Q. One of our customers returned some hardcoat anodized (Type III) parts that we processed about two months ago. The original specification called for no seal, but the customer now requests that we seal the parts, which have been in storage. Is it possible and practical to do this? Y.B.
A. As you probably know, unsealed anodized parts will tend to self-seal over time. The coating will absorb moisture from the air and will at least partially seal. Hydration of the anodic pores is the most common method of sealing the coating. How much the parts seal depends largely on the humidity and ambient temperature present during their storage period. A complete seal might be achieved over a long period of time, but that might not have happened in two months
It never hurts to put the parts in a seal tank to seal or re-seal them. Doing this will continue the sealing process to its normal end point. So, if you seal the parts as you normally would, it should work out well. This sealing could take place in deionized (DI) water close to the boiling point (205-208°F, 96-98°C), or the parts could be sealed using a mid-temperature nickel acetate bath at approximately 185°F (85°C). Seal at least 10 minutes or longer if possible. If you use straight DI water for sealing, be sure to add about a 1/4 lb (100-200 g) of sodium acetate to the bath to help stabilize the pH.
Bear in mind that any fingerprints or other handling marks may be accentuated after sealing. If you think this will be a problem, you could put the parts in the cleaner tank for about 10 minutes to help minimize the handling marks (as long as it is a non-etching cleaning bath), rinse thoroughly and then seal as normal.