Home anodizing-Truths from the experts?

A. 12 V is not high enough. You need 15 - 18 V.

Q. Sorry I have 2x 12 V batteries, so 24 V

Another question: I have mixed 4 liters of 32% battery acid [affil links] to 20 liters distilled water is this correct?

Cheers, tony

A. Hi Tony. It's much too weak. You are looking for 15% by weight, which means that a 1:1 mix of 32% battery acid and distilled water would be quite close.

Regards,

A. Hi David. Nothing wrong with youtube videos, I watch them all the time. But I see conflicting youtube info on the simplest stuff like replacing washers on shower faucets, whereas anodizing is an industrial science with 1500-page textbooks, at least two trade associations / educational associations, annual conferences, and people who have devoted their entire careers to it. Obviously it's tough to condense such a subject into a quick video or a couple of paragraphs of text.

Nevertheless, a good amperage for standard anodizing (as opposed to low current density hobby anodizing) is about 12-18 ASF -- which means 2.25 Amps for your 18 square inch part and 7.5 Amps for your 60 square inch part. At the beginning of the cycle it will take little voltage to generate the 18 ASF; as the anodizing builds and the conductivity of the surface drops, the voltage will climb to 12-15 Volts if you hold 18 ASF.

The 720 Rule will tell you how long to anodize for depending on the thickness you want. It takes 720 Amp-minutes / square foot (or 90 Amp-minutes for an 18 square inch part) to build 0.001" thick anodizing. If you anodize at 12 ASF, that's 60 minutes; at 18 ASF, that's 40 minutes. But 0.001" is pretty thick ... you probably want half of that.

Your acid is probably okay for casual use. How do you know you are not getting any anodizing, or that the material isn't already anodized? And what is happening? What do you see? How much current is flowing? Do you see gas bubbles on the anode and cathodes?

Regards,

A. Ted's advice is sound, David. One of the things he stated is very important, and that is are you sure the part isn't already anodized (some very thin coatings can be hard to see). A simple continuity test with an ohmmeter can confirm. Touch one lead to your Ti rod, and the other to your part, and see if you have continuity. If not, your piece is coated with something, and that coating needs to come off. You can also use a continuity test to see if you're being successful in your attempts to anodize.

Your power supply sounds adequate for the parts you're trying to coat. If you coat per Ted's advice, in approx 20-25 minutes, you'll have grown approx .0005" of coating, which should be fine for what you're trying to accomplish. Set you're voltage at 30, and your amperage at 0. Turn the unit on, and slowly (around 2 minutes) increase your amperage to the appropriate amount for the work you're trying to coat, in this case, 7.5 amps.

The other thing I noted was your acid concentration. You should be around 15%-20% of acid to water. The way you described your set up, (1 gal of water to 1.5 gal of concentrated acid)is way too strong.

You also mentioned "coloring". That's another step done after the initial oxide is grown. I think home anodizers have had success with RIT dye. And finally, there is a sealing step done, which can be accomplished in your home with boiling water (distilled).

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Ed. note: We have a vested interest, but we suggest Robert Probert's "Aluminum How-To" .

A. Hi Riido. Starting from the top of your posting --

Professionals use inhibited chemistry for cleaning, not straight NaOH. Try cleaning the parts by scrubbing with powdered pumice and rinsing first, then dipping into your NaOH for just a couple of seconds, using it strictly as a minimal etch, not a cleaner.

I'm personally not familiar with anodizing in sodium bisulphate and would suggest 10% by volume / 15% by weight sulfuric acid instead. You can probably add battery acid about 1 part to 1 part water for this.

I'd try one part at a time instead of all four until you've learned a bit more. I figure they'll draw about 10 Amps each.

Ideally you should have anodes on both sides rather than one side and the bottom, but I don't think that's one of your major problems.

24V is way too much starting voltage; or you need to limited amps; a battery charger isn't very appropriate for anodizing.

Your aluminum mig welding wire isn't cutting it for carrying the amperage. Maybe take some of that excess aluminum anode sheet and try to "wire" up one spacer with it.

The rainbow coloration probably indicates that you have an anodizing layer of partial wavelength thickness, probably about 1/10 to 1/20 of what you need. Good luck.

Regards,

A. Hi again. Battery chargers are not great power sources for plating or anodizing. One problem is that raw aluminum is highly conductive (you're even using aluminum wire to carry power to your spacers) but anodized aluminum is highly insulating / very high electrical resistance. Since A = V/R, too much current can flow in the beginning, causing burning, and too little in the end, so the coating never gets thick enough. Ideally you either start with low voltage and ramp it up slowly, or you anodize at constant current. But if you determine the maximum current the charger can put out, and compare it to the 10 Amps per spacer which I estimated, maybe you can get by.

Whether you can practically bend your anode material to make a good connection, I don't really know. But what you could do is go to an electrical store or building supply store and buy the right kind and gauge of aluminum wire. In the USA aluminum wiring is used on service entrances to circuit breaker boxes (but not in house wiring). If it's the same in Estonia, maybe you can find 10-gauge aluminum wire instead of your MIG welding wire. Of course, 8-ga or 6-ga could be used if you can't find 10-gauge. I don't know how you'll avoid "rack marks" where the wire attaches to the spacer, because you need a solid, fixed, connection because otherwise the wire and the spacer will anodize at the connection point and you'll lose contact.

Regards,

(courtesy of www.build-electronic-circuits.com)

A. Hi Mitchell. You've got a smart rectifier there, which is doing what it's supposed to, lowering the voltage to maintain constant amps so you don't burn the parts when you first start and resistance is close to zero. Although you setting it to 20 volts instead of about 12 volts seems quite a bit too high. What alloy are you anodizing?

The electricity is splitting H2O into hydrogen at the cathode and oxygen at the anode, so you should expect twice as much gassing at the cathode or more. You didn't mention the size of the parts, but if you are trying to anodize at 12 - 18 ASF, then a 5 A rectifier can only anodize a total surface area of about 5/18 to 5/12 of a square foot (counting both sides), say a single piece 5" x 5", so a 200 L tank sounds pretty big.

Regards,

A. Mitchell,
Use amps to anodize, not volts. Go with Ted's advice of 15 - 18 ASF. Acid concentration, aluminum content, anode to cathode distance affect resistance which in turn has an effect on voltage for a given current. Set your voltage knob to Max, and control the current to achieve the desired current (density).

Your terminology suggests you may have the polarity reversed. The lead slab is the cathode and the aluminum part is the anode. Positive terminal goes to the anode.

A. Hi Willie. Thanks for catching what I obviously should have caught, that the anode (the aluminum workpiece) should be connected to the positive lead :-)

Regards,

A. Hi Mark. Have you successfully anodized thicker materials? Trying to run before you walk might be a mistake :-)

Thin sheets can warp if the anodizing is thick -- topic 44542 addresses a problem with warpage of 0.040 thin wall tubing, and topic 50143 talks about warping of thin sheets. To do any anodizing you must get current to the work and, yes, with thin sheets careful design of multiple contact points will be very necessary; in his October 2, 2018 posting on the latter thread Robert Probert suggests a method of study. Best of luck.

Regards,