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topic 1574

Aluminum anodizing: Constant current vs. controlling the voltage

A discussion started in 1998 but continuing through 2018


Q. I'm thinking about starting Anodizing and at this moment I would like to know how much voltage is usually applied in the process for both large and small materials . . . Can someone Help me out in this subject?

Thank's in advance,

Fridrich Grundinger
Lager's Inversiones C.A.


A. I work at a shop that anodizes aluminum parts for air craft. Our solution is a Boric (7.5g/l) Sulphuric (45g/l) mixture. Our tanks are 48 feet long so we can run large and small parts. The voltage we use is 15 ± 1 Volt. Since we do aircraft, we are rigid about our voltage staying within this range; failure to adhere causes decreased fatigue life, burns, etc. Our voltage range doesn't change regardless of the size of the part.

Voltage has never been a problem for us, rather, the problem is with the amount of amps we draw, i.e., work load. We've gone to a 10,000 amp rectifier to remedy this.

I guess it depends on what type of anodizing you are trying to do. With other techniques, such as hard anodizing or chromic, the voltage ranges are different.

James A. Corier


A. It has been my experience that if you can figure your work load, you are better off running by amps/sq.ft. Voltage can vary because of many different reasons ... Loose contacts, tank temperature, chemical concentration, size of work load, etc.

Voltage will change depending on what kind of anodizing you are doing.

Darrell Barker


A. Architectural sulphuric acid anodizing requires 1.0-1.5 Amperes per and you will need 15-17 Volts DC when your solution contains 160-180 gpl sulphuric acid and 5-10 gpl Al(+3) between 18-20 °C. Lower the acid concentration or raise the Al content it requires more voltage which means loss of efficiency, a situation not wanted.

Timur Ulucak
aluminum extrusions & finishing - Istanbul, Turkey


A. Good anodizing is done using current density (amps/sq. ft.). Voltage is merely the "driving force" and is dependent upon many parameters in the process. For this reason, it is not a good way to anodize.

Those using constant voltage apply an anodic oxide that is formed from decaying current density because the oxide is a resistor and needs the "driving force" to maintain the required amperage.

This constant voltage anodizing can cause soft coatings, poor dyeability, poor seal (poor corrosion resistance) and usually greatly extends the anodize time.

My advice is to learn to anodize using current density and you will be far ahead.

Charlie Grubbs
- Alpharetta, Georgia

Anodizing thickness vs. time


Q. There is an equation that tells a person if they are anodizing using constant current as opposed to voltage anodizing, the time it would take to achieve various thicknesses. Can you share this with me?

Thank you,

Edward f. Yost
line operator quality control - Ramsey, Minnesota, USA


A. Anodizing is theoretically by Current Density. I know a lot of anodizers that these many volts at this much time on this type of alloy will get you this thickness of coating; these numbers vary per shop and all ref. current densities

Chris Snyder
plater - Charlotte, North Carolina


A. The following equation may help you: H = 0.4 * W * T * J/F

where H is layer thickness in microns
W is Anodic efficiency
T is time in minutes
J is total current (amperage) in amperes
F is surface area in

(Anodic efficiency is around 65%)

Jan Prichystal
Audio-visual - Denmark


A. I would suggest using the 720 rule. Try using this formula:

Square feet x Amps / Square foot = Anodizing Amps
Example (500ft2 x 20 amps/ft2 = 10,000 Anodizing Amps)

Square feet x Mill Required
Anodize Amps x 720 = Anodize Time

500ft2 x .70 Mills Required
10,000 Anodize Amps x 720 = 25 minutes Anodize Time

This should get you very close depending of course on tank temp or part thickness.

Bruce Pfaff
- Wausau, Wisconsin

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.

Control of Type III Anodize by Current Density Versus Voltage


Q. Hello,

I'm the quality manager at a small (15 people) metal finishing job shop. We are in the throes of gaining NADCAP approval for several processes. The process that's causing me the most extra work right now is Type III anodizing. Can you recommend some journal articles that spell out the advantages of control by current density instead of voltage? I found the reference books you recommend on this site, but haven't seen much on this specific subject other than "you need to do it this way". I'm looking for some justification.

Thanks in advance for your help. You have a fascinating and useful site. I've already found answers to some other quality problems here.

Jan Harris
metal finishing shop - Tucson, Arizona

Ed. note: Thanks so much for the kind assessment, Jan.


