WO2020219704A1 - Process and composition for passivating metal surfaces - Google Patents

Abstract

Aqueous compositions useful as a passivation treatment prior to painting to reduce corrosion. The compositions includes water, a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having a plurality of carboxylic functional groups, an organo-functional silane, and an organopolyphosphonic acid. A process for treating a metal surface includes contacting the surface with such aqueous compositions. The compositions and the processes provide benefits in comparison to the zinc phosphate metal pretreatment thought to be the standard in the industry.

Description

PROCESS AND COMPOSITION FOR PASSIVATING METAL SURFACES

RELATED APPLICATION

[0001] This application claims the benefit of priority to U.S. Provisional Patent Application

Serial Number 62/839,003, filed on April 26, 2019, all the contents of which are incorporated in this application by reference.

TECHNICAL FIELD

[0002] This invention relates generally to compositions and the use of such compositions for passivating metal surfaces. The composition may be used as a passivation treatment and is intended to be used to treat a range of metals including zinc, iron, copper, brass, magnesium, aluminum, cadmium, or alloys thereof. More particularly, this invention relates to an aqueous composition, suitable for use as a dried-in-place coating for metal, that comprises a bivalent chromium phosphate salt, a phosphonate, a carboxylate polymer, and a silane. The composition may be used as a pretreatment prior to painting and is useful to prevent the formation of white rust on zinc containing metals, without the use of hexavalent chromium.

BACKGROUND OF THE INVENTION

[0003] Galvanized coatings are used extensively for corrosion protection of metal workpieces.

One of the commonly encountered problems with galvanized coatings of all kinds is“white rust” or“white storage stain.” It is manifested as a bulky, white, powdery deposit that forms rapidly on the surface of the galvanized coating under certain specific conditions. White rust can cause considerable damage to the coating and is always detrimental to the galvanized coating’s appearance.

[0004] Because the surface of galvanized coatings is almost 100% zinc, it is the durability of the zinc that provides the outstanding anti-corrosion performance for metals such as steel. Yet zinc is a relatively“reactive” metal. It is the stable oxides that form on the zinc’s surface that determine its durability, and these oxides are formed progressively as the zinc is exposed to the atmosphere. Carbon dioxide in particular is a contributor to the formation of these stable oxides.

[0005] With newly galvanized steelwork, the zinc’s surface has been subjected to little oxidation and is at its most vulnerable. For this reason, a passivation treatment is desired to be used in conjunction with galvanizing operations to provide protection to the galvanized coating during the“youth” period of the coating. This passivation coating provides short-term protection to the zinc to give the stable oxides time to form on the surface.

[0006] Hexavalent chromium compounds have been used in traditional passivation processes to produce the desired conversion coatings to improve the metal surface’s corrosion resistance and paint adhesion. Although the aforementioned coatings enhance corrosion resistance and paint bonding properties, the coatings have a serious drawback. Hexavalent chromium shows toxicological effects and has been determined by the Environmental Protection Agency as a risk to the environment and by the Occupational Safety and Health Agency as a health risk.

Moreover, chemistries based on hexavalent chromium are classified as carcinogens by these agencies.

[0007] Within the past few decades, various compositions and processes, not relying on hexavalent chromium, have been described and used for treating metal surfaces. One such example is described in U.S. Pat. No. 4,169,741 to Rausch et al., which describes a dried-inplace method using a composition comprising, among other elements, trivalent chromium, phosphate ion, and dispersed silica.

[0008] It is highly desirable to provide coatings and processes which are free of hexavalent chromium, but still capable of improving corrosion resistance of metal surfaces, such as zinc, which are comparable to conventional hexavalent chromium-based coatings. There is also a need to provide protective coatings having excellent corrosion resistance with lowered resistivities and adequate coating weights.

BRIEF SUMMARY OF THE INVENTION

[0009] To meet these and other needs, a composition is provided for treating a metal surface to improve corrosion resistance, paint adhesion, and/or maintain low electrical contact resistance.

In one embodiment, the composition comprises water, a trivalent chromium salt with the formula

Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functional silane, and an organopolyphosphonic acid or salt thereof.

[0010] In another embodiment, the composition consists essentially of water, a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functi onal silane, and an organopolyphosphonic acid or salt thereof.

