MX2007009800A

MX2007009800A - Process for sealing phosphoric acid anodized aluminums.

Info

Publication number: MX2007009800A
Application number: MX2007009800A
Authority: MX
Inventors: Craig A Matzdorf; William C Nickerson Jr; Erin N Beck
Original assignee: Navy
Priority date: 2005-02-15
Filing date: 2005-11-14
Publication date: 2007-09-27
Also published as: AU2005327547A1; KR101215772B1; DK1853750T3; JP2008530362A; BRPI0519983A2; EP1853750A4; KR20070121674A; CN101146929B; CA2598390A1; WO2006088520A3; EP1853750B1; WO2006088520A2; JP4805280B2; US20060191599A1; EP1853750A2; DE602005027616D1; CN101146929A; ES2365403T3; ATE506469T1

Abstract

Process of seal coating phosphoric acid anodized aluminum and aluminum alloys to improve the corrosion resistance and maintain the adhesive bonding properties. The process comprises post-treating phosphoric acid anodized aluminum and its alloys with an acidic aqueous solution comprising, per liter of acidic solution, from about 0.01 to 22 grams of a water soluble trivalent chromium compound, about 0.01 to 12 grams of an alkali metal hexafluorozirconate, about 0.0 to 12 grams of at least one alkali metal tetrafluorosilicate and/or an alkali metal hexafluoroborate, from about 0.001 to 10 grams of at least one water soluble divalent zinc compound and from 0.0 to 10 grams of a water soluble thickener and/or water soluble surfactant.

Description

PROCESS FOR SEALING ANODISED ALUMINUM WITH PHOSPHORIC ACID 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 payment of any royalties on it or for it. BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a process for depositing a film or coating on aluminum and its alloys that have been anodized with phosphoric acid. The coating system comprises aluminum anodized with phosphoric acid, a supplementary subsequent treatment or seal coating, optimally, an adhesive bond primer or other supplementary coatings. The anodized aluminum coatings with phosphoric acid are very porous and, therefore, have low resistance to inherent corrosion. These coatings, however, have excellent adhesive properties. Consequently, these anodized coatings would benefit from a subsequent treatment or seal coating that improves corrosion protection without adversely affecting the adhesion properties. The performance characteristics of this invention allow phosphoric acid-anodized coatings to be used in unpainted applications that are currently not feasible; replace anodized aluminum with chromic acid and FPL-engraved, both of which contain chromates for fatigue-sensitive applications, prone to corrosion; in all adhesive bonding applications where the transition is made to non-chromed bond primers; and, in general purpose applications to reduce fatigue flow rate and coating weight compared to other general purpose anodizing coatings. This invention relates to a process for treating anodized aluminum (s) with phosphoric acid to maintain and improve the corrosion-resistant properties. More specifically, this invention relates to the process of sealing anodized aluminum with phosphoric acid and anodized aluminum alloys. The subsequent trivalent chromium (TCP) treatment process comprises an aqueous acidic solution containing effective amounts of at least one trivalent chromium compound soluble in water, an alkali metal hexafluorozirconate, at least one alkali metal tetrafluoroborate and / or hexafluorosilicate, at least one divalent zinc compound, and effective amounts of water soluble thickeners and / or water soluble surfactants. Anodized aluminum is generally sealed or treated after anodizing by processes that employ a variety of sealing processes and compositions. The current high performance back treatments or sealants for anodized aluminum are based on hexavalent chromium chemistry. Hexavalent chromium is highly toxic and a known carcinogen. As a result, the solutions used to deposit these protective coatings and the coating itself are toxic. These films or coatings, however, provide good adhesion and improved corrosion resistance to anodeized aluminum. Typically, seal coatings are deposited on the anodized coating at elevated temperatures and are usually applied by immersion or sprinkling processes. Subsequent treatments may be required by military and commercial specifications that regulate each coating that is being treated. As such, there is no single "after-treatment" specification for all anodized aluminum as there is for "conversion coating" aluminum.

