First Aluminum Anodizing attempts (long w/ pictures)

Greetings All,
Before I tell the story, here's a link to the results:
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I was wandering thru the local hardware store when I found a bottle of
concentrated sulphuric acid, which happened to be labeled as "Drain
Cleaner"...... I chuckled a little as I thought to myself "That's not
drain cleaner, that's anodizing electrolyte" :)
Anyway, I picked up my quart of sulphuric acid, and a liquid bottle of
"RIT" black dye, inconveniently labeled as fabric dye.
Back at the house, I filled a 1 gallon plastic bucket half way with
tap water, and stuffed an aluminum baking tin down one side of the
bucket and attached it to the ground leg of my batter charger.
Next I added the electrolyte (maybe a pint) to the gallon bucket half
full of water. The decision on how much electrolyte to add was based
solely on the feeling of unrest that came as the bucket of solution
started getting REALLY warm (maybe 120F).
After that I took this little aluminum threaded web cam adapter I made
in the morning, and attached it to a piece of aluminum filler wire.
The threaded adapter and filler wire went into the ultra-sonic cleaner
for a few minutes while I went and re-read the instruction for
anodizing I found on the web.
About 10 minutes later, feeling full of confidence, I pulled the part
of the ultra-sonic cleaner and gave it a quick tap water rinse. Then
I hung the part in the still rather warm bucket of electrolyte, and
connect the positive cable of the battery charger to the aluminum wire
holding the part.
With a little trepidation, and the 12v batter charger on the 10 amp
setting, I plugged in the battery charger. Both the part and the
aluminum attached to the ground leg starting "Fizzing", and the
battery charger gauge was pegged (BTW, the battery charger lacks an
actual amp gauge, but instead has a silly gauge that instead of being
labled in amps, is labeled in percent battery charge)
At this point, I know I'm doing something, but the question is how
long should I let this run? In my research I've read conflicting
answers to this question. Some sites said 10-15 minutes for small
parts. Other sites said to let it run until the current flow pretty
much stops, as the aluminum oxide which makes the anodizing coating is
non conductive, so a fully formed coating shouldn't conduct..
Not knowing how long to let the process run, I went with the "More is
better" theory and let it run till the current dropped off and the
bubble formation was nearly non existent.
While the part was "Cooking", I went and mixed up the dye solution. I
used about 1 quart of water to half a bottle of dye. (again, staying
true to the "More is better" theory)
It took about 40 minutes for the part to "cook" completely, IE very
little current flow, and very little bubble formation. I then removed
the part, and gave it a cold water rinse. At this point the part had
a gray / green tint, and lost ALL the luster it had prior to being
After the rinse, I put the part in the dye solution, and stirred it
around a few times. About 10 minutes later I pulled it out, and it
hadn't really changed color. So I let it set for another 10 minutes,
still no color change. Having read that some alloys of aluminum need
a little heat to help the dye penetrate, I gently warmed the dye on a
hot plate, and let the part soak for about 30 minutes.
30 minutes later I pulled the part out of the now warm dye solution.
Now I had a speckled black part, but mostly it was unchanged.
Thinking that since a little heat helped, more heat should help even
more, so I turned the hot plate up.
Now I had the part sitting in a gently boiling solution of black dye,
which thankfully I had the fore thought to do out in the shop,
especially about the time the dye foamed up like boiling milk and
attempted to make a mess out of my already messy shop.
After another 30 minutes of bouncing around in the boiling dye
solution, the part pretty much looked the same as it had earlier, so I
turned the heat off, and wondered where I'd gone wrong.
As an experiment I found a scrap piece of aluminum angle laying on the
floor, and decided to try anodizing it. This time however, I decided
to only "Cook" it for 10 minutes. Amazingly this scrap piece came out
nearly perfect, as seen in the pictures!
So the question is, why did my threaded web cam adapter fail?
Here' my current thinking, which I hope to explore tomorrow after
cleaning the part up on the lathe.
#1 The electrolyte solution was rather warm when I hung the part in
it. perhaps the warm electrolyte was sealing the part as the anodizing
coating was being made. By the time I tried coating the scrap piece
of angle, the electrolyte solution was nearly room temperature.
#2 I "Cooked" the part too long. 40 minutes may have been WAY too
much, and perhaps caused most of the pores in the anodizing layer to
become too small to accept the cheap dye I used.
#3 The unknown alloy I used to fabricate the threaded part may not
like the anodizing process, or the selection of dye I used.
#4 Some combination of the above.
BTW, the wire brush marks near the threads on the web-cam adapter were
done after the part had been boiling in the dye for 30 minutes. For
some reason the dye sort of "Chunked up" on the sides with the
threads. I couldn't wipe the chunks of dye off, so I tried it with
the wire brush, which knocked the big stuff off...
BTW #2, I hope and pray that I can make the web cam adapter look as
good as that stupid piece of scrap I did, because I'm amazed how well
that piece of scrap turned out!
BTW #3, as an experiment, I tossed a piece of the same scrap angle
into the dye, without "Cooking" it first, and the dye just wiped off
In any event, It was a pretty good way to spend a day off :)
Take Care,
James Lerch
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(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
Reply to
James Lerch
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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...
-- "California is the breakfast state: fruits, nuts and flakes." Website:
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Reply to
Tim Williams
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.
Reply to
Last night after posting, I found this site, which is hands down the best descriptive site I've found to date:
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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
formatting link
(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
Reply to
James Lerch
I seem to recall that there are some aluminum alloys that do not anodize well.
Reply to
jim rozen
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.
Reply to
Don Foreman
< a whole bunch of stuff deleted>
I posted this URL in 2001. It is still active and has good info on anodizing aluminum.
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Reply to
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.
Reply to
Harold and Susan Vordos

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