Introduction

Metals are widely used in many applications including transportation, civil engineering, and oil and gas sectors due to their impressive mechanical and thermal properties. Nevertheless, many metals and their alloys have certain drawbacks (i.e., poor resistance to corrosion and wear), limiting their area of possible applications.1,2,3 The main cause of their degradation is their continuous use in aggressive damp environments. Although corrosion is unavoidable, its effect can be minimized by using protective coatings exhibiting superhydrophobicity or strong water repellency.4,5 Polymeric/silane-based coatings, with stable Si–O–Si backbone, are one of the most prominent examples of such materials widely used for corrosion protection as they display low surface energy and superior resistance to thermal and chemical attack. Qing et al.6 developed an approach to enhance the corrosion resistance of stainless steel by using superhydrophobic (SHP) coatings composed of nano titanium dioxide (TiO2) and polydimethylsiloxane (PDMS). The application of these coatings resulted in increased corrosion potential of the steel compared to its untreated state, thereby providing an increased level of protection against corrosion. The application of a SHP coating can delay the oxidation process as it prevents the metal surface from coming into contact with aggressive electrolytes during exposure to humid and salty environments. Hence, SHP coatings provide corrosion protection by creating a barrier layer, delaying the diffusion of corrosive ions and dissolution of metal ions from the substrate.

SHP coating materials, characterized by contact angle (CA) of \(>{150}^{o}\) and sliding angle (SA)/tilt angle (TA) of \(<{10}^{o}\), have attracted significant interest because of their utility in applications requiring corrosion resistance, de-icing, oil/water separation, antireflection, smart membrane, and ship hull protection.7 SHP surfaces in contact with oil/water mixtures can retain an interface air layer and exhibit extreme water-repellency and oftentimes superoleophilicity.8 This phenomenon is based on the combined low surface energy and hierarchical surface features, leading to the formation of a trapped air layer that minimizes physical contact between the corrosive medium and coated-metal substrate. This air barrier adds a protection layer, further preventing electrolytes from coming into contact with the coated substrate.9 Despite the alluring properties of polymeric/silane-based organic coatings, their performance is hindered by their wear resistance. However, this drawback, as well as the corrosion protection properties, can be significantly improved by incorporating dispersed nanoparticles such as titanium oxide (TiO2),6,10 zirconia (ZrO2),11,12 silica (SiO2),13,14 carbon nanotubes (CNT),15,16 aluminum oxide (Al2O3),17,18 zinc oxide (ZnO),19,20 and others.21,22,23,24,25,26,27,28

Several methods have been employed to surface treat metals with SHP polymer/silane organic based materials. For instance, sol-gel dip coating,13,29,30,31,32,33 plasma treatment,9,34,35 chemical vapor deposition (CVD),36,37,38 spin coating,39 layer-by-layer deposition,40,41,42,43,44 electrospinning and electro spray,45,46,47 and micro-arc oxidation48,49,50 have been employed. Although plasma treatment methods have been widely used in producing SHP coatings on metallic substrates, the performance of the resulting coatings is limited by their nonuniform thickness and poor substrate adhesion. In addition, plasma-sprayed coatings suffer from poor mechanical properties and film nonuniformity, especially on complex surface geometries.9,51 The advantages and disadvantages of various coating deposition methods for SHP coatings are summarized in Table 1. A newer method, micro-arc oxidation (MAO) treatment, hereafter referred to as plasma electrolytic oxidation (PEO), is rapidly emerging as an effective process for improving adhesion and overcoming the weakness of plasma spraying. This treatment is surface-deposition-based, which produces dense ceramic oxide coatings on metal and alloy surfaces. PEO is relatively low cost and possesses the ability to produce uniform coatings over complex geometrical surfaces.52,53 Due to its facile control of morphology, rapidity, and less complex setup, PEO treatment has the potential to deliver coatings with unique mechanical properties, adhesion strength, and corrosion resistance. In addition, treating the PEO layer with a low surface energy coating or infusing it with additional ceramic particles further provides the substrate with lower wettability and improved barrier properties.48 While numerous studies have delved into this subject, including simple anodization and plasma-based techniques,54 the mechanisms governing the inclusion of these materials into PEO porous layers have not been intensively discussed. In addition, a dedicated review focusing solely on the incorporation of low surface energy polymers (e.g., silicones, fluoropolymers, and the like), silane-based organic compounds, and/or ceramic particles into PEO-fabricated layers have not been reported. Thus, this review exclusively centers on the preparation of SHP coatings from these precursors with PEO as the initial surface treatment method and addresses the constraints and challenges of designing these coatings for scale-up and industrial applications.

