Patent Grant
11846028
The present disclosure concerns molybdate-based composition embodiments and conversion coating embodiments formed from the composition, as well as methods for making and using same.
Metals having a high strength to weight ratio that are resistant to corrosion are useful in aerospace and other industries. Addition of alloying elements to such metals increases their strength but also can lower their corrosion resistance. For this reason, metal surfaces used in such industries, such as aluminum, generally are coated to improve corrosion resistance. A widely used conventional coating is a chromate conversion coating (or “CCC”). The corrosion inhibitive nature of chromates is known and has been shown to be very effective when used on aluminum alloys. By exposing the alloy to a dichromate solution, the increase in susceptibility to corrosion and pitting can be reduced.
The source of chromate used in chromate conversion coatings is usually chromic acid or potassium dichromate, both of which contain chromium in its hexavalent state, a form known to be carcinogenic. In the United States, the Environmental Protection Agency and the Occupational Safety and Health Administration lowered the permissible exposure limit to 5 μg/m3, while the Restriction on Hazardous Substances directive in Europe has an outright ban on the use of hexavalent chromium. As such, there is a need in the art for improved conversion coatings that do not contain chromium.
Disclosed herein are embodiments of molybdate-based compositions for forming molybdenum-based conversion coatings (or “MoCCs”). In some embodiments, the molybdate-based compositions comprise unique combinations of precursor components, such as a combination of a molybdenum component and a fluorine component (or a combination of fluorine components) in addition to a redox oxidizing component and/or a sulfur component. Compositional components and amounts of such components are described herein.
Also disclosed herein are embodiments of MoCCs that comprise molybdenum-containing ions, fluorine-containing ions, ions from the redox oxidizing component, and/or sulfur-containing ions. In some embodiments, the MoCCs can comprise a mixture of any one or more of MoO2, Mo2O5, MoO42−, and MoO3, and the fluorine-containing ions, ions from the redox oxidizing component, and/or sulfur-containing ions.
Also disclosed herein are objects coated with MoCCs formed from composition embodiments described herein. Further, methods for making the MoCCs and methods of coating the objects are described.
The foregoing and other objects and features of the present disclosure will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.
The following explanations of terms are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. As used herein, “comprising” means “including” and the singular forms “a” or “an” or “the” include plural references unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise.
The present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. Any theories of operation are to facilitate explanation, but the disclosed devices and methods are not limited to such theories of operation.
Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed devices, materials, and methods can be used in conjunction with other devices and methods. Additionally, the description sometimes uses terms like “produce” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art.
In some examples, values, procedures, or devices are referred to as “lowest,” “best,” “minimum,” or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, or otherwise preferable to other selections.
Examples are described with reference to directions indicated as “above,” “below,” “upper,” “lower,” and the like. These terms are used for convenient description, but do not imply any particular spatial orientation.
Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting, unless otherwise indicated. Other features of the disclosure are apparent from the following detailed description and the claims.
Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise indicated, implicitly or explicitly, the numerical parameters set forth are approximations that can depend on the desired properties sought and/or limits of detection under standard test conditions/methods. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited. Furthermore, not all alternatives recited herein are equivalents.
To facilitate review of the various embodiments of the disclosure, the following explanations of specific terms are provided:
Anodic Inhibitor (Anodic Inhibition): An anodic inhibitor is a substance that inhibits the anodic reaction or process of corrosion. In some embodiments, it forms a protective oxide coating on the surface of an object, such as a metal object (and thus promotes anodic inhibition).
Barrier Layer: As used herein, a barrier layer refers to any layer that acts as a physical or chemical barrier on a metal object to other species that promote corrosion of the metal object. Solely by way of example, the MoCC can act as a barrier layer on metal objects to invading chloride ions that attack metals and/or their alloys.
Conversion Coating: A protective layer or coating formed on an object, typically a surface of a metal object, which is created by chemical reactions between metal object and a molybdate-based composition as described herein. In some embodiments, the conversion coating can be formed on a surface of the object such that it is in direct contact with the surface, or it can be formed on the surface such that it is not in direct contact with the surface. In representative embodiments, the conversion coating is formed on a surface of the object such that it is in direct contact with the surface.
Open circuit potential (OCP): OCP refers to the potential of a coated metal (or alloy thereof) surface in an electrolyte as measured against a reference electrode (e.g., Ag/AgCl) and is characteristic of the interface (e.g., the surface chemistry of the solid and the liquid electrolyte). In some independent embodiments, aluminum has a different OCP compared to representative Mo-based coatings in a given electrolyte.
Repassivation: As used herein, repassivation refers to the ability of an object comprising a representative MoCC coating to regain its open circuit potential (completely or substantially, such as to regain greater than 50%, such as 60%, 70%, 80%, 85%, 90%, 95%, 99% of its open circuit potential) after the coating is damaged. In some embodiments, repassivation can be determined by measuring the OCP (or by determining the Icorr value) of a MoCC-coated object before and after damage has occurred. In some embodiments, repassivation can result from migration of Mo (or ions thereof) into the damaged region. This ability to spontaneously repair the damaged area is referred to as ‘self-healing’.
“Substantially Covers”: As used herein, the phrase “substantially covers” refers to embodiments where the disclosed conversion coating and/or the composition that provides the conversion coating covers less than 100% of surface area of the object to which it is applied, such as at least 50% of the surface area of the object, such as 60%, 70%, 80%, 90%, 95%, or 99% of the surface area of a substrate.
To prevent the corrosion of certain metal alloys, such as aluminum alloys, a chromate conversion coating (CCC) can be applied to the surface. However, chromate is a carcinogen. The development of chromate-free and environmentally-friendly replacement coatings is therefore desired. The present disclosure describes environmentally-benign, molybdate-based conversion coatings (MoCCs) for the protection of objects, such as metal-based objects used in various industries typically employing metal or metal alloy components (e.g., aircrafts, cars, boats, etc.).
