Anode to Cathode Ratio, Size, Spacing, Placement in Anodizing

A. Hi Katerina,
If I am understanding you, you expect your work load to be around 14.52 sq. ft, so if you target a 3:1 anode to cathode ratio, you'd like to have about 4.84 sq. ft. of cathode area. But since your cathodes are 1.1438 sq. ft. each, that would be about 4.23 cathodes.

4 cathodes would be ideal, but 3 would be fine too -- it's simply not a critical issue at all. :-)

Luck & Regards,


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

A. Hi again K.,
The cathode size will have no effect on the 720 Rule ... which is simply a specific case of Faraday's Law with constants specific to anodizing inserted into his law.

Don't fret over the cathode area, just put in 4 and forget the removal & shielding. Ideal cathode size has nothing to do with the one individual load, but rather it has to do with optimizing a long-term equilibrium issue. We'll see what other readers say but I have personally never even heard of adding & removing cathodes for different load sizes. Run as many parts as can fit and not exceed your rectifier capacity.

As for 'celebrity', I'm the blabbermouth because I'm the operator of the forum, so naturally I don't like to see unanswered questions :-)

adv.
If you decide to retain personalized help, Robert Probert and Drew Nosti are supporting advertisers who are experts in anodizing.

Luck & Regards,

A. The ratio of the anode to the cathode of 3:1 is a guideline not an absolute. In a perfect world it would be great to always have the 3:1 ration, but in most if not all anodizing it is not practical to hold that ratio. Trust me, I have customers who are using 3:1 and some who use 1:3. From my perspective, having cathodes evenly spaced in your tank will be more important than trying to hold the 3:1 ratio.

Schematic of a Haring Cell
Source: www.pfonline.com/articles/an-investigation-of-the-effect-of-nile-blue-on-the-electrodeposition-of-nickel-on-brass-from-a-watts-type-bath

Remember that the solution has resistance (although chromic acid is fortunately very conductive). You can measure the resistivity of the solution with a Haring cell.
but for a first cut, say it's about 4 ohm-cm.

The resistance of the solution is the resistivity x length / cross sectional area. So if the anode-cathode spacing is 18 inches, or 45.72 cm, then the solution resistance you will encounter in anodizing a square foot of work is 4 ohm-cm x 45.72 cm / 929 cm2, or 0.197 ohms.

So if you anodize at 12 ASF, then according to Ohm's Law the voltage drop across the solution will be 2.4 volts. This is approximate, and it's actually somewhat lower because the current doesn't follow only the horizontal straight lines ⇨

A. By type II I hope you mean sulfuric anodise. If you do there is no problem - I ran some jobs for home in the big tank total anode area approx. 16 inches square and the cathode area 36 square foot a ratio of 1 to 324 and the jobs came out fine.

A. Cathodes for anodizing -- a subject near and dear to my heart and wallet. Cathodes should be aluminum, 6063 T6, and set up on both walls of the tank with the parts centered as best as possible between them. A ratio of 3 to 1 is best (3 Sq Ft anode, 1 Sq Ft cathode) or as close as possible, understanding a job shop environment. Cathodes should not extend below the parts in the tank (again remember the job shop variables). Bolting is recommended over welding if you follow good judgment in adequate contact surface area and torque. Never put joints below or close to solution level. Use a conductive sealant.

A. From the anodizing job shop side of things (as installing excess cathodes would unnecessarily lighten MY WALLET), 3:1 anode to cathode is a bit extreme and I've read in various text books that this can be as high as 10:1. The only difficulty comes in guessing the anode area on that next job coming in -- if you are indeed a job shop, that's always a crap shoot. In that case, I'd go with 7:1 (Come on 7's) ... good luck, don't crap out!

A. Aluminum is more conductive, takes less watts, saves on cooling. Lead puts some lead into the waste stream. Purists say use 6063T5, but just recently a shop (who claims no stray current or galvanic current) got less than 6 months. Another shop that I set up got 4 years out of 6061 1/8 inch thick! For type II the ratio is NOT critical, but 30 to 1 may be a problem. Do not do it.

If you are talking a lead lined tank, then use plastic to mask the bottom and ends. My client that got 4 years with 6061 -- NEVER removed the cathodes. There was no stray current. The heat exchanger was outside. Use one side facing the work.Wow, 1/16 might be a bit thin. But do not line the tank. Have no cathode on the bottom and no cathode on the ends.

A. The use of lead in the form of sheet or tubing is both more expensive and less efficient than aluminum. To avoid the aluminum dissolving readily in the anodizing bath the use of 6063T6 is mandatory. It is not necessary to remove them when not anodizing. Try to stay as close to a 3 to 1 ratio (3 anode to 1 cathode sq. ft.). If you have more questions I would be glad to help.

