COMPOSITION AND PROCESS FOR PREPARING PROTECTIVE COATINGS ON METAL SUBSTRATES ORIGIN OF THE INVENTION The invention described herein was made by employee (s) of the Government of the United States and may be manufactured and used by and for the Government for governmental purposes without the payment of any royalties on it or for it. REQUEST FOR CONTINUATION This Application is a Continuation In Part of the
Copending Application Series No. NC-96,343, filed: April 21, 2005. BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to compositions and to a process for using said compositions to prepare protective coatings on various metal substrates. The process comprises treating the metal substrates with effective amounts of an aqueous acidic solution containing at least one trivalent chromium compound, at least one fluorozirconate, at least one carboxylic compound and / or polyhydroxy compound, at least one corrosion inhibitor, and optionally effective amounts
of fluorometallic compounds such as fluorotitanatos, fluorotantalatos, fluoroboratos, fluorosilicatos, divalent zinc compounds, surfactants, wetting agents and / or thickeners. More specifically, this invention relates to stable aqueous acidic solutions and the process for treating various metal substrates including precoated metal substrates such as anodized aluminum to improve the bonding properties by adhesion and corrosion resistance of metal substrates. The process comprises treating the metal substrates with a stable aqueous acidic solution containing effective amounts of at least one water-soluble trivalent chromium salt, at least one water-soluble hexafluorozirconate, at least one water-soluble poly- or monocarboxylic compound. and / or polyhydroxy compound, and at least one anti-pitting or water-soluble corrosion inhibitor. In addition, compounds that can be added to improve the placement and stability of the acid solutions in small but effective amounts include at least water-soluble hexa- and tetra-fluormetal compounds, divalent zinc salts, and effective amounts of water-soluble thickeners. and / or water soluble surfactants. This invention comprises a range of solutions or
aqueous compositions of specific chemical products and with processes for depositing coatings derived from these chemicals to a variety of metal substrates including pre-existing metal-coated substrates. For example, the compositions or solutions are particularly useful for coating aluminum and aluminum alloy, i.e., aluminum conversion coatings to improve corrosion protection and paint adhesion; to seal anodic coatings to improve corrosion protection; for treatment of titanium or titanium alloys for improved paint adhesion, for treatment of magnesium alloys for improved paint adhesion and corrosion protection; for coating steel for improved paint adhesion and rust inhibition; and for further treatment of phosphate, zinc, zinc-nickel, tin-zinc coatings, and sacrificial cadmium coatings on iron alloys and other metal substrates, e.g., steel for improved paint adhesion and corrosion protection . Many of the current pre-treatment, post-treatment and sealant solutions are based on the use of hexavalent chromium chemistry. Hexavalent chromium is highly toxic and a known carcinogen. As a result,
The solutions used to deposit these coatings and the coatings themselves are toxic. The films or coating of hexavalent chromium, however, provide noticeable paint adhesion, good corrosion resistance, low electrical resistance and can be easily applied by immersion techniques, spray or rubbed. However, environmental laws, executive orders, and local regulations on occupation, security, and health (OSH) are encouraging military and commercial users to search for alternatives. In addition, the use of hexavalent chromium coatings is becoming more expensive as regulations are tightened and costs become prohibitive with future PEL restrictions improved by EPA and OSHA. In addition, certain processes such as spraying chromate solutions are forbidden in some installations due to the risk of OSH, forcing the use of less optimal alternative solutions. In summary, hexavalent chromate coatings are technically surprising, but from a life cycle, environmental and OSH cost perspective, alternatives are highly desirable. Consequently, research is under way to develop alternative processes for metal finishing that are technically equivalent or superior to those
