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. Depending on the relationship between alkoxide/water and the kind of catalyst, 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.
1. Brief Introduction of Sol–Gel Processing
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 [1]. From the above studies, inorganic oxide coatings can protect metal substrates but do not significantly replace chromium-based corrosion barrier coatings (CBCs) [2]. 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.
2. Silanes
Silanes are hybrid organic–inorganic compounds used to help alleviate the problem occurring in the traditional sol–gel technology.
Table 1 gives some chemical formulas of silanes/bis-silanes used to start the discussion
[3]. These CBCs based on these precursors have proven effective in recent years
[4]. Their hybrid structure maximizes their effectiveness against corrosion and their properties for primary technological needs.
3. 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 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 (-CHOCH
2) as an essential component of their monomer, i.e., they consist of an organic root and an oxygen atom joined to two carbon atoms
[29]. 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
[29]. 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 1 shows the mixing procedures of the components and their corresponding concentrations schematically.
Figure 1. An example of CBC synthesis using the ORMOSIL method. Z 6020: N-(2-aminoethyl)-3-(trimethyloxypyl) propylamine, GY-257: phenol 4,4′-(1-methylethylidene) bis-, HM 943: triethylenetetramine.
Figure 2, Figure 3 and Figure 4 show the individual reactions.
Figure 2. Hydrolysis of ORMOSIL.
Figure 3. Epoxy group reaction with primary amine.
Figure 4. Synthesis of epoxy resin—ORMOSIL.
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 5 shows the reactions occurring in this process.
Figure 5. Attachment of a hybrid coating to the surface of the metal.
Figure 76 shows the EIS diagram of the bare sample (Sample 1), ORMOSIL coated (Sample 2), ORMOSIL coated + CeO
2 (Empty) (Sample 3), and ORMOSIL coated + CeO
2(5-ATDT) (Sample 4)
[30][31]. The measurement showed that the ORMOSIL coating offered protection to the HDG metal. The metal had an
Rp = 1.73 E
+03 Ohm cm
2, while the metal protected with ORMOSIL had a protected metal with ORMOSIL
Rp = 1.55 E
+04 Ohm cm
2. When CeO
2 (empty) nano-containers were added to ORMOSIL,
Rp = 9.32 E
+04 Ohm cm
2 and increased to
Rp = 3.05 E
+05 Ohm cm
2 when the nanocontainers were loaded with 5-ATDT corrosion inhibitor
[30][32]. The effect of coating the metal with ORMOSIL + CeO
2 (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 + CeO
2 (EMPTY), respectively. This sample exhibited the “self-healing” phenomenon
[32].
Figure 6. FRA of the samples HDG, HDG@ORMOSIL, HDG@ORMOSIL@CeO
2 (EMPTY), and HDG@ORMOSIL@CeO
2 (5-ATDT) exposed to a 0.5 M NaCl corrosive environment at ambient temperature
[32].
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 HNO
3 [33]. 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
[33]. 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
[34].
4. Water-Based Commercial Sol-Gel CBCs
Chemetall GmbH developed the product OXSILAN AL-0500 based on silane for 2024, 2219, 5083, and 7075 aluminum
[35]. 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
[35].
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
[36].