KR20080000565A - Composition and process for preparing chromium-zirconium coatings on metal substrates - Google Patents

Abstract

This invention comprises an acidic aqueous solution for treating metal substrates to improve the adhesion bonding and corrosion protection which comprises effective amounts of water soluble trivalent chromium compounds, fluorozirconates, fluoro-metallic compounds, zinc compounds, thickeners, surfactants, and at least about 0.001 mole per liter of the acidic solution of at least one polyhydroxy and/or carboxylic compound as the stabilizing agent for the aqueous solution.

Description

COMPOSITION AND PROCESS FOR PREPARING CHROMIUM-ZIRCONIUM COATINGS ON METAL SUBSTRATES

Origin of Invention

The present invention has been made by employees of the US government and can be manufactured and used by the US government for public purposes without payment of any royalties.

Continuous application

This application is part of a pending application of pending US patent application NC-96,346, filed February 15, 2005.

The present invention relates to composites for providing chromium, zirconium coatings on various metal substrates and methods of using the composites. The process optionally comprises fluorotitanium, fluoro, using an amount of an effective acidic aqueous solution comprising at least one trivalent chromium compound, at least one fluorozirconate, at least one carboxyl compound and / or a polyhydroxy compound. Treating the metal substrate using an effective amount of fluorometallic compounds including compounds such as tantalum, fluoroborate, fluorosilicate, divalent zinc compounds, surfactants, wetting agents and / or schners. More specifically, the present invention relates to a process for treating a variety of metal substrates including conventionally coated metal substrates and to a stable acidic aqueous solution to improve the adhesive bonding and corrosion resistance of the substrates. The method comprises treating the metal substrates using a stable aqueous solution comprising an effective amount of at least one water soluble trivalent chromium compound, at least one water soluble fluorozirconate and at least one water soluble carboxyl compound and / or polyhydroxy compound. It includes. In addition, other compounds that may be added to the acidic solution in small but effective amounts include one or more water soluble fluorometallic compounds, divalent zinc compounds, and effective amounts of water soluble thickeners and / or water soluble surfactants.

The present invention relates to methods for depositing coatings derived from these compounds on a variety of metal substrates, including a wide range of aqueous solutions or specialty chemical compounds, including conventional metal coated substrates. For example, the compounds or solutions may be used for aluminum and aluminum alloy conversion coatings to enhance corrosion protection and paint adhesion; To seal the anodized coating to improve corrosion protection; To treat titanium or titanium alloy to improve paint adhesion; For the treatment of magnesium alloys for improved paint adhesion and corrosion protection; For coating steel for improved paint adhesion and rust inhibition; And post-treatment of phosphate coatings for improved paint adhesion and corrosion protection, post-treatment of cadmium sacrificial coatings on aluminum, zinc, zinc-nickel, zinc-tin, iron alloys and other metal substrates such as steel, for example. have.

Most current pretreatment, post-treatment and sealing solutions are based on the use of hexavalent chromium chemicals. Hexavalent chromium is highly toxic and carcinogenic. As a result, the solution is used to deposit these coatings and the coatings are themselves toxic. However, these films or coatings have excellent paint adhesion, good corrosion resistance, low electrical resistance and can be easily applied by immersion, spraying or wipe-on techniques. However, environmental laws, administrative orders and local occupation, safety and health regulations (OSH) are calling on military and commercial users to study alternatives. In addition, the use of hexavalent chromium coatings is becoming increasingly expensive due to strict regulations and future PEL restrictions imposed by EPA and OSHA. In addition, some processes, such as spray chromium solutions, are banned in some facilities due to OSH risks and forcing the use of other optimal solutions. In summary, hexavalent chromium coatings are technically superior, but in terms of life-cycle cost, environment, and OSH, alternatives are greatly desired. Thus, research is underway to develop alternative methods for technically equivalent or better metal finishing with hexavalent chromium coatings without environmental and health deficiencies.

Many of these alternatives, regardless of the composition and method of application, have a tendency to precipitate solid metals from solution, especially after heavy use. Such precipitation, after time, can weaken the effect of the coating solution, such as precipitation of the active compound as a non-soluble solid. In addition, the solid precipitation has the potential to occlude filters, lines, and pumps for immersion and spray applications. Accordingly, there is a need for better composites to stabilize acidic solutions for storage and process applications that do not interfere with the deposition process or subsequent performance of the deposited coating.