In addition, environmental laws, executive orders and local occupation, safety and health (OSH) regulations are encouraging military and commercial users in the search for hexavalent chromium-free treatments. In the case of anodized aluminum, the anodizing film and the base metal are relatively non-toxic. With the addition of the hexavalent chromium treatment required, these coatings become toxic. While some other compositions used to coat anodized aluminum may not contain hexavalent chromium, their technical performance is inferior to coatings based on hexavalent chromium. In addition, the use of hexavalent chromium treatments is becoming more expensive as regulations are strengthened. Costs can be made prohibitive with future restrictions imposed by the EPA. In this way, while the existing hexavalent chromium treatments are notorious in their technical operation in that they provide improved corrosion protection and bond bonding, eg, with coatings such as paint at a low application cost, from a From life cycle, environmental and OSH cost perspective, hexasvalent chromium coatings are detrimental to both, people and the environment.

With respect to the adhesive bond, phosphoric acid anodizing is being implemented as an alternative to anodizing with chromic acid. Anodizing coatings with phosphoric acid provide excellent bonding performance, but fail to adequately protect the base aluminum from corrosion. While anodizing sealants are typically applied to various other anodizing coatings, to improve performance against corrosion, they generally do not apply to phosphoric acid anodizing coatings because the adhesive bonding performance is significantly reduced. As a result, the corrosion protection of an anodized coating with phosphoric acid is provided by chromate bonding primers or general purpose primers. The anodizing coatings of phosphoric acid are characteristically columnar and porous, promoting excellent adhesive bonding performance. However, the porous, colloidal structure also promotes corrosion making the anodizing coatings of phosphoric acid particularly difficult to protect against corrosion. For example, the "honeycomb" core anodized with phosphoric acid, commonly used in military aircraft, quickly corrodes in service with its protective coating.