Hexavalent chromate coatings without the environmental and health disadvantages. Many of these alternatives, regardless of composition and application methods, have a tendency to precipitate solid material from the solution especially after heavy use. This precipitation, over time, can weaken the effectiveness of the coating solution as the active compounds precipitate as insoluble solids. Additionally, solid precipitations have the potential to plug filters, lines and pumps for both immersion and spray applications. Therefore, better compositions are needed to stabilize the acid solutions for storage and process applications that will not interfere with the deposition process or the subsequent operation of the deposited coating. COMPENDIUM OF THE INVENTION This invention relates to compositions and processes for preparing resistant coatings. corrosion on various metal substrates including precoated metal substrates such as phosphate coatings or anodized coatings, which comprises treating the metal substrates with a solution
Aqueous acid comprising trivalent chromium III compounds, fluorozirconates, corrosion and pitting inhibitors, e.g., triazoles, and stabilization compounds. Optionally, one or more fluorometallic compounds, surfactants, thickeners, and divalent zinc compounds can be added to the acid solutions. This invention can be used to improve the adhesion of various other coatings such as paint to the metal surface and to prevent pitting and corrosion of the metal surface such as aluminum, steel, galvanized surfaces and the like. More specifically, the acid solutions of this invention also contain effective amounts of at least one water-soluble compound, which inhibits corrosion or pitting together with stabilizing agents consisting of polyhydroxy compounds and / or water-soluble carboxylic compounds containing one or more functional carboxylic groups having the general formula R-COO- wherein R is hydrogen or an organic radical of lower molecular weight or functional group. Solution stabilizers, ie, carboxylic compounds can be used in the form of their acids or salts. In some cases salts of carboxylic stabilizers work better than their acids. For example, organic acids such as
Formic, glycolic, propionic, citric and other short chain or low molecular weight carboxylic acids that naturally buffer in the mildly acid pH range can be used as the solution stabilizers. The advantage of adding polyhydroxy or carboxylic stabilizers to the acid solution is the shelf life and improved working stability of the solutions. The acid solutions with the addition of the stabilizing agents did not have substantial precipitation after more than twenty-four months of shelf life evaluation and without any degradation of the coating performance as deposited. Figures 1-6 show the improved performance of an aluminum alloy coated with the triazole-containing solutions of this invention as compared to the same coatings without the corrosion inhibiting triazoles. Therefore, an object of this invention is to provide a stable acidic aqueous solution comprising trivalent chromium compounds, fluorozirconates, polyhydroxy compounds or carboxylic compounds and an effective amount of an inhibitor for coating metal substrates including precoated substrates to improve the
adhesion properties and corrosion resistance of the metal. Another object of this invention is to provide a stable acidic aqueous solution having a pH ranging from about 1.0 to 5.5 comprising trivalent chromium compounds, fluorozirconates, anti-pit compounds and at least one polyhydroxy compound and / or carboxylic compound for treating metal substrates with or without a pre-existing metal coating. Another object of this invention is to provide a process for treating metal substrates to provide coatings with an identifiable color, good adhesion and improved corrosion resistance. A further object of this invention is to provide a stable aqueous acidic solution having a pH which varies from about 2.5 to 4.5, comprising trivalent chromium compounds, hexafluorozirconates, corrosion inhibitors and at least one carboxylic or polyhydroxy compound for treating substrates of metal at ambient and higher temperatures wherein the acid solutions do not substantially contain hexavalent chromium. These and another object of the invention will become apparent by reference to the detailed description
when considered in conjunction with the accompanying Figures 1 to 6 (photographs). DESCRIPTION OF THE DRAWINGS Figure 1 (photograph) shows the corrosion performance of the aluminum alloy panel (2024-T3) with a conversion coating derived from the composition of Example 1 without the triazole inhibitor. Figure 2 (photograph) shows the corrosion performance of the aluminum alloy panel (2024-T3) with a conversion coating derived from the composition of Example 1 with the benzotriazole inhibitor. Figure 3 (photograph) shows the corrosion performance of the aluminum alloy panel (2024-T3) with a conversion coating derived from the composition of Example 2 without the triazole inhibitor. Figure 4 (photograph) shows the corrosion performance of the aluminum alloy panel (2024-T3) with a conversion coating derived from the composition of Example 2 with the benzotriazole inhibitor. Figure 5 (photograph) shows the corrosion performance of the aluminum alloy panel (2024-T3) with a conversion coating derived from the composition of Example 3 without the triazole inhibitor.
