Hybrid sol-gel coating formulations doped with corrosion-inhibiting pigments

Abstract

The corrosion-inhibiting coating comprises a hybrid sol-gel polymer matrix formed from a mixture of an organometallic compound, an organoalkoxysilane, a metal-complexing agent, and at least two corrosion-inhibiting compounds or species.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 239,494, filed September 1, 2021, U.S. Provisional Patent Application No. 63 / 239,485, filed September 1, 2021, and U.S. Nonprovisional Patent Application No. 17 / 822,293, filed August 25, 2022, which are incorporated by reference in their entireties. [Background technology]

[0002] The present disclosure relates to sol-gel coatings, and more particularly to hybrid sol-gel coatings doped with corrosion-inhibiting species or pigments.

[0003] Corrosion damage is a problem for metals exposed to the environment. Metal corrosion is a coupled electrochemical reaction involving the oxidation of an anodic metal and the reduction of an anodic oxide. Metallic materials corrode in a variety of gaseous and aqueous environments, such as atmospheric moist air. Metallic materials are particularly susceptible to corrosion due to galvanic coupling, i.e., when two materials (e.g., dissimilar metals) of different electrochemical potentials are electrically connected in the presence of an electrolyte (e.g., water containing dissolved salts).

[0004] Corrosion protection can take a variety of forms, including the use of corrosion-resistant metal alloys, the isolation of dissimilar metals, the application of conversion coatings, plated metals, and the application of coatings (e.g., paints, epoxies, polyurethanes). During use, additional moisture barriers, such as viscous lubricants and / or protectants, may be added to the corroding surface. Traditional surface treatments for metals sometimes use hexavalent chromium as the active corrosion-inhibiting component.

[0005] Coatings generally protect the underlying metal from corrosion by isolating the metal from the environment. If the integrity of the coating is compromised, for example, because the coating does not adhere well to the metal or because the coating is damaged (cracked, scratched, etc.), the underlying metal may be exposed to corrosive conditions. To compound the threat of corrosion due to loss of coating integrity, coatings are typically opaque, masking the metal surface. Thus, corrosion initiated due to loss of coating integrity may be hidden and may proceed unnoticed. Summary of the Invention

[0006] According to embodiments, the corrosion-inhibiting coating comprises a hybrid sol-gel polymer matrix formed from a mixture of an organometallic compound, an organoalkoxysilane, a metal-complexing agent, and at least two corrosion-inhibiting compounds or species.

[0007] According to another embodiment, a corrosion-inhibiting coating comprising a hybrid sol-gel polymer matrix formed from a mixture of an organometallic comprising zirconium (IV), aluminum, and titanium (IV) in a molar ratio of about 1 to about 3: about 0.5 to about 2: about 0.25 to about 1, an organoalkoxysilane, acetic acid, and at least two corrosion-inhibiting compounds or species.

[0008] According to some embodiments, a method of making a corrosion-inhibiting coating includes mixing organometallic compounds to form an organometallic mixture, mixing a metal-complexing agent with the organometallic mixture, mixing the organometallic mixture and the metal-complexing agent with an organoalkoxysilane to form a hybrid polymer mixture, and mixing at least two corrosion-inhibiting species or compounds with the hybrid polymer mixture.

[0009] Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the present invention, together with its advantages and features, reference is made to the description and drawings.

[0010] For a more complete understanding of this disclosure, reference is now made to the following brief description in conjunction with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional side view of a corrosion-protective hybrid sol-gel coating on a substrate. [Figure 2] 1 is a flow diagram of a method for making a corrosion-protective hybrid sol-gel coating. [Figure 3] 1 is a graph showing the results of corrosion resistance testing for coatings according to an alternating current (AC) impedance test. [Figure 4] 1 is a graph showing the results of corrosion resistance testing of coatings by direct current (DC) anodic polarization. DETAILED DESCRIPTION OF THE INVENTION

[0012] As mentioned above, metal surface treatments include chromium as an active corrosion inhibitor. Traditionally, these treatments have been based on hexavalent chromium (Cr(VI)), a known carcinogen that presents environmental challenges. The most promising alternatives are based on trivalent chromium (Cr(III)). Although Cr(III) presents significantly lower health risks than its hexavalent counterpart, there is ongoing regulatory pressure to reduce the permissible heavy metal exposure limits for manufacturing and repair personnel.

