WO2006128154A1

WO2006128154A1 - Trivalent chromium conversion coating and method of application thereof

Info

Publication number: WO2006128154A1
Application number: PCT/US2006/020876
Authority: WO
Inventors: Leonard L. Diaddario, Jr.; Gregory Storer; Bradley J. Proper
Original assignee: Pavco Inc
Current assignee: Pavco Inc
Priority date: 2005-05-26
Filing date: 2006-05-26
Publication date: 2006-11-30
Anticipated expiration: 2007-11-26
Also published as: US20060266438A1; MX2007014838A; CN101223302A; BRPI0611418A2

Abstract

The present invention is directed to a high protection, trivalent chromium coating composition that is particularly useful as a conversion coating on substrates in need of corrosion protection. The conversion coating composition comprises chromium (III) ions, cobalt (II) ions, nitrate ions, and sulfate ions. The invention further provides methods of applying a conversion coating to a plated article and articles with a conversion coating applied thereto.

Description

TRIVALENT CHROMIUM CONVERSION COATING AND METHOD OF APPLICATION THEREOF

CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of U.S. Provisional Application No. 60/684,939, filed May 26, 2005, which is incorporated herein in its entirety.

FIELD OF THE INVENTION

The present invention is generally related to the treatment of a metal surface for improving the properties thereof, particularly the corrosion resistance of the metal surface. More specifically, the invention is related to conversion coatings, preferably coatings useful for applying to a metal plated surface.

BACKGROUND

Conversion coatings are commonly used in the metal finishing industry to provide improved properties to metal surfaces, particularly metal plated surfaces. Conversion coatings are generally known to be particularly useful for providing improved corrosion resistance to the metal and also for improving adhesion properties of the metal in relation to additional coatings that may later be added, such as paint or other finishes. Multiple types of conversion coatings are known and used in the industry, and one type that is particularly known to be beneficial is chromating.

Chromate conversion coatings can be applied at various thicknesses and can provide varying levels of corrosion protection, as well as other properties. For example, such coatings can range from a very thin "blue-bright" finish to a very thick "olive-drab" finish. Blue-bright finishes are transparent with a slight blue tint and high luster. Such a finish not only imparts a corrosion-resistant coating to the surface of the substrate but also aesthetically enhances the substrate and articles made therefrom. Heavier chromate conversion coatings are considerably more protective than the bright finishes, but they do not meet the aesthetic criteria that are characteristic of the bright coatings. These heavier coatings are well-recognized by their yellow, bronze, or olive-drab finishes, which correspond in general order to increasing film thickness. While chromate conversion coatings are particularly useful for providing corrosion resistance, such coatings historically have been prepared from hexavalent chromate (i.e., chromium (VI) ions). This makes such chromate conversion coatings unfavorable since chromium (VI) is generally recognized as being toxic. The National Institute for Occupational Safety and Health (NIOSH), for example, considers all chromium (VI) compounds to be potential occupational carcinogens. The Centers for Disease Control (CDC) and NIOSH report an increased risk of lung cancer has been demonstrated in workers exposed to chromium (VI) compounds. Other adverse health effects associated with chromium (VI) exposure include dermal irritation, skin ulceration, allergic contact dermatitis, occupational asthma, nasal irritation and ulceration, perforated nasal septa, rhinitis, nose bleed, respiratory irritation, nasal cancer, sinus cancer, eye irritation and damage, perforated eardrums, kidney damage, liver damage, pulmonary congestion and edema, epigastric pain, and erosion and discoloration of the teeth. Given these findings, the waste from a hexavalent chromium based solution creates significant environmental concerns, and hexavalent chromium baths require special treatment prior to disposal.

Other types of treatments for forming passivation coatings that are non- chromium containing are known in the art, but these are generally unsatisfactory for enhancing the corrosion resistance of a plated substrate, particularly when the treated substrate is subjected to a humid environment. Such treatments typically include phosphate treatments and a bright dipping step, followed by a coating step using a transparent lacquer. Each step provides an additional barrier layer to corrosive conditions, but not a cohesive film forming a chemical bond between a film-forming element and the coated substrate as is achieved by hexavalent chromium processes. Accordingly, phosphate treatments provide barriers that are porous, permitting moisture to pass through to the coated substrate. Phosphate coatings are also not bright in appearance, but provide dull, opaque, paint-like overcoats lacking the aesthetic attributes of hexavalent chromium conversion coatings.