The present invention provides trivalent chromium III compounds, fluorozirconates, and stabilizing compounds; And optionally, treating the metal substrates with an acidic aqueous solution comprising one or more fluorometallic compounds, surfactants, thickeners, and divalent zinc compounds. A method and composites for providing corrosion resistant coatings on various metal substrates, including coated metal substrates. The present invention can be used to improve the adhesion of coatings such as paint to metal surfaces to improve the corrosion protection properties of metal surfaces such as aluminum, steel, galvanized surfaces and the like. The acidic solution of the present invention is a polyhydroxy compound and / or water-soluble carboxyl compounds wherein the general formula R-COO-, R is hydrogen or one or more carboxyl functional groups having a low molecular weight organic group or functional group An effective amount of at least one water soluble stabilizer or compound consists of. The stabilizers, ie carboxyl compounds, can be used in the form of their acids or salts. In some cases, the salts of the carboxyl stabilizers perform better than their acids. For example, natural buffers such as form, acet, glycolic, propionic, citric acid and other mono- or low molecular weight carboxylic acids in the acidic pH range can be used as the acidic solution stabilizer. The advantages of adding polyhydroxy or carboxyl stabilizers to acidic solutions are improved shelf life and operational stability of the solutions. The acid solution with the stabilizer added did not precipitate after 24 months or more during the evaluation of the shelf life and there was no attenuation of the deposited coating performance. 1-6 show improved performance of aluminum alloys coated with the composite of the present invention as compared to conventional coatings without stabilizing compounds.

Accordingly, it is an object of the present invention to provide trivalent chromium compounds, fluorozirconates, polyhydroxy compounds and / or carboxyl compounds for the coating of metal substrates including precoated substrates to improve adhesion and corrosion resistance properties. It is to provide a stable acidic aqueous solution containing.

Another object of the invention is a pH range of about 1.0-5.5 comprising a trivalent chromium compound, a fluorozirconate and one or more polyhydroxy compounds and / or carboxyl compounds to treat metal substrates with or without existing coatings. It is to provide a stable acidic aqueous solution of.

It is yet another object of the present invention to provide a method of treating metal substrates to provide coatings with identifiable color, good adhesion and improved corrosion resistance.

It is another object of the present invention to include a trivalent chromium compound, hexafluorozirconate, and at least one carboxyl and / or polyhydroxy compound for treating metal substrates above room temperature and the acidic aqueous solution is substantially hexavalent chromium. It is to provide a stable acidic aqueous solution in the pH range of about 1.0-5.5 does not include.

These and other objects of the present invention will become more apparent with reference to the following detailed description when considered in reference to the accompanying Figures 1-6 (photos).

Figure 1 (photo) shows the corrosion performance of a conversion coated aluminum alloy (AA2024T3) panel derived from the composite of Example 4 (TCP / R-COO-) of the present invention, in which the bottom of the panel was not treated.

Figure 2 (photo) shows the corrosion performance of a conversion coated aluminum alloy (AA2024T3) panel derived from a conventional composite (TCP) without the (R-COO-) carboxyl stabilizer untreated at the bottom of the panel.

Figure 3 (photo) shows the corrosion performance of a conversion coated aluminum alloy (AA7075T6) panel derived from the composite of Example 4 (TCP / R-COO-) of the present invention, with the bottom of the panel untreated.

Figure 4 (photo) shows the corrosion performance of a conversion coated aluminum alloy (AA7075T6) panel derived from a conventional composite (TCP) without the (R-COO-) carboxyl stabilizer untreated at the bottom of the panel.

Figure 5 (photo) shows the corrosion performance of an aluminum alloy panel with a coating derived from an acidic aqueous solution of the present invention (pH 3.55) containing 0.1 mol of glycerol per liter of solution after 25 days in neutral salt fog. have.

FIG. 6 (photo) shows a coating derived from an acidic aqueous solution of the present invention (pH 3.90) comprising 0.1 mole of glycerol per liter of solution after 25 days in neutral salt mist, in which the lower part of the panel (FIGS. 5 and 6) is untreated. Corrosion performance of the aluminum alloy panel having a.

