Aluminum anodizing: Constant current vs. controlling the voltage

A. To effectively run constant current/ASF anodizing you will need at least 60 volts to cover most alloys. Most alloys cap out somewhere in the 60s for voltages. To run ASF anodizing you will need to accurately calculate the SQ FT of the part tricky but not impossible. Take the SQ FT and multiply by 24 for 2000/7000 alloy and 30 for the rest. Set your voltage to full with AMPS at zero and slowly raise the AMPS to the total AMPS needed over 5-8 min. Hold for 24 min for 1 mil and 48 min for 2 mils. You should be close to thickness.

A. I work at a shop that anodizes aluminum parts for aircraft. Our solution is a Boric (7.5 g/l) sulfuric (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.

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.

A. Architectural sulfuric 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 sulfuric 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.

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.

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

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%)

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

Example-
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.

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.

Ed. update: We also loved Anodizer's Plaza, but it no longer exists :-(

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

(note: this book is two volumes)

on eBay or

AbeBooks

or Amazon

(affil links)

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

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).

Regards,

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, many 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. Where it can help of course is if you were dealing with constantly changing load sizes which would require constantly changing amperage settings. Good luck.

Regards,

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 / MIL-PRF-8625 [on DLA]).

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.

Stepwise voltage is sometimes called "ramp control". Per my posting just before this one, some people believe in very complex 'ramp control', other believe in the simpler 'constant current', but this is based on their personal experience (and possibly prejudices) ... I don't think we yet have a solid theoretical basis for choosing ramp control over constant current. However, if load sizes vary, voltage ramp control can accommodate this whereas it might be a hassle to keep changing the current setting for each load.

Regards,

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 :-)

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 sulfuric acid, so you are probably a bit high for jobshop hardcoating, although perhaps okay if you run only one kind of part and you can 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 readers don't have to qualify everything with a dozen "ifs, and, and buts" that don'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!

Regards,

"Surface Area Formulas and Tables"
by Tom Rochester

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 better suited to experienced anodizers than beginners.

The truth is that, almost by definition, surface area is absolutely 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!

Regards,

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 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,

"The Metallurgy of Anodizing Aluminum"
by Runge & Kaufman

on AbeBooks

or eBay or

Amazon

(affil links)

A. Hi Tony. If your previous experience is limited to plating, you need to know how anodizing is different: it builds a resistive oxide coat which impedes the flow of electricity and will cause the current to drop at any constant voltage.

If you have previous anodizing experience and still find what is happening to be unusual, the first thing to check is probably the meters on the rectifier. It's possible they're wrong; on a new installation you might even have the wrong shunt feeding the ammeter. Do you have a thickness gauge to check what is going on with the anodizing?

Luck & Regards,

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 :-)

A. First use Mooney's advice above. then let's check what you say about "180 g/L", that is 60.6 gallons of standard commercial sulfuric acid in a 600 gallon anodizing solution.. Is that what you started with? Now if you used some sort of battery acid ⇦this on eBay or Amazon [affil links] , then tell us the label and we will calculate that for you. Now when you start the current are you chilling the solution to hold the temperature to about 72 °F?

A. It seems easy problem according to your explanation. Most likely source of problem is rectifier or rectifier's setting. You should focus on rectifier with electrical technician in order to find problem. He can measure input/output of rectifier at voltage with a device under voltage. I am sure you will find the problem. I skipped the chemistry of bath because I assume your data is true, checked by a chemist.

A. Check to see which terminal is hooked up to the cathodes (Negative) And the parts (positive). I had this same problem with a big customer in Romania and we needed to reverse it to get anodizing.

On second thought, I bet Drew's exactly right :-)

Luck & Regards,

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

A. Hi Tony,
It sounds like you've just had this issue with a fresh tank but have been successful before? Your question doesn't sound like this is a brand new process for you.

Your aluminum is a little low. Normally you'd decant an old tank to get closer to 10 g/L, using the dissolved Al in the heel of the old tank to get right back to work with no fuss. But what I want to put out there is another thing to check -- is your racking, and your connection to the flight bar, still clean and tight? Are you stripping racks between uses? Are the techs scotch-brighting the flight bars before clamping spines to them? Are the undersides of the spines clean? Are the cradles also free of oxidation? Just general question on connectivity throughout the system.

Lots of good advice here and this is one more thing to check on.

A. What's the voltage/amperage capacity of your rectifier? What amps/ft2 (amps/dm2) are you running at? If you're going by amps you must set the voltage at max on your rectifier so the power is there if needed to keep the required amperage as the non-conductive anodise film builds up. In my experience calculating amps per part via the surface area, 20-24 volts max is usually more than enough (depending on work loads) to keep a good stable amperage (very rarely goes above 20).