Treatments that are free of hexavalent chromium have been developed utilizing trivalent chromium (i.e., chromium (III) ions). For example, multiple U.S. Patents describe solutions or processes for treating metal surfaces, wherein the solution comprises chromium ions, substantially all of which are in the trivalent state (see, generally, U.S. Patent No. 4,349,392; U.S. Patent No. 4,359,345; U.S. Patent No. 4,359,346; U.S. Patent No. 4,359,348; U.S. Patent No. 4,367,099; U.S. Patent No. 4,384,902; U.S. Patent No. 4,578,122; and U.S. Patent No. 6,096,140). In each of the cited U.S. patents, an oxidizing agent is included as an essential ingredient in the hexavalent chromium-free solution. It is known in the art that the presence of oxidizing agents may lead to some conversion of trivalent chromium to hexavalent chromium during the formation of the conversion coating. The presence of this hexavalent chromium may then lead to some or all of the previously described health and handling problems. Furthermore, the inclusion of an oxidizing agent may add additional costs and steps to the formation of the conversion coatings.

Accordingly, it would be useful to have a hexavalent chromium-free conversion coating that provides similar attributes of traditional conversion coatings that are based on hexavalent chromium. Furthermore, it is desirable that such a hexavalent chromium-free coating is substantially impervious to moisture and is able to provide excellent corrosion resistance for an extended period of exposure. Preferentially, the coating prepared without hexavalent chromium should also simultaneously enhance the appearance of the substrate by imparting an attractive, brightly polished finish.

SUMMARY OF THE INVENTION

The present invention provides a high protection, trivalent chromium coating composition that is particularly useful as a conversion coating on substrates in need of corrosion protection. The invention also provides methods for applying a conversion coating to such substrates, and coated articles prepared according to the method. The trivalent chromium coating composition is particularly useful in that it is free of hexavalent chromium and is also free of components that may facilitate conversion of trivalent chromium to hexavalent chromium. Further, the trivalent chromium coating composition is beneficial in that it provides improved corrosion protection, particularly extending the time to which a substrate having a conversion coating according to the invention can be exposed to a corrosive environment without substrate corrosion occurring. In one aspect of the invention, there is provided a conversion coating composition. In one embodiment, the composition comprises chromium (III) ions, cobalt (II) ions, nitrate ions, and sulfate ions. Preferably, the chromium (III) ions are present at a concentration of at least about 0.1 moles/L, and the nitrate ions are present at a concentration of at least about 0.4 moles/L. In one particular embodiment, the conversion coating composition comprises at least about 0.01 moles/L of cobalt (II) ions, and at least about 0.1 moles/L of sulfate ions.

Preferably, the conversion coating composition of the invention is free of components that may reduce the corrosion resistance properties of the conversion coating formed of the inventive composition. For example, in one embodiment, the conversion coating composition particularly avoids the use of components that may provide free fluorides. In another embodiment of the invention, the conversion coating is substantially free of chelators, in particular, carboxylic acid based chelators (such as di-oic acids). The conversion coating composition of the invention can further comprise one or more additional components useful for imparting desirable properties to an article with a conversion coating applied thereto. Non-limiting examples of additional components that may be useful in the compositions of the invention include aminocarboxylic acids or salts or derivatives thereof, silicates, such as clays (particularly nanoparticulate clays), and halogenated derivatives of acetic acid, or salts or esters thereof.

The conversion coating composition of the invention is particularly useful in that it can be prepared as a concentrate. The concentrate provides ease of storage and transportation, and it can easily be diluted at the time of use to provide the conversion coating composition of the invention.

According to another aspect of the invention, there is provided a method for applying a conversion coating to an article having an exposed surface. In one embodiment, the method comprises contacting the exposed surface of the article with a conversion coating composition comprising chromium (III) ions, cobalt (II) ions, nitrate ions, and sulfate ions. In one particular embodiment, the chromium ions are present at a concentration of at least 0.1 moles/L and the nitrate ions are present at a concentration of at least about 0.5 moles/L. In one particular embodiment according to this aspect of the invention, the exposed surface of the article has been plated with a metal, such as zinc or a zinc alloy. Accordingly, the method of the invention can comprise plating an article and contacting the plated surface of the article with a conversion coating composition according to the invention. The method can comprise further steps, such as surface preparation steps prior to the plating step, or rinsing and drying of the article after contacting it with the conversion coating composition.

The invention also encompasses articles having a conversion coating applied thereto. In one embodiment, the invention provides an article having an exposed surface with a conversion coating applied thereto, wherein the conversion coating is applied according to the method of the invention. Such coated articles are particularly beneficial for the anti-corrosion protection provided to the article by the applied conversion coating. In one embodiment, the article with the conversion coating applied thereto exhibits anti-corrosion protection arising from the conversion coating such that the article with the conversion coating applied thereto can withstand a salt spray according to ASTM testing method B 117-03 for a time of at least about 200 hours before formation of white salts corrosion products.

