This environmental-quality invention is in the field of sealing the surface coatings produced by anodizing aluminum and aluminum alloy substrates (for example, aerospace, commercial, and architectural products). The invention produces sealed anodization coatings exhibiting good corrosion resistance performance while maintaining acceptable levels of paint adhesion performance.
The International Agency for Research on Cancer has identified both chromium and nickel compounds along with many other pollutants as confirmed human carcinogens. The Boeing Company (Boeing), along with many other companies, has voluntarily agreed with the U.S. Environmental Protection Agency (EPA) to reduce the use of the seventeen most hazardous pollutants which include these compounds. Currently, the only approved sealing solution for the coating produced by the boric acid-sulfuric acid anodizing process is a dilute (45-75 ppm) chromate seal solution. The purpose of the chromate sealing solution is to hydrate surface oxide while entrapping the hexavalent chromium. The hexavalent chromium acts as a corrosion inhibitor to further enhance the corrosion resistance of the anodized coating. Using this dilute chromate seal solution, production operations can use the boric acid-sulfuric acid anodizing process on aluminum alloys 2024, 6061, and 7075 and produce parts that pass a two-week salt spray test and meet the requirements for paint adhesion. Unfortunately, the dilute chromate sealing solution is a hazardous pollutant.
The unsealed aluminum oxide produced by anodizing is usually modeled as two oxide layers on an aluminum substrate. The inner layer is a thin continuous barrier layer of less than 500 angstroms thickness. The outer layer is a discontinuous coating with pores that may penetrate from the outside surface to the barrier layer. These pores are the source of potential corrosion pitting problems that occur in salt spray and other atmospheric environments. In the dilute chromate seal solution process, these aluminum oxide pores are hydrated with entrapped hexavalent chromium. This filling of the pores enhances the corrosion protection of the anodized coating on the aluminum substrate.
In B. Yaffe, Metal Finishing, May 1990, vol. 41 (1990), the author reviews the known methods of sealing anodized aluminum, such as sealing in steam and hot water, nickel acetate, dichromate, and various cold sealing methods. Some of the newer sealing methods have been developed due to environmental concerns and the desire to lower costs. Cold sealing in nickel fluoride has been introduced to lower these costs. However, health hazards have been observed recently for nickel salts, which can cause allergic contact dermatitis. In NASA Tech Briefs, May 1995, a sulfuric acid anodizing process with a lower temperature nickel acetate seal is described. This process produces thin anodized layers that are not detrimental to the fatigue properties of the aluminum substrate, but does not address the health hazards due to the use of nickel salts. In Boeing's boric acid-sulfuric acid anodizing process, anodized layers of about 1 .mu.m thickness are produced, which are then sealed using a dilute chromate solution (as described in Boeing Process Specification BAC 5632, "Boric Acid-Sulfuric Acid Anodizing").
In a study to develop an overall corrosion protection system for aluminum alloys, co-inventor Mansfeld developed a treatment for commercial aluminum alloys using two rare earth metal salt solutions that produced surfaces with excellent resistance to pitting (see Mansfeld et al. U.S. Pat. No. 5,194,138, "Method For Creating A Corrosion-Resistant Aluminum Surface"). For commercial aluminum alloys having a high copper content, co-inventor Mansfeld developed an additional pre-treatment to remove copper from the outer surface to further enhance corrosion protection (see Mansfeld et al. U.S. Pat. No. 5,582,654, "Method For Creating A Corrosion-Resistant Surface On Aluminum Alloys Having A High Copper Content").