Figure 6 (photograph) shows the corrosion performance of the aluminum alloy panel (2024-T3) with a coating derived from the composition of Example 3 with the benzotriazole inhibitor. DETAILED DESCRIPTION OF THE INVENTION This invention relates to stable aqueous acidic solutions and to the process for using said aqueous solutions having a pH ranging from about 1.0 to 5.5, and preferably from about 2.5 to 4.5, or 3.4 to 4.0. for preparing zirconium-chromium coatings, e.g., a conversion coating on metal substrates including, for example, pre-coated substrates such as anodized aluminum or phosphate-coated substrates to improve bonding by adhesion and strength properties to the corrosion of metal. Phosphate coatings known in the art include, for example, coatings of zinc phosphate, iron phosphate, manganese phosphates and mixed calcium-phosphate zinc coatings. The process comprises using the aqueous acidic solution at temperatures ranging up to about 49 ° C (120 ° F) or higher, e.g., up to about 93 ° C (200 ° F). The solutions comprise from about 0.01 to 100 grams and preferably from about 0.01 to 22 or 5.0 to 7.0
grams per liter of the acid solution of at least one water-soluble trivalent chromium compound, e.g., chromium sulfate, about 0.01 to 24 grams and preferably about 1.0 to 12 or 1.0 to 6.0 grams per liter of the solution of at least one fluorozirconate, e.g., an alkali metal salt of H2ZrF6, an effective amount sufficient to inhibit corrosion ranging, for example, from about 0.001 to 4.0 grams per liter and preferably about 0.25. to 2.0 grams or 0.25 to 1.0 gram per liter of a water soluble corrosion inhibitor or pitting compound such as benzotriazole, and from about 0.001 to 2.0 grams and preferably from 0.001 to 1.0 or 0.01 to 1.0 moles per liter of the solution of at least one water-soluble stabilizing agent or compound selected from the group consisting of carboxylic compounds, polyhydroxy compounds and mixtures of these stabilizing compounds in any ratio. If needed, each of the compounds of this invention can be used up to their solubility limits in aqueous acid solutions depending on the metal surface being treated. The metal surfaces treated in accordance with the present invention can be any metal substrate including, for example, surfaces of iron, zinc, magnesium steel,
including galvanized steel, aluminum and aluminum alloys. Any metal surface, including metal surfaces containing a protective or pre-existing metal coating may be treated with the compositions of the present invention. The coatings are applied after cleaning and deoxidizing or stripping the metal substrate, e.g., aluminum substrate through conventional mechanical or chemical techniques. The acid solution of this invention is applied at about room temperature to the metal substrate through immersion, spray or rub techniques similar to the process used for other metal treatments. The residence time of the solution varies from around 1.0 to 60 minutes or longer. With this solution, the dwell time of 1.0 to 40 or 1.0 to 10 minutes provides an optimal film for color change, paint adhesion and corrosion resistance. The residence time of 1.0 to 10 minutes provides appreciable color change to the coating depending mainly on the chemical composition of the aqueous solution. The remaining solution is subsequently rinsed from the metal substrate with tap water or deionized water. In some processes, depending on the
physical characteristics of the metal substrate, eg, the physical size of the steel or aluminum substrates, the addition of a thickener to the solution helps in optimal film formation during spray and rub applications by slowing the evaporation of solution . This mitigates the formation of powdery deposits that degrade paint adhesion. The addition of thickeners also helps in the formation of suitable film during large area applications and mitigates the diluting effect of rinse water remaining on the substrate during the processing of previous stages. This particularity of the process provides films or coatings that do not have stripes and are an improvement in both, coloration and corrosion protection. Water-soluble thickeners such as cellulose compounds may be present in the aqueous acidic solution in amounts ranging from about 0.0 to 20 grams per liter and preferably 0.5 to 10 grams, e.g., about 0.1 to 5.0. grams per liter of the aqueous solution. In addition, depending on the characteristics of the metal substrates, an effective but small amount of at least one water-soluble surfactant or wetting agent may be added to the acid solution in amounts ranging from about 0.0 to 20 grams and preferably
