Corrosion Barrier Coatings: Progress and Perspectives of the Chemical Route

Abstract

Improved corrosion barrier coatings (CBCs) to protect metals will allow future metal structures to operate for extended periods, ensuring improved safety by reducing environmental pollution and maintenance costs. Many production methods and design of corrosion barrier coatings (CBCs) have been developed. This review focuses only on CBCs made with chemistry techniques. These CBCs can be passive and active with remarkable performance. Today, most of the work focuses on the discovery and application of “smart nanomaterials,” which, if incorporated into “passive CBCs,” will turn them into “active CBCs,” giving them the phenomenon of “self-healing” that extends their service life. Today, many efforts are focused on developing sensors to diagnose corrosion at an early stage and CBCs that self-diagnose the environment and respond on demand. In addition, recent technological developments are reviewed, and a comprehensive strategy is proposed for the faster development of new CBC materials.

1. Introduction

Heavy metal compounds to protect metals from corrosion, such as Cr(VI) and Cr(III), exhibit toxicological properties, and their replacement with ecological corrosion barrier coatings (CBC) is of high technological and social importance. Furthermore, vapors and dust containing Cr (VI) inhaled by living organisms cause DNA modification in living cells, resulting in brain and lung cancer [1,2]. Assuming that approximately 2 g Cr(VI) is used per car to protect against corrosion, the automotive industry consumes at least 30 tons Cr(VI) per year at the European level. Many of these quantities are later released uncontrollably into the environment by waste disposal [3]. An environmental study in Denmark revealed that at least 2–4 tons of Cr(VI) per year are emitted into the water from surface treatment or wastewater from related industries, and 16–33 tons are released into the soil [4]. By extension, much more than 100 tons of Cr (VI) per year are still released in Europe [4]. In 2006, European legislation called “REACH” was developed, obliging companies managing hazardous chemicals not to consume more than 1 ton per year [5]. Consumption above this limit imposes a special authorization by the European Chemicals Agency (ECHA) valid for a set period [6]. As a result, companies have started to look for other eco-friendly protective coatings instead of chromium(VI) compounds. As realized by hexavalent chromium, innovative technologies must ensure competitive costs and high-efficiency anti-corrosion performance. The replacement of hexavalent chromium can be done with hexafluorozirconate (ZrF₆²⁻) [7,8,9,10,11,12,13]. However, the absence of hexavalent chromium within the coating reduces self-healing properties [13]. The Alodine T 5900 RTU by Henkel and the Socosurf TCS by Socomore conversion coatings and surface treatments are based on trivalent chromium and are REACH compliant alternatives to Cr(VI) [14,15]. The production process of these products leads to the partial oxidation of the Cr³⁺ to Cr⁶⁺ with a percentage not exceeding 0.1 wt% that satisfies the REACH Directive [16,17,18,19]. Chromium chloride (CrCl₃) and chromic oxide (Cr₂O₃) have been shown to be genotoxic in vitro, where chromosomal aberrations (CAs) were found in human lymphocytes [20,21]. The significant DNA migration caused by CrCl₃ was found in human lymphocytes [22]. The diet supplement chromium(III) tris(picolinate) (Cr(pic)₃) may cause CAs hprt mutations and single-strand breaks [23,24,25]. A study showed that workers’ exposure to trivalent chromium could indicate detectable DNA damage in peripheral lymphocytes [20]. Cerium conversion coatings were produced on aluminum alloy 2024-T3 to offer corrosion resistance because they tend to form hydroxides at a pH of >10, preventing cathodic reactions on corrosive surfaces. The cerium obtains the Ce⁴⁺ form. In an acidic environment, these oxides are dissolved, and the coatings do not provide any more protection [26]. Lately, Zirconium based CBC were developed [27], which meets the chromium replacement requirements and thus offers excellent potential for further applications. Many patents show the maturity of this technology [27,28,29,30], which the automotive industry has exploited since 2005 [27,31,32]. In addition, there are numerous references in the literature that summarize this emerging technology [31,32,33,34,35,36]. Zirconium-based conversion coatings (ZrCC) offer many advantages because they have low environmental impact in addition to being economical [37,38,39]. Recently, Zr and Ce combinations (ZrCeCC) have been studied, which have been shown to provide a synergistic effect in terms of barrier and self-healing for corrosion inhibition [26]. The conversion coatings are created from chemical procedures [40], atomic layer deposition (ALD) [41], and chemical vapor deposition (CVD) [42] methods on metal surfaces. The production of CBC will limit this review only to chemical techniques for space reasons.

Chromium(VI) replacement started with alkoxide-based sol–gel coatings on metals, but the disadvantages became apparent, and the idea was abandoned [43]. Then, ORMOSIL-type coatings were developed with impressive results [44]. Furthermore, the development and evaluation of inhibitors required extensive experimental and theoretical work [45,46]. The development and integration of nanocontainers loaded with inhibitors and their incorporation into polymeric CBC present a significant milestone in the field [47,48]. These integrated systems exhibit the self-healing phenomenon [49]. The self-healing phenomenon was discovered for the first time in polymers by Malinskii in 1969 [50]. Wool considered healing phenomena in polymers that restore deformation to the original state [51,52]. In 2001, White reported a polymeric material stored in nanocontainers that can autonomously heal cracks [53]. Today, investigators are focusing on finding even intelligent nanostructures that react to external stimulations, e.g., pH change exhibiting multifunctionality. This significant progress occurred via the close collaboration of scientists from materials science, chemical engineering, and corrosion science. The success of the new CBC technology imposes close collaboration of scientists with diverse talents collaborating closely with the paint industry and metal end users for the successful implementation of this technology. This paper looks back at this technology’s developments and further sets milestones to achieve in science in this field. This publication will not deal with the substrate because this was the subject of another recent review [40]. The development of intelligent CBCs through chemical techniques is a multidimensional task that requires many individual stages, such as the development of corrosion-resistant coatings, the finding and evaluation of corrosion inhibitors suited for each metal surface, the development of nanocontainers efficient to protect the metallic surface, and finally the development of sensors that will automatically diagnose the condition of the coatings. Figure 1 shows the individual tasks to find ecological CBCs that exhibit the phenomenon of “self-healing”.

2. Sol–Gel Processing

The sol–gel method produces glass and ceramic materials at lower temperatures than the corresponding products molded by melting. This method uses alkoxides, water, catalyst, and solvent [54]. Depending on the relationship between alkoxide/water and the kind of catalyst [54,55], one can obtain a one-dimensional, two-dimensional, or three-dimensional network where one takes fibers, thin films, or bulk materials. Some years ago, scientists attempted to produce coatings on metals to protect against corrosion [43,56]. Using a multiple immersion technique in the sol solution followed by a densification process, they deposited on copper, nickel, iron, and aluminum metals protective ZrO₂, SiO₂, TiO₂, and B₂O₃-3SiO₂ coatings. They were using the dip-coating method of metals in the sol solution, where they were first left to dry and next heat-treated slowly (5 °C/min) up to the temperature of 500 °C. The effectiveness of the sol–gel coatings against corrosion in the early studies was examined with weight loss measurements by exposing them to salt solutions. These studies revealed that the protection offered by the coating depends on its thickness. The increased thickness led to cracks during their thermal densification treatment or the alternating temperature during operation [56]. From the above studies, inorganic oxide coatings can protect metal substrates but do not significantly replace chromium-based CBCs [43]. One needs to reduce the densification temperature to 200 °C to avoid the metal’s destruction and develop flexible coatings that will adapt their physical properties (e.g., anticorrosion protection, the thermal expansion coefficient of the coating) to the properties of the metal. To realize that requirement, one needs precursors consisting of two primary components, the one organic and the other inorganic, the one responsible for the flexibility of the CBC, and the other for the anticorrosion properties.

3. Silanes

Silanes are hybrid organic–inorganic compounds used to help alleviate the problem occurring in the traditional sol–gel technology.

Table 1. The structures of some silanes/bis-silanes.

Material	Structure
Vinyl trimethoxy silane
Bis-l,2-(trimethoxysilyl) ethane
Bis-[trimethoxysilylpropyl]amine
(3-Aminopropyl)trimethoxysilane
3-Glycidyloxypropyl- trimethoxysilane
3-(Trimethoxysilyl)propyl methacrylate
3-(2-Aminoethylamino)-propyldimethoxymethylsilane
1,2-Bis(trimethoxysilyl)ethane

gives some chemical formulas of silanes/bis-silanes used to start the discussion [57]. These CBCs based on these precursors have proven effective in recent years [44]. Their hybrid structure maximizes their effectiveness against corrosion and their properties for primary technological needs.

