All anodizing processes produce coatings with pores in their surfaces. These pores are necessary for the addition of dyes and printing, as well as for the retention of lubricants. However, they also provide in-roads for corrosion. To eliminate this potential for corrosion, and to lock in printed graphics and dyed colors, anodized coatings can be sealed. Sealing also increases the coatings’ stain resistance. However, sealing will reduce the coatings’ lubricity and abrasion resistance.
A simple method of sealing anodic coatings is prolonged emersion in boiling deionized water or steam. Oxides are converted to their hydrated forms, which results in swelling that reduces the porosity of the anodized surface. This method, though common, is not completely effective. Abrasion resistance can be reduced by as much as 20%.
Another common method of sealing anodic coatings is cold sealing. In this process, pores are closed by impregnation of a special sealant chemical in a room-temperature bath. This method is popular as it provides significant energy savings over the boiling water process.
Chemical seals are also an effective way to seal the pores in anodic coatings. Cobalt acetate, nickel acetate, Teflon, hot sodium dichromate, and potassium dichromate are commonly used for this purpose.
Mil spec MIL-A-8625 requires sealing of thin anodized coatings (Type I and Type II); sealing is allowed as an option for thick coatings (Type III).