[0011] In still another embodiment, a process is provided for treating a metal surface. The process includes the step of contacting the metal surface with a composition comprising water, a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functional silane, and an organopolyphosphonic acid or salt thereof.

[0012] In a further embodiment, a process is provided for treating a metal surface comprising the steps of: (1) cleaning the metal surface to form a cleaned metal surface; (2) rinsing the cleaned metal surface with water to form a rinsed metal surface; and (3) contacting the rinsed metal surface with a composition comprising water, a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functional silane, and an organopolyphosphonic acid or salt thereof.

[0013] In a still further embodiment, the process additionally comprises, after contacting the rinsed metal surface with the composition, again rinsing the metal surface with water and then sealing the metal surface.

[0014] The processes may additionally comprise, before the first contacting step, the step of cleaning the metal surface with an alkaline cleaner and rinsing. The processes may further comprise, after contacting the metal surface with the pretreatment composition, the steps of rinsing the metal surface with water and then painting the surface of the metal. The pH of the aqueous pretreatment composition comprising water, a trivalent chromium salt with the formula

Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functional silane, and an organopolyphosphonic acid or salt thereof, is preferably acidic and more preferably has a pH of between about 1 and about 4.

[0015] It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWING

[0016] The invention is best understood from the following detailed description when read in conjunction with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: [0017] FIG. 1 is a photographic image of the results of a 50% dilution of Composition 1, identified below, applied to two separate thick gauge hot dipped galvanized panels, and exposed to bare salt spray for a period of 72 hours;

[0018] FIG. 2 is a photographic image of the results of a 35% dilution of Composition 1, identified below, applied to two separate thick gauge hot dipped galvanized panels, and exposed to bare salt spray for a period of 72 hours; and

[0019] FIG. 3 is a photographic image of the results of Composition 1, identified below, and

10%, 20%, and 30% dilutions of TRI-CLPS®6500ST, a trivalent chromium/organic coating chemical, used to produce on zinc alloys a clear, nearly colorless, Dried-In-Place (DIP) coating, applied to four separate thick gauge hot dipped galvanized panels, and exposed to bare salt spray for a period of 72 hours.

DESCRIPTION OF THE INVENTION

[0020] The present invention is directed to compositions and processes for treating a metal surface. Compositions according to the present invention comprise water, a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functi onal silane, and an organopolyphosphonic acid or salt thereof.

Although such a composition could include additives, it excludes halide ions which promote undercutting. If undercutting occurs, reduced performance could follow.

[0021] Processes according to the present invention include contacting a metal surface with the above composition, including: water, a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functional silane, and an organopolyphosphonic acid or salt thereof. [0022] Aqueous compositions of the present invention are used as a passivation treatment and may be used as a pretreatment prior to painting and to inhibit corrosion, such as white rust, in the uncoated (unpainted) condition. As a result, the composition may be applied to a metal surface after cleaning but before some final coat may be applied to the metal surface. Such an application may contribute to at least one of the following: (1) improving the corrosion resistance of the metal surface; (2) improving the paint adhesion of the metal surface; and (3) maintaining or reducing the resistivity of the metal surface. Compositions of the present invention include compositions which significantly improve one or two of these characteristics, even though at least one of the others is worsened to a lesser extent. The improvement could be due to the compositions alone or the compositions in combination with other process steps. Such compositions are referred to in this document or in the metal treatment industry as pretreatment compositions, conversion coatings, or working compositions.

[0023] Definitions

[0024] As used in this document,“resistivity” is defined as resistance per unit surface area; typical units of resistivity are microhms per square inch.

[0025] As used in this document, the term “hexavalent chromium compound" means compounds, namely salts, of chromium in which the chromium has a valence of plus 6. A wide range of anions could be used, and more than one hexavalent chromium compound could be used. Preferably, the hexavalent chromium compound is anhydrous chromic acid (CrO₃), chromic acid (H₂CrO₄), or chromium chromate (Cr₅O₁₂).

[0026] As used in this document, the term“metal,” used for example in the phrase“metal surface,” includes copper, brass, magnesium, aluminum, iron, zinc, cadmium, or alloys thereof.

Each metal listed includes both the elemental metal and alloys thereof; for example, the term “aluminum” means aluminum and aluminum alloys. The term“alloy” is a metal in which the primary element has a higher content than every other element or a content equal to the highest content of every other element (e.g., an aluminum alloy is a metal in which aluminum is present in an amount at least equal to that of any other element). Iron alloys include cold rolled steel, electro-galvanized steel, and hot-dipped galvanized steel. Preferably, compositions of the present invention are used to treat a range of metals including alloys of copper, brass, magnesium, aluminum, and iron.