[0013] Siloxane-based sol-gel derived coatings are a promising alternative, offering favorable physical barrier properties and adhesion of organic coatings to metal substrates. However, sol-gel coatings can only provide barrier protection against corrosion and often do not exhibit the self-healing properties of hexavalent chromate conversion coatings.

[0014] Thus, described herein are sol-gel coating compositions including hybrid organosilicon / metal oxide (e.g., specific ratios of zirconium (Zr), titanium (Ti), and aluminum (Al)) coating compositions incorporated with multiple corrosion-inhibiting pigments (or species), and processes for preparing such coatings. In some embodiments, the corrosion-inhibiting pigments include two or more of cerium citrate, cerium acetate, magnesium silicate, zinc molybdate, aluminum orthophosphate, and lithium carbonate.

[0015] Adding two or more low-solubility corrosion-inhibiting pigments to a hybrid sol-gel coating can be challenging for a variety of reasons. For example, the pigments (e.g., cerium- and molybdenum-based compounds) can interfere with the crosslinking of the hybrid sol-gel and can react with each other, ultimately resulting in an inability to inhibit corrosion. However, in the coatings described herein, the ratios of components, including the metal ratio, organosilicon-to-metal ratio, and acid-to-metal ratio, when combined with multiple pigments, provide a corrosion-inhibiting hybrid sol-gel coating.

[0016] According to embodiments, the hybrid sol-gel coating formulation comprises a hybrid polymer matrix that is the reacted form of a composition comprising an organoalkoxysilane, an organometallic compound embedded in the hybrid polymer matrix, and a complexing agent, and at least two corrosion-inhibiting pigments or species.

[0017] Examples of organoalkoxysilanes include, but are not limited to, tetraethoxysilane, 3-glycidoxypropyltrimethoxysilane, or 3-aminopropyltriethoxysilane. According to one or more embodiments, the organoalkoxysilane is 3-glycidoxypropyltrimethoxysilane.

[0018] Organometallic compounds include metal alkoxides and metal complexes. Non-limiting examples of organometallic compounds include organozirconium, organozirconate, zirconium alkoxide, zirconium oxide, organotitanium, titanium alkoxide, organoaluminum, and aluminum alkoxide.

[0019] According to one or more embodiments, the organometallic compounds are zirconium(IV) propoxide (i.e., tetrapropyl zirconate), aluminum tri-sec-butoxide or aluminum isopropoxide, and titanium(IV) isopropoxide (i.e., tetraisopropyl orthotitanate). In other embodiments, the molar ratio of zirconium(IV), aluminum, and titanium(IV) (Zr:Al:Ti) in the hybrid sol-gel coating formulation is about 1 to about 3: about 0.5 to about 2: about 0.25 to about 1. In some embodiments, the molar ratio of Zr:Al:Ti is about 1 to about 1.5: about 0.5 to about 1.5: about 0.5 to about 1.0. In one or more embodiments, the molar ratio of Zr:Al:Ti is about 1.2: about 0.8: about 0.5.

[0020] The molar ratio of silicon from the organoalkoxysilane to the metal from the organometallic, or silicon to the metal, in some embodiments is about 2 to about 5. In other embodiments, the molar ratio of silicon to the metal from the organometallic is about 2.6 to about 3.0.

