Sealing Methods for Aluminum Anodizing

By Ronny Kalyango

Aluminum is a type of metal that cannot rust.
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Anodizing solutions are well-known for producing pores in the coating of the anodizing aluminum. These pores absorb dyes and also retain lubricants, if any are applied. In addition, they also provide areas through which the metal can corrode easily. To increase the corrosion resistance and dye retention, sealing is usually applied. Several sealing methods that are used include the use of boiling water, potassium dichromate and nickel acetate.

The Boiling Water Method

This process involves a long immersion of the anodized aluminum in boiling hot water, which has been deionized or is in steam form. This method is not very expensive, as it reduces abrasion resistance only by 20 percent. The oxide is converted into a hydrated form, and the swelling that results reduces the surface porosity.

The Nickel Fluoride Method

This process has been used as an alternative to the hot sealing methods, which, although preventing corrosion, are known to make the anodized aluminum softer; it is a cold sealing process, involving the addition of fluoride nickel to the anodized aluminum. The fluoride ions enter into the pores, which act as the place for the exchange mechanism. Once in the pores, the ions cause a shift in pH, which causes the nickel ions to precipitate. The nickel hydroxide formed then blocks the mouth of the pores, effectively sealing the film. A slow step occurs next, whereby water from the atmosphere diffuses into the film, causing the pores to block, and eventually a film is produced, which is effectively sealed.

The Nickel Acetate Method

The use of nickel acetate works based on the same principle as the use of nickel fluoride, since the nickel ions are still incorporated into the pores. The nickel ions then block or fill the mouths of these pores. However, in this method, the anodized aluminum extrusions have to be heated to a temperature of approximately 100 degrees Centigrade (212 degrees Fahrenheit). This method takes place through hydrolysis, whereby the nickel hydroxide precipitates in the pores of the coating.

Potassium Dichromate

For better resistance to corrosion and saline conditions, the anodic coatings are usually sealed with a 5 percent solution of potassium dichromate. The solutions usually operate at a boiling point, and the immersion takes place for about 15 minutes. At a pH of about 5 to 6, the chromate ions absorption takes place, allowing for the hydration of the coating. The dichromate-coated sealants are not so resistant to staining compared to the other sealant methods.