Table 1 Summary of surface treatment methods for depositing SHP-based coatings
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This review is divided into several sections: key processing parameters for fabricating SHP coatings by PEO; PEO treatment methods for SHP coatings, including the advantages and limitations; various strategies for designing PEO coatings; systemic mechanism of designing PEO–SHP coatings by deposition from dispersions; and constraints and critical issues related to the fabrication of PEO-assisted SHP coatings and suggested directions for future research. This review, however, does not cover discussions on low surface energy materials and wettability due to the availability of comprehensive review articles covering fundamentals and theories of SHP surfaces.55,56,57,58 Various aspects of the theory and mechanisms defined to explain the basic aspects of wetting can be found in many informative reviews.59,60,61,62

Superhydrophobic corrosion-resistant coatings requirements

The design of SHP corrosion-resistant surfaces necessitates adherence to specific minimum criteria governed by numerous factors.7 The key quality prerequisites include surface chemistry and roughness, porosity, material surface free energy, and surface topology, all of which collectively shape the resulting properties of a coated product.24 Surface roughness and surface free energy are of tantamount importance in the design of superwetting surfaces as obtaining the highest CA and lowest TA possible requires the combination of these two characteristics. In addition, to obtain anti-corrosive properties within acidic or basic environments, a SHP surface should ideally possess porosity, crystallinity, substrate adhesion, cohesion strength, and phase stability. A hierarchical structural feature promotes water-repellency, which is further amplified by the surface roughness. Various porous network structures can also yield varying adhesion outcomes for SHP coatings, and currently there is no unanimous agreement on the degree of pore size to achieve adherent SHP coatings. In general, the characteristics of super-nonwettable coatings are dependent upon many factors. The requisites for SHP coatings providing long-term effectiveness are outlined in Table 2.

Table 2 Key requirements in designing SHP-based coatings66
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Techniques for the preparation of SHP-based coatings

Numerous studies have focused on designing coatings that resist corrosion over extended periods.8,52,72 The deterioration of metallic materials represents a primary challenge, imposing substantial economic burdens on various industries. Knowing that metal corrosion is an inevitable process, one effective mitigation approach is the application of organic coatings.65,73,74 These materials preserve the mechanical properties of metals, thereby extending their practical applications, while reducing maintenance costs. Surface treatment methods are also employed to bolster the mechanical resilience of metals when exposed to harsh environments.

The preparation of SHP surfaces falls into two main categories: one involves designing rougher surfaces, and the other entails modifying existing rough surfaces with hydrophobic or low surface energy materials. These treatments substantially influence the surface protective characteristics resulting in enhanced corrosion resistance for the underlying metal. Numerous modification techniques encompassing physical, wet-chemical, and hybrid approaches are currently in development for SHP surface preparation (as summarized in Table 1). This section provides a concise overview of progress in the PEO method for preparing SHP coatings on metallic surfaces, along with various strategies used to improve their resistance to corrosion. Notably, coatings applied on PEO-generated surfaces exhibit superior protective properties due to the strong substrate adhesion offered by PEO-layers. The complex topography and controlled surface composition make them an ideal platform for fabricating SHP composite coatings.

Surface treatment methods for SHP-based coatings

PEO SHP-based coatings

PEO is a technique used to create micro/nanostructural layers on metal surfaces. It offers the advantage of easy adjustability of metal surface topography by modifying the electrolyte composition and operational parameters. PEO is particularly well-suited for generating robust, adherent layers of around 250 μm thick on bare metallic components. In PEO processing, the metal is melted and ejected from the substrate, while oxygen is released from the electrolytes. Subsequently, oxidation occurs due to the interaction between the metal and oxygen, leading to micro-pore and crack formation on the metal substrate. The resulting surface topography, characterized by porosity and open voids, can serve as transport conduits for the infiltration of low surface energy coating materials. PEO facilitates the production of coatings with enhanced corrosion, wear, and thermal resistance due to the formation of durable protective layers on the metallic substrates. Moreover, PEO process is known for its simplicity, environmental friendliness, and capability to create uniform coatings on complex geometries.75,76

PEO-assisted fabrication SHP-based coatings

Surface defects that arise in PEO-coatings on metals prove advantageous in the formation of SHP coating such that the pores and cracks generated during PEO processing serve as the matrix for accommodating low surface energy polymer materials. The outer PEO porous layer greatly facilitates the infiltration of organic or polymeric particles, aiding in the formation of composite layers and mechanical interlocking sites. This, in turn, results in coatings with high protective properties and adhesion compared to the pristine PEO-layer. The density, distribution, and interconnectivity of pores on the PEO coating surfaces increase the effective surface area. This promotes the adsorption of particle additives or polymer from the solution onto the surface, leading to pore sealing, formation of composite layers through chemical interaction, and enhanced corrosion resistance. However, the high temperature of the PEO-plasma micro-discharge ranging from 10,000 to 20,000 K during PEO processing,75 makes in situ incorporation of these additives into PEO porous layers impractical. This is because it can alter the mechanical characteristics or cause thermal degradation of the polymeric material during PEO operation. To address this, a two-step approach is used to prepare SHP composite coatings via the PEO process for water repellency and corrosion protection. In the first phase, PEO is employed to create porous oxide layers that serve as anchors for the incorporation of organic polymers or polymer nanocomposites in the succeeding stage. In the second phase, the treated layer is exposed to low surface energy materials via immersion or suspension in a polymeric solution. The key to producing highly effective SHP coatings via the PEO route is to attain a rougher structure on the existing blank PEO-layer and subsequently deposit low surface energy polymers onto the hierarchically roughened surfaces. The morphology prior to PEO treatment, micro-nanostructure achieved through PEO, pore diameter, and subsequent treatment with low surface energy materials all play vital roles in achieving SHP surfaces.

