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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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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.
Senior chemist - Southampton, Hampshire, England
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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 H Probert
Robert H Probert Technical Services
Garner, North Carolina
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.
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?
plating shop employee - Mumbai, Maharashtra, India
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