[0021] The solution mixture of organoalkoxysilane and organometallic compound prior to reaction to form the sol-gel coating is called a sol solution and / or sol. A sol solution is a colloidal solution of small particles containing metal oxide species. Hybrid sol-gels are formed by solution-gelation condensation of one or more organic and inorganic metal species in solution. The metal species are hydrolyzed and condensed to form metal oxide bridges and a gel network. When the metal species contains organic groups, the gel network is a hybrid organic / inorganic polymer. When used as a coating on a metal substrate, the metal portion of the organometallic species may interact, react, adhere, and / or bond with the metal substrate and / or the metal oxide layer on the metal substrate.

[0022] The sol solution may include a sol carrier solution in which the metal species is dissolved, suspended, emulsified, and / or dispersed. The sol carrier solution may be an aqueous solution, a polar organic solution, and / or a non-polar organic solution. For example, the sol carrier solution may include one or more of water, alcohol, propanol, ether, glycol ether, dipropylene glycol dimethyl ether, and dimethyl ether. Additionally or alternatively, the sol solution may include several other components, including, for example, an organic component, a non-polar component, a surfactant, an emulsifier, and / or a pigment.

[0023] Because sol-gel reactions are generally slow, the sol solution includes a hydrolysis catalyst to accelerate the hydrolysis of the metal species and / or stabilize the hydrolysis rate. Non-limiting examples of catalysts include acids and bases. According to one or more embodiments, the catalyst is a carboxylic acid catalyst, such as acetic acid or succinic acid. The catalyst, in some embodiments, maintains an acidic pH of about 2 to about 3.

[0024] Unlike conventional hybrid sol-gel catalysts, in some embodiments, the catalysts used herein have a dual function as both a catalyst to promote hydrolysis and a metal complexing agent. The catalyst functions as a complexing agent, complexing and stabilizing metals from organometallics that would otherwise precipitate from solution. Given this dual function, the molar ratio of catalyst / complexing agent plays an important role. According to one or more embodiments, the molar ratio of catalyst / complexing agent to metal is about 4 to about 10. According to some embodiments, the molar ratio of catalyst / complexing agent to metal is about 5 to about 8.

[0025] In some embodiments, the component used as the complexing agent is the same component used as the catalyst, and they are added in two separate steps. In other embodiments, the component used as the complexing agent is different from the component used as the catalyst. In some embodiments, acetic acid functions as both the complexing agent and the catalyst. In other embodiments, acetic acid is added as the complexing agent or catalyst, and a different acid or base is added as the complexing agent or catalyst, such that the complexing agent and the catalyst are different.

[0026] The hybrid sol-gel coating formulation further includes at least two corrosion-inhibiting species and / or compounds. The corrosion-inhibiting compounds are added to the composition as nanoscale pigment particles or as individual species or components synthesized in situ to form nanoscale pigment particles. The pigment particles must be sufficiently small, typically nanoscale, in size or they will precipitate from solution. Non-limiting examples of corrosion-inhibiting species include cerium(III), zinc(II), molybdate, silicate, phosphate, and lithium species. Non-limiting examples of corrosion-inhibiting compounds include cerium citrate, cerium acetate, magnesium silicate, zinc acetate, zinc molybdate, aluminum orthophosphate, and lithium carbonate. In some embodiments, the corrosion-inhibiting pigment is formed in situ after adding the individual components to the hybrid sol-gel composition. For example, a cerium(III) salt and citric acid form cerium citrate in situ. In another example, zinc(II) salt and molybdenum oxide (MoO3) form zinc molybdate (ZnMoO4) in-situ.

[0027] In some embodiments, the hybrid sol-gel formulations include combinations of two or more, three or more, four or more, or all of Ce(III), zinc(II), molybdate, silicate, phosphate, and lithium species. In other embodiments, the hybrid sol-gel formulations include combinations of two or more, three or more, four or more, or all of cerium citrate, cerium acetate, magnesium silicate, zinc molybdate, aluminum orthophosphate, and lithium carbonate.

