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topic 3437

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





NMF Canada - Montreal , Canada

Schematic of a Haring Cell
A. Hello Ziane. There is no particular minimum or maximum. Just not so close that the work is in danger of touching the cathode and shorting out, and not so far that excessive voltage is required and power thereby wasted.

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 .197 ohms.

So if you anodize at 10 ASF, then according to Ohm's Law the voltage drop across the solution will be 1.97 volts. This is approximate, and it's probably actually somewhat lower because the current doesn't follow only horizontal straight lines:

Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


Q. For type II anodizing are there any problems associated with running an anode/cathode ratio much smaller than 3:1, say 1:5? Like if you did a small run in a large anodizing tank.

Michael Peppard
- Detroit, MI, USA


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.

Martin Trigg-Hogarth
Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England

To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.


Q. How critical is cathode placement in a type II anodizing tank? I realize cathode area is important, load type, shape of parts to be anodized, etc. Is it better to have one cathode running the full length of the tank and up both ends? or two on the sides only? what is the optimum setup generally speaking?.


Steve Power
anodizer - Nelson, New Zealand

Surface Treatment & Finishing of Aluminium and Its Alloys
Wernick, Pinner & Sheasby


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.

drew nosti
Drew Nosti, CEF
Anodize USA - Ladson, South Carolina


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!

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

Anoplate banner

January 30, 2014

Q. Dear friends,
How to estimate the maximum surface area of parts that can be processed with respect to cathode and tank size?

Aijazullah Tajir
- Abu Dhabi, UAE

August 10, 2014

Q1. What is the effect of more or less (than the standard 3:1 ratio) anode:cathode ratio on the quality & quantity of anodic coating?
Q2. How can we decide proper current density, required for type II anodising, with respect to the sulfuric acid and aluminium contents and the temperature of anodising bath?
Q3. How to restrict the solidification of caustic sludge in caustic etch bath used for etching of aluminum?

Rajabhau Kalbandhe
Service in Electroplating Industry - Nagpur, Maharashtra,India

August 2014

Thank you Martin.
Thank you Drew.
Thank you Milt.
Readers: Now it's your turn. Please help us try to build a shockingly good forum.

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

October 4, 2014

We purchased a hobby anodizing kit, using 7 gallon buckets. My question is on setting up the anodizing anode and cathode.

I'm using a titanium bus bar and 8" x 8" lead cathode, and supporting the parts with titanium wire that came with the kit. Doing several trial runs with aluminum (6061), I'm not seeing much in voltage increasing by the end of process.

Q1. Would it be better to use 6063 aluminum 1/8 rod to support the parts from the bus bar?

Q2. I'm using the 720 rule calculator and 4.5 current density (found an app on iPhone):
34 sq in, .8 mil, 4.5 A/sq ft, set current to 1.06 A for 128 minutes. I have a 15 VDC power supply. Should I be seeing the voltage increase as the resistance of the part increases? If so, should it be a lot or a little.

Q3. Next would using the hot seal method for clear coat be better than the nickel-acetate sealant that comes with the kit?
We normally have parts farmed out but are using it when we have time constraint.

Mark Atkinson
- Syracuse, New York U.S.A

October 2014

Hi Mark. We only offer technical comments in this forum; we don't judge the pluses and minuses of particular brands, or critique 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 priced 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 quite a substantial thickness, exceeding the standards for architectural anodizing. Most people would not anodize to that thickness unless they are doing black, 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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

October 4, 2014

Q. Thank you for replying so quickly.
All connections have been checked with the exception of using SS bolts for connecting to the lead cathodes and titanium bus bar anode. My suspect was the small gauge wire being used to support the part. Yes I know it is not the industry way but if it works in a pinch with the same results when on a time constraint we have no problem with it. I wasn't sure on the mil size when I did the calculation. I since know the parts that we have professionally done is .4 mil. We did purchase a handheld device for checking the anodize thickness. The parts are used in a inspection device for the Pharma Industry. I will see about getting a heavier gauge titanium wire if not we can strip the aluminum. Only a couple parts are being done monthly.

In regards to the hot seal will it hold up just as well compared to the nickel acetate? Again thank you for all and any future help.

Mark Atkinson [returning]
- Syracuse, New York

probert book
Aluminum How-To

by Robert Probert
$89 New
The Chromating - Anodizing - Hardcoating Handbook

October 2014

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 less than what those champions tell you -- my bet is it won't be significant :-)


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

October 5, 2014

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 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.

Marc Green
Marc Green
anodizer - Boise, Idaho

Ed. note -- Readers: Please see also letter 34413, "Anodizing anode to cathode ratio"

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