Due the health problems that chromate presents, it would be useful to find a less toxic alternative. Disclosed herein are embodiments of a composition that can be used to provide a conversion coating on an object, wherein the conversion coating has properties and performance characteristics suitable for use in applications and industries requiring coatings that are resistant to corrosion and degradation. The disclosed composition embodiments provide a coating that can replace conventional chromate conversion coatings as the inventive coating provide similar or improved performance as compared to chromate conversion coatings and advantageously is not toxic or hazardous. The disclosed composition embodiments comprise a unique combination of components that provide coatings capable of repassivation (also referred to herein as “self-healing”), anodic inhibition, and combinations thereof. The disclosed composition embodiments comprise molybdenum (typically in ionic form, such as a molybdenum-containing species and/or in an oxide form) and thus also is referred to herein as a molybdate-based composition. The disclosed composition embodiments provide a unique coating that exhibits properties that cannot be achieved by simply applying molybdate-based paints and/or coating films as the disclosed coating is able to self-heal when damage occurs to the MoCC such that any cracks or pits formed in the MoCC due to environmental corrosion or other damaging forces are repassivated and thereby “healed.” Furthermore, the coating embodiments described herein provide different layers of molybdenum oxide species and/or molybdenum-containing species, which lends to their ability to resist different levels of corrosion. Solely by way of example, even if one layer of a deposited molybdenum oxide species and/or molybdenum-containing species formed from the disclosed composition embodiments were to be damaged by corrosion, one or more additional layers of the coating are able to resist such damage thereby providing an undercoat or barrier layer that resists corrosion damage.
The molybdate-based composition embodiments described herein comprise a molybdate component that provides molybdenum ions for the coatings described herein. In some embodiments, the composition can further comprise an iron component, a redox oxidizing component, a fluorine component, a sulfur component, or any combinations thereof. In some embodiments, multiple different species of each component can be used. For example, using a fluorine component can comprise using a single fluorine-containing species, or a mixture of such species (e.g., potassium hexafluorozirconate alone or in combination with one or more of NaF or KBF4). In particular disclosed embodiments, the composition can consist essentially of a molybdate component, an iron component, a redox oxidizing component, a fluorine component, and/or a sulfur component. In such embodiments, the composition is free of any components that would deleteriously affect the properties of the resulting coating formed from the composition (e.g., components that would reduce the ability of the coating to self-heal or provide anodic inhibition) and/or that would increase the toxicity of the composition or a coating made therefrom. In some embodiments, the composition can comprise, consist essentially of, or consist of a molybdate component, a fluorine component, and a sulfur component or a redox oxidizing component. In some independent embodiments, the composition can consist of a molybdate component, a fluorine component and a redox oxidizing component or a sulfur component.
In particular disclosed embodiments, the molybdate component is a molybdate precursor, such as X2MoO4 (or a mixture of molybdate precursors); the iron component is an iron ion precursor, such as a species comprising Fe3+, Fe2+, or a combination thereof (e.g., X3Fe(CN)6); the fluorine component is a fluoride ion precursor, such as XnYmFp, wherein Y is selected from B, Al, Ga, In, Zr, Ti, or Tl, n is an integer ranging from 1 to 4, such as 1, 2, 3, or 4, m is an integer ranging from 0 to 3, such as 0, 1, 2, or 3, and p is an integer ranging from 1 to 8, such as 1, 2, 3, 4, 5, 6, 7, or 8. With reference to formulas comprising an “X” variable, each X independently can be selected from a suitable counterion, such as potassium, sodium, hydrogen, lithium, cesium, rubidium, or any combination of these counterions. In some embodiments, the fluorine component can have a formula XF, X2ZrF6 or XBF4, wherein X is sodium or potassium. The redox oxidizing component can be a manganese-containing species (e.g., a Mn2+—, Mn3+-, Mn4+-, Mn6+-, Mn6+-, or Mn7+-containing species, such as iron permanganate, ammonium permanganate, barium permanganate, or any combination thereof); a chlorate-containing species (e.g., a perchlorate-containing species, such as NH4ClO4, HClO4, KClO4, NaClO4, or any combination thereof); a technetium-containing species (e.g., a pertechnetate-containing species, such as LiTcO4, NaTcO4, RbTcO4, KTcO4, CsTcO4, TlTcO4, NH4TcO4, or AgTcO4); a rhenium-containing species (e.g., a perrhenate-containing species, such as NH4ReO4); a vanadium-containing species (e.g., a vanadate-containing species, such as LiVO3, NaVO3, KVO3, CsVO3, NI4VO3, (NH4)3VO4, or any combination thereof); or even sulfuric or nitric acid. The sulfur component can comprise sulfur oxides and/or anions, including sulfates, sulfites, sulfides, and thiosulfates.
In some embodiments, the composition can comprise, consist essentially of, or consist of potassium ferricyanide, potassium hexafluorozirconate, sodium molybdate, and potassium permanganate. Such composition embodiments can further comprise a sulfate, sulfite, sulfide, and/or thiosulfate species, such as sodium sulfate, potassium sulfate, hydrogen sulfate, lithium sulfate, rubidium sulfate, cesium sulfate, sodium sulfite, potassium sulfite, bisulfate, lithium sulfite, rubidium sulfite, cesium sulfite, sodium sulfide, potassium sulfide, hydrogen sulfide, lithium sulfide, rubidium sulfide, cesium sulfide, sodium thiosulfate, potassium thiosulfate, hydrogen thiosulfate, lithium thiosulfate, rubidium thiosulfate, cesium thiosulfate, or any combinations thereof. In some embodiments, the composition can comprise, consist essentially of, or consist of potassium ferricyanide, potassium hexafluorozirconate, potassium tetrafluoroborate, sodium molybdate, sodium (and/or potassium) sulfate, sodium (and/or potassium) sulfite, sodium (and/or potassium) sulfide, and sodium (and/or potassium) thiosulfate. Such embodiments can further comprise potassium permanganate. In yet additional embodiments, the composition can comprise, consist essentially of, or consist of potassium ferricyanide, potassium hexafluorozirconate, potassium tetrafluoroborate, sodium molybdate, potassium permanganate, sodium (and/or potassium) sulfate, sodium (and/or potassium) sulfite, sodium (and/or potassium) sulfide, and sodium (and/or potassium) thiosulfate. In some embodiments, the disclosed composition embodiments do not comprise (that is, exclude) chromium.