DETAILED DESCRIPTION OF THE INVENTION The present inventions now will be described more fully hereinafter with reference to specific embodiments of the invention. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. As used in the specification, and in the appended claims, the singular forms "a", "an", "the", include plural referents unless the context clearly dictates otherwise.

In various embodiments, the invention may be described herein in terms of being "substantially free" of certain compounds, elements, ions, or other like components. Accordingly, as used in describing the invention, "substantially free" is intended to mean that the compound, element, ion, or other like component is present, at most, in only trace amounts {i.e., a concentration so minute that the presence of the compound, element, ion, or other like component will have no adverse affect on the desired properties of the coating). Preferably, "substantially free" indicates the specified compound, element, ion, or other like component is completely absent or is not present in any amount measurable by techniques generally used in the art.

The conversion coating composition of the invention is substantially free of chromium (VI) ions. Accordingly, the composition contains, at most, only trace amounts of chromium (VI) ions in such a minute concentration that the presence of the chromium (VI) ions will have no affect on the desired properties of the coating and will have no adverse health or environmental impact. Most preferably, the inventive composition contains no chromium (VI) ions. The conversion coating composition of the invention is also substantially free of oxidizing agents. Such oxidizing agents are generally understood to include agents, such as peroxides and persulfates, that are known to effect conversion of trivalent chromium to hexavalent chromium. Most preferably, the inventive composition contains no oxidizing agents capable of converting trivalent chromium to hexavalent chromium.

The conversion coating composition of the invention generally comprises chromium (III) ions, cobalt (II) ions, and nitrate ions. In further embodiments, the conversion coating composition also comprises sulfate ions. The mixture of the various types of ions, particularly in specified concentrations, is useful for forming a conversion coating on an article having an exposed surface, thereby providing improved corrosion resistance for the article.

According to certain embodiments of the invention, the chromium (III) ions present in the conversion coating composition can be characterized as being present in at least a minimum concentration. For example, in one embodiment, a chromium (III) source is provided in an amount such that the conversion coating composition includes chromium (III) ions at a concentration of at least about 0.1 moles/L. Preferably, the composition includes chromium (III) ions at a concentration of at least about 0.15 moles/L, or at least about 0.2 moles/L. The chromium (III) ions may also be characterized, according to the invention, as being present in a preferable concentration range. For example, in one embodiment, the composition includes chromium (III) ions at a concentration of about 0.1 moles/L to about 0.4 moles/L. In further embodiments, the chromium (III) ions are present at a concentration of about 0.15 moles/L to about 0.35 moles/L, about 0.15 to about 0.30 moles/L, or about 0.15 to about 0.25 moles/L. In one particular embodiment, the chromium (III) ions are present at a concentration of about 0.20 moles/L.

A cobalt (II) source is preferably provided in an amount such that the conversion coating composition comprises cobalt (II) ions at a concentration of at least about 0.01 moles/L. Preferably, the composition comprises cobalt (II) ions at a concentration of at least about 0.02 moles/L, at least about 0.03 moles/L, or at least about 0.04 moles/L. Typically, the conversion coating composition comprises about 0.01 moles/L to about 0.10 moles/L of cobalt (II) ions. In further embodiment, the conversion coating composition comprises about 0.02 moles/L to about 0.09 moles/L, about 0.03 moles/L to about 0.08 moles/L, or about 0.04 moles/L to about 0.08 moles/L of cobalt (II) ions.

The chromium (III) source and the cobalt (II) source can both be any source capable of providing free chromium (III) ions and cobalt (II) ions in solution. For example, in one embodiment, the chromium (III) ion source and the cobalt (II) ion source are chosen from various salts of the metals. According to such an embodiment, it is understood that the source can also introduce ions other than chromium (III) ions or cobalt (II) ions. It is preferred that the chromium (III) source and the cobalt (II) source not include components that could be detrimental to the corrosion resistance properties of the conversion coating composition.

In certain embodiments of the invention, it is beneficial for the conversion coating to comprise further ionic components that, in solution, enhance or improve the corrosion resistance ability of the composition. One example of such further beneficial ions is sulfate (SO₄ ^"2) ions. Without being bound by theory, it is believed that sulfate ions function as film formers on zinc plated surfaces. Sulfate ions can also act as a buffer, facilitating control of solution pH while also enhancing the stability of the solution. Beneficially, such desirable further ionic components can be added to the inventive composition without the need for additional ingredients, which could otherwise increase the cost of preparing the composition or be detrimental to the effectiveness of the composition. Accordingly, it is useful for the chromium (III) source and the cobalt (II) source to comprise compounds that also include the further desirable ions. For example, in one embodiment, the chromium (III) source and the cobalt (II) source can be Cr₂(SO₄)₃ and CoSO₄, respectively.

