Aluminum vs. lead cathodes for anodizing tanks
Q. I am looking for advice on using aluminum vs. lead cathodes for anodizing. We currently use lead cathodes, but are considering switching to aluminum, based on supplier recommendations. The case for aluminum cathodes is that less current and less cooling is required do to the greater current carrying capacity of aluminum vs. lead. A more even coating thickness across the workbar and elimination of lead from the waste stream are also given as benefits. Does anyone have experience with this matter ? Any other information (recommended alloy of aluminum, proper anode to cathode ratio, recommended way to connect to copper bussing, etc.) would be greatly appreciated.Keith Rosenblum
plating shop - St. Paul, Minnesota
A. Charlie Grubbs wrote several articles on this subject.
"Products Finishing" or Clariant may have copies of this article. He recommends 6063 (
6061^6101 a fair substitute).
Albright and Wilson Americas has some fairly strong opinions on this also.James Watts
- Navarre, Florida
Charlie Grubbs' article on aluminum cathodes can be found in the Nov. 1981 issue of Plating & Surface Finishing.
Ted Mooney, P.E.
Pine Beach, New Jersey
As stated above, 6063 T6 or T5 is the recommended alloy for aluminum cathodes. Alloy 6101, NOT 6061, is a substitute! Do not use the overaged 6063 T52 temper as it will dissolve rapidly. All welds should be made with 5356 alloy welding rod, not 4043, which is a common welding material.
Anode:cathode ratio should be approx. 3:1.
Lead cathodes or SS cathodes are not as good conductors as aluminum (Al = approx. 60%). Lead has a conductivity of approx. 9%; SS has a conductivity of approx. 4.5%, therefore, much of the energy from the power supply is utilized overcoming the high resistance of lead or SS, whereas the aluminum conducts the energy to the anode (parts) for better anodizing response.
In many instances, a 2-3 volt saving is noted when aluminum is used. Depending upon the total amperage being used, this could add up to considerable energy savings.
I would suggest that you bring the aluminum cathodes out of the tank and connect to either flexible copper cable or aluminum bus conductor (if the distance from the tank to the rectifier is not too far). Keep the copper below the top of the tank to minimize the possibility of copper dissolution into the anodize tank.
Hope this answers some of your questions.Charlie Grubbs
- Alpharetta, Georgia
A. I have 30 years of experience running anodizing lines. I have used both lead and aluminum. I prefer lead by far. I recommend lead cathodes connected to aluminum buswork. I have found that copper corrodes in a sulphuric acid atmosphere.
If you wish to discuss this further, I will be happy to give you pros and cons.Raymond Hendrix Troy, Tennessee
A. As a major producer for the lighting market, we find that aluminum cathodes work for us.Gerald Janssen
aluminum coil anodizing - Streamwood, Illinois
We have been using aluminum cathodes and busbars for 25 years. Purer the aluminum, better the conductivity for the busbars. AA6063 with minimum alloying elements concentration recommended. For the cathodes, even EC grade extruded sections can be used. Cathode/Anode ratio is 1:2 to 1:4 (not so important, my opinion). If busbars in aluminum, 1 amp/sq.mm is the min. section area for good conductivity. Connection of aluminum to copper needs care to prevent corrosion by cell-effect later. There are some proprietary compounds to put between Cu and Al for such purposes.
aluminum extrusions & finishing - Istanbul, Turkey
A. The Sanford Process supply to their licensees the process tank with graphite cathodes.Leonid Lerner
A. To choose a cathode material for an anodizing system we have to take into consideration a number of factors, such as conductivity, reactivity, corrosion resistance, maintenance convenience, cost and mechanical properties, and so on.
Each kind of cathode has its own advantages and disadvantages. In general, the voltage drop on the cathodes resulting from electric resistance is negligible in the order of millivolts, relative to the whole cell voltage in the order of volts to 100 volts.
The voltage drop on the cathodes is principally attributed to the cathodic reaction resistivity. In the case of anodizing aluminum in sulfuric acid solution, for example, major cathodic reaction is hydrogen evolution. As indicated in many references, the hydrogen evolution resistivity on lead, stainless steels, or graphite is much larger than that on aluminum. As a result, the voltage drop on aluminum cathodes is the least among the cathodes mentioned above.
From the point of view of energy saving, it is natural to recommend aluminum acting as a cathode material for aluminum anodizing. To further reduce the voltage drop on cathodes, one can design the configuration of cathodes to increase their specific surface areas or develop alloys containing catalytic elements.Ling Hao
- Grand Rapids, Michigan
6063 T6 isn't a readily available alloy in the T6 form. To get certified material usually you have to have it extruded at a minimum of 1000 lbs. That's a lot of material for most anodizing operations. The good news is we have our special extrusion certified to 6063 T6 in stock at all times. No minimum. We also have it in 1.5" and 2" X 4" as header bars to attach the extrusion to.
Ladson, South Carolina
June 2, 2009
Q. Well, this is an old thread, but I have a new question. I replaced some stainless cathodes with aluminum cathodes. I followed Charlie's advice for the materials. I used 1/4" x 4" 6063 T5 flat bar stock and welded it into a grid/lattice pattern with 5356 rod. The aluminum dissolved completely in 2 months. What gives? The only thing that I can think of is that the tank wasn't used much which means that it didn't spend much time cathodically protected. I guess I'm forced to go back to stainless or lead, but I really wanted aluminum to work. Comments?