The present invention provides a pH of about 1.0 to 5.5 and a method for preparing zirconium-chromium coatings, such as conversion coatings, on metal substrates including precoated substrates such as anodized aluminum or phosphate coated substrates to improve adhesion and corrosion resistance of the metal; Preferably it relates to a stable acidic aqueous solution having 2.5 to 4.5 or 3.4 to 4.0 and a method of using said stable acidic aqueous solution. Phosphate coatings include, for example, zinc phosphate coatings, iron phosphide, manganese phosphate and mixed calcium phosphate-zinc coatings. The method includes using the acidic aqueous solution at temperatures ranging from about 120 ° F. or even higher, such as about 200 ° F., wherein the acidic aqueous solution is acidic of one or more water soluble trivalent chromium compounds, such as chromium sulfate, for example. About 0.01-100 grams per liter of solution and preferably 0.01-22 or 5.0-7.0 grams, such as about 0.01-24 grams per liter of one or more fluorozirconate solutions, such as alkali metal salt H ₂ ZrF ₆ , and preferably about 1.0-12 or 1.0-6.0 grams and 0.001-2.0 grams per liter of a solution of one or more water soluble stabilizers or compounds selected from the group consisting of carboxy compounds, polyhydroxy compounds and mixtures of any proportion of these stabilizing compounds and preferably Comprises 0.001-1.0 or 0.01-0.2 moles. If desired, each of the compounds of the present invention can be used up to the solubility limit of the acidic solution, depending on the metal surface to be treated.

The metal surface treated according to the invention can be any metal substrate including steel surfaces, including, for example, iron, zinc, magnesium, galvanized steel, aluminum and aluminum alloys. Any metal shaving, such as a metal surface comprising a protective metal coating, can be treated with the composite of the present invention.

After cleaning and deoxygenating or pickling a metal substrate such as an aluminum substrate by conventional mechanical or chemical techniques, the acidic solution of the present invention is immersed, sprayed or wiped-on, similar to the method used for metal treatment. on) is applied to the metal substrate at approximately room temperature. Solution residence time ranges from about 1.0 to 60 minutes. In this solution, a residence time of 1.0-40 or 1.0-10 minutes will yield an optimal film for color change, paint adhesion, and corrosion resistance. A residence time of 1.0-10 minutes provides a color change applicable to the coating, mainly depending on the chemical composition of the aqueous solution. The remaining solution is then rinsed off the metal substrate with tap water or deionized water.

In some processes, depending on the physical properties of the metal substrate, such as the physical size of the steel or aluminum substrates, a thickener is added to the solution to slowly slow solution evaporation to optimize the film during spraying and wipe-on applications. Helps in formation This reduces the formation of powdery deposits that dampen paint adhesion. In addition, the addition of the seekner aids in proper film formation during large area application and reduces the dilution effect of rinse water remaining on the substrate during processing from previous steps. The feature of the method is to provide a stripeless film or coating and to improve coloring and corrosion protection. Aqueous salts, such as cellulose compounds, may be present in the acidic aqueous solution in an amount ranging from about 0.0-20 grams per liter of aqueous solution and preferably from 0.5-10 grams, such as from about 0.1-5.0 grams. Furthermore, depending on the nature of the metal substrate, an effective small amount of one or more water soluble surfactants or wetting agents may be added to the acidic solution in an amount ranging from about 0.0-20 grams per liter of acidic solution and preferably in the range of 0.5-10 grams, such as 0.1-5.0 grams. Can be added. There are many water soluble surfactants known in the art and therefore for the purposes of the present invention the surfactants can be selected from the group consisting of nonionic, cationic and anionic surfactants.

Trivalent chromium is added to the solution as a liquid or solid, and preferably as a trivalent chromium salt, as a water-soluble trivalent chromium compound. In particular, in shaping the acidic aqueous solution of the present invention, the chromium salt can be easily added to the solution in water-soluble form whose valency is +3. For example, some of the preferred chromium compound is _{Cr 2 (SO 4) 3,} (NH 4) Cr (SO 4) 2, Cr (NO) 3 -9H 2 O or KCr (SO _{4) 2} and any mixtures of these compounds It is included in the solution in the form. Preferred trivalent chromium salt concentrations are in the range of 5.0-7.0 grams per liter of aqueous solution. It is known that particularly good results are obtained in these methods when the trivalent chromium compound is present in the solution in the preferred range.

