Reddish-orange deposit on anodizing tank cathodes?

A. Sounds like mostly copper to me, which is present in most Al alloys, getting dissolved in the bath during the anodizing process, and is getting plated onto the cathodes.

A. It is copper that has dissolved out of the parts when anodizing initially starts. It normally results when you anodize 2,4,7xxx alloys which have a high copper content. A better desmut step can reduce this, but removal of the cathode from the tank and wet brushing it will also work which is cheaper but messy and labor intensive.

A. My guess: copper metal.

Does this stuff dissolve in water? HCl? Nitric acid?

If the answers are 'no, no, yes' it's likely Cu.

A. Apparently you have been running some 2000 alloys and are plating the 5% copper onto the cathodes. Sulfuric Acid and copper sulphate ⇦ this on eBay or Amazon [affil links] are (1) a standard copper plating solution and (2) standard anodizing solution. Years back, Metal Finishing published a research paper whereby controlled and varied periodic reverse was used in a copper containing anodizing solution to alternate between positive and negative and deposit copper direct on aluminum without a zincate and without a stannate. I do not think the process ever caught on.

Make a zigzag cathode, try 1 and 2 amps per square foot and see if you can electrolytically plate out the copper. The science writers claim the limit is around 100 mg/L, Boeing even says 40 mg/L. (1) You cannot hold it that low (2) who am I, but I have never been able to see any evidence that it causes any harm.

Q. I learned that the cathodes are made up of 6101T6 aluminum.
The cathodes have an orange coating on them but I was told the flakes may not be coming from the cathodes, but possibly from the parts. I think I will collect some and see if they dissolve in concentrated nitric acid. This would indicate if they are made of copper I believe.

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January 22, 2009

A. Thomas

Do you use a copper buss bar on your tank?
Does your anodize solution have a blue tint to it?
Do you process 2xxx series aluminum?
Has it been a while since you did a decant or dump?
Does it disappear over the weekend and show up after processing parts?

If yes, copper is a likely suspect

Willie Alexander
- Colorado Springs, Colorado


January 22, 2009

A. Thomas,
Sounds like immersion copper.

Trent Kaufman
electroplater - Galva, Illinois

A. If you are running 2000's alloys you have a copper plating solution in the tank. When the rack broke electrical contract, in a bi-polar way, it became negative with respect to the other racks, so it copper plated in the high current density areas.

A. This looks like a copper immersion deposit; Most likely the parts were burned enough so very little current was in those regions allow an immersion deposition to take place.

If you dip the part in nitric I imagine the copper will come right off; if your copper is too high leave some thick aluminum panels (not aluminum foil; preferable not 2XXX series)in the bath overnight with no current, the same phenomena will happen. You will replace your dissolved copper contamination will increased dissolved aluminum though.