Strategies for designing corrosion resistant PEO–SHP based coatings

Choice of low surface energy polymeric/organic materials

The choice of polymeric/organic materials and their concentration in the solution play a critical role in influencing the morphology, wettability, and corrosion resistance of PEO-coatings.77 Commonly used low surface energy polymers and organics for secondary modification of PEO-porous layers on metallic substrates include triethoxysilane,61 3,3,3-trifluoropropyl-trimethoxysilane,78 hexamethyldisilazane,79 1H,1H,2H,2H-perfluordecyl-phosphate,80 1H,1H,2H,2H-perfluorooctyl-trichlorosilane, tetrafluoroethylene,81 trichlorooctadecylsilane,82 polymethyl methacrylate (PMMA),83 polytetrafluoro ethylene (PTFE),84 poly(L-lactic acid) (PLLA),85 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFOTES),86 perfluorocaprylic acid (PA),87 and perfluoroalkylethyl-triethoxysilane.88 Most of the ongoing developments on PEO–SHP coatings for metals are focused on corrosion protection evaluation.

Yang et al.86 developed a porous-oxide coating on Al 6061 by PEO using an electrolyte composed of Na2SiO3 and KOH, operated at 380 V, 60 Hz, for 0.5–30 min and at 22.12 A dm−2. Subsequent 5-min dip coating in 1H, 1H, 2H, 2H-pefluorodecyltrietoxysilane (PFOTES) was performed. The pristine PEO surface displayed a morphology characterized by craters, pores, and nodular structures with CA of 9°. In contrast, the PFOTES-modified surface exhibited higher hydrophobicity with CA of 150°. The lower CA observed on the bare PEO layer was attributed to the pores and cracks on the surface. Potentiodynamic polarization (PDP) test revealed that PEO-PFOTES coated layers showed higher corrosion resistance compared to the bare PEO-oxide layer. Meanwhile, Wang et al.89 examined the influence of adding PTFE on the tribological properties of PEO-Al2O3 coatings. A base electrolyte containing NaAlO2 was used to create PEO coatings at 4  dm−2 for 60 min, frequency of 50 Hz, and electrolyte temperature below 35 °C. Interestingly, the addition of 100–170 nm PTFE particles did not block the PEO coating pores; instead, it increased the thickness of the PEO porous ceramic layer and formed a compact film on the PEO oxide coating. The PEO coatings’ tribological properties were determined on a ball-on-disc tester at dry sliding conditions. Results showed a substantial improvement in the tribological properties compared to PEO coatings without PTFE. However, no information was provided on the anti-corrosion properties of the obtained coatings.

Similarly, Gnedenkov et al.84 employed a two-step approach using silicate-fluoride electrolytes containing Na2SiO3 and NaF for the formation of PEO micro-porous rough layers. Following this, the PEO-layers were exposed to 15 wt.% superdispersed PTFE powder suspension in isopropanol for 10–15 s. The corrosion resistance was determined by PDP and impedance measurements in a 3 wt.% NaCl test solution. The presence of PTFE significantly enhanced the samples’ corrosion resistance. The Icorr of the PTFE-coated surface (5.4 × 10−11 A cm−2) decreased by more than three magnitude orders compared to the PEO coating (2.5 × 10−7 A cm−2) alone, indicating the effective pore and defect blocking in the PEO-coating and uniform distribution of a superdispersed polymer layer on the sample surface. Results from salt spray, a qualitative method used to simulate accelerated corrosive attacks on coated substrates, further supported the data obtained from PDP and EIS studies. The samples were exposed in a 5 wt.% NaCl solution for a week. Figures 1a and 1b revealed that the untreated blank substrate and PEO-layer exhibited severe pitting and extensive corrosion products, attributed to chloride ion penetration through defects. Conversely, Fig. 1c and 1d depicted PEO surfaces sealed with superdispersed PTFE showing no corrosion destruction zones on the surface, indicating the durability of the PEO-sealed coating.84 Scanning electron micrographs of the hybrid coatings with various polymer concentrations and different types of fluoropolymers, as depicted in Figs. 1e–1k, were also acquired to corroborate the results obtained from corrosion testing.90

Fig. 1
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Adapted from 91

Photos of MA8 alloy and composite layers before and after 168-h salt spray test (a) PEO-film, (b, c) composite films, (d) localized corroded area of the composite film. 84 Morphology of (e) bare PEO-film and composite films with different application of SPTFE (f) single (f) double, and (h) triple.90 Morphology of PEO-film (i) and composite films produced by telomeric solution and (j) SPTFE suspension (k).

