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topic 0524

Suggested cathode material for chromic acid anodizing

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Good morning everyone,

I'm looking for some suggestions on materials to be used for cathodes in a chromic anodizing process tank and for anode/cathode surface ratio.

I would be more than happy to year your suggestions and or your experiences.

Best regards,

Hugues Vendette


Hmmm--never really thought about that one. But the The Canning Handbook says stainless steel cathodes with a surface area of 1/10 to 1/5 of the anode area.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


Typically the tank serves as the cathode in chromic acid anodize. Very often tanks are constructed of steel. In order to prevent the build up of excessive amounts of trivalent chromium, the tank walls should be shielded to reduce the effective cathode area. The cathode area required to produce satisfactory anodic coatings is only about 5 percent of the anode area.

Reference: American Electroplaters Society illustrated lecture series -- "Anodizing Aluminum and Its Alloys", M. Schwartz, 1985.

Blair Smith
aerospace - Windsor Locks, Connecticut


I used strips of 300 SS with a ratio of 8 to 1 part to cathode and 22 volts and it worked fine as long as the normal analysis was done. Virtually idiot proof. If you have not done chromic anodizing, be ready for the first few loads to produce green colored parts, until it gets some aluminum in it and then they will turn the normal grey. Do not let your temperature get high. Anodizing generates a lot of heat and it is a tight temp range for optimum results. Remember to calibrate your pH meter with a 1.0 standard. you are on an asymptotic tail at that pH. Make sure your probe will work at that pH and use a double junction probe or one that you can refill because you will contaminate it very fast with a normal probe. There are some good cheaper ones on the market. Doing a little homework on that will save several hundred dollars a year on replacement probes. P.S. do not leave it in the sample any longer than is necessary.

James Watts
- Navarre, Florida


Just some thoughts on this thread..

I am wondering if there is any benefit to using stainless steel plates rather than the tank itself as the cathode as far as bath longevity is concerned...

Our setup uses a mild steel tank as the cathode. We run a porous pot to keep the trivalent levels in check, and cation exchange the bath to remove other impurities.

I have also read about hanging stainless steel plates as the anode for "activating the acid" when mixing a new bath..I have no idea what this is supposed to do, It was written in a spec from Hartzell Propeller Products... I have never tried it. Maybe someone else does..


Dennis Warner
propellers - Tucson, Arizona


We processed Chromic Acid Anodize using lead coils for the cathodes and they also served as the cooling coils, a very ideal situation that we were very happy using for many years. Cathode/Anode ratio 1/8

With the changing environmental regulations, about a year ago we switched to aluminum cathodes with a Cathode/Anode ratio about 1/6. We find that the higher conductivity of the aluminum could have allowed easily for a 1/10 cathode/anode ratio. We are very pleased with the aluminum cathodes and even if there was not an environmental concern, aluminum cathodes would be our choice. The higher conductivity of the aluminum permits a very efficient Chromic Acid Anodizing process that we process everyday.

Joe Hillock
anodizing shop - Philadelphia, Pennsylvania


Re: Dennis' comment. Why end up using a porous pot more than is necessary by using a great excess of cathode. I have seen steel tanks that they put glass that has wire on the inside and covered the bottom and sides of the tank with it. The glass cracks and exposes "the right amount of tank". Not my cup of tea, but it worked quite well. I can see dummying the bath with aluminum part to break it in and avoid the green startup color but do not see what a SS anode would do other than put chrome, nickel and iron in it. Re: Joe's comment. I had always wondered about using aluminum as it is outstanding in sulfuric tanks with its superior conductivity. I was afraid that it would dissolve in the chromic acid. Had not heard of aluminum when I was doing chromic. Lead works, but puts lead chromate in the tank and my preference is to keep dirt and trash out if at all practical. Also the disposal site was excessively interested and wanted more money.

James Watts
- Navarre, Florida



Just to tell you that your cathodes can be made of titanium, lead or stainless steel., well I hope you have success., greetings from Mexico by the way.

Ivan lopez Diez
- Mexico City, Mexico

November 29, 2008

Hi, folks. The subject of cathode material for chromic acid anodizing has come up repeatedly. So, for more insights see letters 19316, 26783, 35181 and 39329.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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