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topic 13822

Aluminum vs. 316L Cathodes for sulphuric acid anodizing


 

Q. Other than electricity costs...can anyone tell me the advantages of using Al cathodes as opposed to 316L cathodes in a standard sulfuric hardcoat process? It would seem to me that the reduced energy costs would be replaced by higher cathode replacement costs? Drew.. I expect you'll be piping up on this one.....

Marc Green
Marc Green
anodizer - Boise, Idaho


 

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!

drew nosti
Drew Nosti, CEF
Ladson, South Carolina


 

Q. Hi everyone,

Well, let me push the question further: Can anyone tell me the disadvantages of using Copper cathodes as opposed to any other material (Al, Pb, etc.) in a standard sulfuric hardcoat process? In fact, Cu is completely^stable in this type of acidic media.

Hocine Djelab
anodizing shop - Verdun, Quebec, CANADA


 

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.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


 

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.

Marc Green
Marc Green
anodizer - Boise, Idaho


 

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.

drew nosti
Drew Nosti, CEF
Ladson, South Carolina

 

! Dear Drew,

Now, as for copper being soluble in 200 grams per liter of sulfuric acid with current:

1) According to the diagram E (volts) vs. pH, Copper is completely stable and not at all soluble in acid even in solution with a negative pH. So, from a thermodynamics point of view Cu cannot oxidize unless its potential is brought to values greater than 0.34 Volts.

2) Since we are considering copper to be a cathode material in the sulfuric acid anodizing tank, its potential can never be positive (anodic).

3) Maybe I am wrong, but I don't subscribe to the idea that a cationic species could be considered as a harmful contaminant in any anodizing bath. In fact, the minute the current is applied these species are very much attracted by the cathode and would never approach the material being anodized (anode).

The only precaution to be taken is to make sure that the load would not stay long in the bath without current. This is a prerequisite. Failure to apply the current rapidly would lead to the following redox couple to take place at the surface of the aluminum:

Cu2+(from the solution) + 2 e => Cu (plated on aluminum) (E0 = 0.337 + 0.03 log (Cu2+) Volts)

Al (anodic material) => Al3+ (to the solution) + 3e (E0 = -1.55 - 0.06 pH Volts)

Metallic ions (cations) are very harmful contaminants in plating baths, where the material of interest is made cathodic.

Reference: M. Pourbaix, "Atlas of Electrochemical Equilibria in Aqueous Solutions" [affil. link to book info on Amazon], Edt. NACE 1974

Kind regards,

Hocine Djellab
anodizing shop - Verdun, Quebec, CANADA


 

! Hi Ted, Marc

First sorry for the typo:

My last sentence should read as follows: "In fact, Cu is completely STABLE in this type of acidic media." For reference see: "Atlas of Electrochemical Equilibria in Aqueous Solutions" [affil. link to book info on Amazon], By Marcel Pourbaix.

Ted,

Copper is not at all soluble in sulfuric acid bath and especially not when it's made cathodic, as it may be noted from the standard potential (E0) for the two following reactions:

Cu ---> Cu+ + 1e (E0 = 0.52 + 0.06log(Cu+)
Cu ---> Cu2+ + 2e (E0 = 0.337 + 0.03 log(Cu2+)

So in order to dissolve copper in baths with pH < 2, the potential (vs. a standard reference electrode) of the copper should be held at values greater than 0.34 Volts, which is never the case when it's made cathodic.

On the other hand, when the system is idle, copper is still stable as its potential is ZERO.

Marc,

You would not have any extra copper in your bath as a result of having your cathodes made of that metal. On the contrary, it is even a way of reducing your dissolved copper (from the treated alloys) by plating it on the copper cathodes as it is used in the Copper Electro-refining process.

Regards,

Hocine Djellab
anodizing shop - Verdun, Quebec, Canada


 

thumbs up signHmmmmm....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?

Marc Green
Marc Green
anodizer - Boise, Idaho


 

thumbs up signDear 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.

drew nosti
Drew Nosti, CEF
Ladson, South Carolina

 

thumbs up signAs 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,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


 

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.