[0027] As used in this document, the term“pretreatment composition” means any composition which improves the corrosion resistance of a metal surface. Aqueous pretreatment compositions are used as a pretreatment and may be used as a passivation treatment to reduce the formation of corrosion in the uncoated (unpainted) condition. Thus, although the composition may be called a pretreatment composition for convenience, it is a composition used for pretreatment (i.e., improving the adhesion of subsequently applied paint) and passivation (i.e., resisting corrosion of the unpainted surface).

[0028] As used in this document, the term“treating” means applying a treatment or cleaning, rinsing, and applying a pretreatment. The pretreatment also functions as a sealant to seal the metal surface, so the term“treating” optionally includes the step of sealing the metal surface.

Further,“treating” optionally can include process steps up through and including painting. For example, treatment steps may also include a step of applying a decorative coating, such as painting. After applying the pretreatment, the pretreatment may be rinsed first or dried-in-place before application of the paint. Each of these steps play a role in a final product’s ability to resist corrosion and minimize paint loss. As mentioned above, the treatment composition can be used as a pre-paint treatment. [0029] Trivalent Chromium Salt

[0030] The composition includes a trivalent chromium salt. Without being bound to any theory, it is believed that oxygen and carbon dioxide produce a defective thin film of metal oxides, and the divalent chromium salt creates a film of its own.

[0031] As used in this document, the term“divalent chromium salt” means compounds, namely salts, of chromium in which the chromium has a valence of plus 3. No hexavalent chromium (or at worst a de minimus, inconsequential amount of it) is present in such compounds. A wide range of anions could be used, and more than one divalent chromium compound could be used. Preferably, the divalent chromium salt is a trivalent chromium phosphate salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2.

[0032] The above chromium phosphate salt may be created from a precursor hexavalent chromium compound. In one exemplaiy embodiment in which the precursor is a hexavalent chromium compound, the hexavalent chromium compound may be anhydrous chromic acid (CrO₃). To create the chromium phosphate salt, the chromic acid may first be reduced in the presence of phosphoric acid to create a trivalent chromium compound, chromium dihydrogen phosphate (Cr(H₂PO₄)₃). Hydrogen peroxide may be the reducing agent. The following pH range has been found to be preferred given certain other conditions: 1 to 4.

[0033] Polymer System

[0034] The composition includes a polymer system having a plurality of carboxylic functional groups. Without being bound to any theory, it is believed that the polymer system creates a physical barrier layer.

[0035] In various embodiments, the polymer system comprises carboxylate functional polymer or a modified carboxylate functional polymer. In certain embodiments, the polymer system comprises a polyacrylic acid (PAA) and/or an acrylic/maleic copolymer. In one embodiment, the polymer component comprises PAA. Preferably, a polyacrylic acid homopolymer is used, such as the type sold by Rohm and Haas under the trademark ACUMER 1510.

[0036] Other examples of suitable polymers included polyacrylates having a molecular weight of from about 1,000 to about 400,000 and polymers based on acrylic acid combined with other moieties. These include acrylic acid combined with maleic acid; methacrylic acid; phosphonate; maleic acid and vinyl acetate; acrylamide; sulfophenol methallyl ether; 2-acrylamido-2- methylpropane sulfonic acid; 2- acrylamido-2-methylpropane sulfonic acid and sodium styrene sulfonate; methyl methacrylate, sodium methallyl sulfonate, and sulfophenol methallyl ether; polymaleates; polymethacrylates; polyaspartates; ethylenediamine disuccinate; and organo polyphosphonic acids and their salts. A methylvinylether/maleic acid copolymer, such as the type sold by International Specialty Products under the trademark Gantrez S-97 BF, may serve to reduce hydrophilicity of the coated metal surface. Furthermore, dispersed waxes may be utilized as suitable agents for reducing hydrophilicity of the coated metal surface.

[0037] Further examples of suitable polymeric components are commercially available from

BASF Corporation under the trade name SOKALAN^®, such as SOKALAN^® CP 5, SOKALAN^® CP 42, SOKALAN^® CP 50, SOKALAN^® PA 25 CL, and PA SOKALAN^® PA 30 CL, SOKALAN^® CP 10 S, SOKALAN^® CP 12 S, and SOKALAN^® 13 S. “CP” generally designates a copolymer whereas“PA” generally designates a polyacrylate.

