Duplex / "double" sealing of anodized aluminum

A. The anodizing is aluminum oxide, the seal is "hydrated aluminum oxide", the dimensional change is zero.

A. I'm not familiar with the Boeing spec, however I will tell you that the sealing of anodizing does not add any thickness to the original coating.

Basically, when sealing anodizing, the only thing we are doing is closing the pores of the anodic coating for increased corrosion resistance. The downside to this, is that the sealing process slightly softens the type 3 coating.

Sealing hardcoat has caused quite a bit of confusion between customer and anodizer over the years, as MIL-A-8625 / MIL-PRF-8625 [on DLA] F does not require that the parts to be sealed, unless requested by the customer. This comes into play when we're talking about Type 3 class 2 coatings (dyed), as the color tends to fade when not sealed.

A. "Double Sealing" seems to mean something different to different people. In my book I offer two kinds of double seal. (1) First Nickel Acetate at about 170 °F for 20 minutes followed by D.I.Water; pH 5.7 with 1 gm/L ammonium acetate, 20 minutes at 170 °F, and this leaves no coloration. (2) Nickel Acetate for 10 minutes followed by Sodium or Potassium Dichromate, and this leaves the coloration you asked about. It is recommended that each application be researched for the conditions of alloy, thickness, end use corrosion requirements, and end use cosmetic requirements.If your customer says "double seal", ask him exactly what he means.

Hi Basavaraj. The way I read MIL-A-8625 is that "duplex sealing" means nickel acetate or nickel cobalt sealing followed by sodium dichromate sealing. But ignoring Mil-A-8625's implication that "duplex sealing" pertains to Class-2 (dyed) coatings, and is intended to improve the corrosion resistance of dyed coatings might be a mistake as well.

It is a very human thing to not want to look stupid; and a sound business decision to not make the customer look stupid. Unfortunately, this creates a strong temptation to simply guess what the customer wants, or to ask others to guess what the customer wants :-)

I think you should straightforwardly ask your customer if nickel/cobalt acetate sealing followed by sodium dichromate sealing is what they want. It just might be that what they have in mind is Probert's "DI water" double sealing as a way to get away from having sodium dichromate on their parts :-)

Good luck.

Regards,

A. Nickel Acetate which is so easily contaminated always benefits from a post sealing in hot d.i. water pH 6.0. The second hot water seal is always good "insurance".

A. Just like you said it is the best you can do, just be aware that low concentration seals are extremely easy to drag in contamination. See my post of 2011 above, Nickel seal first then hot water, but you might raise the temp on up another 10 °F.