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Figures 1e and 1f indicate that both the pristine PEO and PTFE-treated layers did not yield a uniform coating. However, increasing the concentration of PTFE particles effectively filled the pores within the PEO-ceramic layer as depicted in Fig. 1g and 1h. Tetrafluoroethylene (TFE) telomer belongs to another class of fluoropolymer that behaves like superdispersed PTFE. Although PEO-TFE coatings are less developed, they can be made more uniform by incorporating additional chains of strongly bonded polymeric TFE particles (i.e., PTFE).89,92 In Fig. 1j and 1k, the uniformity of PEO-coated surfaces with TFE and superdispersed PTFE had improved in comparison to the bare PEO-ceramic oxide layer.91 Both TFE and PTFE are composed of the same fluoro-molecules, but due to differences in the atom arrangement in TFE telomers, a partial destruction of organo-fluorine compounds led to subsequent aggregation and multimodal surface roughness in PEO-TFE coatings.93 Interestingly, coatings produced by PEO via dip coating in a superdispersed PTFE suspension were characterized by lower CA values of 140 ± 2°,69 coupled with enhanced corrosion and wear resistance. On the other hand, composite layers obtained through PEO with subsequent treatment in TFE telomer solution demonstrated superior SHP properties with CA = 171 ± 2°, thereby improving the properties of the bare PEO-ceramic layer.90 Comparatively, coatings produced by PEO, followed by spray-coating surface treatment with superdispersed PTFE are more hydrophobic than those produced by dip-coating.91,92 The higher CA observed in the spray PEO-oxide-TFE coating compared to the PEO-oxide-PTFE coating was likely due to a more developed surface and the processing method employed.

Nadaraia et al.81 introduced a composite coating method involving both PTFE and TFE on a PEO-porous layer, employing a combination of dip coating and spray method. A silicate-fluoride electrolyte composed of Na2SiO3 and NaF was used, at 40–240 V, 300 Hz frequency, and 12 °C for 200 s. Following the PEO treatment, the ceramic layer was further processed by immersion in an alcohol superdispersed PTFE suspension. To produce the TFE layer, a telomer solution in acetone was sprayed over the PEO-PTFE composite layer. The resulting coating on a Mg-Mn-Ce substrate exhibited a higher CA of ~ 173°. These SHP composite coatings significantly increased the corrosion resistance, effectively protecting the underlying alloy from harsh environments. The protection performance of these coatings was also examined in a salt spray chamber subjected to 5 wt.% NaCl at 35 °C for 40 d. Figure 2 shows the optical micrographs taken before and after exposure to salt spray, while Fig. 3 displays the SEM images of the coatings after a 30 d exposure. The morphologies displayed notable differences, particularly after 30 and 40 d of exposure. Prior to the salt spray test, the layer appeared featureless as shown in Fig. 2. However, after 30 and 40 d of testing, pitting corrosion became noticeable in the optical images of the coated layers. The layers exhibited severe cracking with enlarged pore sizes due to the formation of amorphous compounds and corrosion process (Fig. 3). In general, the incorporation of TFE polymer resulted in higher hydrophobicity and corrosion resistance.94

Fig. 2
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Adapted from 81

Salt spray test images of the modified composite films before and after exposure for different number of days.

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Fig. 3
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Adapted from 81

SEM morphologies of cross-sectional (a, c) and modified composite coating surface (b, d) for 30 (a, b) and 40 days (c, d) of salt spray test.

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In addition, the CA of PEO-PTFE/TFE was reasonably higher than those of PEO-PTFE and PEO-TFE coatings. This result can be attributed to its short oligomer chain length, enabling the creation of a more developed hierarchical structure on the surface. This shows that the challenge of achieving a high CA can be effectively addressed with TFE. Apart from the atomic arrangement difference, the choice of solvent for applying the fluoropolymer plays a significant role in the degree of polymer deposition.77,95 Preparing TFE telomers in acetone yielded the highest CA when compared to other solvents.95

Several recent studies have explored SHP PEO coatings on various metals including Mg,93 Al,97 Ti,98 and carbon steel.99 In these studies, PEO-oxide films, PEO-hydrophobic layers, and PEO–SHP surfaces produced were compared using silicate-fluoride solutions, where PEO–SHP surfaces exhibited superior corrosion resistance in NaCl solution. This result can be attributed to the strong bonding between the hydrophobic layer molecules and PEO-coating components.96 In addition, Zeng et al.85 reported the use of PEO/PLLA composite coating in enhancing the corrosion resistance of Mg alloy with strong substrate adhesion. However, the PLLA coating later detached from the PEO coating. This degradation primarily occurred at the interface between the PEO coating and substrate. The accumulation of corrosion products and hydrogen bubble formation contributed to the swelling and blistering of the PEO/PLLA coating.