[0028] The corrosion-inhibiting pigment compounds are generally poorly soluble in water and aqueous solvents, but can be dissolved in a compatible solution and / or solvent and can be suspended, emulsified, and / or dispersed in the compatible solution and / or solvent. Suitable solutions and / or solvents for dissolving, suspending, emulsifying, and / or dispersing the corrosion-inhibiting compounds include, for example, one or more of polar solvents, non-polar solvents, water, and surfactants.

[0029] According to one or more embodiments, the corrosion-inhibiting species is added to the hybrid sol-gel composition dispersed in a mixture of surfactants and acrylic monomers. Non-limiting examples of surfactants include anionic surfactants, nonionic surfactants, cationic surfactants, amphoteric surfactants, or combinations thereof.

[0030] The corrosion resistance of a corrosion-preventing coating can be evaluated by various methods. In some embodiments, corrosion resistance is evaluated using an AC electrochemical impedance test, which measures impedance (ohms-squared centimeters) as a function of frequency (Hz) in response to exposure. The higher the impedance, the better the barrier / corrosion resistance of the coating. According to some embodiments, the corrosion-preventing coating has an alternating current (AC) impedance of at least 5×10E+06 ohms-squared centimeters at a frequency of 0.01 Hz.

[0031] In another embodiment, the corrosion resistance of the corrosion-protective coating is evaluated using DC anodic polarization, which is expressed as log current density (amperes per square centimeter (Angstroms per cm 2 The potential (voltage (V) relative to a saturated calomel electrode (SCE)) is measured as a function of the corrosion current density. The lower the corrosion current density, the greater the corrosion resistance. According to one or more embodiments, the corrosion-protective coating has an open circuit potential of about -450 to about -600 mV / SCE and a dielectric constant of 1 x 10E-11 to 10E-12 Angstroms / cm. 2 has a current density of less than

[0032] 1 is a cross-sectional side view of a coated substrate 100 including a corrosion-inhibiting hybrid sol-gel coating 102 thereon. The corrosion-inhibiting hybrid sol-gel coating 102 is coated onto, deposited on, and / or cured on the surface of the substrate 102. A second layer of another coating can be deposited on top of the hybrid sol-gel coating 102.

[0033] The substrate 102 is a metal substrate. In some embodiments, the substrate 102 is part of an aerospace structure. Non-limiting examples of metals for the metal substrate include aluminum, copper, magnesium, and alloys thereof. The aerospace structure may be, but is not limited to, an aircraft, a space shuttle, or a missile.

[0034] 2 is a flow diagram of a method 200 for making a corrosion-inhibiting hybrid sol-gel coating. The hybrid sol-gel coating formulation comprises a hybrid polymer matrix that is the reacted form of a composition comprising an organoalkoxysilane, an organometallic compound embedded in a hybrid polymer matrix, and a complexing agent, and at least two corrosion-inhibiting pigments or species.

[0035] The method includes mixing organometallic compounds, as shown in box 202. The organometallic compounds are mixed in a predetermined ratio and under vigorous agitation. According to some embodiments, a first organometallic compound is combined with a second organometallic compound and stirred therewith to form a precursor mixture including the first and second organometallic compounds. Subsequently, a third organometallic compound is combined with the precursor mixture and stirred therewith to obtain a precursor mixture including the first, second, and third organometallic compounds. In some embodiments, the first organometallic compound is a zirconium compound (e.g., zirconium(IV) propoxide), the second organometallic compound is an aluminum compound (e.g., aluminum-tri-sec-butoxide), and the third organometallic compound is a titanium compound (e.g., titanium(IV) isopropoxide).

[0036] The method includes mixing a complexing agent with an organometallic compound, as shown in box 204. The method includes mixing an organoalkoxysilane with the organometallic compound and the complexing agent in a predetermined molar ratio of silicon to total metal, as shown in box 206. The hybrid sol-gel is formed by solution-gelling condensation of one or more metal species in solution. The metal species are hydrolyzed and condensed to form metal oxide bridges and a gel network. If the metal species contain organic groups, the gel network is a hybrid organic / inorganic polymer.