The concentration of silanes, the water amount, the pH of the solution, the temperature of the reaction, and the aging of the solution affect the reaction’s kinetics, the extent of hydrolysis, and the concentration of bis-l,2-(trimethoxysilyl) ethane (BTSE) in the solution. The speed of hydrolysis increases with the amount of water, while the speed of condensation is a function of BTSE concentration [44,58]. CBCs have been produced with BTSE [59,60] using methanol solutions due to the low solubility of BTSE in water [61]. Methanol cannot be used in industrial processes due to its health dangers. This process leads to the formation of many monomers that living organisms can absorb. Thus, water-based chemistry was developed to alleviate the problem. The question arose whether both chemistries produce the same coatings. In an extensive study [58], the CBCs produced with and without methanol solutions were examined concerning the microstructure [62]. The aqueous chemistry yields more high molecular weight species, indicating that the aqueous solution BTSE is less reactive and newsworthy without health hazards. The resulting structures are similar in both cases. Another aspect of the silanes is their ability to be mixed with two or more substances, such as epoxy resins, polymers, and others. The procedure allows for economical upscaling. The silane CBC facilitates tiny pores allowing the diffusion of the electrolyte and the accumulation of corrosive species on the interface coating/substrate, resulting in corrosion. Researchers attempted to create a single layer with a larger thickness that was brittle, but the solution’s stability during deposition was small [63]. It is worth noting that silanes are not electrochemically active substances in solution or solid-state and are not reduced or oxidized unless they have electrochemically active groups. Therefore, they act as barrier coatings. In summary, the silane coatings are hydrophobic and exhibit barrier properties. Their barrier properties prevent water, oxygen, and corrosive ions from reaching the metal substrate [64]. However, a silane film alone cannot effectively protect the metal from corrosion for a long time due to its small thickness, which quickly becomes saturated by the corrosive solution and thus ceases to act as a barrier [65]. This drawback has led to the search for complex systems such as metal/silane/organic coatings to reach greater efficiency and limit the migration of metal ions through the coating [66]. The resulting systems from these works resulted in protective coatings that were more stable with exposure to corrosive environments. The silane acts as a barrier to improve the adhesion between the metal and the organic coating, as in the case of epoxy resins [67], polyamides [68], and the mixture of epoxy/polyamide [69]. One research paper showed resistance to corrosion of metal AA2024-T3 silane-coated bis-[3-(triethoxysilyl)propyl] tetrasulfide (BTESPT). A water silane/DI/methanol solution equal to 4/5.5/90.5 vol% was used to immerse the metal. Afterward, blowing air at 120 °C for 40 min dried the coating. The same metal was pretreated with the standardized commercial (VI)-based TURCO^TM Accelagold used as a reference sample. Electrochemical studies were performed using a 0.1 N NaCl solution. The silane membrane protected the substrate better than the reference chromate pre-treatment offered. In addition, the adhesion of BTESPT to the metal was facilitated by sulfur in the silane molecule [70]. One study showed that the overlap of (3-Glycidyloxypropyl) trimethoxy silane (GPTMS)–tetraethyl orthosilicate (TEOS)–ammonium persulfate based on silanes showed good protective properties for steel [71]. Many researchers reported electrolytic polypyrrole (PPy) coatings being deposited on steel in the presence of oxalic acid with spectacular results for metals in the iron series [72,73,74,75,76,77]. Although it is not entirely clear in the literature, due to the interaction of metal and polymer through galvanic coupling and upward polarization of the substrate, PPy provides anticorrosion protection for steels [78,79,80,81]. Finally, we must point out the difference between bis-silanes and silanes. Silanes have one Si atom per molecule, while bis-silanes have two Si atoms per molecule associated with three OR groups and six OR groups. Bis-silanes and silanes produce six SiOHs and three SiOHs per molecule during hydrolysis, respectively. After condensing the SiOH groups, bis-silanes create a denser network than do silanes.

4. ORMOSIL

a. 

Precursors and Reactions

ORMOSILs are prepared as organically modified silicate solutions that react with organic groups derived from an epoxy resin and a hardener. A series of solutions is prepared using the above ingredients in different proportions to select a suitable final product with the most satisfactory properties.

b. 

Organically Modified Silane

The reported silica alcohols or “silanes” are in the form of R’-Si(OR), where R’ is an organic group reacting with an organic polymer. In contrast, group R is an alkoxy group capable of participating in hydrolysis and condensation reactions. For example, a silane that can be used has the trade name Z 6020, while in the literature, it is found as N-(2-aminoethyl)-3-(trimethyloxypyl) propylamine with a molecular weight of 222.

c. 

Epoxy Resin

Epoxy resins are compounds, which contain at least one epoxy ring (-CHOCH₂) as an essential component of their monomer, i.e., they consist of an organic root and an oxygen atom joined to two carbon atoms [82]. The ring mentioned above is very active and is responsible for the vast number of products resulting from its reaction with several hardening agents. There are three classes of epoxy resins that are cycloalephatic, where there are six-membered rings in the organic part. The epoxy oils contain inseparable fatty acids and glysidic resins, whereas the organic part is a combination of hydrogenic polybasic acids or poly-hydro phenols [82]. They belong to the category of thermosetting polymers, i.e., their original properties are altered irreversibly via heating above a specific temperature.

A basic resin used to create the ORMOSIL membranes can be the bisphenol A diglycidyl ether (DGEBA) condensed with epichlorohydrin. The result of condensation is the creation of an epoxy group at the end of each polymer. The trade name of the product is DGEBA ή Bisphenol A or GY-257, and the name found in the literature is phenol 4,4′-(1-methylethylidene) bis-, or phenol 4,4′ isopropylidenedi- and belongs to the third category of epoxy resins. The presence of the benzene ring in the DGEBA molecule confirms that the material becomes more rigid and stiffer in a short time when applied in a suitable environment.

d. 

Hardener

As its name suggests, the hardener is a substance that hardens some other compounds. For example, one can use diethylenetriamine as a hardener with triethylenetetramine (TETA) or HM 943.

e. 

Solvents

The solvents used to prepare this solution are a mixture of pure ethyl alcohol and acetone. The presence of both solvents ensures the proper dissolution of the precursors at the molecular level and ensures the necessary homogeneity. Therefore, the purity of solvents should be 99.9%.

f. 

Preparation of ORMOSIL

Figure 2 shows the mixing procedures of the components and their corresponding concentrations schematically.

Figure 3, Figure 4 and Figure 5 show the individual reactions.

The surface-cleaned metals are immersed in the finished solution with or without the nano-containers six times at, e.g., a 32 cm/min speed and remain in the diathesis for one minute. Then, the coatings are heated at 70 °C for four days. Figure 6 shows the reactions occurring in this process.

Figure 7 shows the EIS diagram of the bare sample (Sample 1), ORMOSIL coated (Sample 2), ORMOSIL coated + CeO₂ (Empty) (Sample 3), and ORMOSIL coated + CeO₂(5-ATDT) (Sample 4) [40,83]. The measurement showed that the ORMOSIL coating offered protection to the HDG metal. The metal had an R_p = 1.73 E⁺⁰³ Ohm cm², while the metal protected with ORMOSIL had a protected metal with ORMOSIL R_p = 1.55 E⁺⁰⁴ Ohm cm². When CeO₂ (empty) nano-containers were added to ORMOSIL, R_p = 9.32 E⁺⁰⁴ Ohm cm² and increased to R_p = 3.05 E⁺⁰⁵ Ohm cm² when the nanocontainers were loaded with 5-ATDT corrosion inhibitor [40,84]. The effect of coating the metal with ORMOSIL + CeO₂ (5-ATDT) was the increase of R_p in this sample by 1033.60, 109.21, and 7.76 from the steel samples HDG, HDG-ORMOSIL, and HDG-ORMOSIL + CeO₂ (EMPTY), respectively. This sample exhibited the “self-healing” phenomenon [84].

The thickness, structure, and corrosion resistance were investigated concerning the type and concentration of the solvent for which they were produced by mixing 11.2 mL of tetraethyl orthosilicate (TEOS), 15.2 mL vinyltrimethoxysilane (VTMOS), and 4.0 mL 3-(trimethoxysilyl) propyl methacrylate (MEMO) with 19.6 mL 0.05 M HNO₃ [85]. Solvent concentrations ranged between 28.5 and 66.5 vol.% and were non-polar solvents (hexane, heptane), aprotic polar solvents (acetone, MEK), primary alcohols (methanol, ethanol, isopropanol, 2-methoxy ethanol, 2-ethoxyethanol, 2-methyl-1-butanol, 3-methyl-1-butanol), secondary alcohols (1-methoxy-2-propanol), and tertiary alcohols (t-amyl alcohol). In summary, corrosion resistance depends on the type and concentration of the solvent. They found that high solvent concentrations lead to coatings with enhanced resistance to corrosion using aprotic solvents. Minor alcohols yield enhanced corrosion protection at low diluent concentrations. High concentrations of large alcohol diluents degrade the corrosion resistance of the ORMOSIl membrane. This work offered a benchmark between structure–corrosion–the type of solvent–the solvent quantity that is useful for a researcher [85]. ORMOSILS were prepared with 3-glycidoxypropyltrimethoxysilane (GLYMO)–TEOS through the sol–gel method using nitric acid for the complexation of GLYMO and TEOS. As a result, a solid state of -¹H−¹³C and ¹H–²⁹Si CP/MAS NMR led to the organic content determining the microstructure, and thus the anti-corrosion capacity of ORMOSIl. Moreover, the microstructure of ORMOSIL and the method of deposition affects the structure and, consequently, the anti-corrosive performance characteristics [86].

