Aluminum vs. 316L Cathodes for sulfuric acid anodizing

A. Marc,

You knew this would be near and dear to my heart. The advantages of using a properly designed and alloy (6063-T6)aluminum cathodes bussing system is: I ) Better current distribution in the tank, therefore a more even oxide.2) No added dissolved metal other than aluminum in the bath( lead,copper,nickel,iron). Very little aluminum from the cathodes get into the bath.3) Because of better current distribution less burning.4)More usable tank space.5) Lighter and easier to handle.6)Overall less cost.7)Your Quality Control manager and I smile when you set it up!

A. Copper is completely what? There is a typo in your response. It strikes me that copper would be "soluble" when made a cathode in a sulfuric acid bath.

A. I'm not sure what the regs are in Canada, but the amount of dissolved copper in the anodize tank would be a waste treatment issue, here in the states. Copper in the aluminum is obviously detrimental to the anodizing process, so I wouldn't be surprised if the same were true if the copper were in the electrolyte.

A. Copper is a contaminant in anodizing and dissolves readily in 200 grams per liter of sulfuric acid with current. NOT an Option! In today's environment making finishing baths last longer, therefore producing more and better quality work is imperative. 6063-T6 aluminum bus bar and racks will save energy and rejects, and will not add copper contamination to the anodizing solutions.

Hmmmmm....me thinks me is going to have to do a little research on this. I find it hard to believe that the copper won't dissolve in the standard H2SO4 anodizing bath. Stay tuned for my results. I did a little weight test over last weekend on some 6063 aluminum, and found that roughly, over the course of a year, that the 6063 will lose approx. 5% of its initial weight (the Al was just sitting idle, in our tank, with no current actually being applied). I'll duplicate the test with some copper and post the results. Hocine...are you currently using copper cathodes in your tank, or just considering the possibility?

Dear Hocine,

I have always considered myself open minded so I am very interested in hearing more about the practical application of this new concept. I am sure as a member of the AESF Light Metal Committee we would love to receive a paper on the practical benefits of using copper cathodes in sulfuric, type II and hard coat type III anodizing baths. Would you do a paper on this subject or share with us some of the details about how long you have been using this technology. What type of production facility you have and the type of work you do. Also as I read your statement;how do you remove the copper cathodes in between loads? What % of copper is in the anodizing bath and does it effect the salt spray? Do you use air agitation? I for one have not come across it before but would like to pursue some of the details.

As I often do when thinking on-line, I forget that the counter electrode in anodizing is the cathode. While a copper anode in sulfuric acid is completely soluble, a copper cathode is not. Sulfuric acid by itself does not attack metallic copper. That does not mean that I would agree that copper is a proper cathode material for anodizing; I don't think it is. But I'll listen to the experts and await some test results.

Regards,

A. Just for my 2 cents to this string we have been using aluminum cathodes in anodizing for years and they work well. But I remember years ago we used to electropolish SS in a strong phosphoric/sulfuric acid mixture and used copper as the cathode material and they worked just fine and this solution ran at 180 °F. So I would think copper would be a good choice for anodizing. Interesting concept.

A. If you are certain the free acid is only 200 g/L, then look for chlorides.

A. Hi Shaymaa. Table salt is NaCl, sodium chloride, and introduces Cl-, chloride ions, into the solution; hydrochloric acid is HCl, and introduces Cl-, chloride ions, into the solution. There may be Cl- in incoming water, or dragged into the process from elsewhere. Cl- is very corrosive and you should analyze or have the solution analyzed for chloride concentration.

Luck & Regards,

A. Aluminium cathodes never dissolve unless anodizing bath runs or too much acid concentration in anodizing bath. We can't mention about aluminium cathodes lasting time because it depends on production density and volt/current value. If anodizing rectifier always works high capacity, lasting duration of cathodes will be shortened. I never heard as you say cathodes dissolve during idle time. How did you find this reaction as you claim? Also when did dissolve these cathodes? You say previous cathodes lasted inside five years so these ones less than five years? When? How long did take lasting these dissolved cathodes?

A. If you observed your cathode in red colour (copper deposition), then your cathode dissolution would be very fast. The copper content mainly come from low grade sulfuric acid or too much 2XXX series anodizing.

A. Hi Shaymaa. Are you sure the cathodes are from the same supplier and are the same alloy. What alloy are they?

Luck & Regards,

A. If you newly make up your anodizing bath and place new cathode, you won't get to see your aluminium cathode in red except your sulfuric acid being used for making up contains too much copper. Additionally, when you make up the bath (if you use concentrated sulfuric acid to dilute) you should run the chiller to cool down the bath first then add in the sulfuric acid, otherwise the solution temperature will go up very high and this will contribute the fast dissolution of the cathode.

Can you share either bath's appearance or dissolved cathodes with us? You can send to Ted (mooney@finishing.com) and we can discuss better over pics.

A. Shaymaa:
if your anodizing bath contains Cu ion, the replacement reaction with aluminium cathode will occur automatically. Bear in mind that with the copper deposition, that mean there must be some aluminium dissolved. The problem is that this copper deposition coating is loose, not hard sticking to the cathode (easy peel off) when you air agitate the bath, when you doing anodizing (hydrogen release at the cathode to cause the copper coating to peel off). So once the old copper coating peels off, it exposes new aluminium surface that would continue to replace the copper ion to copper (red colour).
From the mechanism I describe here, we can conclude that as long as sulfuric acid grade low and contains high copper ion and because you need to top-up the bath (it means you continue to supply the copper ion), then the dissolving of the aluminium continues to happen.