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Uniformity problems in vertical aluminium anodizing

May 14, 2010

Q. We have a new vertical anodizing plant for vertical hanging aluminium profiles. The profiles are 7 meters long, like the cathodes, and the tank is 8 meters deep (20% sulphuric acid). The problem is that the oxide thickness is greater than 10 microns in the lower part of profile, and less than 3 microns in the upper part, out of uniformity specifications. What is the cause? How could we solve/minimize the problem ?
We know about some "socks" to put on the cathodes in the bath, could they help? how?

Francesco Zanetti
engineer - Italy

May 19, 2010

A. It sounds very much like you have a temperature stratification in your tank.
I would check the temps with a thermocouple device attached to a rod that you can raise and lower to see what the temp is.
If you have tank agitation, is it uniform over the entire length of the tank?

James Watts
- Navarre, Florida

May 21, 2010

A. Make the cathodes shorter than the profile. This may help improve the "thickness greater than 10 microns at the lower part of the profile "

SK Cheah
- Penang, Malaysia

May 21, 2010

Q. Next week we are going to make some temperature measurement. I think you are right, our agitation system is lacking in uniformity: we have 2 water inlet points only, on the tank bottom (whose dimensions are 1 x 5 m) and the recirculation water flow is 50 lt/h. Can any cathodes shielding, protection socks or similar help the uniformity?

Francesco Zanetti [returning]
- Verona, Italy

May 21, 2010

A. Yes, the cathodes can be masked off with a suitable material or they can be cut off. Also, part of your electron movement bounces off the bottom of the tank, so you get a heavier anodize at the bottom of the part. Normally, an anodize tank is somewhat self controlling as to where the anodize builds up, which is why you do not have as bad a dumbbell effect as you do with plating.

James Watts
- Navarre, Florida

May 26, 2010

Q. Ok, it is likely the agitation is lacking. As the cathodes cannot be modified, we only can add parts: have anybody ever used any kind of "socks" for cathodes protection, masking, in order to control the reaction locally? does it works? what kind of effect can be obtained? Are there any further disadvantages?

Francesco Zanetti [returning]
- Verona, Italy

October 17, 2018

Q. Hi, it has been 8 years since. I wonder if your problem has resolved? Here we did use socks to cover up cathode, and control agitation using circulation pump. So far we still get 3 micron thicker at the bottom part max.

Zack Effron
Anodized Aluminum - KL, Malaysia

October 2018

A. Hi Zack. If you have put insulating socks over the bottom of the cathode, blocking electricity flow and effectively shortening the cathode, and yet you still have thicker anodizing at the bottom, it sounds like it must be a temperature difference, with higher temperatures near the top of the tank causing faster dissolution in the acid.


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