5. Water-Based Commercial Sol-Gel CBCs

Chemetall GmbH developed the product OXSILAN AL-0500 based on silane for 2024, 2219, 5083, and 7075 aluminum [87]. These panels underwent multiple coatings with a solvent-free epoxy primer type MIL-DTL-53022, an epoxy primer without water type mil-DTL-53030, and a coating of polyurethane lymphatic type MIL-PRF-85285. As a result, the effects of accelerated corrosion were acceptable in some cases [87].

Boeing developed an anticorrosive sol–gel CBC using Zr(IV) n-isopropoxide and 3-glygodopropyltrimmethoxysilane (GLYMO) as organosilane compounds. Acetic acid was the catalyst. The anticorrosive sol–gel CBC exhibited excellent adhesion to the metal. Accordingly, the CBC with the trade name “Boegel” provides exceptional barrier properties to the metal without needing a further anticorrosive layer [88].

6. Inhibitors

6.1. Inorganic

Parallel to the advancement in the chemistry of coatings, considerable progress was made to add functionality to the CBCs by incorporating corrosion inhibitors, which can be organic, inorganic, or a combination of both.

The protection of the metal substrates from corrosion with the aqueous cation Ce(III) is done by creating an oxide–hydroxide membrane at the cathodic spot of the metal. The cathode reaction mechanism depends on the pH. In a neutral and basic environment [89], the reactions that take place in the presence of oxygen are:

O₂ + 2H₂O + 4e⁻ → 4OH⁻

(1)

O₂ + 2H₂O + 2e⁻ → H₂O₂ + 2OH⁻

(2)

2H₂O + 2e⁻ → 2OH⁻ + H₂

(3)

In contrast, in an acidic environment, one obtains the following reactions:

O₂ + 4H⁺ + 4e⁻ → 2H₂O

(4)

2H⁺ + 2e⁻ → H₂

(5)

The simplified reaction describes the anodic metal sites:

M → M_n⁺ + ne⁻

(6)

Reactions lead to dissolved metal cations whose stability depends on the pH of the solutions and the presence of, e.g., chlorine ions. For example, when the cerium is found on a metal surface, e.g., aluminum, and the pH is increased to 8, the following reaction takes place [90,91,92]:

Ce(OH)₂²⁺ + OH⁻ + e⁻ → Ce(OH)₃

(7)

At the same time, oxidation occurs in the solution:

Ce³⁺ + 2H₂O → Ce(OH)₂²⁺ + 2H⁺ + e

(8)

The oxidation of Ce(III) in Ce(IV) occurs near the surface of the metal if the pH is above 8.7 [93].

The X-ray absorption method (XANES) was used investigate cerium-containing films on aluminum and 5052 alloys [90]. The research was based on the analysis of the Ce L3 (6548 eV) and Ce L1 (6548 eV) absorption edges, which are characterized as 3 and 4 valent cerium. When aluminum was immersed for five days in a bath with Ce³⁺ ions at neutral pH, it was found to be Ce³⁺. After seven days of submergence in the same solution, Ce³⁺ oxidized to Ce⁴⁺. The valence state of the cerium depends on the time of exposure to the solution. In a zinc and iron metal, the deposition of cerium oxide above the substrate presupposes the formation of the hydroxide of the metal itself, which is rapidly removed and has a lower deposition of the compounds of cerium [94].

The addition of compounds such as Ce(NO₃)6H₂O and CeO₂ fine powder was also evaluated in the silane chemistry [95,96]. These enhancers, as one calls them in silane sol–gel chemistry, significantly improve the barrier property of sol–gel coatings combined with the anticorrosion properties of cerium ions [97,98,99,100,101,102,103,104,105]. The anticorrosion action of the cerium ions occurs via the passivation of the imperfections or cracks. In addition, the cerium ions act as cathode-like corrosion inhibitors by the precipitation of insoluble cerium hydroxide in regions of high pH [106,107]. The anticorrosion action of the cerium compounds occurs via the cerium III and IV reactions with the hydroxyl ions to form hydroxides in cathodic regions and act as a barrier of oxygen [103,108]. The literature has many references to the structure of cerium in coatings, reduction of porosity in coatings via cerium incorporation, reactions of cerium depending on the chemical environment, etc. Corrosion of the hybrid sol–gel coating in the X13VD martensitic stainless steel was investigated as a function of cerium concentration [96]. EIS showed that the coating’s best cerium concentration was 0.01 M. When the concentration passed this amount, then the barrier effect of the sol–gel coating was reduced. The increase in the cerium content of this limit increased the hydrophobic character of the coating. Concentrations of cerium above 0.01 M led to the modifications of the sol–gel network, thus reducing the anticorrosion abilities of the material.

Here, further discussion is not necessary. The reader can refer to the references regarding the beneficial effects of cerium on CBCs [109,110].

6.2. Organic

Corrosion control of metal structures to prevent metal dissolution in corrosive environments can be done using organic inhibitors. In corrosive solutions, heteroatomic (O, S, N, and P) and electrons in the coupled form function as outstanding corrosion inhibitors for metals. Organic inhibitors are usually incorporated into CBCs. The literature has extensively analyzed which inhibitors are suitable for which metals. The protective effects of 3-mercaptopropyl-5-amino-lH-l,2,4-triazole (A), 3-mercaptobutyl-5-amino-lH-l,2,4-triazole (B) and 3-mercapto(3-methyl butyl)-5-amino-lH-l,2,4-triazole (C) were studied on brass against corrosion using electrochemical methods [111]. When inhibitor concentrations exceed 0.10 mM, the protection degree is greater than 99%. The protection is attributed to the formation of the molecules of the inhibitors with zinc and copper. The effectiveness is explained by the assistance of quantum mechanical calculations where the HOMO and LUMO states were calculated. The B3LYP/6-311 DFT method with the G(d, p) basis set calculated HOMO, LUMO, ionization potential, electronegativity, absolute hardness, and softness in eV [111]. The difference between HOMO and LUMO gives the HLG gap calculated for A, B, and C equal to 5.75, 5.77, and 5.73 eV, respectively. The smaller the HLG gap is, the higher the reactivity of the inhibitor to bind to the metal, giving a stronger inhibition effect [112]. The experimental and theoretical calculations are in excellent agreement.

Linear sweep voltammetry studied the inhibition of 2-mercaptobenzothiazole (MBT) on copper in ethanol solutions. The presence of MBT shows one anodic process correlated with the oxidation of MBT, which leads to the formation of a film on the electrode. Surface-enhanced Raman spectroscopy (SERS) demonstrated oxidation of MBT forming polymeric complexes with copper and the ionized thiol form [113].

The inhibition abilities of 2-(2-aminophenyl)-N phenylhydrazinecarbothioamide (AP4PT), N,2-diphenylhydrazinecarbothioamide (D4PT), and 2-(2-hydroxyphenyl)-N-phenyl hydrazinecarbothioamide (HP4PT) were investigated for mild steel in H₂SO₄ experimentally as well as by quantum mechanical methods. According to gravimetric and gasometric experiments, the inhibition efficiency follows the order: AP4PT > HP4PT > D4PT. Furthermore, the quantum chemical parameters agree with the experiment-derived inhibition efficiency [114].

The corrosion protection mechanism of 2-mercaptobenzothiazole (MBT) in cerium chloride (CeCl₃) for the treatment of AA 2024-T3 was investigated to determine any synergistic effect between the two inhibitors. Several methods were employed, including image-assisted electrochemical noise, the split-cell technique, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). As a result, 2-MBT was shown to protect AA 2024-T3 via the protective layer on its surface from corrosion in a 3.5% NaCl solution. Moreover, there was no synergy between 2-MBT and the Ce³⁺. Then, in the test period, 2-MBT offered better corrosion protection to AA 2024-T3 than CeCl₃ and 2-MBT+ CeCl₃ [115].

