First Aluminum Anodizing attempts (long w/ pictures)

Neato!

Ah, must be in a red bottle? Or possibly a store brand.

Yep, sulfuric acid makes a nice amount of heat when dissolved. Taking precautions, I've tried to make it splatter to see if it happens, but so far it's just gotten warm, no boiling yet. Given the amount of heat it makes, I don't doubt it can and will splatter, I just haven't seen it yet.

*shrug*

I would say until the current stops.

Indeed, the same process is used in millions of electrolytic capacitors. You're using some right now, inside your computer and monitor, indeed they are almost ubiquitous with electronics. These consist of a coil of aluminum foil and paper, the paper being soaked with a proprietary electrolyte which has low resistivity. (Apparently, exact composition is a tightly guarded industry secret.)

Anyways, these capacitors degrade with time if unused - the oxide layer decomposes and the voltage rating drops. Instead of throwing them out, you can "reform" them by applying a small current which anodizes it (slowly so as not to overheat it). After a long time, maybe a few hours if you are impatient (depends how hard you push it), it'll be back up to rated voltage. Electrolytics go up to 500V, you can imagine how thick that anodized layer must be :-)

Fair enough. You can also start at a low voltage, say 1-5V range, and use a variac or adjustable power supply or steps of voltages to climb up to say 20 to 50V. To make a more consistent deposit you'd want a current-limited supply: set it to top out at say 30V, supplying a constant 1 or 20 or 50 amperes (for however big your workpiece is) all the while until it gets there. Consider two things: electrons (and thus current) flow as long as there is enough voltage to overcome whatever (namely, the oxide layer), and each number of electrons (namely, something like 26.7Ah per mole, aluminum needs three) oxidizes a certain amount of aluminum such that it forms the anodized layer. You can see that 1. it will take a certain number of electrons to get to a given voltage, and 2. it takes a certain voltage for a certain total transfer, or whatever.

I would guess hard anodizing is as high as 100V, unless there's a different chemistry at work in that process. As I said, you can theoretically go up to 500V, and personally I don't see any reason why you can't go beyond, except for practical reasons of course.

That long probably isn't necessary, although I doubt it hurts anything. I once was messing with a similar weak sulfuric acid solution myself, something I tried was anodizing aluminum, I hooked it up and immediately the power supply grunted with effort as probably 20-30A flowed through the small fragment, then within a few seconds, it diminished until I couldn't tell a difference in the current level, dipping and removing the piece to check. That was something like 6V.

Sounds about right.

I can't comment on the dyeing...

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

What you need to do is find the process description for decorative anodizing. There's literally hundreds of different processes. At one time I ran process control in an anodizing plant where we did decorative anodizing. IIRC, we tried to keep the concentration of acid down below 10%, if it got above that, the oxide cells started getting soft and flimsy and didn't soak up the dye. The temperature had to be controlled, too, too hot and the same thing happened. We tried to keep it as cool as we could, usually below 80 degrees, when the chillers or circulation pumps went down, we were out of business. The dye we used ran about $50/pound and was specially formulated for the process. Dying usually took about 15 minutes if the concentration was correct, pH was important on that bath. After treatment was immersion for about

30 minutes in hot nickel acetate solution, pH was adjusted with glacial acetic acid. The power supply was a constant current one, topped out at 10000 amps. A full rack of extrusions usually ran about 6000 amps if the acid concentration was correct. I have no idea what the voltage ran, current was what was specified in the process manual, so much per square foot for each different process. The tank was lead-lined, the racks were titanium.

Alcoa used to have a book on the different processes, I've seen it on the shelf in some university libraries.

Stan

Last night after posting, I found this site, which is hands down the best descriptive site I've found to date:

I may be experiencing both problems, too high an acid concentration, as well as excessive heat.

I wonder if using my Miller 180SD welder in DC stick mode as a constant current power source would violate any warranties?

One other question, since I don't know the pedigree of the material I made that threaded adapter out of, what if it wasn't aluminum, but actually magnesium? (The two materials are difficult to tell apart, yes?)

Take Care, James Lerch

(My telescope construction, Testing, and Coating site)

Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge

I seem to recall that there are some aluminum alloys that do not anodize well.

Jim

Anodizing electrolyte: battery acid from NAPA (about $5) diluted 3:1 (or less) with distilled water.

Temperature: cold is better but room temp works OK.

Current: about 30 mA per square inch for about 90 minutes. An anodize film made at higher current density or higher temperature doesn't accept dye as well. Voltage is typically about 12 volts but make it whatever it needs to be to get the current you need. Battery charger driven by a Variac, electronics bench lab supply or simple home brew. I use a dirt-simple adjustable current source, email me if you'd like a schematic. You could build one with $10 worth of parts from Radio Shack. Then you don't need a variac -- or a handful of fuses for when (not if) you get shorts and blow the fuse in your multimeter being used to measure current.

Be sure you have a good tight connection that will stay connected for the duration.

After anodizing, check work with an ohmmeter. It should show no conductivity.

Dye: RIT works OK for most colors except black. For black, best results are had with real anodizing dye. It's available in small qty from Caswell Plating and perhaps elsewhere. Dye at 140F for

20 minutes or so.

Seal: just boil in distilled water for 30 minutes or so. There are sealants available, but just boiling in water seems to work OK.

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I posted this URL in 2001. It is still active and has good info on anodizing aluminum.

No, not really. Magnesium is much lighter and machines totally differently. It crunches as it cuts, and is quite abrasive, dulling HS tools fairly quickly. 7075-T6 aluminum displays similar machining qualities, but even it can be discerned from magnesium once you've machined it.

From your description, I think what you did was hard anodize your part. The color is a dead give-away. As far as I know, it won't accept a dye finish, regardless of your attempts. Check it with a file on a corner that isn't critical and see if it files. Hard anodizing is much harder than a file, so it won't touch it.

Gorton used to hard anodize the drive pulleys on their mastermils, the bottom of which was also the spindle brake. They'd run for years in an industrial setting before needing to be turned and re-anodized. Hard anodize, unlike other anodizing, can be applied quite thick, and from your description of the amperage falling off, I think that's exactly what you accomplished.

I'd love to hear the results of the file test, assuming you're willing to perform it.

Harold