[0038] The compositions of the present invention may also contain a polymer blend having a plurality of carboxylic functional groups and a plurality of hydroxyl groups. The polymer blend may be a mixture of a first polymer having carboxylic functional groups (-COOH) and a second polymer having hydroxyl groups (- OH). In this embodiment, the first polymer may be, for example, polyacrylic acid or polymethylvinylether-co-maleic acid, and the second polymer may be polyvinyl alcohol. As is well-known, carboxylic functional groups and hydroxyl groups tend to undergo an esterification reaction under certain conditions. Thus, the term“polymer blend” encompasses blends where little or no ester formation occurs, and blends where some appreciable amount of such a reaction occurs.

[0039] Typically, polymer blends for use according to the invention are made by simply mixing at room temperature the first and second polymers, for example polyvinyl alcohol (PVA) and polymethylvinyl ether-co-maleic acid (PMVEMA), with water at a ratio of carboxylic acid to hydroxyl equivalents of about 1.0 to achieve a concentration of about 2.1 wt. %. Using this method of preparation, even if there is some degree of ester formation, not all of the carboxylic acid functional groups and the hydroxyl groups react to serve to cross-link the polymer chains and form water. Thus, many (or even all or substantially all) of the carboxylic groups and hydroxyl groups remain free on the polymeric chains.

[0040] The amount of the first and second polymers utilized, and the relative concentrations of the reactants which form the polymeric blend, can vary over a wide range. The operable ratio of the first polymer to the second polymer can be expressed as the ratio of carboxylic functional groups of the first polymer to hydroxyl groups of the second polymer. The range of acceptable ratios of carboxylic acid to hydroxyl functional group equivalents is from about 0.3:1 to about

3.5:1.

[0041] Silane

[0042] The composition includes a silane. In one embodiment the silane is an organo- functional silane. Without being bound to any theory, it is believed that the organo-functional silane serves to bond with, or assist in bonding among, either the other constituents in the treatment composition or the constituents of other compositions or the metal surface itself or some combination thereof.

[0043] As used in this document, the term“silane” has the same meaning as defined in U.S.

Pat. No. 5,393,353 to Bishop, which is incorporated in this document by reference. As used in this document, the term“organo-functional silane” has the same meaning as defined in U.S. Pat.

No. 6,126,997 to Rivera et al., also incorporated in this document by reference. Specifically, the term“organo-functional silane” means a compound having: (1) a silane radical (e.g., silyl (-

SiH₃), disilanyl (-Si₂H₅), etc ); (2) an organic group (such as an alkyl, an aryl, or an alkoxy group); and (3) a functional group. Such functional groups include, but are not limited to, amino, epoxy, vinyl, and mercapto groups. Exemplary organo-functional silanes which can be used according to the present invention include aminopropyltriethoxy silanes, mercapto silanes, and epoxy silanes. Various silanes may be suitably employed by the present invention. Preferably, the organo-functional silane is an aminopropyltriethoxy silane, such as that sold under the trade name AMEO by Degussa AG of Dusseldorf, Germany, or under the trade name Silwet^® A-1100 by Crompton Corporation of Greenwich, Connecticut.

[0044] Organopolyphosphonic Acid

[0045] The composition includes an organopolyphosphonic acid. As used in this document, the term“organopolyphosphonic acid” means an organic compound comprising two or more phosphoric acid moieties per molecule or a salt thereof. Such compounds include bisphosphonic acids and their salts. Preferably, etidronic acid is used, such as the type sold by ClearTech

Industries, Inc. under the trademark DEQUEST 2010 or VANQUEST 2010, based on HEDP (1- hydroxyethane-l,l-diphosphonic acid), having the working empirical formula of C₂H₈O₇P₂. [0046] A wide variety of organopolyphosphonic acids may be used in the passivation treatment coating composition of the present invention. In one embodiment, the organopolyphosphonic acid is selected from the group consisting of alendronic acid, ibandronic acid, incadronic acid, pamidronic acid, risedronic acid, zoledronic acid, clodronic acid, tiludronic acid, and etidronic acid.