The conversion coating composition of the invention can comprise sulfate ions at a concentration of at least about 0.1 moles/L. Preferably, sulfate ions are present at a concentration of at least about 0.2 moles/L or about at least 0.3 moles/L. In further embodiments, sulfate ions are present at a concentration of about 0.2 moles/L to about 1.0 moles/L. Moreover, sulfate ions may be present in the composition at a concentration of about 0.25 moles/L to about 0.90 moles/L, 0.30 moles/L to about 0.80 moles/L, or about 0.35 moles/L to about 0.80 moles/L. The chromium (III) ions and cobalt (II) ions can also be provided through other sources, such as other metal salts. While sulfate salts of chromium (III) and cobalt (II) are particularly useful, as described above, any chromium (III) salt or cobalt (II) salt could be used provided the salt does not contribute components that could be detrimental to the anti-corrosive properties of the conversion coating composition. Non-limiting examples of other metal salts that could be used in the invention include inorganic salts, such as nitrate salts or chloride salts, and organic salts, such as acetate salts.

Preferably, according to one embodiment, a source of nitrate ions is provided such that the composition comprises at least about 0.4 moles/L of nitrate ions. In further embodiments, the conversion coating composition comprises at least about 0.5 moles/L of nitrate ions or at least about 0.6 moles/L of nitrate ions. Typically, the conversion coating composition comprises nitrate ions at a concentration of about 0.4 moles/L to about 1.5 moles/L. In yet further embodiments, the conversion coating composition comprises about 0.5 moles/L to about 1.3 moles/L of nitrate ions, or about 0.6 moles/L to about 1.2 moles/L.

Any source capable of providing a sufficient concentration of nitrate ions without introducing additional components that could be detrimental to the ability of the composition to impart corrosion resistance to an article can be used according to the invention. Further, such a source should not be or form an oxidizing agent capable of converting trivalent chromium into hexavalent chromium. In one embodiment, NaNO₃ is used to provide nitrate ions to the composition. In a further embodiment, nitric acid may be used as a source of nitrate ions for the conversion coating composition. Preferably, if nitric acid is added to the solution, the amount of nitric acid added is below a level resulting in oxidation of trivalent chromium to hexavalent chromium. Other non-limiting examples of nitrate sources that could be used according to the invention include potassium nitrate, chromous nitrate, cobalt nitrate, and ammonium nitrate.

In light of the concentrations provided above for the chromium (III) ions, the cobalt (II) ions, and the nitrate ions, the conversion coating composition of the invention can be particularly defined in terms of the mole ratio of nitrate ions to the combination of chromium (III) ions and cobalt (II) ions [NO₃V(Cr⁺³ + Co⁺²)]. In one embodiment, the mole ratio of nitrate ions to combined chromate and cobalt ions is greater than about 1.5:1. Preferably, the ratio is at least about 1.75:1, more preferably at least about 2:1, and most preferably at least about 2.5:1. In a further embodiment, the mole ratio of nitrate ions to combined chromium and cobalt ions is at least about 3:1, at least about 4:1, at least about 5:1, at least about 6:1, or at least about 7:1. The conversion coating composition of the invention can include further ingredients generally recognizable by one of skill in the art as being useful for enhancing or improving corrosion resistance or for being beneficial in a conversion coating composition in particular. For example, specific compounds known as useful for inhibiting corrosion can be included in the composition. Non-limiting examples of such corrosion inhibitors include aminocarboxylic acids and salts or derivatives thereof. One specific example of such compounds 5-(dimethylamino)-2,2- diphenylpentanoic acid, which is commercially available under the tradename HALOX^® 510 from HALOX₃ a division of Hammond Group, Inc., Hammond, IN. Further non-limiting examples of additional components useful in the conversion coating composition of the invention include silicates. As used herein, silicates are understood to refer to compounds containing silicon, oxygen, and one or more metals with or without hydrogen, including silicate esters formed by the combination of silicon and oxygen with one or more organic groups. Moreover, the silicates can include compounds wherein the oxygen is partially or completely replaced with one or more different atoms, such as fluorine (e.g., hexafluorosilicates). Silicates can be useful for increasing corrosion protection when applied to an article as part of the conversion coating composition. While not intending to limit the scope of silicates that may be used according to the invention, in particular, silicates useful in the conversion coating include talc, mica, and clays, such as nanoparticulate clays. For example, the composition can include one or more LAPONITE^® clays, available from Rockwood Additives, Ltd. Specifically, the composition may include LAPONITE^® RD (comprised of SiO₂, MgO, LiO₂, and Na₂O) or LAPONITE^® RDS (comprised of SiO₂, MgO, LiO₂, Na₂O, and P₂O₅). Another example of silicates useful in the invention includes magnesium hexafluorosilicate (MgSiF₆^oH₂O).

