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process for anodizing aluminum with limited equipment? Answered

Recently I have learned the basics of anodizing aluminum, and from what I understand the basic process is to clean of the aluminum bit to be anodized, stick it in a bath of a dilute solution of sulfuric acid, and do some electrolysis with the anodizing bit connected to the anode or positive of a supply, then pulling it out some time later and cleaning it off with soapy water and rinsing, and coloring the metal in a thin runny die, then wiping off excess and boiling the part.

So first of all, do I need sulfuric acid? Any substitutes? Will HCL work, or a weak acid like vinegar or even salty water?  If the purpose of the sulfuric acid is to allow free ions to flow in the water, I do not see why it would not work. However, I am not a chemist, and my experimentation with electrolysis is limited to 6th grade science and dropping 9V batteries in salty water. (My favorite experiment was at home when using copper  and coins as the electrodes and watching blue-green corrosion develop and sink until the copper wire was made brittle and thin. Caused some AA rechargeable batteries to explode in the process though lol!)

Also, will any die work? For really small parts, will it be possible to evenly die them with sharpie, standard paper markers, and/or highlighters? What dies will work well? Should the dies be water based, or based on some other thinner?


I was about to write a complete Instructable one day for this, but here is the short version:
Drain cleaner (sodium hydroxide) is used on the clean aluminium to remove the existing oxide layer as otherwise there will be no reaction in the galvanising process.

Titanium wire or marine grade stainless steel is required to hold the aluminium, while lead can be substituted for the other pole.

Sulfuric acid at battery acid strenght is the only thing that wroks well, phosphoric acid does too but the results are far from perfect (on a hobby level).

The time it takes depends on the size and current used.

Having a constant current power supply helps a lot to prevent using too much amps, which will ruin the process.

There are ways to calculate the exact amp rate depending on the surface area of a part but for hobby use this is not too important.

Once finnished the part goes into the cloring bath, aniline dyes work best, but some food colorings and printer inks work well to - one has to test this first of course to avoid parts with no color.

When the color is good the part goes into steam for a few minutes and then into the boiling water for about 30 minutes to seal the oxide.

Nickel acetate works best but I don't like it as it stinks and is not really healthy, so for my projects water it is.

Be aware that certain types of aluminium simply fail to color at a hobby level, cast aluminium, aluminium with a silica content and certain pressed/rolled aluminium parts for example.

With the later there is often the problem of imurities or oils worked into the metal that prevent the forming of an even layer of oxide, polishing those parts before the bath in sodium hydroxide can help a lot here.

When you say that some things work better than others, can you tell me what you mean by 'better'? As in a more even/thick coating? Less time for later steps, etc?

Certain type of aluminium emit "smuts", weird corrosion products, into the bath during etch out. I can't remember which alloys are bad, but 6061 is pretty good IIRC. There are chemicals called "De-smuts" which dissolve the smuts and leave a clean surface.

You know a lot about anodizing, what work do you do that involves anodizing? And would you by chance know about electropolishing aluminum again with some electrolisis thing? Applied Science did a video on that a while back.

I owned a company which designed and made scientific instruments. We would make all our own parts, and I would often anodise them in house, because they were small and in very low quantities. I don't know anything about electropolishing....

Most important is good cleaning, first with acetone and than in sodium hydroxide solution - the concentration is not too critical.
Gloves are a must unless you handle the parts with stainless steel tools (or others with rubber over the surface).
Good contact is next step.
If is can use a stainless steel bolt and a tap the aluminium with a thread, otherwise use 3mm titanium wire and bend it so it holds the aluminium like a clamp.
I have seen people just hanging the parts on the wire - the result is useless as the forming oxide is an insulator, the reaction stops before there is enough oxide produced.

A clean acid bath is important too, I use a small fish tank pump with filter in mine.
Never let any iron or other metal get into your acid it will ruin the process.

I usually don't bother with calculations for my small parts and start at 2A, for bigger parts at 3A.
If the current does not drop significantly within 30 min than I reduce slightly.
The thickness of the oxide layer depends on the time and current used in the acid bath.
I noticed that for good coloring slightly higher amps are better, while for a very thin but strong protective layer small amps work better with more time.
You will see the anodizing is complete when your current drops to zero.

As with the oxide itself on certain types of aluminium....
I prefer to do a test with a scrap piece first to find out what works best, after all a bit of sodium hydroxide bath removes the oxide quickly to start over.
Pressed aluminium, like tubes or square profiles can be tricky due to oil and metal conamination, a slight snading and polishing can really help here, especially if you sand wet with acetone.
During the hydroxide bath on non colored aluminium you might want to check with a colored scrap piece - if it is back to silver the other one should be too.
Double check with a multimeter: Only with slight pressure and without using the pointy tips you should have a short wherever you check ther alu.
Once you have done that a couple of time you get an eye for the right time.

Sulphuric, or phosphoric, HCL poisons the process.

Battery acid is just right, strength and type.

The acid must be cold, and kept cold - we dropped icepacks in our tank, otherwise the dissolution of the anodic film keeps up with the deposition of it.

Use a lead plate for the cathode, and unless you have access to titanium wire, lead worked as a connector to the job.

Everything must be spotlessly greasefree.

NO SOAPY WATER on the anodised parts. Rinse in distilled water, let DRY completely DO NOT heat to dry.

Dye ? We used a commercial anodising dye, but a strong solution of clothing dye works well too.

Failing a tank of nickel acetate as a sealer, then steaming the job in distilled water works well.

Let me know how the sharpie idea pans out - I wanted to try it too.

"Sulphuric, or phosphoric, HCL poisons the process."
What about NaCL? or other commonly available salts?

"Use a lead plate for the cathode"
Will some old 60/40 solder work?

"steaming the job in distilled water works well."
This guy used a big pot of boiling water and some solution sealer thing mentioning water alone will also do the job. He did also did use sharpies, but only to make marks, rather than on the entire piece.

I mean a solution of salt in water. Salty water, instead of battery acid. Battery acid is nasty stuff, and I would have to acually buy some.

SAlty water and aluminium don't mix.

Battery acid should be easy enough to source - ask a garage if you can decant some.

And you need to etch the aluminium with Sodium hydroxide anyway.

Lye?? Where does it come into the process?

You lightly etch the surface to clean it, and remove impurities.

So that is basicly for the cleaning step before anodizing. Got it.