Anodizing ampere-time calculations, surface testing and general questions

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A. Hi Tobby. Robert Probert suggests 165 g/l to 180 g/l in his "Aluminum How To" ... your range sounds far too permissive. But in terms of calculations, the concentration doesn't control the thickness -- the ampere-hours does.

The thickness you seek is quite conventional but you really need a hall-effect thickness meter rather than trying to guess at thicknesses based on ampere-hours because things can go wrong like uneven thickness, broken contacts to the whole rack or just some objects on it, mistakes in logging anodizing time, etc.

(True that this particular thread didn't formerly concern itself with sulphuric acid concentration; it focused more on the 720 rule, but it's only one of 60,000 threads in this forum. Please search the site for any subjects that interest you :-) Luck & Regards,


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

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A. Hi Daniel. The biggest area of expertise that people are usually lacking is experience with government regulations on the operation of anodizing shops (which you reference in question 7). Look into that before you go much further. Another key area is that you have to acquire an anodizing library; and anodizing training if you are serious should come next.

1, 2. Abrasion resistance can be tested with Taber abraders or other methods, but that is something you can probably farm out. A simple hall effect thickness tester is probably the tool you most need. After that, lab glassware and supplies for chemical analysis, seal quality tests, etc. Then cross sectioning equipment and a microscope. Others may give you a different list. Corrosion resistance is critical, and it's tested with a salt spray cabinet, but this can also be farmed out initially.

3. You can make a table and chairs for your shop by slinging a piece of plywood across sawhorses, and sitting on inverted plastic buckets, but you know you shouldn't. Similarly, a battery charger should not be used as an anodizing rectifier no matter how small your operation. As for the sizing, please see question 5.

4. Either cool the anodizing tank right or don't cool it at all. Right means a chiller, temperature regulators, and Teflon or titanium cooling coils -- an expensive proposition.

5, 6. Four ASF is way too low, 12 ASF is somewhat low. 40 ASF is way too high. I'd figure 15-20 ASF. The anodizing thickness vs. time depends on the alloy, but it shouldn't take 2 hours to anodize your parts. I'm not accustomed to using that 720 rule and I apologize that I can't run the thickness calculation for you right now, but my gut feel is about a half hour to forty five minutes will do it.

7. Manitoba is different than the USA, and I can't quote your regulations, but there will be wastewater limitations which require wastewater treatment.

Good luck.

A. Dear Mr Daniel.

I'll try to clear some points, with my poor English, and I hope that I can.

1 - anodizing creates heat so you have to cool your solution, less than 22 °C, and the cooler the better. You can use lead cathodes and lead cooling coils.
2 - anodizing --oxygen sticks to aluminum, and the product of Al+O is Al2O3 WHICH IS ALUMINIUM OXIDE which is very hard, as hard as chrome, not conductive, and porous. That is why you need to increase the voltage slowly, whenever the current goes down, to keep current steady (the current can reach the aluminum surface only through the porosity). To get up to 35 microns you may need 12-15 ASF, voltage can go from 22 volts up to 40 and more volts (depending on thickness required), and up to 45 minutes. After anodizing, if you deform your part you'll notice cracks all over the deformed surface.
3 - after anodizing a proper deionized, or good quality water wash is very important
4 - NOW if you seal the porosity by boiling in hot water for 10 minutes or so, or sealing in other solutions, or dipping in some coloring, you'll have colored or plain corrosion resisting alum surface. But if you kept the part after anodizing without sealing at room temp for a day or more, or you heated in an oven, you'll have a very hard coat.
5 - regarding thickness control and the formula you mentioned, I'm not familiar with it, but I think that would be tricky because the current goes down and increasing or if the surface of the part gets hot you may dissolve your anodizing -- and that's why cooling and agitation is very important. Mr Daniel I liked the way you explained your situation and and I'll be glad if this helped.