Still further example of silicates useful according to the present invention are natural or synthetic phyllosilicate clays. Illustrative of such materials are smectic clay minerals such as montmorillonite, nontronite, beidellite, bentonite, volkonskoite, laponite, hectorite, saponite, sauconite, magadite, kenyaite, stevensite and the like, as well as vermiculite, halloysite, aluminate oxides, hydrotalcite and the like. These layered clays generally comprise particles containing a plurality of silicate platelets having a thickness of about 8-12A tightly bound together at interlayer spacings of 4 A or less, and contain exchangeable cations such as Na⁺, Ca⁺², K⁺ or Mg⁺² present at the interlayer surfaces.

In further embodiments, the conversion coating of the invention may also comprise one or more halogenated carboxylic acids, or salts or esters thereof, provided such carboxylic acids do not also substantially function as chelators. For example, the composition may comprise halogenated acetic acid compounds, or salts or esters thereof, such as trifluoro acetic acid (CF₃CO₂H), sodium trifluororoacetate (CF₃CO₂Na), trichloro acetic acid (CCl₃CO₂H), sodium trichloroacetate (CCl₃CO₂Na)₅ and the like.

While the preferred components of the conversion coating composition are generally described above, the coating composition of the invention can also be characterized in that it is preferably substantially free of certain specific compounds, elements, ions, or the like. In one embodiment, the conversion coating composition of the invention is substantially free of fluoride ions. Preferably, the conversion coating composition is totally free of fluoride ions. This preference is not intended to limit the use of fluorine-containing compounds generally, such as trifluoro acetic acid. Rather, the composition should not include free fluoride ions. The conversion coating composition of the invention can include other, non- fluoride containing, halogen components; however, such halogen components are not specifically required according to the invention. For example, in one embodiment, the conversion coating composition can include chloride ions at a concentration up to about 1 moles/L. In another embodiment, chloride ions are present at a concentration up to about 0.75 moles/L. hi yet another embodiment, chloride ions are present at a concentration up to about 0.5 moles/L.

It is also preferred that the conversion coating composition of the invention be substantially free of chelators. A chelator is generally recognized in the art as a compound, often an organic compound, capable of forming two or more coordination bonds with a central metal ion. Chelators can be capable of coordinating metals in general, or may be more specific for metal of certain valences (e.g., divalent cation chelators). According to the present invention, it is preferred that the conversion coating composition be substantially free of chelators generally. In one preferred embodiment, the conversion coating composition is substantially free of chelators, in particular, carboxylic acid based chelators. Such carboxylic acid based chelators would be recognized as generally encompassing carboxylic acid compounds including two or more carboxylic acid functional groups (e.g., "di-oic acids").

The conversion coating composition of the invention can be prepared in a properly diluted form that is ready to use. Alternately, the conversion coating composition may be provided in a concentrated form that is diluted prior to application of the composition to the article. In one embodiment of the invention, a concentrate is provided wherein, upon proper dilution (such as with water), the diluted concentrate forms a conversion coating composition according to the invention. According to one, non-limiting example, a concentrate is provided wherein upon dilution of about 25% by volume (i.e., 25 parts concentrate to 75 parts diluent), the diluted concentrate forms a conversion coating composition comprising about 0.1 moles/L to about 0.4 moles/L of chromium (III) ions, about 0.01 moles/L to about 0.10 moles/L of cobalt (II) ions, about 0.4 moles/L to about 1.5 moles/L of nitrate ions, and about 0.2 moles/L to about 1.0 moles/L of sulfate ions.

The conversion coating composition of the invention is particularly useful in a method for applying a conversion coating to an article with an exposed surface to provide the article with corrosion resistance. In one embodiment, the method comprises contacting the exposed surface of the article with a conversion coating composition according to the invention.

