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Delay Sealing of Sulfuric Acid Anodize?

Regarding sulfuric acid anodize per MIL-A-8625 / MIL-PRF-8625 [on DLA], type II, is it permissible to dry parts after anodize, and at a later time hot water seal the parts?

Ray Handwerker
anodizing - Bensalem, Pennsylvania, USA
2004

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2004

Theoretically yes, practically NO WAY. "Sealing" is hydration of the aluminum oxide molecule. If the metal is wet coming out of the final rinse, then in the process of drying some hydration takes place, more with heat, less at room temperature, but some sealing happens. You might test with an impedance meter or with a dye stain test and interpret the "degree of sealing", then after your requested time delay, go back and see if it will take any more "seal".

Double sealing taken immediate does work provided the first seal is incomplete, therefore it follows that you may very well develope a system of "double sealing" whereby there is a delay. I doubt if anyone has tried this yet.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Ray,

We often sulfuric acid anodise parts in small (although not to same specification as you do), and have found that parts can be dried following the anodise process and then sealed later. We have found that if left for more than an hour the sealing process is not as effective. I must press that most customers require sealing operations directly after anodise as this gives the best final result. If left too long after drying the anodise coating can become discoloured during the sealing process. If the parts you are anodising are aircraft parts (as I suspect they may be) I would suggest sealing straight after the anodise process, as this will enable you to provide the best corrosion resistance possible for your customer.

Hope this information is useful to you.

Stuart Haynes
- Manchester, England
2004