[0047] In one exemplary embodiment, a polymethylenephosphonic acid according to Formula

I may be used:

(HO)₂P(O)CH₂— R¹— CH₂(O)P(OH)₂ (Formula I)

wherein R¹ is a divalent organic radical which may comprise additional phosphonic acid groups and/or other functional substituents.

[0048] In another exemplary embodiment, a bisphosphonic acid according to Formula P may be used:

H₂PO₃— CR¹R²— PO₃H₂ (Formula II)

wherein R¹ and R² are each separately a hydrogen, a hydroxyl group, an alkyl group, an alkylamine, an aryl group, a substituted aryl group, a nitrogen-containing heterocyclic group, and/or other functional substituents.

[0049] Additional Components

[0050] Additional components may be included in compositions of the present invention. For example, wetting agents, such as fluorosurfactants, may be included to improve wetting. In some cases, thickeners might also be included if an application requiring a higher viscosity is needed. Finally, if necessary, a compatible biocide, such as a 1,2-benzisothiazolin-3-one biocide sold under the trademark NIPACIDS BIT 20 by Clarion of Charlotte, North Carolina or a product sold under the trademark NUOSEPT 495 by ISP Chemicals of Calvert City, Kentucky, can be included to inhibit biological growth in a working bath.

[0051] In an alternative embodiment, the composition of the present invention consists essentially of water, a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functi onal silane, and an organopolyphosphonic acid or salt thereof. Although such a composition could include additives, it excludes halide ions which promote undercutting. If undercutting occurs, reduced performance could follow.

[0052] In another exemplary embodiment, the composition of the present invention consists of water, a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functi onal silane, and an organopolyphosphonic acid or salt thereof.

[0053] Concentration Ranges

[0054] Appropriate concentration ranges of the various components are dependent upon their solubility. At concentrations too low, there are insufficient amounts of the constituents to cover the metal surface and perform their functions. Above the solubility limits, the solute may begin to come out of the solution. Formulating compositions according to the invention in light of these constraints is well within the ability of the person of ordinary skill in the art. In one embodiment, the concentration of Component 1 can range from anywhere between about 5% and about 50%, based on volume of the composition, with the balance comprising DI water. In another embodiment, the concentration of Component 1 can range from anywhere between about

15% and about 25%, based on volume of the composition, with the balance comprising DI water. [0055] The compositions given above are of the working bath. It is desirable, of course, to transport the product in the form of a concentrate, namely up to a 10 to 100 fold increase in concentration of the above working bath concentrations. The upper limit of such concentrates is the solubility limit of the first constituent to meet or exceed its solubility limit.

[0056] PH Ranee

[0057] The pH of the compositions is preferably, when the composition is used to treat galvinized steel, from about 1 to about 4. In one embodiment, the pH of the composition may be from about 1.5 to about 3. More preferably, the pH is about 2.

[0058] Method of Manufacture

[0059] Compositions according to the invention may be made by mixing the ingredients in any of a number of sequences. In one non-limiting embodiment, a concentrate (i.e., master batch) is created by combining water and a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2. A polymer system having carboxylic functional groups may then be added to the solution. Next, an organo-functional silane may be added to the solution. Finally, an organopolyphosphonic acid or salt thereof may be added to the solution. The concentrate can then be diluted, preferably with deionized water, to create the desired concentration at the metal treatment site prior to use.

[0060] In a non-limiting embodiment, upon completion of the reaction to create the trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, some residual hydrogen peroxide may remain. Conversely, in another non-limiting embodiment, the hydrogen peroxide is fully utilized. Preferably, upon completion of the reaction the hydrogen peroxide concentration is within the following range: from 0 to about 0.85 grams H₂O₂ (35%) per gram of