According to the method of the invention, the conversion coating composition is particularly useful for applying a conversion coating on an article wherein the exposed surface of the article is plated with a metal coating by electrodeposition. In one particular embodiment, the exposed surface of the article is plated with zinc or a zinc alloy. Non-limiting examples of zinc alloys useful as plating the exposed surface of the article include ZnSn alloys, ZnNi alloys, ZnFe alloys, and ZnCo alloys. The article can be plated according to any plating method generally recognized in the art as being useful for plating zinc or zinc alloy. For example, the conversion coating, and method of application thereof, can be used in combination with plating techniques, such as cyanide, alkaline non-cyanide, sulfate-zinc, and chloride zinc plating methods. The underlying article with the plating and conversion coating applied thereto is not limited by the present invention. Rather, any article recognizable by one of skill in the art as being subject to plating and as benefiting from the corrosion resistance provided by the conversion coating of the invention can be used in the method of the invention. Specific metals, such as iron-containing alloys, are particularly susceptible to corrosion and are therefore particularly benefited by the method of the invention in terms of corrosion resistance. Accordingly, while the method of the invention may be exemplified herein by reference to iron-containing articles, such as steel articles, the invention is not limited to such specific embodiments. In an exemplary metal treatment operation, an iron-containing alloy, such as steel, is subject to multiple steps in preparing for and carrying out application of the conversion coating. The steel article is prepared for plating by cleaning the article and, optionally, acid treating the article. Preferably, the article is rinsed after the cleaning and optional acid treating steps. The article is then electroplated with zinc or a zinc alloy. After plating, the plated metal is rinsed, optionally exposed to a mild inorganic acid (such as nitric acid or sulfuric acid) to oxidize the surface, and rinsed again. The conversion coating is applied according to the invention, and the article with the conversion coating applied thereto is rinsed and dried. It is understood the method of the invention can include only some of the above steps, in addition to the step of applying the conversion coating.

The conversion coating composition can be applied to the plated article according to any method generally recognized in the art as being useful for applying a solution to an article to allow for chemical bond formation. For example, the application of the conversion coating composition can be by spraying, dipping, immersing, rolling, or other similar methods. In one particular embodiment, the article is immersed in a bath comprising the conversion coating composition of the invention.

When a bath is used for immersing the article for application of the conversion coating, it is understood that the bath, in addition to the conversion coating composition of the invention, can further include additional components standard in the industry that may be necessary to formation of a bath but do not necessarily affect the conversion coating applied to the article. For example, it is understood that such a bath would include water.

Specifically, a bath of the conversion coating composition is preferentially prepared using a clean tank. The tank, or the tank lining, is preferably made from a material inert to the conversion coating, such as polyethylene, polyvinyl chloride (PVC), stainless steel, or the like. In one particular embodiment, water is first added to the tank. Then, while mixing, the appropriate amount of the concentrated form of the conversion coating composition is added. Finally, the rest of the working volume of the tank is filled with water making the conversion coating composition properly diluted and ready for application to the article. Application of the conversion coating to the plated article is preferably carried out under specified conditions. For example, in one embodiment of the invention, the pH of the bath containing the conversion coating composition is maintained within a certain range. Preferably, the pH of the conversion coating composition is acidic (i.e., less than about 7, and more preferably, less than about 4). In specific embodiments, the pH of the coating is about 2 to about 3, about 2.2 to about 2.8, or about 2.4 to about 2.8. According to another embodiment of the invention, the conversion coating is applied to the plated article at a temperature of at least about ambient temperature. In one preferred embodiment, the temperature of the conversion coating composition during application to the article is elevated above ambient temperature. Such elevated temperature is particularly useful in that it has been found to improve the ultimate corrosion resistance of the conversion coating applied to the article when applied at the elevated temperature. In certain embodiments, the temperature during application of the conversion coating composition is between about 20 ⁰C and about 70 ⁰C, about 30 ⁰C and about 60 ⁰C, or about 40 ⁰C and about 50 ⁰C. The period of time during which the conversion coating composition is applied to the article can vary depending upon the other method parameters, such as the method of applying the conversion coating composition to the article, the dilution of the composition, and the temperature of the composition. In one embodiment, the conversion coating composition is applied by immersing the article in a bath including the composition. According to this embodiment, the article is immersed in the bath for a period of up to about 90 seconds. Specifically, the article may be immersed for about 15 seconds to about 75 seconds, about 20 seconds to about 70 seconds, or about 30 seconds to about 60 seconds.

In light of the conversion coating composition and the method of application thereof, as described above, the invention further provides an article having an exposed surface that has a conversion coating applied to the exposed surface thereof. Such an article coated with the conversion coating of the invention is particularly useful in that the conversion coating applied thereto is effective for providing anti- corrosion protection that greatly exceeds the protection provided by the plating alone. The anti-corrosion effects provided by the conversion coating of the invention are easily evaluated by salt spray (or salt fog) testing performed according to the standards of the American Society for Testing and Materials (ASTM) designation Bl 17-03 (October 2003 version). The compositions provided by the invention may be evaluated by other known testing methods and are intended to provide coatings capable of meeting or exceeding performance requirements that may be described in terms of one or more different testing methods. For the sake of simplicity, however, the ability of the compositions of the present invention to resist corrosion is described herein in terms of the noted testing method. Accordingly, a material coated with a coating composition within the boundaries of the present invention should not be viewed as being outside the realm of the invention simply through evaluation with a method different from ASTM Bl 17-03. According to ASTM B 117-03 , the testing apparatus consists of a fog chamber, a salt solution reservoir, a supply of a suitably conditioned compressed air, and atomizing nozzles. Using the apparatus, a salt solution comprised of about 5 parts by weight (pbw) NaCl in 95 pbw water is sprayed onto specimens for continuous prolonged periods to cause corrosion. Depending upon the specimen used, time to corrosion can be evaluated.