A. Check out, it may have what you're looking for. "Anodizer's Plaza" --> 100 Q and A --> question 28.

Jim Gorsich
Accurate Anodizing Inc.
supporting advertiser
Compton, California, USA
accurate anodizing banner

Ed. note: sorry, but Anodizer's Plaza no longer exists at that URL; if anyone knows where to find it, please advise]

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


May 21, 2009

A. Hi Jan

Perhaps you will have difficulties with finding a book, even though I think this is mentioned in Wernick, Pinner and Sheasby's bible =>
It can be explained very easy by the resistance of the formed oxide layer.

If you are doing voltage control Ohm's law U=R*I will show you that building up the oxide layer which increase the R, for the same "U", your current will decrease over time. The decrease will depend on many things such as concentration, alloy, temperature and so forth.

If you on the other hand choose current control You will increase during the formation of the oxide layer (R)

If you have enough voltage, current control will always be the best and most economic way to go.

I have some articles which go into this subject because that is one of the important features when Pulse Anodizing.

Sunny regards

Anne Deacon Juhl
Anne Deacon Juhl
- San Diego, California, USA

June 12, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. For hard anodizing process, I want to know for sulfuric acid bath what should be current increment or how to reach higher limit of current and how do we calculate it?

Nilesh [last name deleted for privacy by Editor]
- Anand, Gujarat, INDIA

A. Hi Nilesh. Most readers are suggesting constant current, rather than increments. The anodized layer in hard anodizing gets very thick (0.002") and consequently very highly resistive; so, depending on the alloy, to hold 24 - 36 ASF, it may take a lot of volts (maybe 48 -90 volts).


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

March 27, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear friends,
for Mil type 2 we consider voltage for anodizing, and for type 3, i.e, hard anodizing we consider amperes ...

So why it is like this, why not vice versa -- and which parameter is good voltage or amperes?

Aijazullah Tajir
- Abu Dhabi, UAE

March 2014

A. Hello cousin Aijazullah.

Most of our readers do not agree with anodizing by voltage for either process. Rather, they suggest anodizing by constant current :-)

Some "rules" of anodizing science are based on "first principles", but at this point in our knowledge, most are still based on empirical personal knowledge. So, while some people may prescribe complicated amperage-time profiles or voltage-time profiles for a given part of a given alloy in a given tank, believing them to be more effective in some way than a simple constant-current profile, there does not seem any consensus that such complications are an improvement in the general case. Good luck.


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

Anodizing with Stepwise Voltage

November 28, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello,

I am trying to anodize a 3D printed aluminum in order to color it black. The best result I could get was using stepwise anodizing. Can anyone help me to understand the effects of stepwise anodizing and how I should control the steps and current density? Or any other tips about this process (anodizing in sulfuric acid at room temperature)?

Payam Ajami
- Germany

December 2017

A. Hi Payam. Anodizing at "room temperature" in sulfuric acid is what people here are calling "Type 2" anodizing (which refers to Type 2 in USA Mil-A-8625 [link is to free spec at Defense Logistics Agency,]).

Whether you do 'stepwise voltage' or 'constant current', the reason and justification is the same: bare aluminum is highly conductive, but the anodized coating is resistive/insulative in proportion to its thickness. If you try anodizing a bare aluminum part at a high voltage, the current will be excessive and it will burn; but high voltage is required later on in the processing because current will decrease or stop if low voltage is maintained as the coating builds up and insulates the part. So whether you operate the process by maintaining a constant current density, or you gradually step up the voltage, you are doing more or less the same thing.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

May 3, 2018

Q. Dear Sir,
If we control current density for this anodizing process at 3 A/dm2, is this for anode surface or cathode surface?
Please advise.
Thanks a lot.

Dallas Sitinjak
- Batam, Indonesia

May 2018

probert book
Aluminum How-To

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

A. Hi Dallas. Such numbers always refer to the anode (the parts you are anodizing). Robert Probert suggests 24 ASF (2.6 Amps/sq. dm) for hardcoat anodizing with proprietary additives, and half that for conventional Type II anodizing with straight sulphuric acid, so you are probably a bit high for jobshop hardcoating, although perhaps okay if you run only one kind of part and you optimize for it =>

Please fully describe your situation (Type II / III / 2-1/2, jobshop or captive, experienced anodizer vs. neophyte, where you got "3 A/dm2", etc.) so we don't have to mention a dozen "ifs, and, and buts" that won't apply to you anyway. Unfortunately, "general" answers can't be covered in the few paragraphs appropriate to a forum, and require book suggestions. Good luck!


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

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