Cr(H₂PO₄)₃. [0061] Application Process

[0062] In a process of the present invention, a metal surface is coated with a pretreatment composition of the present invention. In this coating step, the composition may contact the metal surface by any number of techniques known in the art. One such method is immersion coating in which the metal is immersed in the bath of pretreatment. Other techniques known in the art including spraying, roll coating, or reverse roll coating, as well as manual application (e.g., brushing). The coating step is done for a time sufficient to achieve the desired coating weight on the metal surface, which can be determined empirically. This desired coating weight will depend on a number of factors well-known in the art. In one embodiment, the amount of coating is sufficient to leave from about 0.1 to about 30 milligrams of chromium, phosphate, polymers, organo-functi onal silane, and an organopolyphosphonic acid or salt thereof per each square foot of dried metal surface as determined by the weight-strip-weight method. In another embodiment, the amount of coating is sufficient to leave from about 1 to about 10 milligrams of chromium, phosphate, polymers, organo-functional silane, and an organopolyphosphonic acid or salt thereof per each square foot of dried metal surface as determined by x-ray fluorescence, and most preferably about 0.5 to about 2.5 milligrams of chromium, phosphate, polymers, organo- functional silane, and an organopolyphosphonic acid or salt thereof per each square foot of dried metal surface as determined by x-ray fluorescence. By using a solution with a higher concentration of the included elements, it is possible to leave the desired amount of the dried coating with less residence time.

[0063] In another embodiment, the coating weight may be between about 0.5 and about 2.5 milligrams of chromium per each square foot of dried metal surface as determined by x-ray fluorescence. [0064] A process for treating a metal surface to improve corrosion resistance, improve paint adhesion, and/or maintain low electrical contact resistance comprises: (1) cleaning the metal surface to form a cleaned metal surface; (2) rinsing the cleaned metal surface with water to form a rinsed metal surface; and (3) contacting the rinsed metal surface with a composition comprising water, a bivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functi onal silane, and an organopolyphosphonic acid or salt thereof.

[0065] The cleaning step may be carried out in any manner known in the art. The types of cleaners suitable for use in the present invention will vary with a number of factors, including the metal being treated, the desired application, and the amount and type of impurities and soils on the metal surface. As such, the preferred cleaners can be determined empirically based on these factors. In one non-limiting embodiment, an alkaline cleaner is used. An exemplary alkaline cleaning agent which can be used in connection with the present invention is Bulk Kleen^®

841MC cleaner, an alkaline liquid cleaner sold by Bulk Chemicals, Incorporated of Reading,

Pennsylvania. In another non-limiting embodiment, a phosphoric acid cleaner is used.

[0066] In general, the cleaning step may be accomplished by contacting the metal surface with a bath of an alkaline cleaning solution to form a cleaned metal surface. The alkaline cleaning solution may be an aqueous solution of an alkaline cleaning agent. The cleaning bath cleans the metal surface by removing oil and other contaminants from the metal surface. The cleaning bath is effective to remove the loose impurities and surface soils. Thus, the cleaning bath removes soils and certain impurities from the surfaces of the metal surface. If the metal surface is heavily soiled, a detergent cleaner additive may be included in the cleaning step. [0067] A metal surface which has been contacted by an alkaline cleaning solution is referred to in this document as a“cleaned metal surface.” It is cleaned in the sense that it has been exposed to a cleaning bath. It may not be completely cleaned, however, in the sense that substantially all of the impurities have been removed such that it is ready to be exposed to a pretreatment composition. In some cases, it may be adequately cleaned, but in other cases, it should first be rinsed with water before being contacted with a pretreatment composition (i.e., substantially all of the impurities are removed by that point).

[0068] The rinsing step is well-known in the art, and deionized water is preferably used. The use of deionized water avoids the introduction of any deleterious ions, such as chloride ions, into the system. The rinsing step can be two-fold, with a first rinsing step done using tap water and then rinsing with deionized water.

[0069] After step (3) above, contacting the metal surface with the composition, the metal surface may be rinsed with water once again, as is well-known in the art. The rinsed metal surface can then be sealed. Any chemical sealing composition well-known in the industry can be used. In a preferred embodiment, the pretreatment composition includes just a composition comprising water, a trivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, a polymer system having carboxylic functional groups, an organo-functi onal silane, and an organopolyphosphonic acid or salt thereof. When a sealing composition is used, an intervening rinsing step is preferably applied.

[0070] After step (3) or any subsequent sealing step, the metal surface may be dried, or rinsed and dried, and then a decorative coating may be applied to it. For example, the metal surface may be painted or lacquered, or first primed and then painted. Such steps, priming and painting, are known in the art as“finishing steps,” and any known and suitable finishing steps may be used. Suitable paints include acrylic paints and fluorocarbon paints, among others.

[0071] As can be inferred, after step (3) above or any subsequent sealing step, the metal surface can be dried and then a decorative coating (a paint layer) is applied, without an intervening rinsing step between these steps. This alternative process is known as a“dried-inplace” pretreatment. Regardless of whether the pretreatment is“dried-in-place” or there is an intermediate rinsing step, any known method of drying may be employed. The coating may be dried by, for example, using an oven, forced air, etc.

