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topic 1574

Aluminum anodizing: Constant current vs. controlling the voltage

Current question:

December 28, 2020

Q. I am anodizing a part in 12.5 V 30 minutes without racking and have different color with anodizing a part for 30 minutes with a racking. Does surface area of racking that cause this difference? How does calculate voltage and anodizing time for different load (I use power supply with adjustable and control voltage)?
thanks for your help

Faisal Aprialdi
- Salatiga, Indonesia
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Previous closely related Q&A's starting in:


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.
^- Reply to this post -^


A. I work at a shop that anodizes aluminum parts for air craft. Our solution is a Boric (7.5 g/l) Sulphuric (45 g/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 sq.dm 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

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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 sq.dm

(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 search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)

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


A. Check out http://www.sic.shibaura-it.ac.jp/~sato/lab/plaza/chap328.html, 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]

affil. link
"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
info on Amazon

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
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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, finishing.com Teds signature
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - 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
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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, finishing.com Teds signature
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - 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

affil. link
"How to Make Money with 3-D Printing"
by Jeffrey Ito
from Abe Books

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
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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 spec at Defense Logistics Agency, dla.mil]).

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
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

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
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May 2018

affil. link
probert book
Aluminum How-To

"The Chromating - Anodizing - Hardcoating Handbook"
by Robert Probert
(Finishing.com has sold 700+ copies without a single return request)

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. While very specific questions can often be answered here, "general" answers often 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
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

March 11, 2019

Q. Hi my name is Tod Robinson
I am trying to anodize a bunch of different sizes, thickness and shapes of aluminum remote control car parts. I have very little knowledge of anodizing and I need to ask how do I find the square surface of all the different parts? I have rings, shafts, blocks, and all different shapes? Do I need to be exact or can I just a rough guess? Thank you

Tod robinson
- East Brady, Pennsylvania USA
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April 2019

affil. link

A. Hi Tod. Although it's usually not recommended, it is possible to anodize by voltage even if you don't know the surface area; further, there is a technique called "The Parallel Circuit Method", illustrated in Robert Probert's "Aluminum How-To" that allows you to calculate the surface area from the way the parts react in the anodizing tank ... but those are probably both for experienced anodizers, not beginners.

The truth is that, almost by definition, surface area is crucial to surface finishing and you need to become comfortable with methods to estimate it.

You can't anodize different alloys at the same time, and the rings, shafts, and blocks are quite possibly made of different materials. I'd suggest picking one part and experimenting until you figure out how to anodize it properly before trying to move on to different shapes and alloys. Good luck!


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

December 9, 2020

Q. We apply sulfuric acid anodizing by using constant voltage control and amperage is allowed to roam freely, but we face some problems during the process including :
• Layer thickness for the inner and outer diameter of the same work piece don't match
• The anodizing layer of small thread is easy to remove
• Anodizing layer thickness of different parts is not homogeneous
• Upon assembly, black powder appears in the magnifying devices.
• Not all Al alloys used produce good anodizing layer like 3XXX,2XXX and 5XXX series.
• The dimensions required for the work pieces are of very high accuracy, so I need to achieve the required thickness

We hope to contribute to solving these problems with your experience in this field.

Sabah elaraby
- Cairo elsalam, Egypt
^- Reply to this post -^

affil. link
probert book
Aluminum How-To

"The Chromating - Anodizing - Hardcoating Handbook"
by Robert Probert
(Finishing.com has sold 700+ copies without a single return request)

December 2020

A. Hi Sabah. You have a lot of problems :-(
You should be anodizing by current density as explained in several responses to this thread which we appended your inquiry to.

• ID and OD thickness not matching isn't a big surprise unless you are taking action unsuccessfully. But you need to be far more explicit about what the parts are and their dimensions.
• Threads shouldn't be anodized.
• Both geometry and material can be interfering with getting constant thickness.
• We'll probably need more hints to guess what that black powder is.
• Different alloys need different etching, desmutting, and anodizing parameters.
• It may prove necessary to machine to different dimensions; anodizing may not be able to achieve what you want in terms of dimensional additions.

Luck & Regards,

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

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