Chromium-free method and composition to protect aluminium

Abstract: An improved method of providing a protective coating on the surface of aluminum or aluminum alloys, comprising: removing contaminants from the surface; exposing the surface to water at 50 to 100°C to form a porous bohemite coating on the surface; and exposing the bohemite-coated surface to an aqueous solution comprising a cerium salt and a metal nitrate at a temperature of 70 to 100°C. Oxides and hydroxides of cerium are formed within the pores of the bohemite to provide the protective coating, which provides corrosion resistance and improved paint adhesion.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a method and composition for providing the surface of aluminum and its alloys with a coating to protect against corrosion or to improve adhesion of paint. In particular, the invention relates to a compositon and method that use cerium salts to provide an improved coating on aluminum and aluminum alloys. 2. Description of the Background Art Aluminum and aluminum alloys are frequently used to form structures, such as for aircraft, in which corrosion resistance is required or in which good paint adhesion is required. Aluminum has a natural oxide film which protects it from many corrosive influences. This natural oxide is, however, not sufficiently resistant to such highly corrosive environments as saltwater, nor is it a good base for paints. Improved films, which are both more corrosion resistant and suitable as a base for paints can generally be formed on the surface of aluminum either by anodizing or by chromate conversion. During the anodizing process, aluminum oxide is formed on the aluminum surface, and provides a very corrosion resistant surface which can be dyed or painted. However, anodizing has the disadvantages of high electric resistance, higher cost, longer processing time, and the need to make direct electrical contact with the part. This latter requirement complicates processing considerably. Chromate conversion coatings are formed by dipping the aluminum part in chromic acid, to provide a coating comprising chromium oxide(s) mixed with aluminum oxide. Chromate conversion coatings are corrosion resistant, provide a suitable base for paint, can be rapidly applied, self-heal when scratched, and are very cheap. Furthermore, chromate coatings are reasonably conductive and can be used in sealing surfaces for electromagnetic interference gaskets. The conductive characteristics provided by chromate conversion coating are not characteristic of anodized coatings nor of most protective coatings. Unfortunately, the hexavalent chrome used in producing these cheap, reliable and useful coatings poses serious health hazards as well as significant disposal problems. Dermatitis and skin cancer have been associated with the mere handling of chromated aluminum parts. Severe damage to mucous membranes and skin lesions called "chrome sores" occur from exposure to the ever-present chrome mist in plating shops. Such health hazards to humans represent a major problem in the use of chrome for protecting aluminum. Thus, it would be desirable to replace the chromating process entirely. A recently developed process which eliminates the use of chromium involves coating aluminum surfaces with a film of aluminum oxyhydroxide (pseudo boehmite), as disclosed in U.S. Patent No. 4,711,667 for "Corrosion Resistant Aluminum Coating". This process yields a coating which is not as conductive as a chromate conversion coating, but is not, however, an insulator. In addition, its corrosion resistance is not as good as that produced by chromate conversion. The details of this known process are discussed in Example 1 herein. In another known method, aluminum has been treated with cerium chloride, CeCl₃, to form a mixed cerium oxide/cerium hydroxide film on the surface, as described, for example, by Hinton, et al., in the publication "Cerium Conversion Coatings For The Corrosion Protection of Aluminum", Materials Forum , Vol. 9, No. 3, pages 162 - 173 (1986). In this process, a coating of cerium oxide/hydroxide is precipitated on the aluminum surface and provides a relatively high degree of corrosion resistance. Unfortunately, this process is slow, taking almost 200 hours. The speed of the process can be improved so that the coverage occurs in 2 to 3 minutes by cathodically polarizing the coupon. However, this leads to a less durable coating, and the process is inconvenient because it requires the use of electrodes. DE-A-2 334 342 discloses a metal rinse for metal coatings, in particular for metal surfaces having a conversion coating thereon, such as chromate or phosphate. First, the metal surface is coated with phosphate, chromate or other conversion coating from an aqueous bath and then the conversion-coated metal surface is rinsed with an aqueous acidic solution of a salt of a rare earth metal. EP-A-348 630 discloses a process for applying corrosion-resistant coatings to aluminum alloys by a multi step process. In a first step the alloy is cleaned . The cleaned alloy is exposed to deionized water at 195°-212°F (92-100°C) for five minutes to form a thin layer of boehmite on the surface . The oxidised alloy is further treated in a solution of 1% aluminium nitrate and 1% lithium nitrate. The rinsed oxide coated alloy is then treated in potassium permanganate and finally sealed in a silicate solution. Thus, it would be desirable to provide a chromium-free process for providing aluminum and aluminum alloys with a protective coating which is rapid and does not involve the use of electrodes. SUMMARY OF THE INVENTION The present invention is directed to a method of protecting the surfaces of aluminum or aluminum alloys with a chromate-free protective coating to provide corrosion resistance or paint adhesion to the treated surface. The method uses a composition comprising a cerium salt and does not involve the use of electrodes which would galvanostatically polarize the contact between the aluminum and the aqueous treatment solution. The method in accordance with the present invention comprises: removing contaminants from said surface of said aluminum or aluminum alloys to provide a cleaned surface, exposing said cleaned surface to deionized water at a temperature within the range of 50 to 100°C to form a porous boehmite coating on said surface and is characterized by exposing said surface with said boehmite coating to an aqueous solution comprising a salt of cerium, from 0.1% to 5% by weight lithium nitrate and from 0.1% to 5% by weight aluminum nitrate at a temperature within the range of 70 to 100°C for a sufficient period of time to form oxides and hydroxides of said cerium within the pores of said porous boehmite coating to thereby provide said protective coating. The resulting coating is resistant to corrosion and has good paint adhesion. Optionally, a silicate sealant layer may be added. The present invention further encompasses the above-noted aqueous solution for treating aluminum or aluminum alloy surfaces to provide a protective coating. The present invention is further directed to a composition for providing the surface of aluminum or aluminum alloys with a protective coating in accordance with the above method. Said composition of the invention consists of aqueous solution comprising from 0.01% to 1% by weight of a cerium salt, from 0.1 to 5% by weight lithium nitrate and from 0.1% to 5% by weight aluminum nitrate. The above-discussed and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the method of the present invention, the aluminum surface to be treated is first cleaned to remove any contaminants on the surface. This first cleaning step may comprise, for example, contacting the surface with an alkaline cleaning composition for a sufficient period of time to remove substantially all the grease inhibitors or other contaminants that might interfere with the coating method of the present invention. Such grease inibitors are located on the surface of the aluminum. In addition, the surface to be treated may be cleaned by treatment with a deoxidizing agent to remove substantially all of the oxide inhibitors which might adversely affect the coating method described herein. These deoxidizing agents also remove any smut from undissolved alloying components such as copper. The oxide inhibitors are located on the surface of the aluminum. Other known processes for removing contaminants from the surface of aluminum or aluminum alloys may also be used in accordance with the present invention. After the surface to be treated has been cleaned to be free of contaminants, the cleaned surface is exposed to deionized water at 50 to 100°C to oxidize the aluminum and form a porous boehmite coating, comprising aluminum oxyhydroxide. Next, the surface with the boehmite coating is exposed to an aqueous solution comprising a salt of cerium, aluminum nitrate and lithium nitrate. The concentration of lithium nitrate is from 0.1% to 5%, preferably about 1% by weight. The aluminum nitrate concentration is from 0.1% to 5%, preferably about 1% by weight. The pH of the aqueous solution of the present invention is maintained in the range of 3.5 to 4, and preferably about 4. The metal nitrates produce further oxidation of the aluminum. While not limiting the present invention to a particular theory of operation, it is believed that the cerium salts penetrate into the porous boehmite structure where they are reacted to form cerium oxides and cerium hydroxide. It is believed that these cerium oxides and hydroxides plug the pores in the boehmite to thereby provide the improved protective coating. The cerium salt used in the present method is chosen from the group consisting of cerium chloride, cerium nitrate and cerium sulfate, and is preferably cerium chloride. The concentration of the cerium salt in the aqueous composition is from 0.01% to 1% by weight, preferably about 0.1%. The temperature at which the surface with the boehmite coating is exposed to the aqueous solution of the cerium salt, aluminum nitrate and lithium nitrate is within the range of 70 to 100°C, preferably about 97-100°C. The temperature may be decreased below the preferred range with corresponding reduction in the rate of