It damages and would greatly benefit from a corrosion protective sealant that does not adversely impact the adhesive bonding characteristics of the anodizing coating. COMPENDIUM OF THE INVENTION This invention relates to a process of post-treatment or sealing of anodized aluminum with phosphoric acid and its alloys at ambient or higher temperatures, e.g., varying up to about 93 ° C (200 ° F). More specifically, this invention relates to the subsequent treatment of anodized aluminum with phosphoric acid and its alloys to improve corrosion resistance and maintain adhesion bonding properties, e.g., adhesion of paint and the like. The trivalent chromium (TCP) post-treatment composition of this invention comprises an acidic aqueous solution having a pH ranging from about 2.5 to 5.5 and preferably 2.5 to 4.5 or 3.7 to 4.0, and per liter of the acid solution, from about 0.01 to 22 grams of a water-soluble trivalent chromium compound, about 0.01 to 12 grams of an alkali metal hexafluorozirconate, about 0.0 to 12 or 0.001 to 12 grams of at least one fluorocompound selected from the group consisting of of an alkali metal tetrafluoroborate, an alkali metal hexafluorosilicate and various combinations or mixtures thereof in any ratio, 0.001 to 10 grams of a water-soluble divalent zinc compound, of about 0 to 10 grams and preferably 0 to 2.0 grams of at least one water-soluble thickener, and 0 to 10 and preferably 0 to 2.0 grams of at least one water-soluble nonionic, cationic or anionic surfactant, or wetting agent. Therefore, an object of this invention is to provide an acidic aqueous solution comprising a trivalent chromium compound, an alkali metal hexafluorozirconate, and a tetrafluoroborate and / or hexafluorosilicate to treat aluminum anodized with phosphoric acid and its alloys to maintain its adhesion and improve its corrosion resistance characteristics. Another object of this invention is to provide a stable aqueous acidic solution having a pH ranging from about 2.5 to 5.5 containing effective amounts of a trivalent chromium salt and a hexafluorozirconate to seal anodized aluminum with phosphoric acid and its anodized alloys. A further object of this invention is to provide a stable acidic aqueous solution having a pH ranging from about 3.7 to 4.0 containing a salt of trivalent chromium and a hexafluorozirconate to treat or seal anodized aluminum with phosphoric acid and its alloys at about room temperature and above where the acid solution contains substantially no hexavalent chromium. These and other objects of the invention will become apparent by reference to the following detailed description when considered in conjunction with the accompanying Figures 1-9 (photographs). DESCRIPTION OF THE DRAWINGS Figure 1 is a photograph of anodized aluminum with phosphoric acid, 2024-T3 without further treatment after exposure to 24 hours (1 day) of neutral salt fog test ASTM-B 117. Figure 2 is a 2024-T3 photograph anodized with phosphoric acid subsequently treated with the composition of Example 5 (10 minutes immersion at around 24 ° C (75 ° F)) after exposure to 96 hours (4 days) of neutral salt mist test of ASTM-B 117. Figure 3 is a photograph of an anodized 2024-T3 with phosphoric acid subsequently treated with the composition of the Example 6 (10 minutes of immersion at about 24 ° C (75 ° F)) after exposure to 96 hours (4 days) of ASTM-B 117 neutral salt fog test. Figure 4 is a 2024-T3 photograph anodized with phosphoric acid subsequently treated with the composition of Example 5 (10 minutes at 38 ° C (100 ° F )) after exposure to 1000 hours (42 days) of neutral salt mist test of ASTM-B 117. Figure 5 is a 2024-T3 photograph anodized with phosphoric acid subsequently treated with the composition of Example 7 (10 minutes) at 38 ° C (100 ° F)) after exposure to 1000 hours (42 days) of neutral salt mist test of ASTM-B 117. Figure 6 is a photograph of an anodized 2024-T3 with phosphoric acid subsequently treated with the composition of Example 5 (40 minutes at room temperature (24 ° C (75 ° F)) after exposure to 1000 hours (42 days) of ASTM-B 117 neutral salt mist test. Figure 7 is a photograph of a 2024-T3 anodized with phosphoric acid subsequently treated with the composition of Example 7 (40 minutes aa Environment (24 ° C (75 ° F))) after exposure to 1000 hours (42 days) of ASTM-B 117 neutral salt mist test. Figure 8 is a photograph of an anodized 2024-T3 with phosphoric acid subsequently treated with the composition of Figure 5 (5 minutes at 66 ° C (150 ° F)) after exposure to 1000 hours (42 days) of the ASTM-B 117 neutral salt fog test. Figure 9 is a photograph of a 2024-T3 anodized with phosphoric acid subsequently treated with the composition of Example 6 (5 minutes at 66 ° C (150 ° F)) after exposure to 1000 hours (42 days) of the ASTM-B neutral salt fog test 117. DETAILED DESCION OF THE INVENTION More specifically, this invention relates to the process of using an aqueous acidic solution having a pH ranging from about 2.5 to 5.5, and preferably from about 2.5 to 4.5 or 3.7 to 4.0 to sealing anodized aluminum with phosphoric acid and its alloys to maintain its bond of adhesion and to substantially improve the corrosion resistance properties of anodized aluminum. The preferred process comprises the use of an acid solution comprising from about 0.01 to 22 grams and preferably from about 4.0 to 8.0 grams, eg, 6.0 grams of at least one trivalent chromium compound soluble in water, e.g., chromium sulfate, about 0.01 to 12 grams and preferably about 6.0 to 10 grams, e.g., 8.0 grams of at least one hexafluorozirconate of alkali metal, about 0.0 to 12 or about 0.001 to 12 grams and preferably about 0.12 to 1.2 grams, e.g., 0.24 to 0.36 grams of at least one fluorocompound selected from the group consisting of alkali metal tetrafluoroborates, alkali metal hexafluorosilicates and various mixtures or combinations thereof in any ratio, and from about 0.001 to 10 grams and preferably 0.1 to 5.0 or 1.0 to 2.0 grams of at least one divalent zinc compound such as zinc sulfate. In some processes, depending on the physical characteristics of the anodized aluminum, eg, the physical size of the anodized substrate, a particularity is the addition of a thickener to the solution that helps in optimal film formation during spray and rub applications slowing the evaporation of solution. It also mitigates the formation of powdery deposits that degrade paint adhesion. In addition, the addition of thickeners, helps in proper film formation during large area applications and mitigates the diluting effect of the rinse water remaining on the substrate during the processing of