Anodising a Small Aluminium Part

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Introduction

Anodising is a process whereby a coating of porous aluminium oxide is formed on the surface of aluminium. This coating is then dyed a chosen colour and the oxide layer sealed, thus permanently colouring the metal and leaving it with a hard, corrosion-resistant finish.

Anodising does NOT hide blemishes; the final result can be no better then the finish on the metal. A matte (shot blasted) finish will result in a matte finish when anodising is complete.

The written description of the procedure makes it look a lot more complicated than it is. It's really quite simple.

In Summary

1. - De-grease and remove oxide or old anodising. The best time to anodise is as soon as machining or polishing has finished and before the surface oxidises.
2. - Anodise.
3. - Dye and seal in the colour.

If you read through a few times all should become clear just don't let the detail bog you down at first.

I strongly recommend reading the web pages listed below for background. They will fill in a lot of blanks and give you confidence. Read the safety pointers with care.

Reagents

Caustic

Spray Oven Cleaner.

Acids

Hydrochloric Acid (Spirit of Salts) about 30%w/v - Drain cleaner.

Sulphuric Acid - Fresh car battery acid, not from old batteries. (Dilute before use - 2 parts of acid to one part of water.)

Safety Note. Always add ACID TO WATER, never water to acid. Add a little at a time and stir with a plastic or glass stirrer to assure complete mixing.

Other

Sodium or Potassium Carbonate - Used for neutralising acid spills.

Inkjet ink - A small bottle of refill ink or cut open an ink cartridge. (Note. Take care as it goes everywhere and it's permanent on a lot of surfaces.) Take about half a bottle of ink or the contents of a cartridge and dilute to about 3/4 pint with water.

Equipment

A length of aluminium welding rod or aluminium wire - Use what you can get most easily.

Aluminium strip - The dimensions are not critical. I used a 1 by 1/16 by 24-inch strip, which I cut to size. If you are desperate you could use aluminium foil.

Power supply - A 12V DC 4A car battery charger. (A 24V supply with higher current output would be better.)

Anodising bath - The only metals that should be used for the bath are lead and aluminium. No other metals should be in contact with acid during the anodising process. I used a miniature plastic crate; usually quality plastic containers are best. Most plastic will be unaffected by acid but TEST the plastic before use.

(Testing: - Working in a sink, pour a little acid into the container and observer the results. If the container is damaged discard it.)

Old tooth brush.

Some bits of electrical insulation, stripped from the copper cores.

Two crocodile clips linked by an appropriate length of cable. The length depends on the size of bath used.

Safety

The process requires the use of a range of chemicals and electricity. Failure to use protective equipment is likely to lead to serious injury.

Personal Protective Equipment (PPE) – Use the following; rubber gloves, plastic apron and eye protection.

Recommended actions

Acids and caustics: - This includes the oven cleaner.

In the event of skin contact - wash off with copious amounts of water.

If acid or caustic get into the eye - flood with water and seek urgent medical help.

If acid or caustic get into the mouth - flood with water and seek urgent medical help.

Spillage and splashes - wipe up immediately, if required neutralise acids with sodium or potassium carbonate.

Ink: - Use PPE as above.

Note. Ink gets everywhere; boiling ink gets everywhere else!

Electricity: - Take care to prevent liquids getting onto the battery charger. Take care to avoid electric shock.

Procedure

Safety - Use PPE throughout this procedure. Safety gear should stay on until the parts are coloured and cool and all chemicals are made safe.

Try to work right through without any delays, at least until you have got the parts in the ink solution.

Preparation of the Anodising Bath

1. - I used a double bath with ice packs and chilled water between the inner and outer bath. Anodising is temperature sensitive and is best done cold.
2. - In opposite corners of the inner bath I put lengths of aluminium strip to act as cathodes. These strips extended to the bottom of the bath and were linked together with cable and crocodile clips.
3. - Do not switch the charger on but connect the black -ve terminal of the charger to one of the aluminium strips.

General notes:-

De-greasing is of paramount importance, the oven cleaner will do it but after the part is de-greased it must only be touched with gloves. The oils on your skin will contaminate it.

Stripping the parts with caustic or acid can cause pitting, so do not continue for too long.

I did the stripping in a plastic bowl in a sink. This enabled me to wash away spills safely and easily.

Preparation of the Part

SAFETY Note. - Always scrub in one direction only, away from your body. This will reduce the chances of your being splashed with caustic or acid.