from 0.5 to 10 grams, e.g., 0.1 to 5.0 grams per liter of the acid solution. There are many water soluble surfactants known in the art, and therefore, for the purpose of this invention, the surfactants can be selected from the group consisting of nonionic, cationic and anionic surfactants. The trivalent chromium is added to the solution as a trivalent chromium compound soluble in water, either as a liquid or solid and preferably as a trivalent chromium salt. Specifically, in formulating the acidic aqueous solutions of this invention, the chromium salt can be conveniently added to the solution in its water-soluble form, where the valence of the chromium is more 3. For example, some of the preferred chromium compounds they are incorporated into the solution in the form of Cr2 (S04) 4, (NH4) Cr (S04) 2, Cr (N0) 3-9H20 or KCr (S04) 2 and any mixtures of these compounds. A preferred trivalent chromium salt concentration is within the range of about 5.0 to 7.0 grams per liter of the aqueous solution. It has been found that particularly good results are obtained from these processes when the trivalent chromium compound is present in solution at the preferred scales. Acid solutions may contain at least
a divalent zinc compound to provide color and also improve the corrosion protection of the metal when compared to other treatments or compositions that do not contain zinc. The amount of the zinc compounds can be varied up to the solubility limits to adjust the color imparted to the coating, which ranges from 0.0 to 100 grams to as little as about 0.001 grams per liter to 10 grams per liter, e.g. , 0.5 to 2.0 grams of Zinc2 * cation. The divalent zinc can be supplied by any chemical compound, e.g., salt that dissolves in water at the required concentration and is compatible with the other components in the acid solution. Divalent zinc compounds that are soluble in water at the required concentrations of preference include, for example, zinc cetate, zinc telluride, zinc tetrafluoroborate, zinc molybdate, zinc hexafluorosilicate, zinc sulfate and the like or any combination of them in any relationship. The treatment or coating of the metal substrates can be carried out at various temperatures that include solution temperatures that vary from ambient, e.g., from around room temperature to about 49 ° C (120 ° F) or superior to around 93 ° C (200 ° F). Room temperature is preferred, however, in that
Eliminates the need for heating equipment. The coating can be air dried by any of the methods known in the art including, for example, oven drying, forced air drying, exposure to infrared lamps, and the like. The following Examples illustrate the stable acid solutions of this invention, and the method of using the solutions by providing color recognition, improved adhesion bonding and corrosion resistant coatings for metal substrates including metal substrates having a metal coating. previously existing metal. EXAMPLE 1 To one liter of deionized water, 4.0 grams of potassium hexafluorozirconate, 3.0 grams of basic chromium III sulfate, 0.12 grams of potassium tetrafluoroborate, and 0.25 grams of benzotriazole are added. The solution is stirred until all the compounds are dissolved. Allow to stand at ambient conditions (21-27 ° C (70-80 ° F)) until the pH reaches 3.70. EXAMPLE 2 To one liter of deionized water, add 4.0 grams of potassium hexafluorozirconate, 3.0 grams of
basic chromium III sulfate, 2.3 grams of glycerol (0.025 moles), and 0.25 grams of benzotriazole. The solution is stirred until all the compounds are dissolved. Allow to stand at ambient conditions (21-27 ° C (70-80 ° F)) until the pH reaches 3.55. EXAMPLE 3 To one liter of deionized water, 4.0 grams of potassium hexafluorozirconate and 3.0 grams of basic chromium III sulfate are added. The solution is stirred until all the compounds are dissolved. The pH is maintained between
3. 25 and 3.50 for 14 days with dilute sulfuric acid and dilute potassium hydroxide, and then adjusted to final pH of
3. 90. 0.25 grams of benzotriazole are added. EXAMPLE 4 The solution is prepared as in Example 1, except that the benzotriazole is replaced with 0.50 grams of
2-mercaptobenzimidazole. EXAMPLE 5 The solution is prepared as in Example 3, except that 0.25 grams of 2-mercaptobenzimidazole are added in addition to the benzotriazole. EXAMPLE 6 The solution is prepared with in Example 1, except