previous steps. This additive provides films that do not have scratches and have better coloration and corrosion protection. ThickenersWater-soluble such as cellulose compounds are known and may be present in the aqueous acidic solution in amounts ranging from about 0.0 to 10 grams and preferably from 0.0 to 2.0 grams and more preferably from 0.5 to 1.5 v. , about 1.0 gram per liter of the aqueous solution. Depending on the characteristics of the anodized aluminum, 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 10 grams and preferably from 0.0 to 2.0 grams and more preferably from 0.5 to 1.5 grams, e.g., 1.0 grams per liter of the acid solution. These water-soluble surfactants or wetting agents are known in the foregoing branch and are selected from the group consisting of nonionic, cationic and anionic surfactants. The trivalent chromium is added as a trivalent chromium compound soluble in water, preferably as a trivalent chromium salt. Specifically, in formulating the acidic aqueous solutions of this invention, the chromium salt may conveniently be added to the solution in its water-soluble form where the valence of the chromium is more 3. For example, some of the chromium compounds preferred ones they can be prepared in solution in the form of Cr2 (S04) 3, (NH4) Cr (S0) 2 or KCr2 (S04) 2 and any mixtures or combination of these compounds. The aluminum substrates are either anodized aluminum with phosphoric acid or anodized aluminum alloys containing about 60% or more by weight of aluminum. A preferred example of trivalent chromium concentration is within the range of about 4.0 to 8.0 grams or 6.0 grams per liter of the aqueous solution. It has been found that particularly good results are obtained when the trivalent chromium compound is present in solution at these preferred scales. The addition of preferred metal fluorozirconate to the acid solution ranges from about 6.0 to 10 grams or 8.0 grams per liter of solution. The treatment or sealing of the anodized aluminum with phosphoric acid can be carried out at low temperatures, e.g., around ambient or room temperature or at temperatures ranging up to about 93 ° C (200 ° F). Treatment at room temperature is preferred in that this eliminates the need for heating equipment. The seal coating can be air dried or by any of the methods known in the art, for example, oven drying, forced air drying, exposure to infrared cameras and the like. For purposes of this invention, the terms aluminum anodized with phosphoric acid and anodized aluminum alloys include aluminum and its alloys anodized with phosphoric acid by methods known in the art. In some treatments, the alkali metal tetrafluoroborates and / or hexafluorosilicates can be added to the acid solution in amounts as low as 0.001 grams per liter up to the limits of solubility of the compounds. For example, about 50% by weight of the fluorosilicate is added based on the weight of the fluorozirconate. In other words, for 8.0 grams per liter of the fluorozirconate salt, about 4.0 grams per liter of fluorosilicate is added to the solution. For example, an alternative is to add about 0.01 to 100 weight percent of the fluoroborate salt based on the weight of the fluorozirconate salt. Preferably, about 1 to 10 weight percent, e.g., about 3% of the fluoroborate salt may be added based on the weight of the fluorozirconate salt. A specific example comprises about 8.0 grams per liter of potassium hexafluorozirconate, about 6.0 grams per liter of basic chromium III sulfate, about 0.1 to 5.0 grams per liter of divalent zinc sulfate and about 0.12 to 1.2 grams per liter of potassium tetrafluoroborate and / or hexafluorosilacate. An important result of the addition of stabilization additives, ie, fluoroborates and / or fluorosilicates is that the solution is stable while the pH remains between about 2.5 and 5.5. However, in some cases the pretreatment solutions may require small adjustments to the pH by adding effective amounts of a dilute acid or base to maintain the pH on the scale of about 2.5 to 5.5 or less, e.g., from around 3.25 to 3.5. The acid composition or solution may also contain zinc compounds to further improve the corrosion protection of phosphoric acid-anodized coatings compared to compositions that do not contain divalent zinc compounds. The components of the solution are mixed together in water and can be used without additional chemical manipulation. Divalent zinc can be supplied by any chemical compound that dissolves in water at the required concentrations ranging from 0.001 to 10 grams and is compatible with the other components in the solution. Compounds that are particularly preferred include, for example, zinc acetate, zinc telluride, zinc tetrafluoroborate, molybdate of zinc, zinc hexafluorosilicate, zinc sulfate and the like or any combination thereof in any ratio. The following Examples illustrate the stable seal-coating solutions of this invention, and the method of using the solutions in maintaining the adhesion properties while improving the corrosion resistance of anodized aluminum with phosphoric acid and its alloys. EXAMPLE 1 TCP5PZ2 A stable aqueous acidic solution having a pH ranging from about 3.45 to 4.0 for further treatment of anodized aluminum with phosphoric acid and aluminum alloys to provide a color-resistant and corrosion-resistant coating thereon comprising , per liter of solution, about 3.0 grams of basic trivalent chromium sulfate, about 4.0 grams of potassium hexafluorozirconate and about 1.0 gram of zinc sulfate. EXAMPLE 2 TCP5B3 A stable aqueous acidic solution for subsequent treatment of anodized aluminum with phosphoric acid or aluminum alloys to form a corrosion-resistant coating thereon comprising, per liter of solution, about 3.0 grams of basic trivalent chromium sulfate, about 4.0 grams of potassium hexafluorozirconate, and about 0.12 grams of tetrafluoroborate of potassium. EXAMPLE 3 TCP5B3Z4 A stable aqueous acidic solution for further treatment of aluminum anodized with phosphoric acid and aluminum alloys to provide a corrosion resistant coating and a color recognized therein comprising, per liter of solution, about 3.0 grams of sulfate of basic trivalent chromium, about 4.0 grams of potassium hexafluorozirconate, about 0.12 grams of potassium tetrafluoroborate and about 2.0 grams of divalent zinc sulfate. Table 1 shows the corrosion classifications of three Examples for post-treatment of anodized aluminum alloys with phosphoric acid of this invention as compared to the coating composition of Example 2. Example 3 (TCP5B3Z4) and Example 1 (TCP5PZ2) as average they had higher classifications.

