Suggested cathode material for chromic acid anodizing

A. Hmmm--never really thought about that one. But the The Canning Handbook [on eBay, Amazon, AbeBooks affil links] says stainless steel cathodes with a surface area of 1/10 to 1/5 of the anode area.

A. 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.

A. 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.

A. 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.

A. 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.

A. Hello,

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.

A. Teflon heaters sound fine but, because the anodizing process usually runs at 68 to 72 °F. (room temperature), and the rectifiers provide plenty of heat in operation, few installations actually have heaters.

Many, but not all, people feel that aluminum cathodes are the way to go because of energy savings.

Titanium coils are okay, as are external heat exchangers; both are widely used. But because both localized cooling & localized agitation are required, I believe that external heat exchangers are gaining dominance.

A. You can use steel cathodes in chromic acid anodizing, you can even use the steel tank for the cathode. I have never heard of using aluminum cathodes in chromic anodizing.

The titanium might introduce a bipolar or galvanic stray current -- so if you do use titanium, tie it (connect it) to the cathode side. Teflon would be better.

Like Mr. Mooney said, especially in Florida, you do not need a heater unless you are somewhere around the latitude of Jacksonville, and then only on Monday morning.

A. Throw the aluminum hooks away. Bend the lead cathodes over and then under the tank lip and make contact with copper bolts thru copper lugs to copper cable or buss. Note: You can use steel cathodes. Also a very high ratio of anode to cathode is favorable, even as high as 20/1. Repeat, that is high anode and low cathode.

A. Although we have never recommended it before our information now says that aluminum cathodes may work in chromic acid anodizing and possibly save electricity. We would very much like to work with you to try our aluminum cathodes on an approval basis.

Drew Nosti CEF
Co-chairperson AESF Light Metal Committee Member AAC Hard Coat Committee

?. What is the chloride content of the make-up water?

A. I have discovered that the problem that you mentioned is actually pretty common for chromic anodizing baths where you use the tank as the cathode. Our shop did this for many years also without failure, then all of the sudden we went through 3 tanks in 3 years. We did not have chloride contamination. I would be willing to bet that the pitting you see is near the solution level and within 6 inches on either side of the corners. I think that the problem has something to do with how the tank distributes current and imperfections in the steel. Your tank will eventually fail. The ultimate solution is to install a PVC or polypropylene liner. You will then need to use steel sheet as the cathodes.

A. Is it unthinkable that the flammable metal is plated-out chromium? Although, even porous iron and chromic oxide could flare. I'm surprised if full-density iron could do this, regardless of surface condition. I'm curious how much visible crusty deposits were on surfaces nearby. Was it near the bottom, middle, or top?

Q. "Crusty" may be as good a word as any. I would say that the surface of the cathodes had a relatively thin "crust" of foreign material, that looked about like medium to course sandpaper, with the coating perhaps no thicker than the largest "grain". I don't think there was a particular top to bottom distribution, but it was all below solution level.

A. British defense standards advice the use of mild steel or stainless steel for cathodic materials in chromic acid anodizing.

A. Hi all. Looking at all these merged threads, it seems that every common metal has been successfully used: steel, stainless steel, titanium, aluminum, lead. On the basis of poor conductivity, high cost, and no particular need, it's probably safe to strike titanium off the list. For environmental reasons, I'd probably personally strike lead.

Regards,

A. One of my clients has used aluminum cathodes since 2005. He does aerospace and other work for Boeing, Bell, et al -- absolutely no problem.