Good luck and GOD BLESS

A. The 720 Rule is: 720=ASF x minutes / mils. Where; ASF is Amps per square foot, mils is the desired coating thickness (1 mil = 0.001").

Rearranging to solve for time: 720 /ASF x mils = minutes

For a current density of 20 ASF, and a thickness of 0.0005" (0.5 mils); 720 / 20 x 0.5 = 18 minutes.

(I'm surprised that Ted isn't familiar with it; it has a solid basis in electrochemistry, being based on the Tafel equation. Its been around for decades).

The actual current you would apply is 1.75 sq,ft. x 20 ASF = 35 Amps. The required peak voltage is 0.95 Ohms per sq.ft / 1.75 sq.ft. = .54 Ohms. .54 Ohms x 35 Amps = 18.9 Volts. This assumes no voltage drops due to bad connections, and you haven't increased the actual surface area with racking.

Regarding Amps and Volts. Confusion occurs when the third entity is ignored, which is Ohms. Ohm's Law states that Amps = Volts / Ohms. (I=E/R) An anodizing setup is an electric circuit, as such it obeys Ohm's Law. In all things electrical; current (Amps) does the work, potential (Volts) serves to overcome the resistance (Ohms). Anodic coatings are formed by current, not voltage.

Relating this to anodizing. An electrical resistance exists when the anodic coating is forming, it is caused by the electrolyte being restricted by the porous anodic coating from reaching the base metal. As the coating grows, this resistance increases. his resistance sets the voltage when a given current is flowing. For 20 ASF, 9.4% electrolyte concentration by volume, and 70 °F., this resistance is about 0.95 Ohms per square foot. Its value is inversely proportional to surface area (2 sq.ft. = 0.475 Ohms). It varies a little with alloy as well.

BTW, "standard electrolyte" is 165 g/l by weight, which is 9.4% by volume. Your electrolyte concentration should be lowered, you are courting dissolution problems.

Ed. note: it is our sad duty to advise of the passing of Paul Yursis in August 2005.
Here is a brief obituary by Mike Caswell.

Thanks, Paul. Actually I think you mean Faraday's Law of Electrolysis rather than the Tafel Equation though :-)

Yes, I am familiar with this law, which says that 96,485 amp-seconds converts one gram equivalent weight of metal to or from ions. I know that you can tack the density conversion factor onto it, and end up with "1140 amp-minutes/sq. foot to deposit a thousandth of an inch of nickel" for example. The Metal Finishing Guidebook has a chart with those Faradays vs. thickness numbers for each different metal, but I don't do anodizing, didn't have the factor "720" for anodizing committed to memory, and didn't realize that it had a name. Thanks for letting me know of this practice!

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A. Hi, Maurice. Some things we know from 'first principles', the rules of electrochemical science. The "720 Rule" (at least as an approximation) is one of those things because it's simply Faraday's Law of Electrolysis multiplied by some conversion factors. That is, you multiply the current density in ASF (amps-per-square-foot) times the anodizing time and divide by a factor to get the anodizing thickness. Specifically, you multiply your ASF times your anodizing time in minutes, and divide by 720 and you will get the approximate thickness in mils.

But in some other areas, including what ASF range will generate satisfactory anodizing, our science isn't advanced enough to give us a number based on first principles; rather we still rely on empirical knowledge -- what we have found will probably work and what probably won't. ASF is empirical information rather than something we can calculate.

36 ASF is usually considered the maximum practical current density, with many experienced practitioners feeling that 12-24 ASF is the number you should use in production to keep out of trouble. You can search the site with the term "hardcoat current density" for many interesting discussions on that topic. Good luck!