Salicylaldoxime, 8-hydroxyquinoline, and quinaldic acid were evaluated as inhibiting organic compounds on the corrosion of 2024 aluminum alloy in 0.05 M sodium chloride solution. A plethora of techniques were employed, including electrochemical impedance spectroscopy (EIS), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy coupled with scanning Kelvin probe (SKPFM). The conclusions are that the quinaldic acid, salicylaldoxime, and 8-hydroxyquinoline offer anticorrosion protection for AA2024 by developing a thin organic layer of insoluble complexes on the surface of the alloy [116].

There is intense activity to find environmentally friendly corrosion inhibitors. One such case is the two food spices such as 2,5-dihydroxy-1,4-dithiane (DDD) and 2,5-dimethyl-[1.4] dithiane-2,5-diol (DTDD), which do not cause pollution of the ecosystem [117]. DDD and DTDD eliminate oxidation products in the interface, while 0.5 M H₂SO₄ is used to combat corrosion products. Electrochemical measurements have shown that DDD and DTDD are cathode-biased Cu corrosion inhibitors in the medium H₂SO₄. Furthermore, DDD was shown to perform better than DTDD. DFT calculations with a 6-311 G++(d,p) basis set determined the energy gap (ΔE = E_LUMO−E_HOMO). As previously said, the ΔΕ determines the anti-corrosion performance of the DDD and DTDD corrosion inhibitor, which was 1.90 eV and 4.46 eV, respectively, supporting the EIS results. In addition, molecular dynamics (MD) simulations were performed on the adsorption of DDD and DTDD on the copper surface, which, together with the copper surface XPS measurements, have shown that adsorption is chemical with a mechanism depicted in Figure 8. However, there are misgivings about the viability of this high-cost study of DDD and DTDD. Nevertheless, the idea points the way to finding corrosion inhibitors from cheap natural products to develop a complete ecological anti-corrosion system.

The same group conducted research using Passiflora edulia Sims leaf extract (PESLE) using copper as a substrate exposed to 0.5 M H₂SO₄, which functioned in ecological corrosion inhibition [118]. This was established after electrochemical and XPS studies. Quantum mechanics studies also supported the experiments.

7. Split Cell Technique

The split cell technique provides evidence of the corrosion inhibition mechanism of organic or inorganic inhibitors. The setup for this device (Figure 9) consists of two compartments connected via a porous membrane that allows electrons to pass. One immerses equal size specimens in the two compartments, and at this moment, the current density is near 0 μAcm⁻². As shown in the above schematic, a zero resistance ammeter (ZRA) connects the AA 2024 T3 specimens (electrodes). When 15 min pass, nitrogen gas is merged in one of the compartments, and this specimen turns into a plain anode as oxygen is depleted, leading to a reduction reaction. In the other compartment, air flows, allowing the other electrode to become an oxygen-available cathode. The current measured at this stage is generally of the order of 0.6 μAcm⁻². One can add that 2-MBT in the anodic compartment induces a rapid current density drop and a potential increase, and in the cathodic compartment, one observes a slight current decrease and increased potential. With the addition of benzotriazole (BZT) into the two compartments in another experiment, one observes that current and potential values are unaffected. Here, 2-MBT decreases the anodic and cathodic reaction rates, preventing corrosion initiation [119]. The method accommodates equivalent circuits and extensive mathematics described in the literature [120]. The method is suitable for finding appropriate corrosion inhibition in an explicit metal substrate and is presented for future use by others.

8. Electrochemical Noise (EN) Method

Recent literature described the electrochemical noise (EN) method very extensively [120]. Figure 10A briefly sketches this method. Two equal electrodes are immersed in an electrolyte (3.5% NaCl) connected to a zero resistance ampere (ZRA) meter. The potentials of each electrode concerning a calomel electrode are recorded using an analog to digital converter. This device allows the corrosion phenomena to occur without electrical disturbance. This setup gives information about the anti-corrosion performance of corrosion inhibitors dissolved in the conductive electrolyte. Figure 10B gives the equivalent circuit of this device and the mathematical equations necessary for the analysis of the measurements [120]. The device has been experimentally validated and offers significant results in the performance of corrosion inhibitors. Initially, segments are exported to the original potential and current datasets. At the first point of the section, one produces the square root of the variance of the potential, σ(V(t)), divided by the variance of the current, σ(I(t)) (Figure 10B). Thus, one obtains the evolution of the values of the noise resistance. Figure 10 shows a standard measurement for a metal AA 2024-T3 without and with 0.15 mmoL Ce(NO₃)₃. From a series of measurements involving Ce(NO₃)₃, Co(NO₃)₃, and Mn(NO₃)₃, Ce(NO₃)₃ offers the best anticorrosion performance [121]. The concentration of the inhibitor plays an essential role in the protection provided by these compounds. This method allows for recording the actual behavior of corrosion phenomena.

9. Scanning Vibrating Electrode Technique (SVET)

The SVET technique maps the local current density on electrochemically active samples, such as a corroded metal within an electrolyte. Such an instrument consists of a probe that measures the potential during vibration. The result is an AC dynamic measurement converted to DC potential with a lock-in amplifier that measures the DC current as a function of the x–y position of the probe. The SVET technique requires precise probe vibration through an accurate piezo vibrator. Figure 11 schematically describes such an SVET device. Figure 11 also shows a typical SVET measurement of metal coated with ORMOSIL doped with CeMo(MBT). The I_anodic and I_cathodic currents become equal to the noise values for longer period of times. Thus, one can suggest that self-healing occurs in the samples (Figure 11).

10. Organic Inhibitors in Nanocontainers

8-Hydroxyquinoline (8-HQ) acts as an inhibitor by preventing active sites of the metal surface [122]. In the above discussion, the inhibition action of cerium oxide has been discussed, forming a layer of pure cerium oxide on the metal surface [123]. In one publication, 110-nm ceria nanocontainers were reported to be loaded with 8-HQ [124]. This 8-HQ–CeO₂ complex is a system of organic and inorganic corrosion inhibitors involving nanocontainers. Figure 12 shows the polarization curves of the complex corrosion inhibitor after 6 (a), 24 (b), and 72 (c) hours of exposure to 0.5 M NaCl with the device depicted in the same picture. The included table gives R_p, I_o, and E_o acquired at 72 h of exposure. The polarization resistance (R_p) and corrosion current (I_o) show that protection of the aluminum panel is present after 72 h. The reduction of the anodic current after 72 h is due to the release of the inhibitor from the nanocontainers. These results show the release of the inhibitor from the nanocontainer, offering protection to the metal [124]. Figure 12 shows polarization curves (right) for bare AA2024 aluminum panels gained in the device (left) at 6, 24, and 72 h in 0.5 M NaCl in the presence of 8-HQ-loaded CeO₂ nanocontainers. After fitting the polarization curves, the R_p, I_o, and E_o values were gained.

This result shows that cerium and 8-HQ act together as inhibitors that prevent corrosion of the metal.

11. Nanotechnology

After the first production of CBCs by the sol–gel method, their further progress turned to developing a generally applicable multifunctional corrosion coating technology based on nanoparticles or nanocontainers as functional design elements. The new nanotechnology not only had to replace the used corrosion protection systems based on toxic and environmentally hazardous compounds of heavy metals such as chromates but at the same time add third functions such as abrasion resistance, low surface tension, antifouling properties, antibacterial properties, self-healing of the anticorrosion properties of CBCs, anti-icing technology, etc. In addition, the nanoparticulate or nanocontainers must release controlled corrosion inhibitors to achieve active corrosion and self-healing protection of CBCs. In recent years, revolutionary nanotechnology has been developed that allows for the simultaneous establishment of active and passive protection against corrosion without using toxic compounds or heavy metals. The development of such technology is a complex process that is shown condensed in Figure 13, consisting of three main stages: the development of materials, the examination of their properties, and other supporting actions, including applying technology to demonstrators.

Cooperation with the industry is essential for the targeted development of multifunctional materials necessary to users. The understanding of phenomena requires a modeling approach to reveal the self-healing mechanism. This new nanotechnology-based approach must include pilot-scale production, industrial processing, and application technology.