In particular disclosed embodiments, the molybdate, iron, fluorine, redox oxidizing, and sulfur components each can be provided in particular concentrations. In some embodiments, the concentration of each component can be selected to tune the ability of the resulting coating to exhibit anodic resistance and/or repassivation. In some embodiments, the composition can comprise 0 mM to 100 mM of the fluorine component (or a mixture of fluorine components), such as greater than 0 mM to 100 mM, or 0.01 mM to 100 mM, or 0.02 mM to 75 mM, or 0.045 mM to 75 mM, or 0.1 mM to 50 mM, or 40 mM to 60 mM. In embodiments comprising a mixture of fluorine components, the fluorine components can be present such that the total amount of the mixture of the fluorine components ranges from greater than 0 mM to 100 mM, such as 0.01 mM to 100 mM, or 0.02 mM to 75 mM, or 0.045 mM to 75 mM, or 0.1 mM to 50 mM, or 40 mM to 60 mM. In some embodiments, the disclosed composition embodiments comprise 1×10−4 mM to 0.1 mM of an iron component, such as 1×10−3 mM to 5×10−2 mM, or 7.5×10−3 mM to 1.5×10−2 mM. In some embodiments, the disclosed composition embodiments comprise 0.1 mM to 150 mM of the molybdate component, such as 1 mM to 130 mM, or 10 mM to 125 mM, or 100 mM to 130 mM. In some embodiments, the disclosed composition embodiments comprise 0 mM to 50 mM of the redox oxidizing component, such as greater than 0 mM to 20 mM, or 0.1 to 25 mM, or 1 to 20 mM, or 1 mM to 15 mM, or 2 mM to 10 mM, or 2 mM to 5 mM. In some embodiments, the disclosed composition embodiments comprise 0 to 100 mM of the sulfur component (or a mixture of sulfur components), such as greater than 0 mM to 50 mM, or 1×10−5 mM to 50 mM, or 1×10−4 mM to 25 mM or 1×10−4 mM to 15 mM, or 1×10−4 mM to 5 mM. In embodiments comprising a mixture of sulfur components, the sulfur components can be present such that the total amount of the mixture of the sulfur components ranges from greater than 0 mM to 100 mM of the sulfur component (or a mixture of sulfur components), such as 1×10−5 mM to 50 mM, or 1×10−4 mM to 25 mM or 1×10−4 mM to 15 mM, or 1×10−4 mM to 5 mM.
In particular disclosed embodiments, the composition can comprise, consist essentially of, or consist of (i) 0.1 mM to 75 mM NaF, or K2ZrF6, or KBF4, or any combination thereof, such as 0.1 mM to 60 mM, or 0.1 mM to 50 mM; (ii) 0.1 mM to 150 mM Na2MoO4, such as 100 mM to 130 mM, or 100 mM to 125 mM; and (iii) 1 mM to 15 mM KMnO4, such as 2 to 10 mM, or 2 mM to 5 mM. In some such embodiments, the composition can further comprise 1×10−5 mM to 50 mM sodium (and/or potassium) sulfate, sodium (and/or potassium) sulfite, sodium (and/or potassium) sulfide, and/or sodium (and/or potassium) thiosulfate, such as 1×10−5 mM to 25 mM, or 1×10−5 mM to 10 mM. Also, in some such embodiments, the composition can further comprise 1×10−4 mM to 0.1 mM K3Fe(CN)6, such as 1×10−3 mM to 5×10−2 mM, or 7.5×10−3 mM to 1.5×10−2 mM.
In yet some additional embodiments, the composition can comprise, consist essentially of, or consist of (i) 0.1 mM to 75 mM of a mixture of NaF, K2ZrF6, and KBF4, such as 0.1 mM to 60 mM, or 0.1 mM to 50 mM; (ii) 0.1 mM to 150 mM Na2MoO4, such as 100 mM to 130 mM, or 100 mM to 125 mM; and (iii) 1×10−5 mM to 50 mM sodium (and/or potassium) sulfate, sodium (and/or potassium) sulfite, sodium (and/or potassium) sulfide, and/or sodium (and/or potassium) thiosulfate, such as 1×10−5 mM to 25 mM, or 1×10−5 mM to 10 mM. In some such embodiments, the composition can further comprise 1×10−4 mM to 0.1 mM K3Fe(CN)6, such as 1×10−3 mM to 5×10-2 mM, or 7.5×10−3 mM to 1.5×10−2 mM and/or 1 mM to 15 mM KMnO4, such as 2 to 10 mM, or 2 mM to 5 mM. In particular disclosed embodiments, the compositions comprise any combination of 0.125 M sodium molybdate, 0.05 M potassium hexafluorozirconate, 0.045 mM NaF, 0.16 mM KBF4, 0.03 mM Na2S2O3, 0.0001 mM Na2SO4, and/or 5 mM KMnO4.
Embodiments of the disclosed composition embodiments may be acidic, meaning they have a pH lower than 7. In some embodiments, the composition can have a pH of 0.1 to 5, such as 0.2 to 4 or 0.5 to 2.5. The pH of the composition may be modified to be acidic by adding an organic or inorganic acid component, such as nitric acid, sulfuric acid, citric acid, malic acid, tetraacetic acid, hydrofluoroic acid, manganic acid, and combinations thereof. In particular disclosed embodiments, nitric acid may be used to lower the pH of the composition. In some embodiments, the disclosed composition comprises 0.0001 to 10 mM of the acid, such as 1×10−2 to 10×10−2 mM of the acid.
In embodiments where an acid is added, the acid does not deleteriously affect the corrosion resistance of the coating made from the composition.
Also disclosed herein are kit embodiments that comprise components of the molybdate-based composition disclosed herein. In some embodiments, the kit can comprise a combination of the composition components described above. In some embodiments, the kit can comprise a container containing the molybdate component, the fluorine component, the iron component, or any combination thereof. Such embodiments of the kit may also further comprise separate containers comprising, independently, the redox oxidizing component and the sulfur component. In yet some embodiments, the kit can comprise a container that contains the molybdate component, the iron component, the redox oxidizing component, and the sulfur component. In yet additional embodiments, the kit can further comprise a separate container comprising an acid (e.g., an inorganic or an organic acid) that can be combined with the components of one or more additional containers. In some embodiments, the kit can comprise the molybdate component, the iron component, the redox oxidizing component, the sulfur component, and an acid.
