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Hard anodizing sealing process

If boiling water and dichromate sealing does not help to prevent dye to penetrate in Dye spot test what will be the problem in the sealing process or anodizing process?

Jasjeet Singh
advantech - India
2006

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Hard to say without more information - how long were you sealing, and what were the conditions (temp, time, volts, amps, square footage, acid concentration) of the anodize?

Jim Gorsich
Compton, California, USA
2006

What are the proper sealing process after hard anodizing 40 microns coating in sul . 20/- oxalic - 10gn/ lt . temp-0-1 25 to 30 amps/+ time 1 hr . at 18-20 volts

Pooran Singh
- Faridabad, Haryana, India
2007

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2007

I haven't used oxalic acid-containing electrolytes but have some concern about the anodizing conditions. ASM Handbook, Volume 5 Surface Engineering, gives operating parameters for the Alumilite 225 and 226 processes. Solution is 12 wt% sulfuric + 1 wt% oxalic, T = 10 °C, and voltage is ramped from 10 to 75 V to maintain current density at 30 A/ft2. The "1 hr. at 18-20 volts" may give a poor quality coating, as the rates of oxide growth & dissolution by the electrolyte become comparable.

Common sealing problems with hard anodizing are crazing (due to greater thermal expansion of the aluminum) and impurities in the seal. Anodize formed at 0 °C should be slowly warmed, e.g., first in flowing cold water rinse, then in 50 °C DI water, then in 70 °C dichromate seal (T can be then raised to 95 °C or simply held at 70 °C for twice as long). DI water should be used for the pre-seal rinse as well as all make-up for the seal(s). Impurities such as phosphate, chloride, silicates, etc. interfere with sealing.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
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