11.1. Silanes and Nanoparticles

The addition of nanoparticles to the silane coating benefits the protection of metals from corrosion. A positive effect has been recognized for commercial and newly developed nanoparticles such as cerium dioxide, titanium oxide, cerium molybdate, aluminum hydroxide oxide, aluminum oxide, zirconium dioxide, strontium aluminum polyphosphate (SAPP), and silicon carbide [125,126,127,128,129,130]. These inhibiting materials have been incorporated into hybrid inorganic–organic hybrid sol–gel matrices to improve their behavior in corrosion protection. Especially, amorphous nanoparticles such as CeO₂ have shown improved performance against corrosion via pore-clogging, and by filling gaps and imperfections of the coatings [131,132,133,134]. Their presence in the coating leads to avoidance of crack formation, yielding higher mechanical durability and anticorrosion protection [109,135]. CeO₂ is very stable in alkaline pH. One study showed that the addition of CeO₂ in water-soluble silane film bis-1,2-triethoxysilolaethane (BTSE) exhibited excellent adhesion to the substrate and excellent anticorrosion properties in Al alloys [133], comparable to those of treatment with Cr(VI) [133,136]. The introduction of TiC, TiN, and TiC/SiC/Si₃N₄ (Ti–Si–C–N) improves the hardness of the coatings [137]. A study examined the size (nano-CeO₂ and micro-CeO₂ powders) and the concentration of particles embedded in various hybrid sol–gel coating systems to protect against corrosion of aluminum alloy AA2024. In the case of micro-CeO₂, the coatings are vulnerable to corrosion. For the same level of doping as nano-CeO₂, better corrosion protection is observed. At the same time, high loads with inhibitors harm corrosion protection [138].

A theoretical model simulated self-healing processes in coatings by combining Nernst equations with Fick’s diffusion law and other necessary equations [139]. The results of these calculations were confirmed by many experiments that rinses the inhibitor from a damaged coating containing cerium nanoparticles that released cerium to scratched metal, inducing corrosion. The model was used to describe the self-healing processes of the coatings as observed by the SVET technique [139]. For the proposed coating, the scale and time scale of the inhibitor release were calculated computationally. The permeability of the coating plays a significant role in releasing the cerium from the nanoparticles. These results suggest that the deviation of the values for such parameters from the nominal ones has a critical effect on the release rate of the inhibitors and, therefore, on the active protection against corrosion. The calculations predicted the optimal inhibitor concentration and the stability of the coating concerning these parameters [140].

The effect of adding carbon nanotubes (CNTs) on the corrosion resistance of a tetrasulfide bis(triethoxysylpropyl)ylpropyl (BTESPT) film to a steel substrate was studied in one procedure. The coating was compact, with significant improvement in its anticorrosion capabilities [141].

11.2. CBCs with Nanocontainers

Protective coatings on metals against corrosion are divided into passive and active (Figure 14). Passive coatings provide a barrier to the corrosive environment until they are destroyed, e.g., scratched. When these coatings contain inhibitor-loaded nanocontainers, they turn into active ones. In the analogous case of damage, corrosion inhibitors are released from the nanocontainers and repair the damaged area of the coating [40]. The legitimate question is why one enters the process of incorporating corrosion inhibitors into nanocontainers. This is answered by the unwanted rinsing of the inhibitors from the coatings and deactivation of the inhibitor due to the complexation with the backbone of the coating that disables them [40]. Corrosion inhibitors incorporated into CBCs also negatively affect their properties [2,142]. The type of destruction depends on the loading CBC matrix with inhibitors. The solution to these problems involves a “smart” release of inhibitors achieved by encapsulation in nanocontainers, allowing them to be active on demand. In recent years, many nanostructures of different shapes, sizes, and compositions have been developed with different levels of effectiveness and with diverse methods of production. The corrosion inhibitors are released from the nanocontainers after changes in the pH, internal or external changes in the temperature, or the nanocontainers’ destruction. The pH change can be attributed to corrosion, i.e., the pH decreases (micro-anodes) in spots due to the dissolution of metals and hydrolysis reactions, while areas are increased (micro-cathode) due to oxygen reduction by the formation of hydroxide ions.

Another parameter for selecting the nanocontainers is their compatibility with the coating chemicals and their homogenous dispersion in the CBCs. The material from the nanocontainer shell must be irresistible from any influence of coating production.

It is recommends to test the nanocontainers’ dispersion in the CBC any time after sample production [143], as shown in Figure 15. In some cases, its surface modification is necessary to ensure repulsion between the nanocontainers during mixing with the coating resins to achieve homogeneous distribution in the paint.

The literature documents many techniques as far as the encapsulation of corrosion inhibitors into the nanocontainers is concerned [144,145,146]. As for nanocontainers formed by coating a template and removing it by combustion, the most effective method is with the help of a vacuum chamber. One places a desired quantity of nanocontainers in a jar inside a chamber connected with a high vacuum pump. In a funnel that communicates with the chamber, one puts enough corrosion inhibitor diluted in a solvent. When a high vacuum in the chamber is observed and consequently transferred to the nanocontainers, one opens the funnel attached to the vacuum chamber, which floods the chamber. The adsorption of the corrosion inhibitor destroys the vacuum inside the nanocontainers. The amount encapsulated in nanocontainers is determined by the thermogravimetric analysis (TGA) method. One measures the weight of the same quantity as a function of the temperature in empty and full nanocontainers. The difference in weight resulting from these measurements is reduced by the lost weight of the encapsulated corrosion inhibitor.

Table 2. Inhibitor loading in the nanocontainers (Adapted with permission from Reference [148]).

Inhibitors	TiO2	CeMo	CeO2
8-hydroxyquinoline	3.56	5.22	5.22
5-amino-1,3,4-thiadiazole-2-thiol	18.90	46.45	21.91
2-mercaptobenzothiazole	6.14	59.44	63.42
1-H-benzotriazole-4-sulfonic acid	-	12.03	-
1-H-benzotriazole	-	-	54.17
p-toluenesulfonic acid	41.27	-	-

shows the w/w% loading of inhibitors in TiO₂, CeMo, and CeO₂ nanocontainers. The table shows that some corrosion inhibitors have a low loading on nanocontainers such as 8-HQ, regardless of the type. In other nanocontainers, the loading with corrosion inhibitors is large enough for one type, but the same nanocontainers do not accommodate a large amount of another corrosion inhibitor. Several reasons can explain this behavior related to the interaction or pore size of the nanocontainer with the corrosion inhibitor. It is a matter of optimization to find the best conditions for maximum loading. This fundamental result suggests that each system must specify this quantity to determine its effectiveness in CBCs before use. Brunauer–Emmett–Teller (BET) measurements can explain the physical adsorption of inhibitor molecules on a solid surface, pores, and the interior of the nanocontainers based on the critical analysis of their specific surface area [147].

Developing the encapsulation technology of corrosion inhibitors in nanocontainers releases the active chemicals at a controlled rate in self-healing coatings. When the nanocontainers are ceramic, the corrosion inhibitor passes via the porous shell wall, assuming all are inside the nanocontainers. Here, the driving forces are the local pH change caused by corrosion, where they form polymers in the cracks of the CBC and heal it through chelation processes [149]. Therefore, one can suggest that the release rate depends on the thickness of the shell, permeability, distance from the crack, size of the crack, and the concentration of the nanocontainers. However, this simplified discussion is not accurate if Figure 16 measured the release of a corrosion inhibitor from nanocontainers [150]. The experimental curve was simulated by three equations indicating three kinetic release mechanisms from the nanocontainers, namely, fast (t₁), medium (t₂), and slow (t₃). At the beginning of the experiment, the corrosion inhibitors come from the surface of the nanocontainers. Later, the inhibitors in the walls participate in the release process. Finally, the inhibitors inside the shell take part in the release process. In other words, one can divide the corrosion inhibitors into three categories depending on their origin in the ceramic nanocontainers [150]. The release kinetics of 1-H-benzotriazole-4-sulfonic acid from the CeMo nanocontainers was fitted using three time constants [150].

This study shows that there are three types of corrosion inhibitions analogously that have enclaved, where their concentration depends on the volume of the nanoparticle. Suppose one assumes that the absorbance phenomena are independent of the nanocontainers’ size for the same shell thickness. In that case, the corrosion inhibitions in the interior, pores, and the surface of the nanocontainers are proportional to the internal volume, porosity volume, and surface area, respectively. The literature confirms this approach, proving experimentally that the degree of self-healing depends on the diameter of the microcontainer [151]. The maximum healing performance depends on the adequacy of the corrosion inhibitor to heal a specific size of the crack. Therefore, the design of the self-healing surface requires the actual quantity of inhibited corrosion agents, size of cracks, and endurance time of the self-healing coating for specific types of damage.

A second instrumental parameter that plays a substantial role in the self-healing of CBCs with corrosion inhibitor-loaded nanocontainers is their distance from the metal. An aluminum alloy AA2024-T3 was coated with two layers, A and B, where A was on the metal, and the B layer was on the A layer (Figure 17) [152]. Porous silicon nanocontainers (MBT@NCs) loaded with 2-mercaptobenzothiazole (MBT) inhibitor were loaded on the B-layer, and vice-versa in another experiment, namely, the A layer loaded with MBT@NCs. Increasing the distance between MBT@NCs and the metal surface benefits the barrier properties but worsens the corrosion inhibition. Inversely, this happens when MBT@NCs are incorporated into the coatings in direct contact in the future [152].