The acidic solution may include one or more divalent zinc compounds to provide color and also improve corrosion protection of the substrate as compared to other treatments or compounds that do not contain zinc. The amount of zinc compound is, in a small amount of about 0.001 grams per liter in the range of 0.0-20 grams per liter, per liter to 10 grams, for example Zinc ² + cation 0.5-2.0 to adjust the color imparted to the coating Up to the gram, can be changed. The divalent zinc can be supplied by any chemical compound such as, for example, a salt that is soluble in water at the required concentration and compatible with the other compounds of the acidic solution. Divalent zinc compounds that are water soluble at the required concentrations preferably include zinc acetate, zinc telluride, zinc tetrafluoroborate, zinc molybdate, zinc hexafluorosilicate, zinc sulfate and any combination thereof. do. The treatment or coating of the metal substrate can be carried out at various temperatures including solution temperatures ranging from about 120 ° F. or more to 200 ° F. at ambient temperatures such as room temperature. However, room temperature, which eliminates the need for a heating device, is preferred. The coating can be air dried by one of the methods known in the art, including, for example, oven drying, forced air drying, ultraviolet lamp exposure, and the like.

The following examples illustrate how to use the solution in providing a stable acidic solution of the present invention, color recognition for metal substrates including metal substrates with existing metal coatings, improved adhesion bonding and corrosion resistant coatings.

Example 1

A stable acidic aqueous solution having a pH in the range of about 3.4-4.0 to treat metal substrates to provide corrosion resistance and color recognition coatings, per liter of solution, about 3.0 grams of basic trivalent chromium sulfate, about 4.0 potassium hexafluorozirconate Gram, about 1.0 gram of zinc sulfate and about 0.2 mol per liter of a solution of an alkali metal salt of formic acid.

Example 2

A stable acidic aqueous solution for coating the steel substrate to form a corrosion resistant coating is, per liter of solution, about 3.0 grams of basic trivalent chromium sulfate, about 4.0 grams of potassium hexafluorozirconate, and per liter of a solution of alkali metal salts of citric acid. About 0.2 mole.

Example 3

A stable acidic aqueous solution for coating steel substrates to provide corrosion resistance and color recognition coatings, per liter of solution, about 3.0 grams of basic trivalent chromium sulfate, about 4.0 grams of potassium hexafluorozirconate, about 2.0 grams of divalent zinc sulfate And about 0.001 mole per liter of formic acid solution.

Example 4

An improved stable acidic aqueous solution is prepared from about 4.0 grams per liter of potassium hexafluorozirconate, about 3.0 grams per liter of basic chromium (III) sulfate, and about 0.01 mol per liter of potassium formate. After about 30 days, the solution has a pH of 3.96. After about 12 months, the solution has a pH of 3.92.

Example 4A

An improved stable acidic aqueous solution is prepared from about 4.0 grams per liter of potassium hexafluorozirconate, about 3.0 grams per liter of basic chromium (III) sulfate, and about 0.1 moles per liter of glycerol.

Example 5

The solution is about 0.01-10 grams per liter of potassium hexafluorozirconate, about 0.01-10 grams per liter of basic trivalent chromium sulfate, about 0.0-10 grams per liter of water-soluble surfactant, about per liter of methyl cellulose 0.0-10 grams, about 0.0-5.0 grams per liter of divalent zinc compound, and about 0.001-0.2 moles per liter of water soluble carboxylic acid salt.

Example 6

In the solution of Example 5, potassium hexafluorozirconate is 4.0 grams per liter, basic chromium sulfate is 3.0 grams per liter, the divalent zinc compound ranges from 0.05-2.0 grams per liter, and the range of water-soluble carboxylic acid salts is 0.005-0.01 moles per liter and the water-soluble surfactant ranges from 0.0-10 grams per liter and methyl cellulose sikinners ranges from 0.0-10 grams per liter.