June 4, 2009
A. Strange that you would get only 2 months life time. What concentration are you running?
I use cathodes supplied by Drew Nosti (the poster above yours) and I get close to 2 years life.
With that being said, I have 2 tanks that I operate at similar concentrations, but at different temperatures, and have noted that I get a much quicker dissolve rate of the cathodes in the tank at the higher temperature. What I've done to combat that, is when that tank is going to be idle for a period of time, I will keep it chilled to 40 F to slow down the dissolve rate.
Another possible solution for you, depending on your use (which sounds very infrequent), would be to store your acid in drums, then pump into the tank as needed.
Lastly, are you sure you don't have any stray current from your rectifier going to the cathodes while the tank is idle? That would also cause them to dissolve quicker.
anodizer - Idaho
June 5, 2009
Thanks Marc. The tank is "thin film sulfuric anodize" and the concentration is much more dilute than type II at around 45 g/L. Temp is 78 degrees. Even though the temperature is higher, I expected less attack because of the dilution. That was probably wrong. The bath is used infrequently because we are waiting on auditors to qualify the process. I can't really store the tank solution in drums because it is 2200 gallons. Energy costs to chill that much solution to 40 degrees during idle periods would be too costly. The life of the anodes was so short, that I have no choice but to switch to either lead or stainless and try to design for the poor conductivity. My curiosity about the aluminum failure is mainly academic at this point.
June 5, 2009
Double check your alloy/temper. I have seen your issue happen twice, both times due to the wrong temper being provided.
- Colorado Springs, Colorado
June 28, 2009
A. I still feel the material you received was NOT 6063 T6. But BOLT NOT WELD. The only catch is that you do not put the connections to the header bar below solution level.
Ladson, South Carolina
July 1, 2009
Drew and I might have to agree to disagree on this. I use Drew's cathodes, but am a proponent of welding the cathodes to the header bar, then coating the top with a fluoropolymer powder coating to prevent corrosion, and an unsightly anodizing tank. This also minimizes cleaning, and eliminates the need to check bolt torque on a regular basis.
anodizer - Idaho
Q. I would like to take this letter further deep and need suggestions from various experts already in this field (who have always been an inspiration to me).
It has been quite a debate, and I have read in many forums/letters/online/offline publications, etc. that Aluminium cathodes are better than S.S./Lead. I agree to this fact after reading about it almost everywhere and getting opinions from experts in this field.
Now my query is, How is graphite in comparison with Aluminium cathodes? S.Wernick, R.Pinner & P.G.Sheasby in their book titled "The Surface Treatment and Finishing of Aluminium and its Alloys" =>
has mentioned in Pg 404, under the heading, 'Cathodes' that, "Stainless steel cathodes are used for chromic acid anodizing and for some integral color anodizing processes and graphite is used in mainly continuous anodizing processes...."
Now I am confused here. What does 'continuous anodizing process' which they are referring to, mean? If someone can throw some light on this, I would be grateful.
Also, Industry experts/Technicians who have used graphite as cathodes could share their experience comparing to Aluminium cathodes (if there is anyone who has experience in using both types)
- Rajkot, Gujarat, India
September 21, 2010
A. 'Continuous anodizing process' is I think known also as 'reel to reel' or 'coil anodizing'Stephen Parkes
Retired MD of Anodising Company - Walsall, England
September 1, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Am setting up a anodizing plant for Alum Strips, Very low spec requirement, but large volumes, (50,000 pieces a month).
Have a FRP Lined tank approx. 10,000 Ltrs, at 16 V, 2000 Amp rectifier, plan to fill with 180gm/ltr of sulphuric ( I hope thats not too much ), ambient temp currently at 25 °C,
The part is a alum strip 6063t4, 1" wide, up to 30 inch long suspended by a rotomac type conveyor approx. 1.5 ft above the tank surface using alum wire hooks.
My question is what material and what configuration/shape do I use for the cathodes.
lead vs alum, everybody I spoke to uses lead, but none of them knew why!!
Any suggestions would be great..
(have the necessary environ clearances and ETP to handle the effluent etc.)
Manufacturer - Chandigarh, India
June 7, 2012
Q. I have a home anodizing system, and the physical constraints of its location prohibit removing the cathodes each time its not in use. The first cathodes were 6061 aluminum sheet, but I'm now removing and replenishing the acid in the tank because they completely dissolved over a period of about 10 months of disuse. My question is whether lead sheet, say 1/16" or 1/8" thick, will last longer. Does lead also dissolve in the sulphuric bath, or will it last a few years?Jack Long
- Austin, Texas, USA
A. The lead cathodes will last longer, but Aluminum is preferred in industry. 6063 cathodes would be a much better alloy choice as opposed to 6061, it is less susceptible to corrosion, and a far better conductor than lead. I get approx. 1 1/2 - 2 years of service out of my 6063 cathodes before they have to be replaced.
While the lead cathodes will last quite some time, you will eventually end up with a anodizing bath that is contaminated with lead, and will/should need proper disposal.
It would seem to me, that a different solution to your problem if your bath is going to be idle for months at a time would be to store your anodizing bath in a poly drum(s) while not in use.
anodizer - Idaho
Thanks for your advice, Marc. I decided not to buy lead sheet due to the disposal problem. There's also an issue of mechanical support in the tank, which is simply something that's specific to my particular installation. Maybe the solution for this is to build a small rack to hang the plates on when they're idle, and not leave them in the bath longer than a week or so at a time. If I had done that originally, then the first set would probably still be useable.
- Austin, Texas
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