As described above, the molybdate-based compositions disclosed herein can be used to form a molybdate-based conversion coating (or “MoCC”). The MoCC typically is formed on an object that needs protection from corrosion and other stresses from a surrounding environment. For example, the MoCC disclosed herein can be used on objects typically used in the aerospace field, the automobile industry, or the nautical industry. For example, the coatings are suited for use on the parts of airplanes, boats, and cars that typically are exposed to stresses that cause corrosion and/or damage to the airplane, boat, or car. In particular disclosed embodiments, the MoCC embodiments described herein can be used on objects to create a surface that is suitable for paint adhesion. Solely by way of example, the MoCC embodiments described herein can be applied to an object, and then a primer and an outer topcoat paint layer can be provided. In particular disclosed embodiments, the object typically is a metal object and comprises aluminum, magnesium, and/or iron and often can be made of or contain an aluminum alloy, a magnesium alloy, an iron alloy, or any combinations thereof. Without being limited to a particular theory, it currently is believed that adding the MoCC to an object's surface improves paint adherence because it can prepare the object's surface to be painted by removing organic impurities on the surface, and it can provide porous surface that increases adhesion leading to more interaction between the MoCC and a primer layer.
In particular disclosed embodiments, the MoCC embodiments described herein can be formed from the compositions disclosed above. As such, some embodiments of the MoCC can comprise molybdenum-containing species (e.g., one or more of Mo2O5, MoO42-, MoO2, and MoO3), fluorine ions, ions formed from the disclosed redox oxidizing agent, sulfur ions, and any combination thereof. In some embodiments, the MoCC can comprise molybdenum-containing species (e.g., one or more of Mo2O5, MoO42+, MoO2, and MoO3), fluorine ions, permanganate ions, perchlorate ions, pertechnetate ions, perrhenate ions, vanadate ions, (and any combination thereof), sulfur ions, and any combinations thereof. Without being limited to a particular theory, it currently is believed that the MoCC is formed via redox reactions of molybdenum-containing species present in the composition and a component of the object being coated, such as a metal object. The redox reaction can promote formation of different forms of molybdenum-containing species which can be reduced as the metal object undergoes oxidation. This reactivity can produce the MoCC, which contains internal layers of different molybdenum oxide species (e.g., Mo2O5, MoO2, and/or MoO3), or molybdenum-containing species (e.g., MOO42−), or any combinations thereof. In particular disclosed embodiments, the MoCC can comprise an initial layer that forms on the metal object. This initial layer can comprise a mixture of Mo2O5, MoO42−, MoO2, and MoO3 species. The MoCC also can comprise a second layer formed on the initial layer that comprises Mo2O5 and MoO42− species. In some embodiments, the coating can form a barrier layer when the coating is applied to a surface of the object. The barrier layer can comprise an outer layer comprising Mo+5 and Mo+6 ions and an inner layer comprising Mo+4.
A representative schematic of a representative MoCC embodiment is illustrated in
As described herein, the MoCC embodiments made using the compositions described herein exhibit the ability to self-heal via repassivation. Without being limited to a particular theory, it is currently believed that the different molybdenum-containing species present in the MoCC can migrate between the different layers of the MoCC. For example, when the MoCC is scratched or damaged, molybdenum-containing species are able to migrate to and passivated the damaged area, thereby preventing any further corrosion. This can be evidenced by monitoring the open circuit potential (or “OCP”) of an object coated with an embodiment of the MoCC in a salt solution (e.g., 0.05 M NaCl) before and after the surface of the MoCC is scratched. Typically, after scratching the MoCC surface, a sudden drop in potential will be observed, which indicates that the surface has become more active since the underlying object (e.g., aluminum) was exposed. Typically, the OCP will then increase back to what it was before the coating was scratched (or substantially close thereto), thereby indicating that the coating has healed itself. In particular disclosed embodiments, the MoCC embodiments exhibit repassivation within a very short time period. For example, some embodiments of the disclosed MoCC may exhibit repassivation in 5 minutes or less, such as 3 minutes or less, or 2 minutes or less, or even 60 seconds or less after the MoCC is scratched. In some embodiments, the MoCC embodiments can exhibit an OCP of −500 to −750 mV versus an Ag/AgCl standard reference electrode, such as an OCP of −550 to −650 mV. For comparison, the OCP of conventional CCC's can typically range from −500 to −600 mV. In yet additional embodiments, the MoCC embodiments can exhibit anodic inhibition, but do not exhibit cathodic inhibition, which typically is exhibited by conventional conversion coatings, such as CCCs.
In a representative embodiment, a MoCC was applied to an aluminum alloy substrate and provided a corrosion potential from −670 mV to −600 mV versus an Ag/AgCl reference electrode. Additionally, the number of corrosion pits was reduced on the MoCC-coated sample when compared to uncoated substrate samples. The repassivation ability of embodiments of the disclosed MoCCs can be examined by scratching the coated sample with a glass tip and measuring the open circuit potentials (OCP). In a representative embodiment, the OCP rapidly dropped after scratching due to the exposed underlying alloy; however, the OCP also rapidly traced back to its pre-scratch potential, indicating that the MoCC possessed the ability to ‘self-heal’ via repassivation.
In particular disclosed embodiments, scanning electron microscopy (SEM) analysis can be used to analyze embodiments of the MoCCs disclosed herein, after application to a substrate. In some embodiments, the SEM result confirmed that the MoCC exhibited a surface morphology having a mud cracked pattern similar to what would be seen on a sample coated with a CCC. In some additional embodiments, ultraviolet-visible (UV-Vis), Raman, Fourier Transform-Infrared (FT-R) and energy dispersive (XRD) spectroscopy methods as well as X-ray photoelectron spectroscopy (XPS) can be used to confirm the presence of the MoCC on the surface of an object. These analytical techniques can be used to confirm that a protective layer of MoCC is in fact deposited and formed on an object after exposing the object to a composition embodiment described herein.
Disclosed herein are embodiments of making a molybdate-based composition and a conversion coating formed from the composition. In some embodiments, the molybdate-based composition can be formed by combining the components of the composition as separate stock solutions. For example, separate stock solutions of the molybdate component(s) and the iron component(s), and the redox oxidizing component(s), and/or the sulfur component(s) can be prepared by combining each component separately with water. Amounts of each stock solution are then combined and mixed to obtain the desired concentration of each component and thereby provide the composition. In some embodiments, the method can further comprise adding an amount of an acid to the composition to lower the pH of the composition to a desired pH as described herein. In some embodiments, 1 M HNO3 can be added until the pH is lowered (such as to a pH of 1.5). The composition can then be allowed to equilibrate prior to exposing an object to the composition.