The photographs (FIGS. 1-4) show the corrosion and pH data of aluminum panels with or without stabilizers in the comparative solutions. The metal substrates AA2024T3 are cleaned for about 15 minutes in non-silicate neutral alkali chemicals, and oxygen is removed for about 5 minutes with iron-containing chemicals and processed for about 5 minutes in TCP. The panels are then placed in an ASTM B117 salt fog. In Figures 1 and 2 (AA2024T3), the bottoms of the panels were not treated to demonstrate the performance of bare-metal on conversion coated aluminum panels treated with solution as in Example 4 of the present invention. The metal substrate (AA7075T6) is cleaned for about 15 minutes in non-silicate neutral alkali chemicals, and oxygen is removed for about 5 minutes with iron-containing chemicals and processed for about 5 minutes in TCP. The panels are then placed in ASTM B117 exposed salt mist. Again, the bottoms of the panels of FIGS. 3 and 4 (AA7075T6) were not treated to demonstrate the corrosion of bare metal on the conversion coated aluminum panels treated with solution as in Example 4 of the present invention. The term "salt haze" refers to the salt spray corrosion-resistance test described in ASTM-B117-61.

The stabilizing carboxylic compounds are water soluble acids and / or water soluble carboxylic acids and adipine, citric acid, acetic acid, citraconic, fumaric acid, glutaric, tartaric acid, ethylenediamine tetraacetic acid or a significant number of carbons that reduce the solubility of the compounds. Carboxylic acid salts with salts such as those provided with hydrocarbon bonds in carboxyl groups that do not include these. Combinations of two or more of the salts and / or acids may also be used to obtain a particular pH. For example, salts such as potassium formate or citrate and / or low molecular weight acids are good stabilizers at concentrations of at least 0.001-1.0 mole per liter. Particularly good results were obtained from the acidic solution prepared by adding about 0.01 mole per liter of potassium formate after 4 days of initial solution preparation. Good results are obtained when the stabilizing agents are carboxylic compounds containing both hydroxy and carboxyl groups, including compounds such as citric acid, glycolic acid, lactic acid, glucolic acid, glutaric acid and salts thereof.

In addition to the carboxyl compounds as stabilizers, small but effective amounts of polyhydroxy compounds can be used as stabilizers in amounts ranging from about 0.001-2.0 and preferably 0.01-1.0 moles per liter. Such compounds include, for example, trivalent compounds such as glycerol and dihydric ether alcohols such as glycol ethers containing alkylene glycol ethers, such as triethylene glycol ether, propylene glycol ether, tripropylene glycol ether, diethylene glycol ether and the like. do. Water-soluble poly (oxyalkylene glycols) with low molecular weights in the range up to about 1000, such as ethylene glycol, propylene glycol, butylene glycol, cyclohexanol and poly- (oxyethylene) or poly- (oxypropylene glycol) Other glycols, including some of low molecular weight compounds, can be used to promote the stability and dispersibility of solids in coating baths or acidic solutions. Other known dihydric and trivalent fatty alcohols include water soluble low alkanes, such as divalent and trivalent alkanesols containing up to 12 carbon atoms. Said classes of divalent and trivalent low alkanolols are, for example, trimethylene glycol, and diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, dibutylene glycol, tributylene glycol, and and Glycols containing up to 10 carbon atoms of alkylene groups such as other polyalkylene glycols may be included and the alkylene radicals contain 2-8 carbon atoms, preferably 2-4 carbon atoms. Polyhydroxy and carboxyl stabilizing compounds or mixtures may be used in any ratio in acidic solutions.

In addition to the polyhydroxy and carboxyl stabilizing compounds, the acidic aqueous solution preferably contains one liter of a solution of one or more fluorometallic compounds, including hexafluorotitanate, heptafluorotantalate, tetrafluoroborate, hexafluorosilicate, and the like. Small but effective amounts of sugar, such as 0.0-24 grams, such as 0.01-12 grams.

In preparing the acidic solution of the present invention, known water soluble surfactants may be added to the trivalent chromium solution in an amount in the range of about 0-20 grams per liter and preferably 5.0-10 grams per liter or 1.0-5.0 grams. have. The surfactant is added to the aqueous solution to give better wettability by lowering the surface tension to ensure complete coverage, and a more uniform film on the metal substrates. The surfactant includes one or more water soluble compounds selected from the group consisting of nonionic, cationic, and anionic surfactants. Some of the well known water soluble surfactants are monocarboxy imidoazolines, alkylsulfate sodium chloride (DUPONOL®), ethoxylated or propoxylated alkylphenols (IGEPAL®), alkylsulfonamides, alkali sulfonites , Palmiticcanol amide (CENTROL®), octylphenyl polyethoxy ethanol (TRITON®), sorbitan monopalmitate (SPAN®), dodecylphenyl polyethylene glycol ether (TERGITROL®), alkyl pyrrolidones, polyal Coxylated patty acid esters, alkylbenzene sulfonates and mixtures thereof. Other known water soluble surfactants include, for example, adducts of ethylene oxide with nonylphenol ethoxylate and patty amines; See John Willy et al., Publication Kirk-Osmus Encyclopedia, 3rd Ed, “Surfactants and Cleaning Systems”.

