h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I am curious how to anodize some aluminum parts I have, not really for added durability, but more for color and appearence. I have heard of anodizing-like dyes that you can buy and use, but I don't know anything more! Please help!

Stand back! Metallurgist coming through!

As you may know, anodizing is a surface treatment, principally for aluminium, but also used on zinc and nickel, which increases surface hardness by up to a thousand times, as well as painting it a nice pretty colour. Anodized parts are, however, particularly prone to cracking under tensile strain.

It's generally not advisable at home, due to H&S hazards, but can be done. You will need a car battery and some sulphuric acid (plus safety equipment and a suitable inert watertight tank) in addition to the dyes, which makes it a bit of a stressful pastime.

The basic procedure is to turn your part into a cathode, which then attracts the dye which has been prepped with sulphuric. Some complex bonding goes on and hey-presto, you have a spanking new anodized part.

All in all, it's only a good idea for highly specialised bits, or on a mass production scale. Buy some paint instead.

B

Or
If your lucky you might find a local firm that will do one off parts for you.
If you want them the same colour(s) as they are using for production work.
I once got some parts for a motorbike done that way.

I think you will find it essential to connect your workpiece to the Anode (+ve) rather than the Cathode (-ve). It is Anode from which the term ANODize derives.

There is a basic method described here:

http://easyweb.easynet.co.uk/~chrish/t-anodis.htm

I've not tried it but it sounds feasable.

Be very careful if you are working from conc sulphuric acid. Always add acid to water (slowly & stirring well), never water to acid. Also, if you get sodium hydroxide in pellet form be careful of the dust. If you get NaOH dust in your eyes you are in no end of trouble.

Have Fun!

Ugi

Yeah, but has anyone ever heard of the way to "dye" aluminum? It isn't really anodizing, but it puts a nice color to the part... Maybe I am wrong, but...

I think the dying is part of the anodizing process - the dye is incorporated into the oxide layer that you form by anodizing. That's how a 70s aluminium kettle etc gets its colour!

The guy using the method at the end of that link is doing it to dye his aluminium telescope parts black - you can apparently use all sorts of water-soluble dyes.

"I think you will find it essential to connect your workpiece to the Anode (+ve) rather than the Cathode (-ve). It is Anode from which the term ANODize derives"

Incorrect!

The anodizing solution is the anode.

B

I don't want to turn this into a rant, but I have to pick you up on that MB...

To quote from the "New Penguine Dictionary of Science", which happens to be the closest book to hand:

"Anodizing A process for increasing the width of the protective layer of an oxide coating, especially on a piece of aluminium, by making it the anode in a electrolytic cell usually containing sulphuric acid..."

You're forming Al2O3 from Al metal in the reaction, so Al -> Al^3+.

2Al + 3H20 -> Al2O3 + 6H^+ + 6e^-

The Al must be losing electrons and this can only happen at the anode because that's where they can go to. The cathode is lousy with electrons already so it's never going to accept any more.

The cathodic reaction will be 6H^+ 6e^- -> 3H2, balancing your redox and your acidity.

Your move...