Once the composition has been prepared, it can be used to coat (or substantially coat) an object. The object is exposed to the composition to for an amount of time sufficient to form the corresponding conversion coating on the object. In some embodiments, the amount of time during which the object is exposed to the composition can range from one minute or less to 15 minutes, or from 2 minutes to 12 minutes, or from 5 minutes to 10 minutes. In particular disclosed embodiments, the object is exposed to the composition for 7 minutes. A person of ordinary skill in the art will recognize with the benefit of this disclosure that the amount of time may increase or decrease depending on the size of the object to be coated.
In some embodiments, the MoCC composition is deposited on an object to form the conversion coating. Exemplary methods for depositing the MoCC composition can include, but are not limited to, spray coating, dipping, sputtering, printing, painting, or submerging, the object in (or with) the MoCC composition. Any other suitable deposition methods also can be used. In some embodiments, the MoCC can form a single continuous (that is, an uninterrupted) layer directly on a surface of the substrate. In some embodiments, the MoCC can be applied so as to form a coating on particular areas of the object and not on other areas, such as by depositing the composition in a pattern on the object. In some embodiments, the MoCC can form a layer that completely covers a surface of the object (e.g., 100% of the surface area) or that substantially covers a surface of the object (e.g., at least 50% of the surface area of the object, such as 60%, 70%, 80%, 90%, 95%, or 99% of the surface area of a substrate). The MoCC will form a separate layer on a surface of the object and it can have a thickness ranging from 10 nm to 10 mm, such as 25 nm to 5 mm, or 50 nm to 1 mm, or 100 nm to 500 nm. After the MoCC has formed on the object, the object can be rinsed with water and dried (either using an affirmative drying step, such as heating the object, blotting the object, or passing an inert gas over the object, or by allowing the object to dry under ambient conditions).
The molybdate-based composition described herein can be used to form molybdate-based conversion coatings on an object, such as a metal object. The object typically is made of aluminum, magnesium, iron, or an alloy of aluminum, magnesium, and/or iron, or any combination thereof. The MoCCs disclosed herein may be used to protect an object from corrosion and/or to reduce the amount of wear due to corrosion. In some embodiments, MoCCs formed from a composition embodiments comprising a a redox oxidizing component, a sulfur component, or any combinations thereof (as described above) can provide superior corrosion protection as compared to MoCCs formed from compositions without these components. In some embodiments, MoCCs formed from compositions comprising a redox oxidizing component, a sulfur component, or any combinations thereof can exhibit Icorr values of less than 0.9 μA/cm2, such as Icorr values of less than 0.8 μA/cm2, or 0.7 μA/cm2, 0.5 μA/cm2, or 0.1 μA/cm2. In some embodiments, MoCCs formed from compositions comprising a redox oxidizing component, a sulfur component, or any combinations thereof can exhibit Icorr values of 0.01 μA/cm2 or less, such as between 0.01 μA/cm2 and 0.0020 μA/cm2, such as 0.0025 μA/cm2.
In some embodiments, methods of protecting a metal surface from corrosion include the steps of preparing a MoCC composition as disclosed herein, and contacting a metal surface with the composition to form a conversion coating on the metal surface. In such embodiments, contacting the object can comprise any of the deposition methods described above. Any of the MoCCs described herein may be used in to protect a substrate from corrosion and/or to reduce the amount of wear due to corrosion. In some embodiments, the disclosed MoCCs can exhibit repassivation at a rate that is the same or that is superior to a CCC.
In particular disclosed embodiments, the composition is used to provide a conversion coating that covers or substantially covers parts of aircraft, vehicles, and/or boats. In exemplary embodiments, the composition is used to provide a conversion coating that protects airplane wings and other components of an aircraft from corrosion.
Disclosed herein are embodiments of a composition, comprising: 0.1 to 75 mM of a single fluorine component having a formula XnYmFp, or a combination of such fluorine components; 1 to 150 mM X2MoO4; and 1 to 15 mM of a redox oxidizing component comprising a permanganate species, a perchlorate species, a pertechnetate species, a perrhenate species, or a vanadate species; wherein each X independently is a counterion selected from potassium, sodium, hydrogen, lithium, rubidium, or cesium; Y is selected from B, Al, Ga, In, Zr, Ti, or Tl; n is an integer selected from 1, 2, or 3; m is an integer selected from 0, 1, 2, or 3; p is an integer selected from 1, 2, 3, 4, 5, or 6; and wherein the pH of the composition ranges from 0.5 to 2.5, and the composition does not comprise chromium.
In any or all of the above embodiments, the composition further comprises 0.0001 to 50 mM Na2SO4, 0.0001 to 50 mM Na2SO3, or a combination thereof.
In any or all of the above embodiments, the composition further comprises 0.03 to 100 mM Na2S2O3.
In any or all of the above embodiments, the composition further comprises 0.0001 mM to 10 mM of an acid.
In any or all of the above embodiments, the composition comprises 0.1 to 75 mM NaF, or K2ZrF6, or KBF4, or any combination thereof, 1 to 150 mM Na2MoO4; and 1 to 15 mM KMnO4.
In any or all of the above embodiments, the composition comprises 40 to 60 mM K2ZrF6; 100 to 130 mM Na2MoO4; and 2 to 10 mM KMnO4.
In any or all of the above embodiments, the composition comprises 0.125 M Na2MoO4; 0.05 M K2ZrF6; and 5 mM KMnO4.
Some embodiments of the disclosed compositions comprise 0.1 to 75 mM of a single fluorine component having a formula XnYmFp, or a combination of such fluorine components; 1 to 150 mM X2MoO4; and 0.0001 to 50 mM X2SO4, X2SO3, X2S2O3 or any combination thereof, wherein each X independently is a counterion selected from potassium, sodium, hydrogen, or lithium; Y is selected from B, Al, Ga, In, Zr, Ti, or Tl; n is an integer selected from 1, 2, or 3; m is an integer selected from 0, 1, 2, or 3; p is an integer selected from 1, 2, 3, 4, 5, or 6; and wherein the pH of the composition ranges from 0.5 to 2.5, and the composition does not comprise chromium.
In any or all of the above embodiments, the composition comprises 0.0001 to 5 mM Na2SO4 and 0.03 to 5 mM Na2S2O3.
In any or all of the above embodiments, the composition comprises 0.1 to 75 mM of a mixture comprising NaF, K2ZrF6, and KBF4; 100 to 130 mM Na2MoO4; and 0.0001 to 5 mM of a mixture comprising Na2SO4 and Na2S2O3.