In another experiment, an aluminum alloy AA2024-T3 was coated with multiple surfaces containing CeMo(8-HQ) nanocontainers and others with nano-traps of water and chlorine (Figure 18) [153,154]. In one study, it was demonstrated that water traps drastically affected the diffusion coefficient of water in the ORMOSIL coatings [154].

After producing CeMo nanocontainers, the pore diameter distribution was measured with the BET technique (Figure 19). From the analysis of isotherms (Figure 19a) from BET measurements, the mean pore radius was calculated to be 24.435 Å with a pore volume of 0.588 cm³/g (Figure 19b).

These measurements are necessary to optimize nanocontainers for better exploitation to accommodate and release corrosion inhibitors. The multifunctional coatings (Figure 20) were loaded with CeMo(8-HQ) nanocontainers and coatings of water and chloride traps exposed to 0.5 M NaCl(aq) for 1 h, 144 h, and 288 h. Figure 20 shows the EIS Bode plots for these samples. From these measurements, the incorporation of traps and CeMo(8-HQ) is highly compatible with the coating ORMOSIL. EIS measurements show coatings to be highly effective in providing anticorrosion protection of metals, as shown by the high barrier properties. The addition of CeMo(8-HQ) and water and chloride traps improve the corrosion resistance of the CBCs amazingly.

SVET measurements demonstrated corrosion inhibition and the development of self-healing behavior. Furthermore, they studied the micromechanical properties of the CeMo(8-HQ)+nanotraps ORMOSIL coatings, showing that adding these pigments beneficially affects their mechanical properties [153].

Layered double hydroxides (LDHs) (Figure 21) are ionic layered solids consisting of hydroxide layer [M^II_1−xM^III_x(OH)₂]^x+ Z [M^II_1−xM^III_x(OH)₂]^x+ structures, where M represents the metal cations, and Z are layers of anions or/and neutral molecules (such as inhibitors, water, etc.). Z anions are softly attached and often replaceable. This property is used for self-healing purposes in cases of corrosion triggered by local changes in pH due to corrosion via an exchange between the inhibitor and hydroxyl anions [155]. The dimensions of polycrystalline particles range between 200 and 400 nm in length and the lateral size between 20 and 40 nm [156]. The incorporation of LDHs into a polymer coating loaded with divanadate anions as corrosion inhibitors were incorporated into a commercial primer on aluminum alloy 2024 [156]. The resulting CBCs exhibit a self-healing effect and give corrosion protection superior to that offered by CBCs on a chromium basis [156]. Furthermore, layered double hydroxides (LDHs) have been incorporated into CBCs executed for various functions. For example, in organic polymeric coatings, LDHs intercalated with nitrate anions act as chloride nanotraps [157]. Their occurrence in the coatings delays the diffusion of "chloride" ions to the metal surface and, consequently, the induction of corrosion processes. The literature makes extensive references to this system [96].

Another study reports the synergy between the layered double hydroxides (LDH) and cerium molybdate (CeMo) hollow nanocontainers incorporated into CBC, the anticorrosion properties of galvanized steel used in the automotive industry [158]. Both nanocontainers were loaded with MBT. The primer coating was loaded with LDH(MBT) or CeMo(MBT) in 4 wt.%. The third sample was produced combined with LDH(MBT) and CeMo(MBT) in a total amount equal to 4 wt.%. The electrochemical behavior was studied by EIS and SVET techniques. Figure 22 shows the SVET results of total anodic and cathodic current obtained for the three coatings. One can perceive that the I_anodic – I_cathodic are close to zero for up to 17 h for the LDH(MBT) sample, and then they start to increase linearly with the time. In the CeMo(MBT) sample, the I_anodic – I_cathodic increases rapidly up to three hours due to corrosion and then drops to zero for times longer than 20 h due to self-healing.

In the last sample consisting of LDH(MBT) + CeMo(MBT), I_anodic – I_cathodic becomes maximal after 3–5 h of immersion in a salt solution. After that, both anodic and cathodic activities are low and do not exceed 5 μA/cm², again due to self-healing. In summary, the inhibition of corrosion activity extends over a long period for the LDH(MBT) + CeMo(LDH) system. These results demonstrated a synergistic effect for corrosion inhibition and the self-healing effect by mixing the two nanocontainers [158].

The LbL system presents another approach to induce self-healing embedded in hybrid epoxy-functionalized ZrO₂/SiO₂ sol–gel coatings deposited onto the aluminum alloy AA2024 [159]. Here, 70 nm SiO₂ particles are multiple coated with poly(ethylene imine)/poly(styrene sulfonate) (PEI/PSS) polyelectrolyte layers. Benzotriazole (BTZ) is entrapped within the polyelectrolyte multilayers during the LbL-assembly. The pH changes during aluminum corrosion trigger the release of the inhibitor (BTZ). The LbL nano reservoir releases the stored inhibitor over a prolonged period of “on-demand” to damaged regions. The methodology appears to be cost-effective with active feedback on the corrosion processes. In another work, instead, SiO₂ particles produced ZnO particles coated with a polyaniline (PANI) polyelectrolyte layer. On it, they coated BZT, which enclaves with polyacrylic acid (PAA). The total particle size reached 950 nm. In this work, the release of BZT was measured with 3, 5, and 7 pH as a function of time. After 8 h, the BZT release in mg L⁻¹/g of ZnO nanocontainer at pH 3, 5, and 7 was 0.87, 0.59, and 0.36, respectively. LbL nanocarriers (5%) were incorporated into alkyd resin, and it was found after an electrochemical study that this system could be helpful for CBCs of marine interest [160]. Figure 23 makes the LbL technology more understandable to the reader.

In another study [161], the LbL particles were based on a cerium zinc molybdate (CZM) core coated with a PANI layer and polyelectrolyte layers mixed with an imidazole corrosion inhibitor. Studies determined the release of corrosion inhibitors as a function of pH and time. These LbL were incorporated into alkyd resins on mild steel and evaluated with electrochemical analysis (Tafel plot), where the improvement of their anticorrosion behavior was certified by incorporating LbL into the coating. In the two works [160,161], there is no discussion about whether these coatings offer the phenomenon of self-healing.

pH-Poly-(methacrylic acid) (PMAA)-sensitive nanocontainers (Figure 24) loaded with 2-MBT and hexafluoro titanic acid (H₂TiF₆) corrosion inhibitors significantly improved the anticorrosion properties of epoxy coatings on aluminum alloy 2024-T3, offering a beneficial effect on the anticorrosion properties of CBC [162].

Poly (methacrylic acid) (PMMA) and CeO₂ were combined to create PMAA@CeO₂ nanocontainers filled with MBT and incorporated into an epoxy coating applied to an aerospace alloy (AA 2024-T3). The innovative coatings were examined with electrochemical impedance spectroscopy (EIS), and the results showed enhanced protection of the AA 2024-T3 substrate derived from the epoxy coating containing the 2-MBT-loaded nanocontainers [163].

Hollow mesoporous zirconia (hm-ZrO) nanospheres were prepared and charged with the 2-mercaptobenzothiazole corrosion inhibitor (2-MBT). Solid silicon nanoparticles as models were used to manufacture the ZrO shell after thermal solidification was removed with NaOH. The nanocontainers had a size of 400 nm. The loading efficiency of 2-MBT was 63%. A 2-MBT release study was carried out for the nanocontainers, and it was found that its release was more remarkable in acidic and alkaline pH conditions than the neutral pH [164]. The same team produced hollow silicon spheres and loaded them with 2-MBT [165]. The 2-MBT kinetics was higher in the alkaline and acid environment than in the neutral pH. The silicon nanocontainers’ inhibitor loading efficiency was more significant than that observed for hm-ZrO and was approximately 72%. The group went beyond the first study and incorporated these nanocontainers inside an organic–inorganic hybrid sol–gel coating made on modified 9Cr-1Mo ferrous steels. Active corrosion protection attributed to the releases of encapsulated inhibitor molecules was observed, thus demonstrating the “self-healing” ability of CS-NI [165].

Another more advanced system was developed consisting of hollow mesoporous silica nanocontainers (HMS) with molecular photo responsive switches such as azobenzene (AZO). The production started with the formation of SiO₂ templates coated by Azo-HMS, followed by the core removal. The containers were loaded with benzotriazole (BTA) released by light activation. The BTA@ Azo-HMS nanocontainers with light-responsive release properties were incorporated into an alkyd coating onto aluminum substrates. Upon UV irradiation, BTA was released, inducing self-healing [166].

