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topic 34125

Anodizing rectifier puzzler


Q. I've got a puzzler here. I have 2 anodizing tanks, both powered by the same make of rectifier. The only difference between rectifiers is one is approx. 6 years older than the other. I place a load in each tank (both tanks have similar concentrations, and Al content) and start both loads at the same time. With both loads being run at the same current density, the older rectifier forming voltage is 26.5, while the newer rectifier forming voltage is approx. 23.

The voltage rise on the older rectifier is more than twice that of the newer one. Checking the coating thickness on both loads at the same time shows no substantial difference in coating thickness (both loads were the same alloy, and new racks were used). It seems obvious that I'm getting some type of resistance somewhere on the older rectifier.

All electrical connections to the bus bar, and cathode bars have been checked, and cleaned.

Anyone have any thoughts/suggestions as to whats going on?

Thanks in advance.

Marc Green
Marc Green
anodizer - Boise, Idaho

simultaneous 2005

A. This may sound a little stupid, but have you checked the voltmeters?

James Totter
James Totter, CEF
- Tallahassee, Florida


A. Hi Marc,

Why don't you ask an electrical engineer?

Have you tried measuring not the voltage but the amperage as it may well have to do something with the loading?

Lastly, try your dawg.

Good luck,

freeman newton portrait
Freeman Newton [dec.]
White Rock, British Columbia, Canada

(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).


A. Have you verified that the volt meters are exactly accurate?

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina


Q. Actually, yes. I had checked the voltage on the "problem" rectifier, and the voltmeter on the rectifier is reading within 2 tenths of a volt of what the multimeter is reading. I went ahead and checked the "good" rectifier with the multimeter, and found that the voltmeter is actually reading 2 volts higher than what the multimeter read. This makes it even more puzzling. I dunno... maybe I'll check with my pooch, as Freeman recommended!

Marc Green
Marc Green
anodizer - Boise, Idaho


A. Just an idea, but could the problem be in the wiring (from the rectifier to the bus bar and or cathode bar) or tank rather than the rectifier? Not the connections, but the actual pieces themselves.

You may want to try switching the connections that go to the rectifiers (I have 2, they are close enough to pull this off - you may not be set up for it though).

Also, depending on how your wiring is set up - do you have cables going to your cathodes or straight copper bar? - you may be able to check the temperature (I would think that it would increase) of the connecting wiring and see if maybe the increased resistance is there rather than in the rectifier itself... We've switched over to straight bar, but I know that back when we were using cable we saw increased resistance once because one of the cables got messed up - crack in the insulation or something.

I don't know if any of that would make as much of a difference as what your seeing though, I'm just pulling ideas out of the air.

Good luck!

Jim Gorsich
Accurate Anodizing Inc.
supporting advertiser
Compton, California, USA
accurate anodizing banner


A. Check the voltages at the "load"- close to the actual anodes and the work pieces. As you state, the two work lots (surface area) are about the same so the two currents should be about the same therefore you expect the voltages to be the same at the tank. If not, the variation is within the tank, not the rectifiers or wiring.

Compare voltages at the tanks to readings taken at the rectifiers (we are using the same hand-held meter here).

If your voltage drops from rectifier to tank do not agree, or vary over time, start checking the individual voltage drops across connections and over lengths of wire. Sounds like you may find the older installation has some high resistance connections.

Dwight Dixon
- Bergenfield, New Jersey


A. Marc

A few months ago we had a similar situation. We raised temperature on a type III tank to double type II capacity. We knew free acid content would allow for some difference but not what we were experiencing.

Have you had an outside lab verify your solution analysis? High solution metal contents will produce VERY similar symptoms.

Also, you could look for a diode/SCR to be out on the lame power supply.

(If your symptom suddenly showed up to an existing process, look for a mechanical/electrical rather than a chemical solution- no pun intended:)

Good luck.

Willie Alexander
- Colorado Springs, Colorado

Sulphuric Anodise Upgrade Problem

June 26, 2020

Q. Hi

I've just upgraded a Sulphuric Anodise Bath from 320 litres to 640 litres.

I've used the same lead cathodes from the old bath and the same jig with the same number of parts on but the parts are being etched (heavily eaten away) rather than anodised?

It's the same rectifier, with the same programme. The only thing that's changed is the volume, this is to increase capacity from one to two jigs.

The anode/cathode configuration is the same on the new bath.

One of the cathodes has a white deposit building up on it?

The problems seems electrical but we've checked the wiring out and all looks okay?

Anyone have any ideas??

Julian Rees
- Redruth, Cornwall UK

June 26, 2020

A. Titrate for the free acid and come back. Obviously the sulfuric acid is too low or the wrong acid.

robert probert

Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

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