In a recent study conducted on titanium alloy, PEO-oxide films were formed in a phosphate suspension, followed by immersion in a 2% solution of a fluoroalkyl-based silane. Results showed that the corrosion potential in NaCl test solution for the uncoated, PEO coated, PEO-hydrophobic, and PEO–SHP coated alloys were \(-\) 0.430, 0.400, 0.068 and 0.6168 V, respectively. The significant shift in corrosion potential for the PEO-coated samples was attributed to the sealing of PEO-porous layers with a hydrophobic agent. This shift also indicated reduced corrosion susceptibility. Moreover, the low-frequency impedance values of the PEO-hydrophobic and PEO–SHP coated layers were \(\sim\) 10 and 256 times higher, respectively, than that of the PEO-oxide layer. Meanwhile, Mashtalyar et al. examined PEO-oxide-containing fluoroparaffin films and the measured impedance modulus and polarization resistance of the composite films increased by two magnitude orders compared to the bare PEO-layer.100 The fluoroparaffin embedded in the PEO-porous oxide layer conferred hydrophobic properties to the surface, resulting in CA ranging from 122 to 137°, depending on the fluoroparaffin type. Similar results were reported for Al alloys.97 Mohammed et al.83 also examined the influence of PMMA sealing layer on the corrosion protection properties of PEO–TiO2 coatings in a fluoride-containing solution. The PMMA layer effectively sealed the micropores and microcracks in the oxide coatings, preventing the diffusion of corrosive electrolytes through the coating.

Chen et al.98 developed a fractal-patterned coating on a Ti–6.5Al–2Zr–1Mo–1V alloy using the PEO method. The PEO coatings were prepared using a solution of sodium silicate, sodium phosphate, and sodium aluminate at 400 V, 600 Hz, and 50 °C. To achieve self-cleaning and corrosion resistance, nanorod structures were grown on micro-patterned PEO layers. This was done by immersing the coated samples in 3 wt.% NaOH at 40 °C for 2 h, followed by immersion in 1, 1, 2, 2-tetrahydroperfluorodecyltrimethoxysilane for 100 min and baking at 140 °C for 40 min. The resulting TiO2 ceramic layer exhibited uniformly distributed micro-pores in the range of 3.2 to 5.5 μm as shown in Fig. 4. The PEO–TiO2 incorporated a hydrophobic agent, revealing a dense inner and loose outer surface layers with a thickness of ~ 15 µm (Fig. 4c). The SHP coating displayed CA of 172.05° and SA of 2.1°, while the corrosion resistance test results in NaCl solution revealed good open circuit potential (OCP) stability for the first 30 min of immersion as depicted in Fig. 5a. Notably, both the polarization potential of the PEO–TiO2 ceramic (M) and PEO–TiO2-superhydrophobic (MKF) coatings were significantly higher than that of the bare alloy, indicating better shielding properties of the obtained layers.

Fig. 4
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Adapted from 98

Surface and cross-section SEM images (a, b), PEO oxide (M), (d, e, f) multi-functional (MKF), and (c) SHP cross-sectional (MKF) films.

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Fig. 5
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Adapted from 98

OCP (a), Nyquist (b), and Bode plots (c, d) of the bare Ti alloy, PEO-coated layers and PEO–TiO2–SHP coating (MKF) in NaCl solution.

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The Nyquist plots, as shown in Fig. 5b, provide insights into the electrochemical behavior of the bare substrate, PEO-oxide layer (M), and PEO-oxide SHP coating. For the titanium alloy substrate, the plot exhibited an oval shape arc, indicating the formation of oxide layers as a result of immersion in NaCl solution. When comparing PEO–TiO2 ceramic and PEO–TiO2 SHP coatings with the PEO-ceramic coating, a notable difference was observed. These coatings displayed three capacitive loops with the PEO–TiO2 SHP coating showing a significantly larger loop (Figs. 5c and 5d), suggesting that fewer electrons passed through the barrier. The high-frequency region of PEO–TiO2 superhydrophobic (MKF) showed that it had the best corrosion resistance performance.

