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Chromic acid anodising problem




July 28, 2009

Q. Recently, a batch of 30 Aluminium sleeves 2024 high copper alloy were anodised on our automatic line here at Eaton. The program is fully computerised, and all pretreatments, and anodising follows a program.These came out light grey as usual and were passed.
A second batch of 30 sleeves, same alloy, same program, were processed. These came out very light, almost as if they were not anodised.
Apart from an interruption of the program, can you think of any thing that could have caused this? Could it be that not enough current was being drawn on the second batch?
I would appreciate some ideas.

Mike Channing
Senior chemist - Southampton, Hampshire, England



"The Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby

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First of two simultaneous responses --

A. Anodizing is a very exothermic process-it generates a good bit of heat. Higher temperatures produce thinner coatings. My guess is that your tank temperature controller failed.

James Watts
- Navarre, Florida
July 30, 2009



Second of two simultaneous responses --

A. Not enough info, but go ahead and study; (1) Our Mil spec does not allow anodizing 2024 in chromic acid "unless otherwise specified". (2) Some shops use the 20 volt system on 2024, (3) Did the ramp of time versus voltage change between loads? Were the loads of equal size and did the amps measure the same. (4) Did the sulphate change between loads (limit 500 ppm) (5) Did the dissolved copper change between loads? (6) Did the dragged in chloride" change between loads? (7) did you notice whether the temperature changed between loads? Come back to us and let;s study this further.

robert probert
Robert H Probert
Robert H Probert Technical Services
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Garner, North Carolina
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July 30, 2009


A. Mike,

How long was it between loads? Are we talking hours, days or weeks? This will give us an idea whether we are talking about potential chemistry changes or more physical changes.

If we are talking hours then I would suspect some sort of jigging or contact issue or a possible interruption in current. If we are talking a longer period this would open up the possibilities.

Brian Terry
Aerospace - Yeovil, Somerset, UK
August 3, 2009


August 5, 2009

A. The following queries gives further case study

1. What is gap between 2 batches ?
2. Whether rectifier is worked properly ? The current observed for both batches are same? If there is the problem in rectifier operation could be the one main reason.
3. Bath temperature and concentration will play important role. All are maintained and monitored properly?

T. Chandra Sekhar Reddy
plating shop employee - Mumbai, Maharashtra, India




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