[0072] Determining the times of treatments of the metal surfaces with the baths of the various steps is well-known in the art. They need only be long enough to permit a sufficient time for cleaning (in the case of the cleaning step) or reaction (in the case of the pretreatment or sealing steps). They can be very short or as long as thirty minutes and depend on the stage of treatment, the type of application (e.g., immersion, spray), the type of metal surface, and the desired coating weight, among other factors. The immersion time of a substrate into the composition solution will vary with the stage, and generally varies between approximately 1 minute up to about 10 minutes. The times for immersion are typically longer than when spray is used as the method of contact. Rinse times in general can be fairly short, e.g., 30 seconds to one minute. The specific times of treatment may vary over wide ranges and can be readily determined by one of ordinary skill in the art.

[0073] The desirable performance characteristics of the present invention can be achieved by the processing steps described above to produce a pretreated metal surface with good paint adhesion and corrosion resistance. These characteristics are obtained on the metal surface without a decorative coating. Accordingly, the treated metal surface can be used in unpainted products and will exhibit corrosion resistance even if there is a delay between the treatment steps and any subsequent painting.

[0074] The compositions and processes of the present invention provide the stated benefits without the use of additional components which affect the basic and novel characteristics of the invention. When added to the composition in sufficient amounts, other components may affect the novel characteristics. For example, certain components may make the compositions unstable. Such components may affect the shelf-life of the treatment. Other components may degrade the performance of the compositions and processes of the present invention.

[0075] In summary, the present invention provides environmentally friendly compositions and processes for treating metal, while still maintaining excellent corrosion resistance and paint adhesion. More particularly, the present invention avoids the use of hexavalent chromium, and its associated health hazards and disposal problems.

EXAMPLES

[0076] The following examples are included to more clearly demonstrate the overall nature of the invention. FIGS. 1-3 illustrate the improved results obtained by employing aqueous compositions of this invention. These examples are exemplary, not restrictive, of the invention.

[0077] Synthesis of Composition 1

[0078] In the below experiments, a batch of Cr(H₂PO₄)₃ was first synthesized via the combination of the raw materials: (1) CrO₃; (2) H₃PO₄; and (3) H₂O₂. The final concentration of the master batch was determined to be 43.84% wt.%. This master batch was then combined with the polymer system having carboxylic functional groups, the organo-functi onal silane, and the organopolyphosphonic acid or salt thereof identified in Table 1 below:

[0079] TABLE 1

[0080] Synthesis of Concentrated Formula

[0081] For shipping purposes it may be desirable to synthesize a more concentrated formula, which may then the diluted at the customer’s location. One embodiment of such a concentrate may be synthesized as follows. First, a batch of Cr(H₂PO₄)₃ is synthesized via the combination of the raw materials: (1) CrO₃; (2) H₃PO₄; and (3) H₂O₂. The final concentration of the master batch was determined to be 46.89% wt.%. This master batch was then combined with the polymer system having carboxylic functional groups, the organo-functi onal silane, and the organopolyphosphonic acid or salt thereof identified in Table 2 below to create the concentrate:

[0082] TABLE 2

[0083] Preparation of Test Panels

[0084] Galvanized steel panels were treated via the following process. First, the panels were wiped by hand with a 100% solution of Bulk Sol™ 27AM. Bulk Sol™ 27AM is a highly concentrated detergent-solvent booster designed to remove difficult soils from steel, galvanized and aluminum surfaces. Second, the panels were rinsed at ambient temperature for 60 seconds. Third, Composition 1 was applied to the panels using a roll coater. Fourth, the panels were dried with hot air.

[0085] Although illustrated and described above with reference to certain specific embodiments and examples, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention. It is expressly intended, for example, that all ranges broadly recited in this document include within their scope all narrower ranges which fall within the broader ranges.

Claims

CLAIMS What is claimed is:

1. An aqueous pretreatment composition for treating a metal surface, the composition comprising:

water;

a bivalent chromium salt with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2;

a polymer system having a plurality of carboxylic functional groups;

an organo-functi onal silane; and

an organopolyphosphonic acid.