In any or all of the above embodiments, the composition comprises 0.125 M Na2MoO4; 0.0502 M of a mixture comprising K2ZrF6, NaF, and KBF4; and 0.03001 mM of a mixture comprising Na2SO4 and Na2S2O3.
In any or all of the above embodiments, the composition further comprises 1 to 15 mM of a redox oxidizing component that comprises a permanganate species, a perchlorate species, a pertechnetate species, a perrhenate species, or a vanadate species.
In any or all of the above embodiments, the composition further comprises 1×10−3 to 5×10−2 mM X3Fe(CN)6, wherein X is a counterion selected from potassium, sodium, hydrogen, lithium, rubidium, or cesium.
Also disclosed herein are embodiments of a coated object, comprising: an object comprising a top surface; and a conversion coating formed on the top surface of the object that covers or substantially covers the top surface of the object, wherein the conversion coating comprises one or more of MoO2, Mo2O5, MoO42−, and MoO3 and exhibits an Icorr value ranging between 0.0020 μA/cm2 and 0.01 μA/cm2.
In any or all of the above embodiments, the conversion coating comprises an outer layer comprising Mo(VI) and Mo(V) and an inner layer comprising Mo(IV).
In any or all of the above embodiments, the conversion coating exhibits anodic inhibition.
In any or all of the above embodiments, the object comprises aluminum, magnesium, iron, or an alloy of aluminum, magnesium, and/or iron, or any combination thereof.
In any or all of the above embodiments, the conversion coating does not comprise chromium.
In any or all of the above embodiments, the conversion coating has an open circuit potential of −500 to −750 mV versus an Ag/AgCl standard reference electrode.
In any or all of the above embodiments, the conversion coating has an open circuit potential of −520 to −700 mV versus an Ag/AgCl standard reference electrode.
In any or all of the above embodiments, the conversion coating exhibits repassivation within 60 seconds after damage to the substrate.
In any or all of the above embodiments, the conversion coating is formed from the composition of any or all of the above composition embodiments.
In any or all of the above embodiments, the conversion coating is formed directly on the top surface of the object.
Also disclosed herein are embodiments of a coating, comprising: one or more of MoO2, Mo2O5, MoO42− and MoO3; fluorine ions; and ions formed from a redox oxidizing component, or sulfur ions, or a combination of ions formed from a redox oxidizing component and sulfur ions.
In any or all of the above embodiments, the coating is made from a composition according to any or all of the above composition embodiments.
In any or all of the above embodiments, the ions formed from the redox oxidizing component are permanganate ions, perchlorate ions, pertechnetate ions, perrhenate ions, vanadate ions, and any combination thereof.
Also disclosed herein are embodiments of a method for making an object comprising a conversion coating, comprising:
exposing an object to a composition according to any or all of the above embodiments for a time period sufficient to form a conversion coating on one or more surfaces of the object; and
removing the coated object from the composition to provide the object comprising the conversion coating, wherein the conversion coating comprises one or more layers comprising one or more of MoO2, Mo2O5, MoO42− and MoO3.
In any or all of the above embodiments, the object comprises aluminum, magnesium, iron, or an alloy of aluminum, magnesium, and/or iron, or any combination thereof.
In any or all of the above embodiments, the conversion coating does not comprise chromium.
Materials:
Large sheets of aluminum alloy AA2024-T6 having the composition listed in Table 1, were cut into coupons that measured 1.5 cm×1.5 cm. The coupons were polished to a finish of 1 μm by wet polishing using a copper-free Buehler Metadi diamond suspension solution.
Coating solutions were made using varying amounts of ferricyanide, hexafluorozirconate and sodium molybdate and in one embodiment the ferricyanide was replaced with titanium dioxide. Stock solutions were made using these three species by combining each one separately with deionized water. Twenty-five (25) mL of each solution was then mixed to get the desired concentration. After the solutions were combined together, 1 M HNO3 was added until the pH was lowered to 1.5. This mixture was left overnight to equilibrate prior to use as a coating bath.
The polished samples were degreased using isopropanol and placed in a tissue wetted with DI water for 10 minutes prior to exposure to the coating solution. The sample remained submerged in the solution until a uniform coating was formed. The coating time for the formation of molybdate coatings is in the same range as the time taken for the formation of chromate conversion coatings, which is listed as 5-10 minutes. The coating formulation time depended in part on the composition of the coating bath. However, the majority of the samples formed a good coating at approximately 7 minutes. Once the coating was formed, the sample was removed from the bath, rinsed with DI water and blotted dry with a tissue.
A total of twelve composition embodiments were initially analyzed, with varying concentrations of the components contained within the solution. The amount of sodium molybdate in the coating formations was fixed at 25 mM. The concentrations of potassium hexafluorozirconate and potassium ferricyanide were varied, as described in Example 1 below.
Electrochemical Analysis:
Each of the twelve compositions of Example 1 were used to coat a polished aluminum alloy AA2024-T6 sample. The coated samples were exposed to a 0.05 M NaCl solution to test the corrosion resistance of each of the coatings. All electrochemical tests were conducted using a flat cell. A platinum coated niobium mesh was used as the counter electrode. All potentials were measured with respect to an Ag/AgCl reference electrode. A solution of 0.05 M NaCl was used as the corrosive media. Electrochemical tests were performed using a PC4/FAS1 and a REF 600 model potentiostat supplied by Gamry Instruments, Inc. Gamry Framework version 4.1 was used to control the potentiostat and Gamry's ECHEM analyst version 1.1 was used to analyze the data. The software was used to perform an open circuit potential (OCP) and potentiodynamic test to obtain the polarization curves. Prior to polarization, the open circuit potential was monitored until the signal stabilized in a range of +/−5 mV which took 20 s to 120 s. Polarization tests were conducted in potentiodynamic mode. Samples were polarized from −0.75 V to +2.0 V versus the OCP at a scan rate of 5 mV/s.
Repassivation:
A sample of 1 cm2 area exposed to 0.05 M NaCl and the OCP was monitored. During this time the sample was scratched with a glass tip and the OCP was monitored.
Optical Microscopy:
An Olympus model PMG3 microscope connected to a computer was used to capture the micrographs of the samples before and after coating, and after corrosion studies. The magnification was maintained at 50× and the surface of the sample, the color of the coating and the number of pits created after the sample had been corroded, was observed.
