The formation of the anodized coating on aluminum is characterized by a series of closely packed hexagonal crystal columns with a more or less cylindrical pore in the center of each crystal. The purpose of the pore during the anodizing process is to allow electrolyte to flow to the aluminum surface and to provide a path for ions to flow and allow the anodizing process to proceed.
After the anodizing process, these pores also act as a receptacle for dyestuffs should they be desired. These pores however, can also act as a conduit for corrosion processes in many circumstances. In most cases, these pores are closed in the final stage of the anodizing process during the “sealing” step.
This sealing step can be accomplished using a variety of agents. The simplest is boiling deionized water. This is, however, a slow process and is presently only used in limited applications. A more common agent and the one most frequently used is a hot 1800 – 2050F solution of nickel acetate. This method is favored in conventional (i.e. not hard coat) anodized components that have been colored with dyes.
The sealing of hard coated (hard anodized) surfaces is more problematic. While hot sealing significantly reduces the potential for corrosion it also measurably lowers the hardness of the coating. An alternative commonly used for hard coatings is to use a solution of either sodium or potassium dichromate in lieu of the nickel acetate bath previously discussed. The advantage of this bath is that it is operated at a lower temperature and provides good corrosion protection. A disadvantage is that the dichromate tends to leach from the film over time. This causes a gradual loss of corrosion protection. To help mitigate this problem a “duplex” seal was developed which combines the nickel acetate and the dichromate processes. This method reduces the migration of the dichromate from the coating but also somewhat decreases the hardness of the hard anodized surface. In most applications, this “compromise” has been acceptable and provides and corrosion resistant and reasonably hard coating.
Recently, however, the use of hexavalent chromium has been under scrutiny and within the RoHS guidelines has been banned. Fortunately, there is an alternative sealing method that provides very good corrosion protection but does not decrease the hardness of the coating. This bath utilizes nickel fluoride at or near room temperature and is used extensively at the D-CHN facilities.
While the use of the dichromate process would help insure maximum corrosion protection, we feel that the low temperature nickel fluoride seal will provide excellent corrosion protection while preserving the hardness of the hard coated surface.