It is established that anodic coatings produced on aluminum are highly porous oxide films, which tend to absorb chemicals from the surrounding environment; therefore, anodized aluminum is subject to attack from environments causing structural and physical damages of the finished aluminum. For this reason a wide range of postsealing processes have been developed for Type Ⅱ anodizing to enhance the corrosion resistance of anodic coatings over the decades. The major objectives of sealing are twofold: enhance the corrosion resistance of anodic coatings and prevent the leaching of dyes and enhance the UV-light resistance of dyes in the micro-pores of the anodic coatings. Tremendous efforts have been contributed to understanding and developing numerous sealing processes. Among the most commonly used seals are hot water (boiling deionized water), nickel acetate solution, and dichromate solution. Low-temperature nickel fluoride sealing has recently been available in North America for saving energy, preventing dry marks, and/or minimizing the loss of hardness and abrasion resistance. Steam sealing has been used in Japan since 1932. A sodium silicate seal is sometimes used in military applications for minimizing hardness and wear resistance reduction without environmental pollution. Additionally, a number of nonaqueous sealants were proposed for sealing applications. New sealing processes continue to emerge.
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