[0037] The method includes mixing a catalyst with the mixture of organometallic compound, organoalkoxysilane, and complexing agent to promote the reaction, as shown in box 208. In some embodiments, the catalyst is the same as the complexing agent in box 204. In other embodiments, the catalyst is different from the complexing agent in box 204. Whether the catalyst and complexing agent are the same or different from the complexing agent, the total molar ratio of the combined catalyst and complexing agent to the metal is about 4 to about 10.

[0038] The mixture of organometallic compound, organoalkoxysilane, complexing agent, and catalyst is thoroughly stirred before adding the corrosion-inhibiting pigment. In one or more embodiments, the mixture of organometallic particles, organoalkoxysilane, complexing agent, and catalyst is stirred for about 18 to about 30 hours, or about 22 to about 26 hours.

[0039] The method includes adding at least two corrosion-inhibiting species or compounds, as shown in box 210. The corrosion-inhibiting compounds are added to the composition either as nanoscale pigment particles or as individual species or components that are synthesized in situ to form nanoscale pigment particles. The size of the pigment particles must be sufficiently small, typically nanoscale, or they will precipitate out of solution. [Example]

[0040] Example 1. Corrosion-preventing hybrid sol-gel formulation The hybrid sol-gel formulation comprises the following composition shown in Table 1: [Table 1]

[0041] Example 2. Corrosion-inhibiting hybrid sol-gel formulation The hybrid sol-gel formulation comprises the following composition as shown in Table 2. The corrosion-inhibiting species forms the corrosion-inhibiting compound in-situ. [Table 2]

[0042] Example 3. AC Electrochemical Impedance and ASTM B117 Salt Spray Exposure Corrosion Resistance Testing The corrosion resistance of the corrosion-protective coatings was evaluated by ASTM B117 testing and AC electrochemical impedance, which evaluates impedance (ohms squared centimeters) as a function of frequency (Hz) in response to salt spray exposure. The higher the impedance, the better the barrier / corrosion resistance of the coating. Figure 3 shows the AC electrochemical impedance test results for various coatings, which are also shown in Table 1 below, including the 168-hour ASTM B117 results (i.e., pit counts). The aluminum (Al) sample was Al6061 (Al-Si-Mg) and contained no coating. The TCP-Al sample was a commercially available trivalent chromium process coated (TCP). The chromate aluminum sample was a commercially available chromated conversion coated (CCC). The aluminum hybrid sol-gel sample was a hybrid sol-gel containing a multi-component mixture of cerium citrate, magnesium silicate, and zinc molybdate, coated in accordance with the present disclosure. [Table 3]

[0043] Example 4. Corrosion resistance test by DC anodic polarization The corrosion resistance of corrosion-protective coatings is evaluated using DC anodic polarization, which is expressed as log current density (amperes per square centimeter (Angstroms / cm) 2 The potential (voltage (V) relative to a saturated calomel electrode (SCE)) is measured as a function of the temperature.

[0044] Figure 4 and Table 4 below show the results of DC anodic polarization testing of various coatings. The sample described in Example 1 was used, where the hybrid sol-gel coating does not contain any corrosion-inhibiting pigments or seeds. [Table 4]

[0045] The corresponding structure, material, acts, and equivalents of all means or step-plus-function elements in the following claims are intended to include any structure, material, or acts for performing the function as specifically claimed in combination with other claimed elements. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or to limit the disclosure to the detailed form set forth. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the present disclosure. The embodiments have been selected and described to best explain the principles and practical applications of the present disclosure and to enable others skilled in the art to understand the present disclosure in various embodiments with various modifications suitable for the particular uses intended. While preferred embodiments have been described, it will be appreciated that those skilled in the art, both now and in the future, may make various improvements and enhancements that fall within the scope of the following claims, which should be interpreted to maintain the appropriate protection for the disclosure as originally described.

