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topic 12130

Current/Voltage for Aluminum Anodizing


Q. Hello Folks,

I have been anodizing small parts to build production machines for the company where I work. I started the anodizing process by my own from reading. I have been using successfully a 35 amps/ 12 vdc rectifier. If I want to expand this process to a larger capacity, what would be the rectifier range to get in order to assure enough current/voltage for any area to anodize? I will appreciate also any suggestion of how to get used anodizing chillers.


Reynaldo Arroyo
- Valencia, California


A. Reynaldo,

I'm assuming you are doing decorative anodizing, as it doesn't sound like your current power supply is enough for hardcoat. You should be anodizing at 12-15 amps per sq ft of surface area for decorative anodizing, so size your rectifier accordingly (20 volts should be more than sufficient). If you want to increase your load size, you'll also want to make sure your cooling system can handle your needs, as well. If, indeed you want to hardcoat your parts.. you'll need to size your rectifier for 25-40 amps/sq ft (you'll probably need 50 volts, or more). Check Metal Finishing magazine, or PF magazine, there are a lot of used equipment ads in there.

Marc Green
Marc Green
anodizer - Boise, Idaho


A. The required voltage is largely dependent on the distance from the anode to the cathodes. If the spacing you presently use will be adequate for the larger parts, then 12 volts will remain adequate, although it sound low to me and I'd feel more comfortable with an 18-volt rectifier.

The amperage required is proportional to the surface area of the load. If it will be 50 times larger, the amperage should be 50 times higher. As a guideline if you don't have enough data to do the proportioning, figure that you will probably not anodize at more than 20 amps per square foot.

Suppliers of used rectifiers can be found in's Equipment Directory.

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


A. Dear Reynaldo

Sizing a rectifier is easy, if you are going into serious production and want to have up to date equipment. For type II you will need 20 to 30 amps per sq ft. at 18-24 volts. For type III it will be 30-40 amps sq ft at 75 volts. Automatic ramp and timer are also essential. Cooling and agitation are extremely important to maintain quality production work. I also highly recommend using aluminum cathodes to save energy and eliminate a lead source

drew nosti
Drew Nosti, CEF
Ladson, South Carolina


? Drew...20-30 ASF for decorative anodizing? hmmmm..sounds a little high to me.

Marc Green
Marc Green
anodizer - Boise, Idaho

October 24, 2020

Q. Hi, I have been anodizing using a fairly robust, but small scale setup. I typically do not ever do parts larger than about 100 square inches. I have just set up a new system and a new constant current power supply with 10A, 30v capabilities. I'm using around 12% acid concentration derived from battery acid at 3:1 distilled water to acid. My question or problem is that I do not seem to be getting enough volts for the amount of amps I want to use, and my volts seem very low if compared to 720 rule. I have reduced acid concentration slightly, removed cathodes, and still find the volts seem low.

I would love lots of input regarding the volts/amps relationship. I do understand electricity, and current, and how its derived: volts x amps = watts, etc. But I am having trouble understanding the variables here, if the watts are a factor, how the voltage affects the outcome of the anodic layer properties, etc.

I have a part running now, are these parameters OK? 44 square inch, 6061 T6, 5 amps and 18v. I think the volts should be higher.

Thanks very much, and I have enjoyed the good info here!

Jason Brown
- Laguna Hills, California

We'd like to give credit for this graphic explaining Ohm's Law (A = V / Ω) but see it on many websites without info of whose work it actually was :-( ohms law
October 2020

A. Hi Jason. Electricity can be hard to understand and visualize because it is invisible and rather abstract.

Amperes is the flowrate of the electricity, actually the number of electrons traveling past a point in a given time period, analogous to the flowrate of water; Niagara Falls would be analogous to very high amperage, the amount of Pepsi flowing through a plastic straw would be analogous to very low amperage.
Voltage is analogous to water pressure; it takes strong pipes and high pressure pumps to provide water to sinks & toilets near the top of skyscrapers, even if only a slow drip of water is wanted, whereas a duct tape patch can hold on an above ground pool even if it holds thousands of gallons.

Although you may understand some aspects of electricity very well, I think you have your understanding of Volts and Amps interchanged based on your question. Voltage does not come into play in the 720-Rule, which is simply Faraday's Law of Electrolysis with a couple of constants related to aluminum factored in. I'd suggest that you review our introduction to Faraday's Law as a starting point for a deeper understanding of it. Your operational parameters sound fairly right. 5A for 44 sq in. equals 16.36 ASF, and that's the number for your 720-Rule calculation. The 18V seems somewhat higher than you ought to need for 5A but maybe not outrageous especially if your meters aren't precise ... but you can't independently control it (voltage and amperage are tied together by resistance).

Luck & Regards,

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

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