Scanning Electron Microscopy:
The surface morphology of the coated samples was examined using a Hitachi S-4700 Field-Emission Scanning Electron Microscope. The scanning electron microscope was operated at a voltage of 20 keV. SEM was operated in analysis mode to provide high resolution images for these samples, as well as for the energy dispersive x-ray spectroscopy, which is included in this particular instrumentation system.
Ultraviolet-Visible Spectroscopy:
A UV-Vis spectrometer supplied by Shimadzu (model UV-2401PC) was used to analyze the coatings to confirm the presence of molybdate. A tungsten lamp was used and the studies utilized wavelengths in the range from 200 to 900 nm.
Fourier Transform-Infrared Spectroscopy:
FT-IR spectra were obtained using a Thermo Nicolet model Magna 760 FT-IR spectrometer in a grazing angle drifts mode with a gold slide as background. Data was collected from averaging 256 scans.
Raman Spectroscopy:
Raman spectra of the coated samples, uncoated samples and finely ground sodium molybdate were obtained using an Almega model with a 785 nm laser supplied by Thermo Electron Scientific Instrument Corporation. Data was collected from averaging 64 scans. High resolutions scans were collected using a 4 μm spot size and laser intensity at 10%.
X-ray Photoelectron Spectroscopy:
XPS analysis was performed using a Kratos Axis Ultra DLD. The samples were analyzed with and without sputtering, which was conducted for 1 minute with Ar+. A polished Al sample, sodium molybdate and a coated sample were analyzed to determine what chemical species were present on the surface of the coated aluminum alloy AA2024-T6.
In this example, twelve molybdate-based compositions, prepared as described above and as shown in Table 2, were applied to an aluminum substrate.
The parameters that were considered to determine the quality of the coating are listed in Table 3. They are as follows; open circuit potential (OCP) measured in millivolts versus Ag/AgCl reference, Ecorr measured in millivolts versus Ag/AgCl reference electrode and Icorr measured in microamperes. The difference between the OCP and the Ecorr is that the OCP values were observed by measuring the open circuit potential of the sample, and Ecorr was determined using potentiodynamic polarization experiments.
Slight variations in the measurement of OCP values can be seen in
Tafel plots and corrosion parameters were obtained from polarization studies, and are shown below in Tables 4-15 for each set of replicates used in the testing of each specific Composition, as indicated. For each Composition, the polarization data was conducted in 0.05 MNaCl using a platinum counter electrode, and potentials were measured against an Ag/AgCl reference electrode.
The potentiodynamic polarization data shown in
Composition 2 was used as an exemplary embodiment of the MoCCs disclosed herein, and used for further analytic studies.
Aging studies were conducted to determine when the coatings are adequately aged. Chemistry of the chromate conversion coatings have been known to change with time, with a corresponding change in corrosion resistance. Typically CCCs have to be aged for at least 24 hours before they exhibit good corrosion resistance.
A study was conducted to determine how long the coating can be aged to obtain high corrosion resistance.
A study was conducted to determine whether the coating is protective to AA2024-T6. In the study, a sample coated with Composition 2 was compared to an as-received uncoated aluminum sample. In this example, the MoCC was aged for 24 hours.
One of the properties that make CCCs particularly useful is their ability to self-heal or repassivate. In order to make a MoCC that can replace CCCs, it should display substantially the same or superior behavior.
Samples that were viewed using the scanning electron microscope included the following: a polished AA2024-T6 sample and an AA2024-T6 sample coated with Composition 2 before and after it had under electrochemical testing. In this example, the MoCC was aged for 24 hours.
An aluminum sample that was coated with Composition 4 was subjected to electrochemical tests and this coating was considered to offer low corrosion protection. SEM imaging was performed to compare it to the coating of Composition 2. In this example, the MoCC was aged for 24 hours. The sample shown in the images of
An AA2024-T6 sample that was coated using Composition 2 and had undergone corrosion testing was imaged, and the images are shown in
The images in
Pits were found on the sample shown in
The composition of the MoCC formed with Composition 2 on the aluminum substrate was also analyzed using energy dispersive x-ray spectroscopy (EDS). The location of the coated sample where the EDS spectrum was obtained, is shown in the SEM of
UV-Vis reflectance spectroscopy was used to determine the chemical species present on the surface of the AA2024-T6 samples. For all scans, the first and last 25 nm showed significant noise, however, the compounds of interest do not exhibit any peaks in those regions. Consequently, they have been omitted from the spectra shown. A sample of polished AA2024-T6 with no coating was used as a baseline for comparison, and that spectrum is shown in
FT-IR spectroscopy was conducted on three samples: (1) uncoated polished AA2024-T6, (2) a MoCC formed using Composition 2 on an AA2024-T6 substrate, and (3) finely ground sodium molybdate powder.
Raman spectroscopy was also conducted on three samples: (1) uncoated polished AA2024-T6, (2) a MoCC formed using Composition 2 on an AA2024-T6 substrate, and (3) finely ground sodium molybdate powder.
The samples that were analyzed using Raman spectroscopy were also analyzed using XPS. The XPS wide scan obtained from the uncoated polished aluminum alloy AA2024-T6 is shown in
The samples were analyzed before and after sputtering. The sputtering depth was calculated as follows using Equation 1:
where
M: molar weight of the target [kg/mol]
ρ: density of the material [kg/m3]
Na: 6.02×1026/kmol; (Avogadro number)
e: 1.6×10−19 A (electron charge)
S: sputtering yield (atom/ion)
jp: primary ion current density [A/m2]
t: time (min), and
z: depth of sputtering (nm).
The values used are defined in Table 19 for the ion of Ar+. Based on these values, the sputtering depth was calculated to be 40 nm.
The Al 2p peak was not distinctly observed in the narrow scan performed at 65 to 86 eV, which is shown in
A difference between the two spectra is that after sputtering, a small peak is seen to start appearing where aluminum would typically be seen on a XPS spectrum. Although a peak starts appearing, it is still very small and barely above background noise. This is supported by the fact that Al 2p forms ˜1.4 atomic % of the analyzed depth. The constant appearance of the aluminum shows that aluminum ions form an integral part of the MoCC coating. This indicates that the coating is a chemically formed Al—Mo composite coating. The larger broader peak at 65 eV is still observed and this peak is consistent with the 4s subshell of molybdenum.