Rick Richardson, MSF
Dayton, Ohio


 

!! Ok... here are my rather informal test results. I took a piece of copper..and lightly etched in in nitric acid to remove any loose burrs, rinsed well, and weighed. The weight was 36.514 grams. I left the piece in an ambient, non air agitated, 165 gr/liter sulfuric (tech grade) solution. After 5 days, I rinsed, dried, and re-weighed said piece, and came out with 36.493 grams. So...assuming I did the math correctly, the piece lost .00058% of its weight over the 5 day period, which, I believe translates into an approx. 4% weight loss over an entire year. Now, from what I understand through e-mails that have been sent to me...one would expect an increase in weight loss due to air agitation...but.. at the same time, I shouldn't see any loss while the current was actually being applied. So.. what does this all mean? Heck..I don't know! :) I think I'll stick with Al just to be on the safe side!

Marc Green
Marc Green
anodizer - Boise, Idaho



Longevity of Titanium Cathodes in Sulfuric Acid Anodizing Bath

April 13, 2015

Q. Some time back, the aluminum sheets I was using as cathodes for anodizing completely dissolved into the bath. I was away from the system for about six months when it happened, and considering there was no current flowing, it hadn't occurred to me that I needed to remove the sheets. Does anyone know if this would also be an issue with titanium sheets? Can they be permanently immersed in a solution of 20-30% acid without dissolving over time? Also, the titanium I commonly see for sale is referred to as Grade 2 or Grade 5. Is this suitable for immersion in an anodizing bath?

Jack Bryant
Hobbyist - Custom Electronics - Austin, Texas USA



Quick dissolution of aluminum cathodes

August 6, 2020

Q. I am using aluminium cathodes 6063 for sulfuric acid anodization. Lately we put new cathodes in and they dissolve rapidly, and I really wonder. The past one lasted five years,. I just found that there is a reaction while the tank is idle and the power contact is off, the acid concentration is 200 gm/l. What factor could cause the rapid dissolution of cathode in its idle state?

shaymaa .A
- cairo Egypt


August 7, 2020

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

robert probert



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


August 8, 2020

Q. Please what do you mean by chlorides?

shaymaa .A [returning]
- cairo egypt


August 2020

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,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading


August 7, 2020

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?

alaattin tuna
- TURKEY Sakarya


August 11, 2020

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 sulphuric acid or too much 2XXX series anodizing.

John Hu
- Singapore


August 11, 2020

Q. Dear alaattin tuna
I prepared the bath and there is reaction around cathode, like you said as if there is high concentration of acid, so I checked concentration and it's in normal range. I observed the cathode the past week and the thickness of the cathode changed -- I noticed it with my bare eyes. Also I hadn't mentioned the the cathode dissolved totally.

Dear John Hu,
Yes, the cathode is red colour but it's new cathodes and we are anodizing 7075 alloy and 7020 alloy; the past one was also red but it didn't dissolve that rapidly? shaymaa .A [returning]
- cairo egypt


August 2020

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

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading


simultaneous August 11, 2020

Q. Dear Ted,
Yes the cathodes are from the same supplier; they are aluminium 6063, and we bought them at the same time and we used it for a little while and stored it and we just use it now.

shaymaa .A [returning]
- cairo egypt


August 12, 2020

A. What do you [mean] all "normal range"? 180 g/L without the presence of chloride will NOT dissolve your cathodes in less than 3 years, 200 g/L will slightly attack. 220 g/L will eat them up. If you consider 220 g/L as "normal" then you had better use stainless steel or lead.

robert probert



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


August 12, 2020

thumbs up sign Dear robert
I don't consider 220 gm/l normal range. I mainly work in in the range 160 to 180 gm/l -- I dilute the tank solution from 200 to 173 lately to avoid dissolving of the cathode rapidly.

shaymaa .A [returning]
- cairo egypt


August 14, 2020

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 sulphuric acid being used for making up contains too much copper. Additionally, when you make up the bath (if you use concentrated sulphuric acid to dilute) you should run the chiller to cool down the bath first then add in the sulphuric acid, otherwise the solution temperature will go up very high and this will contribute the fast dissolution of the cathode.

John hu
- Singapore


August 14, 2020

Q. Yes I noticed that the cathodes in red color ... but how come the sulphuric acid contains too much copper? And I also ask you to explain to me what is the relation between high copper content and quick dissolution of cathodes?
Thanks john!

shaymaa .A [returning]
- cairo egypt


August 14, 2020

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.

Alaattin Tuna
- TURKEY Sakarya


August 17, 2020

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 sulphuric 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.

John hu
- Singapore

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