that 0.25 grams of 2-mercaptobenzimidazole and 0.25 grams of 2-mercaptobenzazole are added in addition to the benzotriazole. EXAMPLE 7 The compositions of Examples 1, 2 and 3 were used to coat aluminum alloy panels (2024-T3) as follows: The process comprises cleaning panels of 7.62 cm (3 inches) by 12.7 cm (5 inches) by 0.762 mm (0.030 inches) in Turkish 425 at 60 ° C (140 ° F) for 15 minutes. It is rinsed in hot running water using double cascade backflow. Immediately, coupons are dipped in Turkish Smut Go for 5 minutes. They are rinsed with running water at room temperature using double backflow in cascade. Immediately, the panels are immersed in the compositions of Examples 1, 2 and 3 for five (5) minutes at 21-27 ° C (70-80 ° F). It is rinsed in tap water at room temperature using double backflow in cascade. The final rinse is on deionized water. Allow the panels to air dry and rest overnight. The coatings are ready for testing or subsequent coating with organic finishing coating eg. (MIL-PRF-23377) epoxy primer. EXAMPLE 8
The test panels were cleaned and coated by the process set forth in Example 7, then placed in neutral salt mist (ASTM B117) at an inclination of 6 degrees from vertical. After 3 weeks (21 days) in salt fog, the operation of the coating is shown in Figures 1-6. Control coatings made from the compositions of Examples 1, 2 and 3 were made without the addition of the triazole sting inhibitors. It is evident from the comparison of Figures 1-6 (photographs) that the addition of the sting inhibitors resulted in a positive effect on the corrosion resistance of the coatings made of the different compositions. The counter bite or corrosion inhibitors are water soluble compounds selected from the group consisting of triazoles, benzimidazoles, benzazoles, benzoxazoles and mixtures of these inhibitors in any ratio. Preferred corrosion inhibitors or pitting compounds include triazoles containing up to 12 carbon atoms, such as alkyl and preferably aryl triazoles. The aryl triazoles contain 6-10 carbon atoms, including compounds such as benzotriazole and tolyltriazole, and the triazoles of
alkyl containing up to six carbons such as methyl or ethyl triazole. Triazoles such as benzotriazole are commercially available under the trade name COBRATEC. The sting inhibitors are dissolved in the solutions in an effective amount sufficient to inhibit corrosion, and preferably in amounts ranging from about 0.001 to 4.0 grams per liter, and more preferably in amounts of 0.25 to 2.0 grams or about 0.25 to 1.0 grams per liter. Other useful triazoles include water-soluble hydroxybenzotriazole, such as hydroxy-4-alkylbenzotriazoles, hydroxy-6-benzotriazole, hydroxy-5-chlorobenzotriazole, hydroxy-6-carboxybenzotriazole, hydroxy-5-alkylbenzotriazoles and the like. Stabilizing carboxylic compounds added to aqueous acidic solutions include water-soluble acids and / or carboxylic acid salts, including water-soluble carboxylic acids and salts such as adipic, citric, acetic, citraconic, fumaric, glutaric, tartaric, or ethylenediamine tetraacetic acid provided that the hydrocarbon chain in the carboxylic group does not contain a significant number of carbons that decrease the degree of solubility of compounds. The combinations of two or more of the salts and / or acides can be
use to obtain a specific pH. For example, lower molecular acids and / or salts such as formate or potassium citrate can be used at concentrations of at least 0.001 to 2.0 moles or 0.001 to 1.0 moles per liter. These compounds are good complete stabilizers. Particularly good results were obtained from acidic solutions prepared by adding about 0.01 mol per liter of potassium format after 4 days of the initial solution preparation. Good results are obtained if stabilizing agents with carboxylic compounds containing both hydroxy and carboxylic groups including, for example, compounds such as citric acid, glycolic acid, lactic acid, gluconic acid, glutaric acid and its salts. In addition to the carboxylic compounds as stabilizing agents for the solutions, small but effective amounts of polyhydroxy compounds can also be used as stabilizers in amounts ranging from about 0.001 to 2.0 and preferably from 0.01 to 2.0 moles or 0.01 to 1.0 mole per liter. The compounds include the trihydric compounds, e.g., glycerol and the dihydric ether alcohols, e.g., glycol ethers including alkylene glycol ethers, such as triethylene glycol ether,