Specifically related to the present invention, corrosion testing is performed on zinc plated steel specimens to which the conversion coating of the invention has been applied. The onset of two types of corrosion products is documented in the test: white salts and red rust. The visual appearance of white salts indicates failure of the conversion coating and corrosion of the underlying zinc plating. The visual appearance of red rust indicates failure of the zinc plating and corrosion of the underlying steel specimen. For purposes of evaluating anti-corrosion protection of the conversion coating of the present invention, the time to formation of white salts corrosion products is used. In one embodiment of the invention, an article having the conversion coating of the invention applied thereto is characterized in that the conversion coating provides anti-corrosion protection such that when subjected to a salt spray according to ASTM testing method B 117-03 described above, the conversion coating is resistant to formation of white salts for a time of at least about 200 hours. According further embodiments, the conversion coating is resistant to formation of white salts for a time of at least about 250 hours, at least about 300 hours, at least about 350 hours, or at least about 400 hours.

In addition to anti-corrosion protection, the conversion coating of the invention is preferably aesthetically pleasing. For example, in one embodiment, the conversion coating applied to an article according to the invention is a bright-blue coating. Furthermore, a colored chromate coating can be achieved by exposing a coated article to a dye, such as the Mordant family of diazo dyes, without degrading the corrosion protection of the coating. For example, a yellow coating can be produced with the use of Mordant Orange 6 dye. A suitable dye exposure time is about 5 seconds to about 40 seconds, whereby the dye is at a temperature between about 20 ⁰C and about 40 ⁰C. The present invention is further advantageous in that the conversion coating can be formed without pre-baking the article to be coated to relieve any hydrogen- embrittlement. Other products require the parts to be baked prior to chromating, which also requires the surface of the parts to be reactivated prior to chromating. Baking usually destroys corrosion protection. Chromating and then baking greatly simplifies the conversion coating process.

EXPERIMENTAL

The present invention is more fully illustrated by the following examples, which are set forth to illustrate various embodiments of the invention and are not to be construed as limiting thereof.

EXAMPLE 1

A series of test parts were produced to evaluate various chromate formulations. The parts were plated in acid-chloride zinc electroplating consisting of 3 oz/gal zinc metal, 18 oz/gal ammonium chloride, 4% (vol/vol) Smart Zinc Carrier (available from Pavco, Inc.), and 0.2% (vol/vol) Smart Zinc Toner (available from Pavco, Inc.). The parts were zinc plated at 20 amps/ft² for 25-30 minutes to produce a zinc thickness of approximately 0.3 to 0.5 mm. After plating, the parts were rinsed in water and then rinsed in 0.5% (vol/vol) nitric acid solution. Various different conversion coating compositions were applied to the plated parts prepared as described above. Application was by dipping in a bath comprising a conversion coating composition according to the invention at a temperature of 50 ⁰C and a pH of 2.6 for 60 seconds. The parts with the conversion coating composition applied thereto were tested for corrosion resistance according to ASTM B 117-03, as described above. Corrosion resistance was determined as a time to formation of white salts. Conversion coating composition ingredients (in moles/L), mole ratio of nitrate ions to combined chromium (III) and cobalt (II) ions, and time to white salts (in hours) are provided in Table 1.

acid] corrosion inhibitor

As a comparative to the above data, a commercially available trivalent chromium based conversion coating was also tested according to the same parameters as described above. A bath containing 14.5% (vol/vol) HyproBlue (available from Pavco, Inc.) at a pH of 2.4 and a temperature of 40 ⁰C was used. A plated part was immersed in the bath for 60 seconds. Three separate parts were tested under salt spray according to ASTM B 117-90. Time to white salts for the three parts was 135.6 hrs, 169.7 hrs, and 115.7 hrs.

EXAMPLE 2

Various test parts were prepared as described in Example 1 and were coated with further conversion coating compositions according to the present invention to evaluate the ability of the coatings to resist corrosion. Application was by dipping in a bath comprising the conversion coating composition a temperature of 40 or 50 ⁰C and a pH of 2.6 for a time of 60 seconds. The parts with the conversion coating composition applied thereto were tested for corrosion resistance according to ASTM B 117-03, as described above. Conversion coating composition ingredients (in moles/L), mole ratio of nitrate ions to combined chromium (III) and cobalt (II) ions, and time to white salts (in hours) are provided below in Table 2.

