Duplex Sealing With Nickel Acetate and Di Water
In past Anodizing Clinics you have talked about duplex sealing using, first, a relatively concentrated (3–5% by weight) nickel acetate bath followed by sealing in deionized water as a good way to seal dyed anodic coatings.
We have our own recipe for this type of sealing that calls for 1.25% nickel acetate followed by hot water sealing. Maybe this duplex seal is what contributes to the light-fastness of dyed coatings more so than the concentration of nickel acetate. We have noticed that duplex sealed (NiAc +DI water) dyed coatings have better light-fastness. Maybe the “duplex” sealing has more to do with light-fastness than the concentration of NiAc.
Is that possible? J.M.
There are lots of ways to seal dyed parts, as you know. One reason for the duplex seal is that NiAc at the appropriate pH and temperature creates an environment at the anodic pore that maximizes the precipitation of nickel hydroxide in the pore. This helps create a good seal with minimal leaching of the dye. You are correct in that the exact concentration of nickel acetate is less important than the duplex process. The important thing is to get nickel in the pore. The reason that the immersion in NiAc is short is to keep sealing smut to a minimum. It gets the nickel in the pore and most of the sealing is done in the DI water.
Nickel concentration can vary. Usually the less nickel, the less problem with smut—unless you add surfactants to the bath. There is nothing wrong with adding surfactants, as long as the anodized and dyed part is not to serve as a base for a caulk, adhesive bond, or paint. The proprietary mid-temperature nickel seals are very good, but they have surfactants. If smut is not an impediment, then the sky’s the limit on what you might do.
Rarely is smut an attribute. Usually, better sealing means better light-fastness. Most organic dyes are not very light-fast to exterior exposure, but a better seal will definitely help preserve the color for as long as possible. Among the dyes with the greatest lightfastness are the black MLW, HBL and H3LW, whereas reds and blues tend not to do so well in this regard.
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The following anodizing process overviews are provided as a means of introduction to aerospace anodizing