1. - Twist short lengths of electrical insulation into any grub screw holes to protect the threads.
2. - Spray the part with oven cleaner.
3. - Scrub the part with the toothbrush making sure you get into all corners and around any grub-screw holes. The caustic will remove the oxide layer and any old anodising. A black layer may form on the metal.
4. - Rinse with water.
5. - Transfer the part to the hydrochloric acid solution and scrub the entire surface of the part. If a dark layer has formed be sure to scrub it off.
6. - Rinse with water.

Electrical Connections to the Part

Note. It is essential that a good electrical connection is made between the part and the aluminium rod or wire and that this connection is maintained through the duration of the anodising process. If this connection is not good or if it fails then the process will fail!

1. - Cut the Aluminium rod or wire to an appropriate length. This will depend on the size of the bath and how the part is to be supported.

2, - Force/screw one end of the aluminium rod or wire into any small hole in the part, preferably in a place that will not be visible after anodising. It may be necessary to flatten or bend the end of the rod to get a good tight fit (see "Other Notes" below). The place where the rod is touching will not be anodised.

Anodising

Safety - This step requires very good ventilation. Hydrogen is liberated from the bath and this is both explosive and noxious, so make sure there are no sources of ignition, flames, sparks or cigarettes nearby. Be careful not to short out your charger by allowing the terminals touch or allowing the part (anode) to touch one of the cathodes. I worked in a room with an outside door open.

1. - Hang the part in the bath in such a way that it is not touching the bottom or sides of the bath; nor must it be very close to either of the cathodes.
2. - Connect the Red +ve terminal of the charger to the aluminium rod.
3. - Fill the bath with the dilute battery acid until the part is covered.
4. - Switch on the battery charger and anodise for about 60 minutes. Gently shake the part from time to time, as this will remove any bubbles that are clinging to it.

What you should see

When you switch on the charger the current flow should be quite high, most of the charging lights on a 4A battery charger will be on. The current flow should fall away slowly and the charging lights go out one by one. This reduction in current flow is caused by the formation of the oxide layer on the metal. Oxide is not a conductor of electricity. Bubbles will rise from both the anode and the cathode. Try not to inhale the vapours and make sure there are no sources of ignition nearby. After a time the part should become an even grey colour.

5. - Switch off the battery charger, remove the part from the bath and rinse with water. Remove the welding rod from the part.

Dyeing the Part

Remember - Ink gets everywhere; boiling ink gets everywhere else! If you can complete this part of the process outside, so much the better.

1. - Put the part in to the ink solution for at least twenty minutes, the ink solution must entirely cover the metal. This the first time you can suspend the process, the part can actually stay in the ink for some time, overnight or longer.
2. - Transfer the part and the ink to an old pan, bring to the boil and simmer for twenty minutes. This seals the oxide layer trapping the ink into the surface of the part producing a permanent and durable finish.
3. - Remove the part from the ink solution, rinse it and allow it to cool.

Finishing

1. - Polish the part using a good quality waxy polish. Some colour will come off at first but you should end up with a nice semi-matte finish.
2. - Clear up and wash down all surfaces. Return the reagents to the storage containers or dispose of them appropriately.

If you have read through this a few times and are still less than fully confident that you can carry out the procedure both safely and effectively, have the anodising done by a specialist!

Other Notes

Some types of aluminium work better than others.

Bending aluminium welding-rod. This stuff will snap if it is just bent over on itself. It can be bent over in stages, softening it and then bending it some more. To soften, heat the rod in a flame and when hot immediately plunge it into water to cool.

You don't have to use black dye as lots of colours work, it just depends on your taste.

All the concentrations of the reagents are approximate.

If you have not worked with chemicals and are concerned about using them safely get and read the appropriate safety data sheets.

Always dispose of chemicals safely.

Web Links

There are a number of good sites out there. They can be found using search engines.

I recommend that at least the following web pages be read through.

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

Anodising Aluminium:- http://freepages.pavilion.net/users/nickfu...ull/anodise.htm

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Disclaimer

In summary:

• -You break it; you get to keep both parts.
• - You damage something; it's your problem.
• - You hurt yourself or someone else gets hurt; it's your problem.
• - If you cannot make it work; it's still your problem!

This procedure involves the use of chemicals and electricity and so must be considered hazardous. Although I have tried to outline a safe procedure you may only use these notes at you own risk.

Neither I, nor anyone else will accept any responsibility for any loss, damage or injury that may occur as a result of your making any use whatsoever of this procedure.

Copyright remains the property of the Author.

This document may be freely reproduced and distributed on a non-commercial basis. Under no circumstances may the disclaimer or the comments relating to safety be removed.

© Ora8i

January 2003