Regards,

A. Thanks Azeem.

Let's see then --

According to your 0.3 rule,
A/dm2 x minutes * 0.3 = microns

And according to the 720 rule,
A/ft2 x minutes / 720 = mils

So let's try plugging your numbers into their 720 rule:
1 A/dm2= 9.29 A/ft2, so ...
1.5*9.29 x 30 / 720 = 0.5806 mils, which is 0.0005806 inches = 14.7 microns

We're pretty close, thanks :-)

Regards,

A. Hi Abdel-Moniem, I think there is a misunderstanding here. The resistance of the solution is not changing significantly; rather, the resistance of the part is changing because the anodized film which you are building on it is highly resistive/insulative, and gets ever higher in resistance as it builds in thickness.

That is why people recommend anodizing by current, rather than by voltage. Set your rectifier current to 12 or 18 amps/sq. foot, or whatever you find that you can succeed with, and then let the voltage vary as necessary to achieve it. Good luck.

Regards,

A. Hi Deepak.

Voltage will be a bit over 12 V depending on the alloy and the distance from workpiece to cathodes, but an 18 volt rectifier is a better bet. As mentioned, the current will be about 12-18 Amps per square foot (the lower end for hobbyist LCD approach). You need to calculate the surface area of the profile you will be anodizing, and recognize that 12 V or 15 V is the terminal or ending voltage. You need to start much lower to not exceed the 12-18 ASF when there is no anodizing on the part yet, such that it's still highly conductive. You gradually up the voltage. That's why it's generally better to use a rectifier with current control than voltage control.

Regards,

A. Hi Rahul. Paul Yursis did that for us already. Look above for his name. If you can't follow it, please quote something he said and explain what the difficulty is. Also, note that others, like Azeem Khan have expressed the same rule in metric units if you are more comfortable with that. Good luck.

Regards,

A. Hi Rishi. Although 2.334 x 10 ^-5 foot square X 15 ASF = 3.501 x 10 ^-4 Amps, I suspect that we're losing something in the translation :-(

Regards,

A. Hi H. Please try to get back to us with the details of your situation rather than an abstract question. Titanium racks or aluminum racks? What type of work? Are the parts alkaline etched?

Anodizing of architectural extrusions, as one example, is sometimes done on aluminum racks with no rack stripping at all, just fresh aluminum screws each cycle (which adds virtually no surface area to the calculation); although the area of the racks is large it doesn't count because it all stays anodized. But if you are anodizing the rack or rack tips as you anodize the parts, yes, whatever area you are actually anodizing is what counts in your 720 rule. Then too if you are doing a heavy etch it can strip the anodizing from the racks.

I think the bottom line is that the 720 rule can help you plan, but you can't use it with such careful nuance that actual thickness measurements become unnecessary :-)

Regards,

A. As Ted Says, "careful nuance". the 720 "rule" is practically approximate in a small range of parameters, but since the anodic film builds exponentially with time and thickness, it is not arithmetically accurate. Use nuance.

A. Hi Nas. If you are a hobbyist trying to familiarize yourself with anodizing, you'll find many of your answers on this thread, and all of them by searching this site (although bouncing between simple things you've heard a hundred times and things too complicated for you to study until you've picked up some intermediate knowledge isn't very efficient). Books are better for tutorial learning, if your library has any, because the author has spent months or years arranging it all in that you can learn in a proper sequence/progression.

But if you are planning on building a production facility you honestly should retain a consultant or at least someone with a good deal of experience. Although I stopped consulting a decade ago I did it for 25 years and must emphasize that it is the early decisions that really matter, not the ones made when it is too late for anything but a Hobsen's choice :-(
It sucks to be left with only enough room for 8 tanks if you ought to have 9, or to have wide shallow tanks when narrow deep tanks would have been better or vice versa. But early decisions include whether you'll run 40-hours / 1-shift vs. more shifts, what size range parts you will accept, whether you want to primarily serve architectural, automotive, aerospace, medical, or consumer markets, whether you'll take low markup commodity work or only the tough stuff, what ancillary services you want to offer such as precision masking, bright dipping, 2-step coloring, etc.

Luck & Regards,

A. There are several consultants on Finishing.com who can walk you through setup. If you send me an email I can at least send over initial information on what you will need.