When large surfaces are not allowed to submerge or when sprayed onto vertical surfaces, thick agents may be added to retain the aqueous solution on the surface for sufficient contact time. The reagents used are known to be inorganic and are preferably used in an effective amount of trivalent chromium solution at a sufficient concentration in the range of about 0-20 grams per liter of acidic solution and preferably in the range of 0.5-10 grams or 1.0-5.0 grams. Organic water soluble thickeners added. Specific examples of some preferred seekers include cellulosic compounds, such as, for example, hydroxypropyl cellulose (Klucel), ethyl cellulose, hydroxyethyl cellulose, hydroxymethyl cellulose, or methyl cellulose and mixtures thereof. Other water soluble inorganic seekers include colloidal silica, bentonite, starch, gum arabic, tragacanth rubber, agar and various mixtures.

After preparing the metal substrate surface to be coated by conventional techniques, the solution can be applied by immersion, spraying or wipe-on techniques. The TCP solution of the present invention can be used at high temperatures in the range of 120 ° F. or higher such as 200 ° F. and is optimally applied through immersion to further improve the corrosion resistance of the coating. The solution residence time is in the range of about 80 ° F. or more, for 1-60 minutes, and preferably 1.0-40 or 1.0-10 minutes. After remaining, the remaining solution is thoroughly rinsed off the substrate with tap water or deionized water. No further chemical manipulation of the deposited film is necessary for good performance. However, application of a strong oxidizing solution can yield a film with additional corrosion resistance. Additional corrosion resistance is assumed to be due to the formation of hexavalent chromium in the film derived from trivalent chromium. The aqueous solution can be sprayed in a spray tank device designed to be replaced by a soak tank.

Although the present invention has been described in terms of numerous specific examples, it is apparent that other changes and changes may be made without departing from the spirit and scope of the invention as specified in the appended claims.

Claims (40)