propylene glycol, tripropylene glycol ether, or diethylene glycol ether. Other glycos include some of the lower molecular weight compounds such as ethylene glycol, propylene glycol, butylene glycol, cyclohexanol, and poly) oxyalkylene glycols) soluble in water, e.g., poly- (oxyethylene) or poly- (oxypropylene glycols), which have weights Lower molecular variables that vary up to about 1000 can be used to promote the stability and dispersibility of solids in the coating bath or acid solutions. Other known di- and trihydric aliphatic alcohols include the water-soluble lower alkanols, such as the di- and tri-hydric alkanols containing up to twelve carbon atoms. This class of di- and trihydric lower alkanols can include glycols containing up to ten carbon atoms in the alkylene group, e.g., trimethylene glycol, and the polyglycols, such as diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, dibutylene glycol, tributylene glycol. , and other polyalkylene glycols, wherein the alkylene radical contains up to eight carbon atoms and preferably two to four carbon atoms. Combinations or mixtures of carboxylic and polyhydroxy stabilization compounds can be used in the solution
acid in any relation. In addition to the polyhydroxy and carboxylic stabilization compounds, aqueous acidic solutions may contain small but effective amounts of 0.0 to 24 grams, eg, 0.01 to 12 grams per liter of solution of at least one fluormetal compound that preferably includes stabilization compounds such as hexafluorotitanate, heptafluorotantalate, k tetrafluoroborate and hexafluorosilicate. In preparing the acid solutions of this invention, known water-soluble surfactants can be added to the trivalent chromium solutions in amounts ranging from about 0 to 20 grams per liter and preferably about 5.0 to 10 grams or 1.0 to 5.0. grams per liter. The surfactants are added to the aqueous solution to provide better wetting properties by reducing the surface tension, thereby ensuring complete coverage, and a more uniform film on the metal substrates. Surfactants include at least one water soluble compound selected from the group consisting of nonionic, anionic and cationic surfactants. Some of the best known water-soluble surfactants include monocarboxyl imidoazoline, sodium salts of
alkylsulfate (DUPONOL®), ethoxylated or propoxylated alkylphenol (IGEPAL®), alkylsulfonamides, alkaryl sulfonates, pallycolcanol amides (CENTROL®), octylphenyl polyethoxyethanol (TRITON®), sorbitan monopalmitate (SPAN®), dodecylphenyl polyethylene glycol ether ( TERGITROL®), alkyl pyrrolidones, polyalkoxylated fatty acid esters, alkylbenzene sulfonates and mixtures thereof. Other known water soluble surfactants include, for example, nonylphenol ethoxylates, and ethylene oxide adducts with fatty amines, see the publication, "surfactants and Detersive Systems", by Johyn Wiley et al., Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed. When large surfaces do not allow immersion or when vertical surfaces are to be sprayed, thickeners may be added to retain the aqueous solution on the surface for sufficient contact time. The thickeners used are known inorganic thickeners and preferably the organic water-soluble thickeners added to the trivalent chromium solutions in effective amounts, eg, at sufficient concentrations ranging from about 0 to 20 grams per liter and preferably 0.5. to 10 grams or 1.0 to 5.0 grams per liter of the solution. Examples
Specific to some preferred thickeners include the cellulose compounds, e.g., hydroxypropylcellulose (Klucel), ethylcellulose, hydroxyethylcellulose, hydroxymethylcellulose, methylcellulose, and mixtures thereof. Other inorganic thickeners soluble in water include colloidal silica, clays such as bentonite, starches, gum arabic, tragacanth, agar and various combinations. After preparing the metal substrate surface to be coated by conventional techniques, the solution can be applied by immersion, sprinkling or rubbing techniques. The TCP solutions of this invention can be used at elevated temperatures ranging up to 49 ° C (120 ° F) or higher, e.g., up to 93 ° C (200 ° F) and optimally applied through immersion to further improve the corrosion resistance of the coatings. The residence time of solution varies from around 1 to 60 minutes, and preferably 1.0 to 40 or 1.0 to 10 minutes at around 24 ° C
(75 ° F) or higher. After standing, the remaining solution is then thoroughly rinsed from the substrate with tap water or deionized water. No additional chemical manipulations of the deposited films are necessary for excellent performance. The solutions
Aqueous can be sprayed from a spray tank apparatus designed to replace the immersion tanks. While this invention has been described by a number of specific examples, it is evident that there are other variations and modifications that can be made without departing from the spirit and scope of the invention as set forth particularly in the appended claims.