Aluminum vs. lead cathodes for anodizing

A. Hi Gordon. Sorry, I'm not sure if that is a refutation of the claims on this page or just a question. But my understanding, as a non-metallurgist, is that 6063 IS a precipitation hardening alloy.
Luck & Regards,


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

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.

A. Charlie Grubbs' article on aluminum cathodes can be found in the Nov. 1981 issue of Plating & Surface Finishing.

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

A. 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 sulfuric acid atmosphere.

If you wish to discuss this further, I will be happy to give you pros and cons.

Q. Hello. Do you have any examples of specifications for lead starter sheets? Is it a pretty standard product? Many thanks, Eric Eric Frawley Partner - New York June 23, 2021 A. Hi Eric. Raymond's posting is very old and the e-mail address we have on file may not reach him, but we'll try. Luck & Regards, Ted Mooney, P.E. RET Striving to live Aloha finishing.com - Pine Beach, New Jersey

A. As a major producer for the lighting market, we find that aluminum cathodes work for us.

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A. Dear Gerald,

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.

Sincerely,

Timur Ulucak
aluminum extrusions & finishing - Istanbul, Turkey

A. The Sanford Process supply to their licensees the process tank with graphite cathodes.

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.

A. Keith,

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.

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.

A. Jon

Double check your alloy/temper. I have seen your issue happen twice, both times due to the wrong temper being provided.

A. I still feel the material you received was NOT 6063 T6. But BOLT NOT WELD [see online library article]. The only catch is that you do not put the connections to the header bar below solution level.

A. 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.

A. 'Continuous anodizing process' is I think known also as 'reel to reel' or 'coil anodizing'

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.

A. Hi Rodger. We added your inquiry to a thread about cathodes for anodizing tanks. As you see, aluminum cathodes are an accepted alternative.

Tanks can be fiberglass, polypropylene, PVC, or PVC lined steel.
Coils can remain lead or be titanium or teflon.
We have dozens of threads with second & third opinions on those subjects if you search the site with patience. Good luck.

Regards,

A. Lead, titanium, and stainless steel will stand up in an EXTERNAL exchanger, but the only metal that can be used for an internal coil is zirconium. You can "get by" for a while with titanium, but tie in to the cathodes for cathodic protection.

A. I have used 316L stainless heat exchangers for external plate and frame units. The other materials may work but why bother. I have never had a failure due to material on the 316 units except when a customer used plain steel fitting when hooking up the unit. And then the unit was still good to go.

! Information from our alliance partner in regard to lead cathodes:

Let me know if you have any follow-up questions.

A. Here is a list of the relative conductivity for various metals for a type II & III anodizing bath: Aluminum 36, carbon 5, lead 4, titanium 2, and stainless steel 1.

When it comes to aluminum 6063 T6 has the slowest dissolution rate compared to several alloys including 5051. Generally, this correct alloy will last between 2-3 years without stray currents in the tank. 6063T6 is a special alloy/temper so it is not available as a stock purchase no matter what a standard aluminum supplier says. It is most likely T52 or T5 so ask for certs before purchase.

Now you can increase the life of any aluminum cathode by adding material but since other alloys dissolve at a faster rate than 6063T6 you will just add more aluminum into the anodizing solution and dilute your acid. Always stick with a reputable supplier who can supply certs to you before you purchase.

A. Yes. You may use ANY wrought alloy aluminum.