reaction. For a treatment temperature of 97-100°C, this process step may be completed in about 5 minutes. For lower temperatures, longer time periods will be required to complete this process step. Optionally, the present method may include the further step of exposing the treated surface to a solution of a silicate compound, such as 10 percent by weight potassium silicate at 90°C to 95°C for about 1 to 1.5 minutes, to provide a final silicate sealant layer, as described in Example 1. The coatings formed in accordance with present invention protect the treated surface to provide corrosion resistance as discussed in Example 1 or to provide improved paint adhesion as discussed in Example 2. Examples of practice of the present invention are as follows. EXAMPLE 1 The method in accordance with the present invention provides an improvement on the known process disclosed in U.S. Patent No. 4,711,667, previously discussed in the "Description of Related Art" herein, and referred to hereinafter as the "Sanchem process". In this example, the corrosion resistance of samples treated in accordance with the present invention is compared to the corrosion resistance of samples treated in accordance with the Sanchem process. The Sanchem process was practiced by treating aluminum alloy coupons type 2024-T3, having dimensions of 3 inches by 10 inches (7.6 cm by 25.4 cm), by the following steps: Step 1: Clean coupon in alkaline cleaner, such as Chemidize 740 (obtained from Sanchem Inc.) at 71°C for 3 minutes. Step 2: Rinse 1 minute with deionized (D.I.) water. Step 3: Deoxidize at 30°C - 35°C for 20 minutes in a mixture of 10% nitric acid and 3% sodium bromate. Step 4: Rinse 1 minute in D.I. water. Step 5: Place in D.I. water at 97°C - 100°C for 5 minutes Step 6: Place in solution of 1% lithium nitrate and 1% aluminum nitrate at 97°C - 100°C for minutes. Step 7: Rinse in D.I. water. Step 8: Place in solution of 0.25% KMnO₄ for 5 minutes at 57°C - 60°C. Step 9: Rinse in D.I. water Step 10: Place in solution of 10% potassium silicate at 90°C - 95°C for 1 - 1.5 minutes. Step 11: Rinse in D.I. water. Step 12: Blow dry. In accordance with a preferred embodiment of the present invention, the aluminum alloy coupons (type 2024-T3) were pre-treated as described in steps 1 through 5 above. Then the cleaned coupon was exposed to the composition of the present invention and dried. Thus, the present process eliminated steps 8 through 11 in the Sanchem process, which required treatment with potassium permanganate and an additional sealing step with potassium silicate. The specific treatment steps used in accordance with the present invention were as follows: 1. Clean coupon in alkaline cleaner (Chemidize 740) at 71°C for 3 minutes. 2. Rinse 1 minute in deionized water. 3. Deoxidize at 30°C to 35°C for 20 minutes in a mixture of 10% nitric acid and 3% sodium bromate. 4. Rinse 1 minute in deionized water. 5. Place in deionized water at 97°C to 100°C for 5 minutes. 6. Place in solution of 0.1% cerium chloride, 1% lithium nitrate, and 1% aluminum nitrate at pH of 4 at 97°C to 100°C for 5 minutes. 7. Blow dry. Aluminum alloy coupons treated by each of the above-described processes were subjected to a salt spray test in accordance with the American Society for Testing and Materials B117 (Standard Method of Salt Spray (Fog) Testing), for 3 days at 95°C. The corrosion resistance of the coupons treated in accordance with the present process was as good as the corrosion resistance of the coupons treated in accordance with the Sanchem process. The quality of the corrosion resistance was determined using the measurement standards of MIL-C 5541 (Chemical Conversion Coatings on Aluminum and Aluminum Alloys). Thus, the present process provides good corrosion resistance while eliminating the steps of treatment with potassium permanganate and with a sealant, to thereby reduce processing time and costs. In addition, various modifications of the Sanchem process and of the present process were made and these modifications are summarized in Table 1. Treatment M1 employed the preferred method of the present invention set forth above. Treatment M₂ was the same as M₁ except only steps 10 and 11 of the Sanchem process were deleted. Similar variations to the Sanchem process are identified in Table 1 as S₁ and S₂. In S₁, steps 8 - 11 of the Sanchem process were deleted. In S₂, steps 10 and 11 were deleted from the Sanchem process. 