. At the same time, simple anti-fouling and self-cleaning tests also proved that the coating has water repellency and low surface adhesion.
Rasitha et al. [51] prepared a superhydrophobic coating composed of hexamethyl-disilazane-modified silica and (3-glycylyoxy propyl) trimethoxy-silane (GPTMS) on the surface of pure aluminum by a dipping and pulling method. The study shows that the WCA of the superhydrophobic coating can be as high as 170°, and the SA is less than 1°. The modified aluminum sample’s corrosion current density is smaller than the blank pure aluminum sample in 3.5 wt.% NaCl aqueous solution and the corrosion inhibition efficiency is as high as 99%, effectively realizing the aluminum’s corrosion protection. Eventually, Table 1 summarizes the different types of coatings on aluminum alloy surfaces and their characteristics, advantages, and drawbacks.
Table 1. Different types of coatings and their characteristics, advantages, and drawbacks.
The types of Coatings Characteristics Advantages Drawbacks References
Chemical Conversion Film Coatings A few micrometers in thickness

As a connecting layer between the topcoat and the aluminum alloy substrate, it increases the adhesion of the coating.		 Low cost

Simple production process

Wear resistant

Good corrosion resistance

Suitable for large-scale industrialization		 Cr (VI) is poisonous and carcinogenic.

Phosphorus-containing wastewater could cause eutrophication of water bodies.		 [11,12,13,14,15,16,17,18,19,20,[21,22,12][23,13][24,25][11]14][15][16][17][18][19][20][21][22][23][24][25]
Anodizing Film Coatings Thickness in micrometers

Composed of alumina in the shape of nano-channels

Post-treatment for sealing holes or use with coatings.		 Good adhesion on the substrate

Suitable for large-scale industrialization		 Once the coating is damaged, the substrate is corroded [26,27,28,29,30,31][26][27][28][29][30][31]
Organic Painting Coatings Consists of three layers, with the layer in contact with the aluminum alloy being a priming coat, the top layer being a topcoat, and the intermediate coat connecting the priming coat and topcoat.

Suitable for substrates of any size and shape.		 Good weather resistance

Anti-fouling and preventing adhesion of marine organisms

Suitable for large-scale industrialization		 It may degrade in aqueous solution. Environmentally unfriendly

Poor adhesion on the substrate		 [32,[32]33,[33]34,35,36,37,38,39,[34]40,[35]41,[42,43,36][37][38][39][4044,]45,46][41][42][43][44][45][46]
Superhydrophobic coatings Non-infiltration.

Synergistic effect of micro-nano graded rough structure and low surface energy material modification.		 Good anti-corrosion and anti-fouling

Self-healing		 Poor chemical stability

Poor durability		 [7,8,[47,8]48,49,50,51][7][47][48][49][50][51]

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