TABLE 1 Corrosion Resistance to Anodize with Phosphoric Acid with Aluminum Alloy Sealer 2024-T3 After 1,000 hours of Exposure to Neutral Salt Spray from ASTM B 117.

EXAMPLE 4 3.0 grams per liter of basic chromium III sulfate and 4.0 grams per liter of potassium hexafluorozirconate are added at a specific volume of deionized water. The pH is maintained between 3.25 and 3.60 for 14 days using dilute potassium hydroxide or dilute sulfuric acid. After 14 days the pH is adjusted to 3.90 +/- 0.05 and it is allowed to settle overnight. The solution is ready for use EXAMPLE 5 3.0 grams per liter of chromium sulfate are added III basic, 4.0 grams per liter of potassium hexafluorozirconate, and 0.12 grams per liter of potassium tetrafluoroborate at specific volume of deionized water. The solution is allowed to stand for about 10 days, or until the pH rises to between 3.75 and 4.00. The solution is ready for use. EXAMPLE 6 To Example 4, 1.0 grams per liter of zinc sulfate is added during the initial mixing. The solution is ready for use. EXAMPLE 7 To Example 5, 2.0 grams per liter of zinc sulfate are added during the initial mixing. The solution is ready to be used. EXAMPLE 8 Post-treatment coatings were applied to anodized aluminum as follows. The process of anodizing with phosphoric acid according to ASTM D 3933, "Conventional Practice for Surface Preparation of Aluminum for Structural Adhesives Bonding (Anodization with Phosphoric Acid) ", was followed completely. after anodizing aluminum panels of 7.62cm (3") by 25.4 cm (10") by 8.128 mm (0.32") aluminum alloys 2024-T3 by the process of Anodizing with Phosphoric Acid, the panels were rinsed completely twice in deionized water. Immediately after rinsing, the panels were immersed in a solution of either Example 6 or 7 for 10 minutes at ambient conditions. The immersion was immediately followed by two rinses with deionized water. The panels were air-dried at ambient conditions before being subjected to neutral salt mist in accordance with ASTM B 117. The coupons were kept on a rack at 15 degrees for the duration of the test. The unsealed phosphoric acid (P7AA) control coupons were tested together with the coatings present. Figures 2 and 3 (photographs) show the operation of subsequent treatments of the compositions of Examples 5 and 6. Figure 1 (photograph) shows an unsealed PAA panel after exposure to neutral salt mist of ASTM B 117. The subsequent treatments of Figures 2 and 3 provide improved corrosion resistance compared to the untreated coating of Figure 1. EXAMPLE 9 Test specimens were anodized as in Example 8. In this example, the compositions (solutions) of Examples 5 and 7 were heated to 38 ° Centigrade (100 ° Fahrenheit) and the panels were submerged for a total of 10 minutes. Figures 4 and 5 (photographs) show the corrosion performance of these coatings after 1000 hours of neutral salt mist according to ASTM B 177. It is evident that the composition of example 7 is an improvement compared to the composition of Example 5 EXAMPLE 10 Test specimens were anodized as in Example 8. In this example, the compositions (solutions) of Examples 5 and 7 were maintained at ambient conditions, around 24 ° C (75 ° Fahrenheit), and the panels were They submerged for a total of 40 minutes. Figures 6 and 7 (photographs) show the enhanced corrosion resistance of these coatings after 1000 hours of neutral salt mist according to ASTM B 117. EXAMPLE 11 Test specimens were anodized as in Example 8. In this example, the compositions (solutions) of Examples 5 and 6 were heated to 66 ° C (150 ° Fahrenheit), and the panels were immersed - for a total of 5 minutes Figures 7 and 8 (photographs) show the corrosion resistance of these coatings after 1000 hours of neutral salt fog according to ASTM B 117. Table 2 compares the results of corrosion resistance of the Examples based on classifications. Numbers of ASTM D 1654. In the ASTM classification method, the best possible marking is 10, meaning that substantially no corrosion is evident in the test panel. The ratings decrease to 1, which represents substantially 100% corrosion of the panel surface. From the data in Table 2, it is evident that the process of this invention is an improvement over previous processes used to subsequently treat or seal anodized aluminum with phosphoric acid and its alloys. TABLE 2 The numerical corrosion classifications of the panels treated with the compositions (solutions) and the process of this invention, based on ASTM D 1654, had classifications as high as 10 (no corrosion) to a low of 1 (completely corroded), where there were no subsequent treatments. The classifications comprise an average of three panels classified for each condition.