7 PROCESS VARIATIONS M₁ Present process (preferred). Addition of 0.1% CeCl₃ to Step 6 of Sanchem process; deletion of Steps 8-11 of Sanchem process. M₂ Present process (Altered). Addition of 0.1% CeCl₃ to Step 6 of Sanchem process; deletion of Steps 10 and 11 of Sanchem process. S₁ Sanchem process. Deletion of Steps 8-11. S₂ Sanchem process. Deletion of Steps 10 and 11. Corrosion resistance provided by the variations of the method of the present invention, M₁ and M₂, were compared with variations, S₁ and S₂, of the Sanchem process. The comparisons were made by subjecting treated aluminum alloy coupons, type 2024-T3, to a salt spray chamber for 8½ days at 95°C. Two tests were performed. In a first comparison treatment, M₁ was compared to treatment S₁. In the first test, the method of the present invention, M₁, gave better corrosion resistance than the S₁ treatment. In the second test, the method of the present invention M₁ gave about the same level of corrosion resistance as the S₂ treatment. These results indicate that the method of the present invention, treatment M₁, can produce the same or even better corrosion resistance than a Sanchem process which has been correspondingly modified to have fewer steps. In addition, the method of the present invention, treatment M₁, was compared to treatment M₂ in which only steps 10 and 11 of the Sanchem process were deleted. The results showed that the additional steps 8 and 9 of the Sanchem process counteracted the corrosion resistance provided by cerium chloride salts introduced in accordance with the present invention. Accordingly, it is preferred that steps 8 and 9 of the Sanchem process be deleted, as has been done in accordance with the present invention. Finally, the process of the present invention treatment M₁ was modified to include steps 10 and 11 of the Sanchem process to provide a final sealant. In addition, the dioxidization of step 3 above of the present process was performed at 24°C (i.e., room temperature) for 40 minutes. The test samples were two aluminum alloy coupons, type 2024-T3. The treated samples were subjected to corrosion testing in accordance with ASTM B117, previously referenced, for a period of 168 hours. Good corrosion resistance was obtained for both samples, as indicated by applying the measurement standards of MIL-C-5541. In addition, the test results for the two test samples were very similar to each other. For comparison purposes, two test samples from the same batch as used above were treated in accordance with the Sanchem process as previosly described and subjected to the same corrosion testing as the samples treated in accordance with the present invention. One of these test samples had corrosion resistance as good as the samples treated in accordance with the present invention, and the other test sample was considerably worse than the sample treated by the present invention. EXAMPLE 2 This example presents data showing that the method of the present invention provides the surface of the aluminum or aluminum alloy with a coating which provides good paint adhesion. Test samples consisting of aluminum alloy coupons, type 2024-T3 were treated in accordance with the present invention as previously indicated in Example 1 in steps 1 through 7. Paint was then applied to the treated test samples. The test samples passed the paint adhesion tests specified in Federal Standard 141 (Paint, Varnish, Lacquer and Related Materials, Methods of Inspection, Sampling and Testing) method 6301, as specified in MIL-C-5541, both before and after salt spray testing in accordance with ASTM B117. In addition, these test samples passed a 180 degree bend test after salt spray testing.

Claims: A method for providing the surface of aluminum or aluminum alloys with a protective coating comprising: (a) removing contaminants from said surface of said aluminum or aluminum alloys to provide a cleaned surface; (b) exposing said cleaned surface to deionized water at a temperature within the range of 50 to 100°C to form a porous boehmite coating on said surface; characterized by (c) exposing said surface with said boehmite coating to an aqueous solution comprising a salt of cerium, from 0.1% to 5% by weight lithium nitrate and from 0.1% to 5% by weight aluminum nitrate at a temperature within the range of 70 to 100°C for a sufficient period of time to form oxides and hydroxides of said cerium within the pores of said porous boehmite coating to thereby provide said protective coating. The method of claim 1 wherein said salt of cerium is chosen from the group consisting of cerium chloride, cerium nitrate, and cerium sulfate. The method of claim 1 wherein said salt of cerium comprises cerium chloride and the concentration of said salt of cerium is from 0.01% to 1% by weight. The method of claim 1 wherein the concentration of said salt of cerium is about 0.1% by weight. The method of claim 1 wherein said removing said contaminants comprises exposing said surface to an alkaline cleaning composition or a deoxidizing agent. The method of claim 1 wherein the pH of said aqueous solution is in the range of 3.5 to 4. The method of claim 1 further comprising after step (c), exposing said surface with said protective coating to a metal silicate solution at a temperature of 90 to 95°C for a sufficient period of time to form a final sealant layer. A composition for providing the surface of aluminum or aluminum alloys with a protective coating in accordance with the method of claim 1, said composition consisting of aqueous solution comprising from 0.01% to 1% by weight of a cerium salt, from 0.1% to 5% by weight lithium nitrate and from 0.1% to 5% by weight aluminum nitrate.

Inventors: KINDLER ANDREW, KINDLER, ANDREW
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1992
Main classification: C23C22
IPC codes: C23C22