Over the past few years, the focus of PEO coatings has primarily centered on valve metals, such as Ti, Al, Mg, and their alloys due to the challenges associated with applying PEO processes to ferrous alloys. The kinetics of oxidizing non-valve metals (e.g., steel and its alloys) by PEO processing is deemed unfavorable as the driving force for oxidizing ferrous materials is lower than that needed for converting hydrogen to water during PEO processing. Consequently, fabricating a corrosion-resistant surface on steel and its alloys through PEO processing has become challenging. However, a noteworthy breakthrough was achieved by Boinovich et al.48, who demonstrated the possibility of preparing a SHP surface using a fluoroalkyl-based silane as hydrophobic agent on bare low carbon steel. The coating films were applied either on PEO or magnetite coatings, leveraging low surface energy and high surface roughness. The key to success lies in promoting the growth of multilayers, which in turn induced the formation of a dense barrier with a textured surface. Subsequently, after PEO treatment with a fluorinated polymer incorporating silica nanoparticles on the oxide film, the resultant surfaces exhibited superhydrophobicity with CA as high as 160°. ElS and PDP were used to evaluate the electrochemical properties of the multilayer coatings, and results revealed that the bilayer coating composed of hydrophobic monolayer on PEO sublayer showed superior protection against corrosion in a 0.5 M NaCl solution. This study confirms that the inclusion of silica within the PEO-coatings significantly improved their corrosion resistance. In general, sealing PEO coatings with silica gel, not only improves their uniformity and morphology, but also enhances overall adhesion strength.14 The research team led by Gnedenkov employed the PEO method to create SHP films on three different metallic substrates, commercial purity titanium (CP–Ti), low-carbon steel, and Mg alloy (MA8), using a fluoroalkyl-based silane as hydrophobic agent.99 The peculiarities of the corrosion process on these three metal samples protected by oxide film, PEO, and SHP nanocomposite coatings were investigated. Results revealed that CP-Ti coatings displayed the highest impedance modulus, Zf = 2.1 × 108 ohm cm2 when compared to the SHP coatings on steel (\({8.3\times 10}^{6}\) ohm cm2) and MA8 alloy (2.5 × 107 ohm cm2). Note that the electrolyte composition used in forming the PEO oxide film was different for each metal, which could explain the observed differences in the performance of the coated layers. There was no significant difference in the wetting characteristics of the fabricated SHP coatings on the three different metals.

While organic or polymer-based coatings typically demonstrate superior anti-corrosion and barrier performance, they can become compromised when they absorb water during prolonged exposure to corrosive environments. Moreover, unsealed micropores and microcracks in organic coatings often result in a reduced coating protection.11,101 Hybrid coatings combining polymer/silane with metal oxides have emerged as a promising solution as these composites harness the advantages of polymer-mixed metal oxides, offering effective corrosion protection.

Inclusion of ceramic particles

Nano ceramic oxide powders including Fe3O4, SiO2, TiO2, antimony oxide (ATO), and Al2O3 have been used as inclusions to further improve the hydrophobicity and corrosion protection of PEO coatings.4,14,102,103 This approach involves blending either polymeric or silane organic solution with metal oxides prior to top-coating onto a PEO layer. Note that the inclusion of oxide particles in PEO coatings helps seal pores and cracks, further preventing the intrusion of aggressive electrolytes. Qiu et al.4 added Fe3O4 particles to PEO coatings on Mg alloy using a specific formulation. Hydrophobic modification was achieved by first dispersing Fe3O4 in hexadecyltrimethoxylsilane (HDMS) solution, followed by curing. The resulting HDMS-coated Fe3O4 was then blended with dopamine hydrochloride and dodecanethiol, and coated onto a pre-existing PEO layer. The resulting SHP coatings displayed higher CA values compared to the bare PEO coating, as shown in Fig. 6.

Fig. 6
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Adapted from 4

SEM morphologies with corresponding water CAs: (a) PEO, (b) PEO-Fe2O3 and (c) PEO-induced.

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As shown in Fig. 6c, the HDMS-coated Fe3O4 nanoparticles were well-connected, forming an array structure. This result demonstrates the role of the dopamine moiety in facilitating the nanoparticle adhesion and formation of SHP coatings. The degrees of corrosion resistance of both the PEO and SHP PEO-Fe2O3 coatings were examined via PDP studies in NaCl test solution. The Ecorr and I corr values of PEO coatings prepared without the addition of Fe2O3 particles were \(-1.35\) V and 67.18 nA cm−2 respectively, as shown in Fig. 7. Incorporating Fe2O3 particles into the PEO coatings led to a slight positive shift in Ecorr from − 1.35 to − 1.33 V and decrease in Icorr from 67.18 to 14.39 nA cm−2. This result indicates that the Fe2O3 particle addition effectively reduced the corrosion rate of the Mg alloy.

Fig. 7
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Adapted from 4

PDP curves of the PEO and PEO/Fe3O4 composite coatings on Mg alloy substrates in NaCl solution.