For the purposes of this invention, water-soluble surfactants or wetting agents can be added to the trivalent chromium solutions in amounts ranging from about 0 to 10 grams per liter and preferably 0.5 to about 1.5 grams per liter of water. the solution of trivalent chromium. The surfactants are added to the aqueous solution to provide better wetting properties by reducing the surface tension, thus ensuring full coverage, and a more uniform film on the coated substrate. Surfactants include at least one water soluble compound selected from the group consisting of nonionic, anionic, and cationic surfactants. Some known water-soluble surfactants having solubility at the required concentrations include the monocarboxyl imidoazoline, sodium salts of alkyl sulphate (DUPONOL®), poly (alkylene-ethanol) of tridecyloxy-alkylphenol ethoxylated or propoxylated (IGEPAL®), sulfonamides of alkyl, alkaryl sulfonates, palmitic alkanol amides (CENTROL®), octylphenyl polyethoxyethanol (TRITON®), sorbitan monopalmitate (SPAN®), dodecylphenyl polyethylene glycol ether, e.g. TERGITROL®, alkyl pyrrolidone, polyalkoxylated fatty acid esters, alkylbenzene sulfonates and mixtures thereof. Other known water-soluble surfactants include alkylphenol alkyloxylates, preferably nonylphenol ethyloxylates, and the various anionic surfactants, which have at least one sulfonate substituent on the phenyl ring, and the ethylene oxide adducts with fatty amines . Other known water-soluble compounds are found in "Surfactants and Detersive Systems", published by John Wiley & Sops in Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd edition. When large surfaces do not allow immersion or when vertical surfaces are to be sprayed, thickening agents are added to retain the aqueous solution on the surface for sufficient contact time. The thickeners employed are known inorganic and organic water soluble thickeners which can be added to the trivalent chromium solutions in effective amounts, eg, a sufficient concentration ranging from about 0 to 10 grams per liter and preferably 0.5 to 1.5. grams per liter of the acid solution. Specific examples of some preferred thickeners include the cellulose compounds, e.g., hydroxypropylcellulose (e.g., Klucel), ethylcellulose, hydroxyethylcellulose, hydroxymethylcellulose, methylcellulose, and mixtures thereof. Some of the less preferred thickeners include inorganic water soluble thickeners such as colloidal silica, clays such as bentonite, starches, gum arabic, tragacanth, agar and various combinations. After preparing the surface that is going to Treat through conventional anodizing techniques with phosphoric acid, the solution can be applied by immersion, sprinkling or rubbing techniques. The solution can also be used at elevated temperatures up to 18 ° C (65 ° F) and optimally applied by immersion to further improve the corrosion resistance of phosphoric acid anodizing coatings. The residence time of the solution is around 1 to 60 minutes, depending on the solution temperature and the concentration of the solution. After the stay, the remaining solution is then completely rinsed from the substrate with tap water or deionized water. No additional chemical manipulations of the deposited film are necessary for excellent performance. However, an application of a strong oxidizing solution can provide a film with improved corrosion resistance. The additional corrosion resistance is assumed to be due to the hexavalent chromium formed in the trivalent chromium film. The aqueous sealant composition can be sprayed from a spray tank apparatus designed to replace the immersion tanks. This concept also reduces the active chemical volume from around 3,785 liters (1,000 gallons) to around 113.55 to 189.25 liters (30 to 50 gallons).