Drew Nosti CEF
AESF Light Metal Committee
Member AAC

A. To answer my own question - a rather basic one I might add. While visiting Mr. Nosti's website I have concluded that the anode:cathode ratio is 3:1. I hope you do not mind Mr. Nosti, I have taken this quote from the AnodizeUSA [a finishing.com supporting advertiser] website, "...the ratio of 3 sq feet of anode to 1 sq foot of cathode gave the most consistent anodizing.". It should be noted that Mr. Nosti is referring to 6063T6 aluminum cathodes. It's funny to think, how we grow we sometimes misunderstand stuff. in fact, I feel quite foolish at the moment. For 6 months now I was under the impression that the anode:cathode ratio was to be 1:3. I was anodizing with decent results utilizing this method. I was also using just simply 1/16 aluminum sheeting of an unknown alloy for cathode material. I have currently reversed that theory thanks to Mr. Nosti and finishing.com. My current setup will soon be recalibrated implementing a 3:1 (anode:cathode) ratio while utilizing 6063T6 aluminum cathodes. Just one more step in the right direction. Thanks people! Cheers!

Thanks Dan, it is nice to hear that correct information is spreading. Remember it is not a perfect world and in anodizing you can get almost anything to "WORK" .The question is what is the best way. When starting or updating any metal finishing line try to travel in a direction that will lead to the best technology, even if it is in small steps. Good luck.

A. We have had a member on another anodizing forum try this unusual (to us anyway) 1:3 cathode/anode area. There is definitely something useful here, we will check this out in detail with properly instrumented anodizing experiments this spring and inform Drew directly of our findings.

Ed. note: it is our sad duty to advise of the passing of Paul Yursis in August 2005.
Here is a brief obituary by Mike Caswell.

A. Hello, the ratio from anode:cathode is 3:1 as a minimal ratio in the project of a tank building for a full charge, maximal anode area or rectifier capability that define the greatest area that can be processed.
If you use in any given moment a 30:1 ratio, no problem; make sure that the rectifier is so adjusted to prevent burning, in the ratio ASF (amp per sq. ft.) that you use conform your bath and Tank construction condition.
I do not remove the cathodes from the Tank, only for cleaning or replacing.

A. Just size for the maximum surface area load using separate cathodes and not using the whole tank for a cathode. Area ratio is not as important on regular anodizing up to 0.0008 inch, however, more critical in hardcoat.

A. Dear Sir,
First of all we need to define the size of anode. I have experienced that undersize anode will definitely heat up at the junction point. It results in loss of energy in form of heat dissipation.

So if you plan to give suppose 2000 amps DC current then the anode bus bar size should be minimum 100 mm width and 20 mm thickness. That is 100* 20 =2000 or you can use 80 mm width and 25 mm thickness. Again the calculation will come up to 2000.
So you can use different sizes with result amounting to 2000 for 2000 amps DC current.

Actually I am using 1:3 anode to cathode ratio. I am still confused whether it's right or wrong. But I am getting consistent anodizing.

Cheers!!
Happy Anodizing.

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

Hi cousin Rajabhau ...

A1. The 3:1 anode to cathode ratio is simply widely used and has been found not to introduce problems with dissolved aluminum concentration or overheated cathodes, while allowing the size of the load (anode area) to vary widely. It's a target to plan for at the design stage rather than something to worry about day to day.

A2. Although some guidelines are based on first principles (like the '720 rule' which was born from Faraday's Law, and the 50:50 rule which stems from the way aluminum combines with oxygen), much of our anodizing knowledge is empirical (certain operating parameters have been found to work better than others). There have been limited attempts to chart how you should try to compensate some of the other parameters if you choose not to adhere to the empirically determined guidelines for some of them, and Robert Probert's "Aluminum How-To" discusses the issue, but I don't think compensation is an ideal approach.

Production floor electrochemistry is complicated, and I think the best advice is from Larry Durney's "Trouble in Your Tank?": Obey the Letter of the Law. Look up the recommended voltage for the alloy in question, so you have it (14.5 V for 5052 through 23 V for 319 & 380) and then anodize at 12 ASF, 15% by weight H2SO4, less than 0.02% Cl, at 70 °F.

A3. If you put "aluminum etch sludge" into the Google custom search engine near the top of our pages you will see several interesting discussions on the topic. It is claimed that gluconate is a valuable addition agent for this, but keeping the solution moving until it can be filtered can also be important. Please continue the sludge topic on one of those threads. Good luck.

Regards,

Hi Mark. We offer technical comments in this forum but we don't judge the pluses & minuses of particular brands or suppliers. But there is a borderline issue here in that you are using a hobby priced system and many of us would not consider hobby components adequate for industrial anodizing. This system apparently uses a technique called "low current density anodizing", which is not to my knowledge endorsed in the industrial sector: You are anodizing at 4.5 ASF whereas most anodizers would use 12 to 18 ASF. So yes, it takes 3 times as long.

A1. I think the titanium wire is a better approach because the aluminum rod will become anodized and can't be used again until it is stripped. The wire must be large enough to carry 1.06 Amps; the conductivity of titanium is only a small fraction of copper, so we're talking substantial wire, not thread.