Nano valves sensitive to pH were installed in the pores of the mesoporous silica nanoparticles (MSNs). Then, the nanocontainers were loaded with benzotriazole and incorporated into CBCs. Finally, the intelligent coatings were immersed in a 0.1 M NaCl solution; the EIS measurements were conducted at frequent time intervals. The impedance modulus of the blank metal decreased with the time up to 20 days of immersion in the salt solution. On the contrary, the impedance modulus of the coating with nanocontainers was 4.7 × 10⁴ Ω cm² and 1.8 × 10⁵ Ω cm² in 1 and 15 days, respectively. This increase with time was attributed to the “self-healing” effect [167].

So far, we have discussed spherical nanocontainers loaded with organic inhibitors. One group created halloysite nanotubes (HNTs) that charged them with cationic corrosion inhibitors such as Ce³⁺ and Zr⁴⁺ [168]. So far, such studies are rare in the literature [169]. The size of the pore of the nanotubes was 10 nm with a length ranging between 0.1 and 0.5 μm. There was an 80% reduction in the empty pore volume of HNT after filling with corrosion inhibitors in the lumen of the nanotubes. Coatings were created with the HNTs, and in this case, the “self-healing” effect was observed [168].

Multiwall carbon nanotubes (MWCNTs) were loaded with benzimidazole (BZ) and incorporated into an epoxy coating. EIS and sacrificial tests evaluated the anticorrosion and self-healing properties of this coating. In addition, the impedance parameters were determined as a function of immersion time in a 3.5% NaCl solution up to 204 h. Based on the equivalent circuit describing the EIS measurements, the R_film parameters were determined to be 2.324 × 10⁴ Ω cm², 2.605 × 10⁵ Ω cm², and 8.365 × 10⁴ Ω cm² for 12, 96, and 204 h of immersion in a salt solution, respectively. The increase of R_film up to 96 h was attributed to the “self-healing” effect. Following that time, the R_film parameter decreased due to inhibitor depletion from the MWCNTs [170].

Low-cost halloysite nanotubes from a commercial source were internally loaded with corrosion inhibitors (2-mercaptobenzothiazole) and then coated with multi-electrolyte hybrid strips. These modified nanotubes were incorporated into the coatings and studied for the protection provided against corrosion of the AA2024 aluminum alloy. Modified halloysite nanotubes do not allow the profuse release of the corrosion inhibitor (2-mercaptobenzothiazole), except on demand due to local pH changes that come from corrosion, and they appear to be a promising technology [171].

Attapulgite (ATP) nanoparticles were assembled with polyelectrolytes and BTA using the LbL technique. The fiber-shaped morphology of ATP was 0.8–1 μm long and 20 nm in diameter. Using multiple coating techniques, an ATP/polyethyleneimine (PEI)/poly(sodium-4-styrene sulfonate) (PSS)/benzotriazole (BTA)/poly(sodium-4-styrene sulfonate) (PSS)/benzotriazole (BTA) structure was manufactured and assessed via electrochemical methods. The contact angle of water on CBC without the nanoparticles, the CNC with ATP, and the CBCs with the assembled ATP was equal to 68.8°, 73.3°, and 88.2°, respectively. The CBCs with the assembled ATP exhibit hydrophobicity, which is important for paints. Another important result presents the BTA release kinetics of the assembled ATP nanoparticle at pH = 7, 4, and 10, equal to 27.2, 34.7, and 86.4%, respectively. The release rates for pH equal to 7 and 4 are within the measurements’ errors, indicating that the system is useless for an acidic environment. The EIS measurements were recorded as a function of time and analyzed using an equivalent circuit consisting of R_sol, R_coat/C_coat, and R_ct/CPE_dl. For 24 h of immersion in a salt solution, R_coat and R_ct were 9.5 ± 0.20 × 10⁸ and 2.1 ± 0.44 × 10⁸, respectively. For 384 h in a salt solution, R_coat and R_ct decreased down to 1.2 ± 0.28 × 10⁸ and 6.0 ± 0.76 × 10⁷ Ω cm², respectively. Indication for “self-healing” for the CBC-assembled ATP system was observed by scanning electrochemical microscopy (SECM), claimed by the same work [2].

To simulate a biomimetic network, self-healing coatings were manufactured in carbon steel to inhibit corrosion using cellulose nanofibers grafted with a corrosion inhibitor (oleic acid (OA)). Cellulose nanofibers have a terminal −OH in the molecular chain where OA is attached and released from the fibers by changing the pH that causes the corrosion inhibitor to be desorbed from the nanofibers to cover the scratched metal providing protection, thus “self-healing” [172].

New generations of CBCs must be intelligent that attain this property from a new generation of nanocontainers incorporated into coatings exhibiting self-healing induced by pH, redox, temperature, light, and magnetic field. For example, the pores of mesoporous silica nanocontainers (HMS) were modified with azobenzene moieties. Azobenzene functioned as light switches that controlled the release of benzotriazole corrosion inhibitors (BTA). Resins on aluminum accommodated these light-sensitive nanocontainers. These CBCs automatically repair the corrosion area during ultraviolet radiation when scratched, where they release the corrosion inhibitors and aggravate the self-healing effect. The UV radiation at 365 nm modifies azobenzene in the cis form, so the pores open. However, when one illuminates visible light (450 nm), the azobenzene molecules that close the pores are irradiated with the pores because the isomer cis turns into trans, leading to the closure of the gates. As a result of this switching, the release kinetics of the erosion inhibitor from nanocontainers responds to radiations of 365 nm and 450 nm [173]. Similar CBC self-healing work has been done with microcapsules stimulated by sunlight [174]. In recent work, mesoporous silicon nanoparticles (MSNs) were surface-modified with tannic acid complexes that released the encapsulated benzotriazole corrosion inhibitor with pH triggering. The CBCs resulting from incorporating these MSNs into resins on metals exhibited self-healing after 20 days of immersion in a solution of 0.1 M NaCl, confirmed by electrochemical spectroscopy. The impedance modulus at 0.01 Hz grew from 4.7 × 10⁴ Ω cm² to 1.8 ×10⁵ Ω cm² after 15 days of immersion in the salt solution [167]. Another approach was made by developing nanocontainers that incorporate and release two corrosion inhibitors with different release mechanisms. 2-Mercaptobenzothiazole (MBT) inhibitor binds covalently to the nanocontainer shell [175]. Polydimethylsiloxane diglyceryl ether is incorporated inside the nanocontainers. MBT first releases from the nano permeable shell that allows the release of the packaged inhibitor and eventually leads to the sequential release of the two inhibitors. This new methodology is in its infancy, but it is very promising [175].

The development of supramolecular chemistry has widened the scope for manufacturing smart containers by installing supramolecular nano valves into their pores [176]. The exterior of the Fe₃O₄@mSiO₂ nanocontainer incorporated supramolecular assemblies of hypiridynium c water soluble pillar(5)arenes, thus creating corrosion potential stimulus-responsive gates. The supramolecular gate keepers engulfed the encapsulated 8-hydroxyquinoline, a corrosion inhibitor, within the Fe₃O₄@mSiO₂ nanocontainers embedded in resins on AZ31B. These innovative coatings demonstrate the phenomenon of self-healing by creating a protective layer on the magnesium stimulated by corrosion potential [177].

12. Corrosion Sensors

The destruction of CBCs can be caused by cracks in the metal or by corrosion. All these adversities must be sensed immediately to obtain measures to protect the metal structure. The automatic diagnosis of these damages reduces the cost of repairs and increases construction safety, especially in airplanes. Corrosion in airplanes can occur in inaccessible places and is extremely difficult to find. Faults can be diagnosed visually by relatively simple corrosion inspection, e.g., with colored products resulting from local change in pH or automated electronic mechanization. Another automation that should be added to airplane metal structures is that of anti-icing for the automatic removal of ice from its wings.

A technique used in a steel and aluminum paint star system is fluorescent corrosion (FCI) used in paint and coating primer systems for steel and aluminum alloys.

7-Diethylamine-4-methylcumarin (7-DMC) (indicator, coumarin derivative) was added in amounts of about 0.05 to 1.5 wt% in the epoxy/polyamide primer stained on the alloy substrate. After this primer, they passed the epoxy top polyamide overcoat. Each dye had a thickness of 30 microns. Indicators change color or fluorescence depending on the pH or changes in the oxidation state of the metal [178,179].

Lumogallion, N,N-bis-(salicylidene)-2,3-diaminobenzofuran (SABF), and Phen Green^TM were incorporated into epoxy/polyamide primer to determine how aqueous solution and corrosion processes affect the Al³⁺, Mg²⁺, and Cu²⁺ ions. Fluorescence microscopy was employed to identify the localized corrosion and corrosion processes occurring. In conclusion, they have shown that fluorescent probes respond to changes in metal ions Al³⁺, Mg²⁺, and Cu²⁺ in an aqueous solution [180].