Another feature of this invention is the ability of this protective seal coating to provide anodized coatings with phosphoric acid with better corrosion resistance or at least equivalent to other known anodic sealed coatings produced with sulfuric, chromic, boric-sulfuric or other known compositions. . This capability has not been available before and offers new potential applications for anodizing with phosphoric acid in corrosive environments that were not previously possible. Aluminum anodized with phosphoric acid have a major advantage over these other coatings in that their coating weights are typically 10 to 50 times lower. This provides significant weight savings and lower fatigue flow rate to structural aluminum alloys. In addition, this invention has the ability to improve the performance of anodizing coatings with phosphoric acid which are currently being implemented as an alternative adhesive bond to anodization with chromic acid. Phosphoric acid anodizing coatings which have not been subsequently treated are known to have lower corrosion resistance, but they are also known to have excellent bonding characteristics. This invention increases the corrosion performance of the anodized aluminum, while maintaining the adhesive bond strength of the coatings. The terms, for purposes of this invention, "solubility" and "water soluble" mean water solubility of the chemical compounds used in the solutions of this invention at least at the concentrations set forth herein. Although 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.

Claims

CLAIMS 1.- Process for sealing anodized aluminum with phosphoric acid and aluminum alloys to improve the corrosion resistance and maintain the adhesive bond strength, which comprises treating the anodized aluminum and its alloys with an acidic aqueous solution having a pH that it varies from around 2.5 to 5.5; the aqueous acidic solution comprising, per liter of solution, about 0.01 to 22 grams of a trivalent chromium compound, about 0.01 to 12 grams of an alkali metal hexafluorozirconate, about 0.0 to 12 grams of at least one selected fluorocompound of the group consisting of an alkali metal tetrafluoroborate, an alkali metal hexafluorosilicate and mixtures thereof, of from about 0.001 to 10 grams of at least one divalent zinc compound, from 0.0 to about 10 grams of at least one thickener soluble in water and from 0.0 to about 10 grams of at least one water soluble surfactant.
2. The process according to claim 1, wherein the pH of the aqueous acid solution varies from about 3.7 to 4.0 and the temperature of the aqueous acid solution varies from about room to 93 ° C (200 ° F) .
3. - The process according to claim 1, wherein the trivalent chromium is a water soluble compound ranging from about
4.0 to 8.0 grams, the alkali metal hexafluorozirconate is a water soluble compound ranging from about 6.0 to 10. grams, and the fluoro compounds are water soluble compounds that range from about 0.12 to about 1.2 grams. 4. The process according to claim 1, wherein the thickener varies from about 0.5 to 1.5 grams and the surfactant varies from about 0.5 to 1.5 grams.
5. The process according to claim 1, wherein the fluorocompound is present in the aqueous acid solution in an amount ranging from about 0.24 to 0.36 grams and the treated anodized aluminum is subsequently washed with water at temperatures ranging up to 93 ° C (200 ° F).
6. The process according to claim 1, wherein the thickener is a cellulose compound present in the aqueous acidic solution in amounts ranging from about 0.5 to 1.5 grams per liter. 1 . - The process in accordance with the 1 claim 1, wherein the trivalent chromium compound is trivalent chromium sulfate. 8. The process according to claim 1, wherein the alkali metal hexafluorozirconate is potassium hexafluorozirconate. 9. The process according to claim 1, wherein the trivalent chromium compound is trivalent chromium sulfate ranging from about 4.0 to 8.0 grams, the alkali metal hexafluorozirconate is potassium hexafluorozirconate ranging from about 6.0 to 10 grams, and the alkali metal tetrafluoroborate or alkali metal hexafluorosilicate varies from about 0.24 to 0.36 grams. 10. The process according to claim 1, wherein the divalent zinc compound is at least one of zinc acetate and a zinc sulfate. 11. The process according to claim 1, wherein the water soluble surfactant is selected from the group consisting of water-soluble nonionic, anionic and cationic surfactants. 12. The process according to claim 10, wherein the zinc sulfate is present in the aqueous solution in an amount varying from about 0.1 to 5.0 grams. 13. The process according to claim 1, wherein the trivalent chromium compound is chromium sulfate present in the aqueous solution in an amount ranging from 4.0 to 8.0 grams, the mixture of alkali metal tetrafluoroborate and metal hexafluorosilicate. alkali are present in the aqueous solution in an amount ranging from about 0.001 to 12 grams. 14. The aluminum anodized with phosphoric acid coated with seal and aluminum alloys according to claim 1.