A2. The voltage should increase substantially; if it's not building to 12 volts, neither is the anodizing. I suspect you have poor connections somewhere. Note that 0.8 mils is a very substantial thickness, exceeding the standards for architectural anodizing. Most people would not anodize to that thickness unless they are doing black, where greater thickness means more dye for greater color saturation -- and if I read correctly, you are doing clear. Most specs would call for more like 0.2 or 0.3 mils for clear anodizing ... where did you get this spec and/or what is the function of these anodized parts?

A3. Nickel acetate sealing is fine, but so is hot water sealing. Hot water would leave you with fewer variables to tame. Good luck.

Regards,

A. Hi again. Supposedly, medium temperature proprietary nickel acetate seals offer somewhat better corrosion resistance than hot water. But proprietary products always have a champion and hot water will never have one; so the difference, if any, will probably be much less than what those champions tell you -- my bet is it won't be significant at all. Where nickel acetate sealing gets more important is in color anodizing.

Regards,

A. I think for your application I would change several things:

1. Use an aluminum buss bar

2. Use aluminum cathodes (6063 is preferred)

3. Use Aluminum welding rod as your method of contact to your part, it's relatively inexpensive, readily available in 1/8" diameter, and carries current much better than Ti wire. I like 1100 series rod, as it bends easily w/o breaking...however 4043, and 5356 will also work. When you're done with the wire, just toss it and recycle it. I wouldn't hassle with the stripping.

4. You didn't mention the amperage output of your rectifier, but as Ted mentioned, 4.5 ASF is pretty low. Using 12-15 ASF, you should be able to achieve your desired .0005" coating thickness in approx. 25 minutes.

5. Your mid temp Ni acetate seal sounds just fine for your application, however I will disagree with Ted in that a high temp (200-212 °F) DI seal offers better corrosion resistance, but it doesn't seem practical in your situation.

Finally, a side note. You need to realize that since your anodizing process appears to be being used in a commercial application, you are probably subject to local and federal environmental laws, and may need the appropriate permits. These permits, the monitoring equipment, and the lab testing of your effluent is not inexpensive...and you may find that it might be better to pay a little OT to your employees in order to get the job completed/manufactured early so that you can use your outsource anodizer, and not miss your shipping deadline to your customer. The fines for not abiding by environmental laws can be extremely high.

A. Hi William. In metal finishing, the positive pole is the anode, the negative pole is the cathode. As you may know, in computer programming a "+" sign is sometimes used to represent the word "and"; so a memory aid I use is to mentally call the positive pole the "andode" and the negative pole the "can'tthode :-)

In anodizing, the part being anodized is made anodic (connected to the positive lead of the power supply). In electroplating the part being plated is made cathodic or negative. For this reason, when you are talking about electroplating, you connect the positive lead to "anodes" made of copper, or zinc, or nickel (or another metal) as the source of the metal; but in anodizing you connect the negative lead to aluminum or lead "cathodes".

No metal is deposited in anodizing. Rather, oxygen is drawn to the anodically charged parts and converts the aluminum on the surface of the part to oxides/hydroxides of aluminum. Good luck.

Regards,

A. Hi Dan. Although metal finishing is a science, a lot of it is empirical knowledge rather than based on first principles of mathematics. Furthermore, some of the advice has to do with secondary effects in production situations rather than whether or not you can successfully electroplate or anodize on a one-time basis. For example, in nickel plating you might polarize a nickel anode if it is too small, or in zinc plating you might have to fight against rising solution concentration if you have too much zinc anode area.

For decades it was traditional to line anodizing tanks with lead as a corrosion-proof lining, then simply connect the negative lead to the lining, making the entire area of the tank the cathode. This probably resulted in anode to cathode ratios of 1:5 to 1:50 and yet the process survived; so I doubt that slightly too much cathode area is a serious problem in anodizing. Too little might cause over-heating, or perhaps excessive corrosion of the cathode, or perhaps limit the speed of the process due to not being able to get enough hydrogen through the boundary layer of a very small cathode, but I think the issue is not critical and it will not be valuable to try to prove wrong those who think a 3:1 anode to cathode ratio is the ideal -- it's a good target if there's no good reason why not, but I doubt that 2:1 would cause any problems.

For the very simplest answer of all, go with a 1:1 -- and you'll never have to worry again about which way is which :-)

Regards,

A. Hi Scott. Those who feel that a carefully controlled anode-to-cathode ratio is important tend to favor the 3 to 1 ratio that you believe you have. So I would not change it.

Regards,

A. Honestly I have seen ratios of cathodes all over the place. The 3:1 ratio is a starting point, I use it to do calculations for a set up. Since most anodizing lines see many different SQ FT of work, it is impossible to keep this ratio constant. So guys think of the 3:1 ratio as flexible and use what you need to get great work. I am though a strong believer in 6063T6 and have documentation/studies to confirm it.

? Hi Khushal. I think you're asking about anode-to-cathode ratio, but the somewhat different rhythm of Indian English compared to United States English leaves me unsure.
Please introduce your own situation rather than trying to cast the question in the abstract, and I think we'll be able to communicate better. Thanks!

But in the meanwhile, anode to cathode ratio is not nearly as important as many others parameters, so don't let it steal your focus :-)

Regards,