A method of coating metal substrates to improve corrosion protection and adhesive bond strength, Treating the metal substrate with an acidic aqueous solution having a pH range of 1.0 to 5.5; Wherein, the acidic aqueous solution contains 0.01-100 grams of at least one trivalent chromium compound, 0.01-24 grams of at least one fluorozirconate, 0.0-20 grams of divalent zinc compound, and 0.0-20 surfactant per liter of solution. Gram, Schner 0.0-20 grams, and an effective amount of one or more stabilizing compounds selected from the group consisting of polyhydroxy compounds, carboxyl compounds and mixtures of polyhydroxy and carboxyl compounds A method of coating metal substrates to improve corrosion protection and adhesive bond strength. The method of claim 1, The metal substrates have a conventional metal coating thereon A method of coating metal substrates. The method of claim 2, Conventional metal-coated substrates are anodized aluminum A method of coating metal substrates. The method of claim 2, The existing metal coating substrate is phosphate coated A method of coating metal substrates. The method of claim 1, The metal substrate is an aluminum alloy A method of coating metal substrates. The method of claim 1, The metal substrate is iron alloy A method of coating metal substrates. The method of claim 1, The carboxyl compound is a hydroxy-carboxyl compound A method of coating metal substrates. The method of claim 7, wherein The hydroxy-carboxyl compound is citric acid and its water-soluble salts A method of coating metal substrates. The method of claim 7, wherein The hydroxy-carboxyl compound is glycolic acid and its water soluble salts A method of coating metal substrates. The method of claim 7, wherein The hydroxy-carboxyl compound is a gluconic acid and a water soluble salt thereof. A method of coating metal substrates. The method of claim 1, The carboxyl compound is formic acid and its water-soluble salts A method of coating metal substrates. The method of claim 1, The carboxyl compound is propionic acid and its water-soluble salts A method of coating metal substrates. The method of claim 1, The acidic shoe solution contains 0.001-1.0 mole per liter of formic acid A method of coating metal substrates. The method of claim 1, The acidic aqueous solution contains 0.001-2.0 moles per liter of the stabilizing compound A method of coating metal substrates. The method of claim 14, The stabilizing compound is glycerol A method of coating metal substrates. The method of claim 14, The stabilizing compound is a carboxyl compound having at least one carboxyl functional group per molecule. A method of coating metal substrates. A composite for coating metal substrates to improve corrosion protection and adhesive bond strength of the metal substrates, The composition is an acidic aqueous solution having a stabilizing compound in the pH range of 1.0 to 5.5; Wherein the stabilizing compound comprises 0.01-100 grams of one or more trivalent chromium compounds, 0.01-24 grams of one or more fluorozirconates, 0.0-20 grams of divalent zinc compounds, 0.0-20 grams of surfactant Selected from the group consisting of 0.0-20 grams of Schner and polyhydroxy compounds, carboxyl compounds and mixtures of polyhydroxy and carboxyl compounds Composite coating metal substrates. The method of claim 17, The stabilizing compound is a carboxyl compound having at least one carboxyl functional group per molecule. Composite coating metal substrates. The method of claim 17, The carboxyl compound is a hydroxy-carboxylic acid and a water soluble salt thereof Composite coating metal substrates. The method of claim 19, The hydroxy-carboxyl compound is citric acid and its water-soluble salts Composite coating metal substrates. The method of claim 19, The hydroxy-carboxyl compound is a glycolic acid and a water soluble salt thereof Composite coating metal substrates. The method of claim 19, The hydroxy-carboxyl compound is lactic acid and its water-soluble salts Composite coating metal substrates. The method of claim 17, The carboxyl compound is formic acid and its water-soluble salts Composite coating metal substrates. The method of claim 17, The carboxyl compound is propionic acid and its water-soluble salts Composite coating metal substrates. The method of claim 17, The polyhydroxy compound is glycerol and the carboxyl compound is a low molecular weight carboxylic acid or a water soluble salt thereof. Composite coating metal substrates. The method of claim 17, The stabilizing compound is a mixture of low molecular weight carboxylic acid and polyhydroxy compound Composite coating metal substrates. The method of claim 17, The stabilizing compound is a low molecular weight polyhydroxy compound Composite coating metal substrates. The method of claim 17, The polyhydroxy compound is glycerol Composite coating metal substrates. The method of claim 17, The zinc compound is a water-soluble zinc salt present in the acidic aqueous solution in an amount of 0.5-2.0 grams. Composite coating metal substrates. The method of claim 29, At least one polyhydroxy compound is glycerol Composite coating metal substrates. The method of claim 29, The polyhydroxy compound is polyalkylene glycol Composite coating metal substrates. The method of claim 17, The pH ranges from 2.5-4.5, the trivalent chromium compound is 0.01-22 grams, the fluorozirconate is hexafluorozirconate at 0.01-12 grams, and the stabilizing compound is 0.001-per liter 1.0 moles Composite coating metal substrates. 33. The method of claim 32, The stabilizing compound is a low molecular weight carboxylic acid or a water soluble salt thereof Composite coating metal substrates. 33. The method of claim 32, The stabilizing compound is a polyhydroxy compound Composite coating metal substrates. 33. The method of claim 32, The zinc compound is 0.001-10 grams Composite coating metal substrates. 33. The method of claim 32, The thickener and / or surfactant ranges from 1.0-5.0 grams Composite coating metal substrates. 33. The method of claim 32, The zinc compound is present in the aqueous solution in an amount of 0.5-2.0 grams per liter of solution Composite coating metal substrates. The method of claim 17, The acidic aqueous solution comprises 0.01-12 grams per liter of at least one fluorometallic compound selected from the group consisting of fluorotitanium, fluorotantal, fluoroborate, fluorosilicate and mixtures thereof. Composite coating metal substrates. The method of claim 38, The fluorometallic compound is fluoroborate, and the fluorozirconate is hexafluorozirconate Composite coating metal substrates. The method of claim 38, The fluorometallic compound is fluorosilicate, and the fluorozirconate is hexafluorozirconate. Composite coating metal substrates.

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