3′,6′-Bis (diethylamino)-2-[(1-methyl)amine] (“FD1”) was incorporated into epoxy coatings for the timely detection of steel corrosion. Steel corrosion products stimulated the FD1 index, and fluorescence prematurely identified them when mixed with the coating precursors in the anode region during steel corrosion. This method can be considered an early corrosion detection method created in steels for timely diagnosis before the metal is severely damaged [181].

Various pH-sensitive microcapsules containing corrosion indicators were synthesized and incorporated into commercially available coatings. The indicators were bromothymol blue, phenol red, neutral red, cresol red, and phenolphthalein blue, coloring the coatings purple, yellow, reddish-purple, and fuchsia, respectively. Preliminary results from salt fog testing of prototypes showed that pH-sensitive microcapsules detect corrosion before visible rust appears, even in hidden areas [182].

Coumarin was chosen as a fluorescent index (FI) incorporated into coatings to monitor the underlying corrosion on various metal substrates where corrosion reactions change the pH [183]. Coumarin initially fluoresces in ultraviolet light but changes to non-fluorescent at the corrosion point. This was used to estimate the corrosion reactions between the metal and the coating. This was associated with the change in fluorescence intensity of the coating. The coating was immersed in a 3.5% sodium chloride solution and examined after 24 h for corrosion sites caused by pH. The energy dispersion spectroscopy of selected samples proved that fluorescence reduction was associated with corrosion points. The method showed that the optimal thickness of the coating is 80 μm. Fi in the coating system can reveal corrosion under specially prepared coatings before any visible damage occurs on the surface of the coating itself [183].

Phenolphthalein (PhPh) was loaded into silica nanocontainers (SiNC) to form efficient pH sensors for early corrosion detection of polymer matrix on AZ31 [184]. The color of PhPh changes to pink upon local corrosion that is observable by visual inspection. The system also exhibited “self-healing”. The investigators noted that their system at the micron range could detect minor corrosion events. With this pH sensitivity via drastic color-signaling, the service life of the metallic structures can increase, yielding a reduction in maintenance costs, combined with frequent inspections [184].

Silicium nanocontainers loaded with phenolphthalein were incorporated into resins of the company Mankiewicz GmbH (Germany) on aluminum and magnesium metals. These coatings were designed to detect pH changes resulting from corrosion via local color change resulting from the interaction of phenolphthalein in active cathodic regions. The tests demonstrate the pH sensitivity of advanced nanomaterials dispersed in aqueous solutions. Experiments have shown that this system may be suitable for corrosion detection [185].

(3-Glycidyloxypropyl) trimethoxy silane (GPTMS), methyltrimethoxysilane (MTMS), and hexamethylmethoxymelamine (HMMM) were crosslinked with FAS-3 (3,3,3-trifluoropropyltrimethoxysilane) and FAS-17(1H,1H,2H,2H-perfluorodecyltriethoxysilane) (

Table 3. Compounds inducing superhydrophobicity.

FAS-3 (3,3,3-trifluoropropyltrimethoxysilane)
FAS-17 (1H,1H,2H,2H-perfluorodecyltriethoxysilane)

) to create a hydrophobic coating of thickness two μm on aluminum metal heated in an oven in 120 °C for 30 min [186]. The coating had enhanced hydrophobicity with an angle of contact of approximately 120°. The coatings were studied with potentiodynamic polarization and electrochemical paralysis (EIS), which showed reduced corrosion associated with water-repellence and an intersecting network of low cross-permeability [186].

The coatings made have the rough morphology, as depicted in Figure 25. Such an interface with entrapped air between metal and electrolyte prevents Cl⁻ ions from reacting with the metal substrate. In addition, the air valleys formed are occlusive, thus increasing the corrosion resistance. Another hypothesis is that the dense, net barrier of the epoxy–HMMM hybrid structure results in a strong adhesion with aluminum metal oxide with hydrophobic groups of perfluoro pointing outwards, making water and Cl⁻ water molecules challenging to penetrate [186].

13. Conclusions with Perspectives

The last twenty years have realized remarkable achievements in terms of CBCs’ investigation techniques and the creation of chemistries for remarkable CBC performances. However, giant leaps have been made to develop active CBCs with nanocontainers of different types, shapes, responses, and aptitudes. Nevertheless, many challenges continue to be endemic in science and the technology of passive and active CBCs, and we are waiting for an exciting period of future research. The main challenges focus on developing multifunctional micro/nano-containers that can accommodate various corrosion inhibitors that will activate on-demand, the development of new nanocontainers of small size but large load capacity, and their compatibility with commercial resins. What we avoid discussing in the literature is what are the requirements of the industry and what should be the performances of these new protective coatings. The question arises of the “industry requirements” for such CBCs.

The new CBCs should be standardized according to their implementation. The only stakes should not be the replacement of chrome with environmentally friendly processes but the extension of the life of the protection offered by the current CBCs. As far as the car industry is concerned, we must beware of the increase in time of corrosion protection for light alloys (aluminum, magnesium), which allows for higher car production use. As far as steel is concerned, the aim is to exceed the ASTM B117 [187], which is 500 h of protection in a salt spray chamber, and to double it. Innovative technology must protect their resistance to scratches, graffiti, and exposure to water. The requirements of modern technology are entirely different in aeronautics. It is a well-known fact that we achieve approximately three thousand hours of salt spraying testing with chromate technology. If we want to classify a new “self-healing” CBC technology as successful, we must double the current maintenance intervals to save their maintenance cost. The new systems to be achieved by the prolonged release of healing agents must also be multifunctional, and in the case of airplanes, a critical activity is anti-icing.

In the case of aeronautics, there is a great need to develop CBCs, which, with some automation of the status of CBCs, can consolidate the condition of the coating. The third objective is the shipping industry; in this case, it is necessary to double the lifespan of existing coating systems. The “self-healing” of CBCs will bring additional benefits. The discovery of new biocides derived from nature and included in eco-technology is imperative to remove the harmful biocides we use today. Today, we are seeing much work to find new nano-containers and new coating CBCs. However, this is not enough, considering that the industries that will use these products are cautious in approaching the new proposed solutions. The literature has a few examples that follow different approaches.

For example, Zn/Al LDH manufactured by anion exchange was used in an EADS primer for aeronautical application and offered a masterful self-healing effect. Electrochemical and enhanced corrosion tests have established this. Developed LDH nanocontainers are promising applicants for the replacement of chromate-based anticorrosion pigments, as they provide comparable or even more excellent corrosion protection properties [156]. Another example comes from the maritime industry. Here, CuO, ZnO, and CeMo nanocontainers were loaded with bromosphaerol (CuO and ZnO), and 8-hydroxiquinoline (CeMo) was incorporated into commercial paints used by Wilkens SA and Re-Turn SA, and two ships were painted to test the technology in seawater. Bromosphaerol comes from the marine environment and is an essential ecological biocide. The ship painted with Wilkens SA paints traveled across the Adriatic Sea, and the other ship painted with the Re-Turn paint traveled around the world, both at 14 knots. After a year, the two ships were taken out from the seawater and inspected for fouling and corrosion. The inspection found that the nanotechnology-painted stripes were in better condition than the same commercial paints used for the rest of the ships [40,188,189,190,191]. Here, it is important to note that large quantities of nanocontainers were produced in pilot-scale production, indicating that our technology can quickly pass on to industrial-scale production. The quantities of copper and CeMo are minimal. The bromosphaerol comes from a Ficus from the sea near Corfu and is grown in a nursery to obtain large quantities. This technology is ecological, and there is no need for licenses to apply it to ships.

Research and development of “self-healing” CBCs have reached enormous magnitudes globally, involving many academic and industrial research organizations. The sponsorship by state and industrial organizations increases over time strongly with an incentive not only to understand the mechanisms of healing but to create some new products. There are a few commercial successes, such as Nissan’s "Scratch Guard Coat" painting system [192]. However, the road is very long because the time from developing a prototype until it comes into production is very long. Nevertheless, this does not prevent the government and the industrial finance organizations from increasing investment because the incentives are plentiful, such as reducing the cost of maintenance and repair of CBCs, especially in remote construction sites. This article has an incentive to review the literature so that it becomes an open discussion of the state of the art and the steps that need to be done to soon have commercial applications. “Self-healing” materials have huge boundaries for increasing the resistance of CBCs to corrosion. There is considerable scope for developing new materials such that these sensors will detect the corrosion onsets automatically. We have many creative years ahead of us to work in this beautiful field of self-healing materials that will be readily available for our everyday uses one day soon.