2. The aqueous pretreatment composition of claim 1, wherein x is 2.

3. The aqueous pretreatment composition of claim 1, wherein the polymer system comprises carboxylate functional polymer or a modified carboxylate functional polymer.

4. The aqueous pretreatment composition of claim 1, wherein the polymer system is selected from the group consisting of: a polyacrylic acid (PAA) homopolymer, an acrylic/maleic copolymer, and polyacrylates having a molecular weight of from about 1,000 to about 400,000.

5. The aqueous pretreatment composition of claim 1 wherein the polymer system comprises acrylic acid combined with at least one additional moiety.

6. The aqueous pretreatment composition of claim 5 where at least one additional moiety is selected from the group consisting of: maleic acid; methacrylic acid; phosphonate; maleic acid and vinyl acetate; acrylamide; sulfophenol methallyl ether; 2-acrylamido-2-methylpropane sulfonic acid; 2-acrylamido-2-methylpropane sulfonic acid and sodium styrene sulfonate; methyl methacrylate, sodium methallyl sulfonate, and sulfophenol methallyl ether; polymaleates; polymethacrylates; polyaspartates; ethylenediamine disuccinate; and organo polyphosphonic acids and their salts.

7. The aqueous pretreatment composition of claim 1, wherein the polymer system is a polymer blend comprising a first polymer having carboxylic functional groups and a second polymer having hydroxyl groups.

8. The aqueous pretreatment composition of claim 7 wherein the first polymer is comprised of a polyacrylic acid or a polymethylvinylether-co-maleic acid, and the second polymer is comprised of a polyvinyl alcohol.

9. The aqueous pretreatment composition of claim 7 wherein the ratios of carboxylic functional groups to hydroxyl functional group is from about 0.3:1 to about 3.5:1.

10. The aqueous pretreatment composition of claim 1 wherein the organofunctional silane is selected from the group consisting of aminopropyltriethoxy silanes, mercapto silanes, epoxy silanes, and mixtures thereof.

11. The aqueous pretreatment composition of claim 1 wherein the organopolyphosphonic acid is a bisphosphonic acid, etidronic acid, or their salts.

12. The aqueous pretreatment composition of claim 1 wherein the organopolyphosphonic acid is selected from the group consisting of alendronic acid, ibandronic acid, incadronic acid, pamidronic acid, risedronic acid, zoledronic acid, clodronic acid, tiludronic acid, and etidronic acid.

13. The aqueous pretreatment composition of claim 1 wherein the organopolyphosphonic acid is a polymethylenephosphonic acid of Formula I

(HO)₂P(O)CH₂— R¹— CH₂(O)P(OH)₂ (Formula I)

wherein R¹ is a divalent organic radical comprising additional phosphonic acid groups.

14. The aqueous pretreatment composition of claim 1 wherein the organopolyphosphonic acid is a bisphosphonic acid of Formula P

H₂PO₃— CR¹R²— PO₃H₂ (Formula II)

wherein R¹ and R² are each independently a hydrogen, a hydroxyl group, an alkyl group, an alkylamine, an aryl group, a substituted aryl group, or a nitrogen-containing heterocyclic group.

15. The aqueous composition of claim 1, further comprising a wetting agent, a thickener, a biocide, or a wax.

16. The aqueous composition of claim 1, further comprising a fluorosurfactant.

17. The aqueous composition of claim 1, wherein the composition is halide ion free and has a pH from about 1.5 to about 3.0.

18. A method for preparing a metal pretreatment composition comprising:

reducing a hexavalent chromium compound, with hydrogen peroxide, in the presence of phosphoric acid to create a trivalent chromium compound with the formula Cr(H_xPO₄)₃, where x is 1.5 or 2, at a pH of between about 1 and about 4; and

after the hexavalent chromium compound has been fully reduced, adding: (1) a polymer system having a plurality of carboxylic functional groups, (2) an organo-functi onal silane, and

(3) an organopolyphosphonic acid.

19. A method for coating a metal surface comprising:

roll coating the metal surface with the composition of claim 1;

spraying the metal surface for a period of up to about 300 seconds in the composition of claim 1; or

dipping the metal surface for a period of up to about 300 seconds in the composition of claim 1.

20. The method of claim 19 wherein the metal surface is selected from the group consisting of zinc, zinc alloys, aluminum, aluminum alloys, steel, zinc coated steel, and zinc with aluminum alloy.