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plating, anodizing, & finishing Q&As since 1989

"Residual current of anodizing tank"

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2005

Hi
I have been anodizing for five to six years now, and have come across some interesting problems. Most I might add I have sorted through the help of others through your excellent web site Ted.
My latest puzzler is after anodizing,I turn my amperage control to zero,and usually there is a rapid drop of amps on my ammeter.After which I turn off the power.Now after turning the amps down there is a very slow drop in amperage almost as though there is a residual current still in the tank.It can take up to15-20 minutes to dissapate the current stored in the tank.
Any ideas?
I have cleaned all my connections,changed my sulphuric acid solution.installed fresh 6063t5 cathodes but no joy.My next line of thought is my rectifier.It is home made but has been desined by a professional electronics firm and still works great.

Thanks for any info

Steve Power
Aircraft Engineer - Nelson, New Zealand

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2005

There is a very strong galvanic voltage between titanium racks and aluminum cathodes. Also betwewen lead and aluminum. And just a little bit between titanium and lead. Get a meter, go measure, and let us all know what you are finding. We would also like to know the concentraiton of the anodizing solution along with the voltages you find.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

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2005

Sorry for the delay, Robert.
I am using alloy wire(1100 series) to connect my parts.A typical anodizing run,say about 30-50 amps at 1.5 amps per square decimeter, My voltage sits at around 14-18 volts,depending on load.Amperage is controlable,voltage is not.I have connected my rectifier to a series of aircraft landing lights to simulate a load,and used the amperage control as a dimmer,the lights dim and shine with the amperage control,so I can assume my rectifier is working just fine.My acid concentration is 190-200g/litre free sulphuric acid.The voltage drops at the same rate as the amps after turning the amperage control to zero.ie very slowly. Hope this info helps
Kind regards,

Steve Power
- Nelson, New Zealand