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How to build up aluminum concentration in Type III bath?

I have the opposite problem as the guy who asked whether it was necessary to decant when the aluminum concentration got too high. My aluminum concentration is about 3.0 mg/l and I have been told that the Type III coating process works best when the Aluminum concentration is in the upper part of the acceptable band - say in the 8 - 10 range. How do I get there quickly?

Mike Palatas
Operator - Los Angeles, California
September 8, 2011

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First of two simultaneous responses -- September 9, 2011

I'm of the opinion that 8-10 gr/lt is a bit too much for hardcoat but not detrimental, for sure. You really don't want to exceed 12 gr/lt.

Anyhow, an addition of Aluminum sulphate will boost the Al content to your desired parameter.

Marc Green
anodizer - Boise, Idaho

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I am not quite sure that you should wish to get to the upper limit of the band so quickly. 2-3g/l should do just fine to start with. Usually when one starts with a completely fresh sulfuric acid solution, 1% aluminium sulphate would kick us off properly. When you discard old solution and make a fresh one, you can retain a small percentage of the old solution (say 10% - depending on the al-content at the time of discarding) to set you on your way.
The initial aluminium is to take the 'extra bite' off the fresh sulfuric acid concentration in the bath. It inhibits aggressive attack on the jobs when the solution is new.

Ravi Rao
- Belgaum, Karnataka, India
Second of two simultaneous responses -- September 10, 2011

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September 12, 2011

Be aware that most commercial available aluminum sulphate is sold for water treatment and is usually loaded with impurities. Reagent grade is prohibitively too expensive.
If you also have a Type II regular anodizing solution you probably see white salt accumulating at the top of the cathodes, scrape it off and use it, then run scrap/strip/run scrap/ strip, etc....You can probably start hard coating well at 4 gm/L. If you use reagent aluminum sulphate start at 6 oz/gal (something you can weigh easily).

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

I took a perforated plastic bucket of clean 6061 chips and hung it in the bath over night for two weeks. Took it out each morning, rinsed it and hung it aside each production day. I know this is a HACK approach, but it worked.

Sam Pearson
- Miami Florida
November 15, 2011