Table 2

a - Also included 7.5 g/L LAPONITE RD b - Also included 0.200 M CF₃CO₂H c - Also included 0.399 M CF₃CO₂H d - Also included 0.400 M CCl₃CO₂H e - Also included 0.600 M CCl₃CO₂H f- Also included 0.018 M MgSiF₆-OH₂O

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

THAT WHICH IS CLAIMED:

1. A conversion coating composition comprising: (a) at least about 0.1 moles/L of chromium (III) ions; (b) at least about 0.4 moles/L of nitrate ions;

(c) cobalt (II) ions; and

(d) sulfate ions.

2. The conversion coating composition of claim 1 , comprising at least about 0.2 moles/L of chromium (III) ions.

3. The conversion coating composition of claim 1, comprising at least about 0.6 moles/L of nitrate ions.

4. The conversion coating composition of claim 1, wherein the concentration of cobalt (II) ions is at least about 0.01 moles/L.

5. The conversion coating composition of claim 4, wherein the concentration of cobalt (II) ions is at least about 0.02 moles/L.

6. The conversion coating composition of claim 1, wherein the concentration of sulfate ions is at least about 0.1 moles/L.

7. The conversion coating composition of claim 6, wherein the concentration of sulfate ions is at least about 0.2 moles/L.

8. The conversion coating composition of claim 1 , wherein said composition is free of fluoride ions.

9. The conversion coating composition of claim 1, wherein said composition is substantially free of chelators.

10. The conversion coating composition of claim 1 , wherein the ratio of nitrate ions to the combination of chromium (III) ions and cobalt (II) ions is greater than 1.5:1.

11. The conversion coating composition of claim 1 , further comprising one or more additional components selected from the group consisting of silicate- containing compounds and halogenated carboxylic acids, or salts or esters thereof.

12. The conversion coating composition of claim 11, wherein said silicate- containing compounds are selected from the group consisting of clays and magnesium hexafluorosilicate.

13. The conversion coating composition of claim 12, wherein said clays comprise nanoparticulate clays.

14. The conversion coating composition of claim 11 , wherein said halogenated carboxylic acids are selected from the group consisting of halogenated acetic acids and salts or esters thereof.

15. The conversion coating composition of claim 14, wherein said halogenated acetic acids comprise trifluoro acetic acid, trichloro acetic acid, or salts or esters thereof.

16. The conversion coating composition of claim 1 , further comprising up to about 1.0 moles/L of chloride ions.

17. The conversion coating composition of claim 1, wherein the chromium (III) ions and the cobalt (II) ions are supplied as metal salts.

18. The conversion coating composition of claim 17, wherein the metal salt comprises a sulfate metal salt.

19. The conversion coating composition of claim 1 comprising:

(a) about 0.1 moles/L to about 0.4 moles/L of chromium (III) ions;

(b) about 0.02 moles/L to about 0.09 moles/L of cobalt (II) ions;

(c) about 0.4 moles/L to about 1.5 moles/L of nitrate ions; and (d) about 0.2 moles/L to about 1.0 moles/L of sulfate ions; wherein the composition is free of fluoride ions and chelators.

20. A method for applying a conversion coating to an article having an exposed surface, said method comprising contacting the exposed surface of the article with a conversion coating composition according to claim 1.

21. The method of claim 20, wherein the exposed surface of the article is plated with zinc or a zinc alloy.

22. The method of claim 20, wherein the pH of the conversion coating composition is between about 2.0 and about 3.0.

23. The method of claim 20, wherein the temperature of the conversion coating composition is between about 20 ⁰C and about 70 ⁰C.

24. The method of claim 20, wherein said contacting step is carried out for a time of up to about 90 seconds.

25. The method of claim 20, wherein said contacting step comprises immersing the article in the conversion coating composition.

26. An article having an exposed surface with a conversion coating applied thereto, wherein said conversion coating is applied according to the method of claim 20.

27. The article of claim 26, wherein the exposed surface of the article is plated with zinc or a zinc alloy.

28. The article of claim 26, wherein the conversion coating applied to the exposed surface is effective for providing anti-corrosion protection such that the conversion coating, when subjected to a salt spray according to ASTM testing method Bl 17-03, is resistant to formation of white salts corrosion products for a time of at least about 200 hours.

29. The article of claim 28, wherein the conversion coating applied to the exposed surface is resistant to formation of white salts corrosion products for a time of at least about 300 hours.