Method for increasing the corrosion resistance of aluminum and aluminum alloys

Abstract

Aluminum and aluminum alloys are protected from corrosion by immersion in an alkaline lithium or alkaline magnesium salt solution. Immersion in the salt solution causes the formation of a protective film on the surface of the aluminum or aluminum alloy which includes hydrotalcite compounds. A post film formation heat treatment significantly improves the corrosion resistance of the protective film.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is as follows: 
     
       1. A method for providing an aluminum alloy containing lithium with a surface coating that protects against corrosion, comprising the steps of immersing a substrate comprised of an aluminum alloy that contains 0.5 to 10 weight percent lithium in an alkaline salt solution having a pH of at least 8 and a concentration ranging from 0.01M to 1.0M wherein an anion of said salt in said alkaline salt solution is capable of forming a salt with said lithium in said aluminum alloy, and drying a film formed on said substrate after said step of immersing. 
     
     
       2. A method as recited in claim I wherein said anion of said salt in said alkaline salt solution is selected from the group consisting of CO 3   2- , SO 4   2- , Cl - , Br - , and OH - . 
     
     
       3. A method as recited in claim 2 wherein said step of immersing is performed when said alkaline salt solution has a temperature ranging from 25° C. to 30° C. 
     
     
       4. A method as recited in claim 1 further comprising the step of heating said film formed on said substrate. 
     
     
       5. A method as recited in claim 4 wherein said step of heating is performed at approximately 150° C. for approximately four hours. 
     
     
       6. A method of protecting aluminum and aluminum alloys against corrosion comprising the step of immersing an aluminum or aluminum alloy in an aqueous solution consisting .[.solely.]. .Iadd.essentially of .Iaddend.a lithium salt .Iadd.to form a corrosion resistant coating.Iaddend.. .Iadd.7. A method of protecting aluminum and aluminum alloys against corrosion comprising the step of immersing an aluminum or aluminum alloy in an aqueous solution which is free of a quantity of tannin sufficient to form a corrosion resistant coating, comprising a lithium salt to form a corrosion resistant coating. .Iaddend..Iadd.8. A method of protecting aluminum and aluminum alloys against corrosion comprising the step of immersing an aluminum or aluminum alloy in an alkaline aqueous solution consisting essentially of a lithium salt to form a corrosion resistant coating. .Iaddend..Iadd.9. A method as recited in claims 6, 7 or 8 wherein the pH is greater than 9..Iaddend..Iadd.10. A method as recited claims 6, 7 or 8 where the pH is 10 or greater. .Iaddend..Iadd.11. A method as recited in claims 6, 7 or 8 wherein the lithium salt is LiCl, LiOH, LiBr, Li 2  CO 3  or Li 2  SO 4 . .Iaddend..Iadd.12. A method as recited in claims 6, 7 or 8 wherein the concentration of the lithium salt 
     
     
        ranges from 0.01 to 0.6 molar. .Iaddend..Iadd.13.  A method as recited in claims 6, 7 or 8 wherein said concentration of the lithium salt ranges from 0.05 molar to 0.1 molar. .Iaddend..Iadd.14. A method as recited in claims 6, 7 or 8 wherein said immersing step is performed when the lithium salt solution has a temperature ranging from 25° C. to 30° C. .Iaddend..Iadd.15. A method as recited in claims 6, 7 or 8 further comprising the step of heating the coating formed on the aluminum or aluminum alloy to improve the corrosion resistance of the coating. .Iaddend..Iadd.16. A method as recited in claim 15 wherein said heating step is performed at approximately 150° C. for approximately 4 hours. .Iaddend..Iadd.17. A method of protecting aluminum and aluminum alloys against corrosion by forming a hydrotalcite coating on a surface of an aluminum or aluminum alloy substrate by immersing said substrate in a solution consisting essentially of a lithium salt solution having a pH which allows hydrotalcite formation, for a length of time sufficient to allow a degree of hydrotalcite formation which provides corrosion 
     
     
        resistance for the substrate. .Iaddend..Iadd.18.  A method as recited in claim 17 wherein said pH is greater than 10. .Iaddend..Iadd.19. A method as recited in claim 17 wherein said length of time is on the order of minutes. .Iaddend..Iadd.20. A method as recited in claim 17 wherein said alkaline metal salt is a lithium salt selected from the group consisting of LiCl, LiOH, LiBr, Li 2  CO 3  or Li 2  SO 4 . .Iaddend..Iadd.21. A method as recited in claim 17 wherein said step of immersing is performed at approximately room temperature..Iaddend..Iadd.22. A method as recited in claim 17 further comprising the step of heating the hydrotalcite coating subsequent to formation to improve the corrosion resistance of the coating. .Iaddend.