The summary for the peak-fitting processes are shown in Tables 22 and 23. To ensure the peak fitting was done correctly, the ratios of the areas corresponding to the j-values 5/2 and 3/2 was calculated to be 1.5, which is the value that would be expected for the 3d subshell.
The results in Table 23 support the presence of a self-healing MoCC, since the mechanism for corrosion protection for CCCs involves Cr6+ and Cr3+, with Cr3+ providing a barrier for protection and the Cr6+ responsible for the self-healing ability that is exhibited by a CCC. Another result indicative of the MoCC showing CCC-like characteristics is that the spectra showed different compositional makeup prior to and after the sputtering process. Before sputtering, the results indicated that on the surface the species present are oxidized Mo5+ and Mo6+. After sputtering, approximately 40 nm were removed from the surface and this resulted in the observation of reduced Mo4+, which was not seen originally. These results are similar to CCCs in which the surface layer is composed of predominantly oxidized Cr6+ and the base layer is composed of mostly reduced Cr3+ Since there are multiple valence states of molybdenum present in this sample, the same mechanism proposed for the self-healing behavior of CCCs is likely active in the self-healing behavior of MoCC that was shown in
The peak that is shown at 235.9 eV in
The XPS spectra obtained in the oxygen is region are shown in
Due to the broadness of the peaks observed in
Table 26 shows the composition of MoCC before and after sputtering in terms of valency of Mo species in the coating. It can be seen that the outer layer is predominantly composed of oxidized forms (Mo5+ and Mo6+) while the inner layer is predominantly composed of reduced species (Mo4+).
This data indicates that the MoCCs described herein are composed of multiple molybdate-based species including MoO2, Mo2O5, MoO4 and MoO3. These MoCCs consist of two layers, with a surface layer primarily composed of oxidized Mo(VI), and an inner layer that is primarily composed of reduced Mo(IV) and Mo(V) species. This is illustrated in
The molybdate-based compositions disclosed herein provide environmentally-friendly corrosion-protective molybdate coatings. Once the MoCCs are formed, tests determined that substrates coated with the MoCCs had improved corrosion resistance as compared to uncoated substrates, and it was shown that the MoCC was not just a superficial layer but was in fact protective of the underlying aluminum alloy substrate via anodic inhibition. Corrosion results are summarized in Table 27.
In this example, a chromate conversion coating (CCC) was prepared and used for comparison with the MoCCs disclosed herein. The CCC-coated comparison sample was prepared as described by D Chidambaram, C. R. Clayton, G. P. Halada, and Martin W. Kendig, “Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings I. Composition and Electrochemical Behavior of the Oxide Film”, Journal of The Electrochemical Society, 151 (11), B605-B612, 2004, and the commercially-available Alodine® chromate conversion coating from Henkel Technologies.
In this example, a MoCC was formed using Composition 2 (as described in Example 1), and its corrosion protection properties were compared with the CCC prepared in the Comparative Example. Specifically, the corrosion resistance of an aluminum substrate coated with MoCC Composition 2 were compared to an aluminum substrate coated with the CCC and an uncoated aluminum substrate, and the results are shown in Table 27.
As shown in Table 27, the MoCC exhibited similar OCP and Icorr values as the CCC. The Icorr value is indicative of corrosion rate.
The data shown in Examples 1-2 indicate that embodiments of the MoCCs disclosed herein possess the ability to self-heal. Using optical microscopy, it was observed that the blue color remained and the number of pits was reduced when compared to an uncoated sample. SEM revealed the surface morphology to consist of a mud cracked pattern that was similar to what would be seen on a sample coated with a CCC. XPS showed the MoCCs include multiple molybdenum-based species. Specifically, multiple valence states of Mo exist in the coating, such as MoO2, Mo2O5, MoO42− and MoO3. The surface of the MoCC is primarily composed of oxidized Mo(VI) and Mo(V), whereas the inner layer also included reduced Mo(IV).
This representative embodiment of the disclosed MoCC exhibits performance that is at the very least comparable to Cr2O3 and CrO42− oxides formed with CCCs in which the surface is composed of oxidized Cr(VI) and the inner layer is composed of reduced Cr(III). In this MoCC, the oxidized molybdates from outer layers migrate to active regions and repassivate any exposed alloy by getting reduced to Mo(IV). This data indicates that a molybdate-based coating can be a suitable replacement for CCCs for aluminum and its alloys.
In this example, MoCCs were formed using permanganate (MnO4)−1 ions, and/or sulfate (SO4)−2 ions, sulfite (SO3)−2 ions, and/or thiosulfate (S2O3)−2 ions. Exemplary composition embodiments of such MoCCs are summarized in Tables 28-30. These MoCCs exhibited corrosion resistance similar to that seen with CCCs, but that also is unexpectedly superior to that exhibited by other MoCC embodiments described herein.
In one example, the composition of Table 29 was used to coat an aluminum alloy substrate by dipping the aluminum alloy substrate in a solution comprising the components of Table 29 for 5 to 10 minutes. The OCP of the coated substrate was −530 mV, as shown in
Comparing the Icorr of the MoCC shown in
Also, the Icorr, of the MoCC coating formed from a precursor solution comprising sulfate (e.g., the composition of Table 29) was lower than that observed for a MoCC coating formed from Composition 2, as was the Icorr, of MoCC formed from a precursor composition comprising permanganate. In particular, the Icorr, of the MoCC formed from the sulfate-containing composition was 2.5 nA/cm2 and the Icorr of the MoCC formed from the permanganate-containing composition was 200 nA/cm2, whereas the Icorr, for the Composition 2 embodiment was 910 nA/cm2. As such, MoCC coatings formed from compositions comprising a redox oxidizing component, such as a permanganate species, exhibited over 4 times better corrosion resistance than MoCC embodiments made from compositions solely comprising K2ZrF6, K3Fe(CN)6, and Na2MoO4. Also, MoCC embodiments formed from precursor compositions comprising a sulfur component provided 360 times better corrosion resistance than compositions solely comprising K2ZrF6, K3Fe(CN)6, and Na2MoO4.
In view of the many possible embodiments to which the principles of the present disclosure may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting. Rather, the scope is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.
This is the U.S. National Stage of International Application No. PCT/US2018/064525, which was published in English under PCT Article 21(2), which application in turn claims the benefit of the earlier filing date of U.S. Provisional Patent Application No. 62/596,550, filed on Dec. 8, 2017; the entirety of each of these prior applications is incorporated herein by reference.
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