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Li et al.103 employed a multifunctional approach to deposit ATO and methyltrimethoxysilane (MTMS) onto the surface of a PEO-coated Mg alloy. Briefly, the deposition of PEO coating on Mg alloy was done using a solution of Na2SiO3, NaOH, and phytic acid at 400 V, 300 Hz frequency, and 20% duty cycle for 5 min. The MTMS deposition on the PEO-coated Mg alloy was performed by dissolving MTMS in both water and ethanol and stirring at 50 °C for 2 h. The doping of MTMS was performed by adding ATO nanoparticles into the MTMS solution and 2 h hydrolysis coupling at 50 °C. Thereafter, the PEO-coated layer was immersed in the prepared solution, followed by a 2-h drying at 120 °C. The resulting ATO-containing PEO/MTMS composite coating exhibited a three-layer structure, with an inner PEO layer thickness of 11 ± 2.16 µm, intermediate MTMS layer thickness of 16.17 ± 0.26 µm, and top ATO layer thickness of 4.77 ± 0.52 µm. Agglomeration of ATO white particles was observed, effectively covering and sealing the coated surface. Electrochemical and hydrogen evolution tests were conducted in 3.5 wt.% NaCl test solution to evaluate the coatings’ protection performance. The ATO-PEO/MTMS layer showed the lowest hydrogen evolution rate (0.96 μL/cm h) compared to the pristine PEO-coated (3.82 μL/cm h) and MAO-MTMS (2.05 μL cm−2 h−1) layers, indicating superior protection for the Mg alloy substrate. In addition, the Icorr of the composite coating was three magnitude orders smaller than that of the bare Mg alloy. This improved corrosion resistance was due to the formation of a conductive network of ATO on the surface layer. The plugging of pores by ATO nanoparticles was also considered responsible for this enhanced corrosion resistance. It is worth noting that while these strategies are promising in improving the barrier properties of PEO coatings, more research is needed to assess their impact on coating durability and mechanical integrity.

Designing PEO SHP-based coatings by deposition from dispersions

Experimental data consistently show that modifying the surface chemistry of PEO-oxide layers by applying particles from a dispersion directly impacts the surface texture of these coated layers. Several factors come into play including the packing density of aggregated molecules, quantity of hydrophobic agents used, and binding agent employed. Surface hydroxyl and alkoxy groups in polymer/silane organic compounds are one of the vital components that determine the formation of a hydrophobic layer on a PEO-oxide surface. The chemisorption and aggregation of hydrophobic agent particles on PEO layers are dependent upon the chemical interaction between the surface polar solvent and PEO sublayer. This interaction can lead to both hydrolysis condensation reactions and formation of hydroxyl groups from organic particles, which can co-exist to seal the micropores and open cracks within the PEO coatings. For instance, silane’s alkoxyl group can be converted to Si–OH that can further chemically react with –OH groups formed on the PEO coated surface (PEO–OH). This reaction results in the formation of hydrogen bonds as depicted in Fig. 8a. The hydrolysis condensation reaction, illustrated in Fig. 8b, contributes to the PEO–O–Si bond formation, strengthening the bond between the organic/silane compound and PEO surface. The density of PEO–OH also dictates the growth mechanisms and barrier performance of these films.104

Fig. 8
figure 8

Schematic representation of the PEO–SHP coating deposition from silane particle dispersion

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The deposited coating texture is determined by the aggregation and self-organization of hydrophobic agent molecules. When these SHP particles are deposited on the surface, they become charged as a result of surface moiety dissociation and ion adsorption from the solution. This induced electrostatic force between the deposited particles and PEO-oxide layer causes the attraction of these particles to the PEO-film. At relatively low ion concentration in the dispersion medium, provided adequate surface charge and particle stabilization, electrostatic forces encourage the formation of a dense hydrophobic agent layer atop the PEO-matrix layer. The interaction of the particles with the PEO-ceramic layer at higher ion concentrations is determined by several sets of forces acting between the particles and PEO film, including structural forces, ion electrostatic repulsion, van der Waals attraction, and sign or magnitude of the peculiarities of the given system.105,106 Differences in the surface density of the polar hydroxyl groups and surface topography defects result in variations in water repellency across the surface. The resulting SHP coating can exhibit unimodal roughness when both the particle concentration and deposited film thickness are substantial, or bimodal when these factors are limited. Multimodal roughness can be effectively controlled by tuning the particle concentration and film thickness.97,107

The interaction between the hydrophobic particles and PEO-layer plays a crucial role in evaluating the coating’s durability when formed via the deposition of a hydrophobic agent from a dispersion. Thus, the ultimate surface texture of the deposited layer can be effectively controlled by varying the particle concentration and charges, deposition rate, dispersion medium polarity and viscosity, solvent volatility, and the like. Thoughtful parameter selection allows for the achievement of either bimodal or multimodal surface chemistry, providing a self-stable SHP layer on metallic surfaces.