Claims

1. 1. A corrosion prevention coating comprising: an organometallic compound; an organic alkoxysilane; a metal complexing agent; at least two corrosion inhibitor compounds; a hybrid sol-gel polymer matrix formed from a mixture comprising 10. The corrosion-inhibiting coating, wherein the at least two corrosion-inhibiting compounds are two or more of cerium citrate, cerium acetate, magnesium silicate, zinc molybdate, aluminum orthophosphate, and lithium carbonate.

2. 10. The corrosion-protective coating of claim 1, wherein the organometallic compound comprises an organozirconium, an organozirconate, a zirconium alkoxide, a zirconium oxide, an organotitanium, a titanium alkoxide, a titanium oxide, an organoaluminum, an aluminum alkoxide, an aluminum oxide, or a combination thereof.

3. 10. The corrosion-protective coating of claim 1, wherein the organometallic compound comprises zirconium(IV) propoxide, aluminum-tri-sec-butoxide, and titanium(IV) isopropoxide.

4. 2. The corrosion-control coating of claim 1, wherein the organoalkoxysilane is tetraethoxysilane, 3-glycidoxypropyltrimethoxysilane, or 3-aminopropyltriethoxysilane.

5. 10. The corrosion-control coating of claim 1, wherein the molar ratio of silicon to metal in the organometallic compound is about 2.6 to about 3.

0.

6. The corrosion-protective coating of claim 1 , wherein the metal-complexing agent is an acid catalyst.

7. A metal substrate comprising the corrosion-inhibiting coating of claim 1 on a surface thereof.

8. an organometallic comprising zirconium (IV), aluminum, and titanium (IV) in a molar ratio of about 1 to about 3: about 0.5 to about 2: about 0.25 to about 1; an organic alkoxysilane; Acetic acid, at least two corrosion inhibitor compounds or species; 1. A corrosion-protective coating having a hybrid sol-gel polymer matrix, comprising:

9. 1. A method for producing a corrosion-protective coating, comprising: mixing organometallic compounds to form an organometallic mixture; mixing a metal-complexing agent with the organometallic mixture; mixing the organometallic mixture and the metal-complexing agent with an organoalkoxysilane to form a hybrid polymer mixture; mixing at least two corrosion inhibitor compounds into the hybrid polymer mixture; and The method, wherein the at least two corrosion-inhibiting compounds are two or more of cerium citrate, cerium acetate, magnesium silicate, zinc molybdate, aluminum orthophosphate, and lithium carbonate.

10. 10. The method of claim 9, wherein the organometallic compound comprises an organozirconium, an organozirconate, a zirconium alkoxide, an organotitanium, a titanium alkoxide, an organoaluminum, an aluminum alkoxide, or a combination thereof.

11. 10. The method of claim 9, wherein the organometallic compounds include zirconium(IV) propoxide, aluminum-tri-sec-butoxide, and titanium(IV) isopropoxide.

12. 10. The method of claim 9, wherein the organoalkoxysilane is tetraethoxysilane, 3-glycidoxypropyltrimethoxysilane, or 3-aminopropyltriethoxysilane.

13. 12. The method of claim 11, wherein the molar ratio of zirconium to aluminum to titanium is about 1.2 to about 0.8 to about 0.

5.

14. 10. The method of claim 9, wherein the molar ratio of silicon to metal in the organometallic compound is about 2.6 to about 3.

0.

15. 10. The method of claim 9, wherein the metal-complexing agent is an acid catalyst, the method further comprising mixing the acid catalyst with the hybrid polymer mixture.

16. 16. The method of claim 15, wherein the molar ratio of the metal-complexing agent to total metal from the organometallic particles is about 5 to about 8.

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