The constraints and critical issues to the fabrication of PEO-assisted SHP coatings

In recent years, there has been a significant focus on improving the deposition of SHP coatings and elucidating the remarkable water repellency of SHP surfaces. The fabrication of PEO coatings with deposited SHP on metallic surfaces requires precise control of processing parameters to achieve superhydrophobicity. These parameters encompass factors such as the choice of starting materials, dispersed particle concentration, surface composition, hydrophobic agents used, solvent for particle dispersion, and surface topography of pre-existing PEO-oxide layers. Key challenges include ensuring strong adhesion, desirable wettability, and corrosion resistance of the resultant coatings. Adopting appropriate processing parameters to fabricate high-quality multifunctional coatings can significantly enhance the anti-corrosion attributes of a PEO–SHP coating. However, the challenge remains in mitigating degradation and maintaining coating adhesion over extended periods. Incorporating polymeric materials such as PLLA does not guarantee long-term adhesion to a PEO-film or the preservation of its integrity properties. In situations where adhesion between the polymeric/silane components and PEO layer is insufficient, particularly in aggressive media, the polymeric material can deteriorate. Addressing this challenge involves employing a combination of strategies to develop stable SHP hybrid layers that simultaneously ensure superhydrophobicity and coating adhesion, as these aspects are interdependent. In addition, the choice of solvent for preparing polymeric/organic solutions plays a crucial role in the deposition of fine layers on porous PEO coatings. Selecting an appropriate solvent is a useful approach to raising the CA and improving a coating’s corrosion protection performance.

PEO-structured coatings modified with hydrophobic polymeric/silane layers offer the advantage of facilitating the formation of multilayer films. These coatings not only enhance the corrosion resistance, but also impart superhydrophobicity. The logical approach to achieving both acceptable anti-corrosion properties and superhydrophobicity involves forming composite SHP coatings by immersing PEO-layers in organic solutions. Numerous studies have consistently shown that SHP PEO/polymeric coatings exhibit higher CA values and superior corrosion resistance when compared to using silane organic compounds. However, it is important to note that the achievement of high CA values alone does not necessarily guarantee enhanced superhydrophobicity and corrosion resistance, as evidenced by extensive severe cracking and localized corrosion observed in SHP coatings after salt spray test (Fig. 3). This raises questions about the direct relationship between high CA values and corrosion resistance (or surface mechanical integrity). In reality, surface wettability is a complex phenomenon that does not bear a direct relation with corrosion resistance, but is rather influenced by various processing parameters, making it challenging to draw definitive conclusions about its impact. Different studies have reported significantly varying values under distinct experimental conditions. Further research in this direction is required to develop SHP and highly corrosion-resistant coatings suitable for commercial applications.

The main challenge in developing SHP films is the high cost associated with the materials used in their synthesis and processing. In addition, there are concerns about the safety and toxicity of organic agents commonly used to seal PEO-micropores. Many of these organic sealants can pose health risks upon exposure. Incorporating particles into PEO coatings can be an effective approach to creating SHP nanocomposite layers and optimizing the surface chemistry of these coatings. However, there are serious concerns about the use of nanoparticles, as inhaling them during processing can lead to serious health issues such as lung inflammation and heart problems. Exploring ways to modify the surfaces of nanoparticles to reduce their health risks could open new research avenues. While SHP coatings are excellent at repelling water, they do not provide resistance against water vapor. Water vapor condensation is a common natural phenomenon and often contributes to the failure of coated products. There is also concern about the application of SHP coatings in environments where temperatures drop to the dew point. Under such conditions, condensation tends to form on the coating, leading to significant wetting and subsequent loss of superhydrophobicity. To address this issue, research efforts should focus on developing mechanically durable anti-condensation coatings with robust water-repellent properties.

Research on SHP coatings has primarily focused on Mg and its alloys, with some attention given to Al, Ti, and steel. Among these, Mg has been the most extensively studied, followed by Ti, Al, and steel. However, there are scarce reports on PEO–SHP coatings for steel and its alloys. Further investigations in these areas are essential to advance the development of engineered metallic components for SHP applications. The comprehensive analysis in this review is aimed at supporting the research and development in this field. Some of the suggested areas for further investigation include: (1) development of hybrid polymeric materials on PEO coatings for long-term corrosion resistance and durability; (2) fabrication of simple, low cost and scalable anti-condensation coatings with robust anti-superwetting properties; (3) processing of low-cost organic materials for SHP coatings to make them accessible for various applications; (4) fabrication of SHP PEO coatings for large-scale applications; and (5) exploring the use of environmentally benign low surface energy materials as alternatives to toxic compounds.

Summary

This review summarized the current progress in the field of PEO-assisted fabrication of SHP coatings. The micropores inherent in PEO coatings offer a unique opportunity for tailoring SHP-based coatings to specific requirements. The concentration and composition of polymeric/silane-based organic/oxide particles play a crucial role in shaping the structure, coating composition, wettability, and corrosion resistance of PEO-coatings on metals. This review also outlined the fundamental principles of SHP coatings and strategies for developing highly effective SHP PEO coatings, where the mechanism of creating PEO–SHP coatings through deposition from dispersion was examined and discussed. The main advantages and challenges associated with the PEO technique for depositing SHP-based coatings, as well as the essential parameters influencing PEO-assisted SHP coatings were also summarized. Furthermore, this review underscored critical challenges and pertinent issues in the fabrication of SHP coatings, concluding with recommendations for areas deserving further investigations.