Anodising Aluminium
Properties of Anodised Aluminium:
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hard scratch resistant surface - aluminium oxide layer
oxide layer does not conduct electricity
oxide layer is porous and can be dyed a wide range of colours.
The Process of Anodising in outline:
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an oxide of aluminium is formed on the surface of the aluminium after the aluminium is formed
to shape - by turning rod, bending sheet, polishing, texturing a surface etc.
the oxide is formed by immersing the finished object in a bath of dilute sulphuric acid and
passing an electrical current through the acid. The object is the ANODE (connected to the
POSITIVE of the electrical supply) and an aluminium plate forms the CATHODE (connected to
the NEGATIVE of the electrical supply) This produces oxygen at the surface of the object to be
anodised which combines with the aluminium to form aluminium oxide.
a voltage of 12 Volts between the anode and cathode is required.
the longer the current is passed, the thicker the oxide layer will be.
the larger the surface area of the object the greater the current that will be drawn from the
supply.
Practical Anodising - what you will need:
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a container for the acid - a 4 litre plastic ex ice cream container is ideal
a 12 volt DC power supply - a laboratory type power supply (current is usually limited to
about 4 amps maximum and would be okay for objects up to about 5 sq. inches surface area)
or a car battery charger delivering up to about 20 amps for larger objects (check current
supplied, varies greatly depending on the model )
Sulphuric acid about 2 M strength. About 3.2 litres to part fill the container. (SEE HEALTH
AND SAFETY NOTE)
aluminium sheet to form the cathode - about 1mm thick and 50 mm wide, long enough to
be shaped to fit the container (down the side, along the bottom and up the other side - a U
shape)
an aluminium rod to suspend the object to be anodised over the tank - 6mm dia bar or sheet
bent to form a bar.
aluminium wire to suspend the object from the bar into the acid - aluminium welding rod or
aluminium wire about 1.5mm to 3 mm dia or a strip cut from sheet aluminium. (a scrap yard is
a good source of suitable aluminium)
insulated wire to form the electrical connections from the power supply to the anode and
cathode.
the object to be anodised must be chemically clean, this is achieved by dipping the object in
a 1M Sodium hydroxide solution for about 30 seconds. (SEE HEALTH AND SAFETY NOTE)
Sodium hydroxide is also known as Caustic Soda and can be obtained from DIY stores
in pearl form - it is used as a paint/drain cleaner. In practice, about 20 grams (4 level teaspoons) added to 1/2 litre of water will give a working solution.
ANODE
Positive 12 volts
CATHODE
Negative volts
SULPHURIC AC
ID
CORROSIVE
4 litre container
It is very important to get good
contact between the suspension hook and the object to be
anodised. Best method is to
drill a hole (or use a hole) and
‘screw’ in the aluminium wire
bent into a U shape at the end,
or for a smaller hole,
hammered into a fishtail shape
and then ‘screwed’ in tight.
Anodising Process:
1. Fix the aluminium hook.
2. Clean the work by brushing using a nail brush with water and washing-up liquid
3. Immerse for 30 seconds in the Sodium hydroxide solution to chemically clean - do not touch
with fingers - wash under the tap - do not touch with fingers, hold by the hook.
4. Immerse in the tank and fix hook over the anode bar - make sure that the acid covers the work
and the work does not touch the cathode.
5. Switch on 12 volt supply - if the process is working you will see gas bubbles given off the
cathode.
6. For a thick oxide layer leave for 70 minutes, for a thinner layer 35 minutes will be satisfactorythe thicker the oxide the deeper the colour will be when dyed.
7. Remove and wash in water - do not touch, hold by the hook.
8. To finish the work it can be left natural aluminium colour or dyed.
Finishing the work:
The anodised layer is porous and can be dyed a range of colours using water soluble dyes.
Dyes that work well are as follows:
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Dylon’s multipurpose cold water clothes dyes. Kingfisher Blue and Emerald Green work
really well, other colours do not. Mix about 1/3 of the can in a 1/2 litre of water.
Ink Jet printer ink (diluted) works well and the 3 colours can be mixed to give a range of
colours. The contents remaining in a used cartridge, squeezed from the foam inside the
cartridge, will give a dark green colour.
Quink fountain pen ink (diluted) gives a grey/blue
A stain used to stain biological specimens works really well and gives a red/orange colour.
The stain is ALIZARIN RED and is obtainable from laboratory suppliers as a powder in 5
gram bottles. 5 grams is a lot of stain, only a very small amount is needed in 1/2 litre of
water.
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The most difficult colour to produce is a pure black. This can be produced by using a water soluble stain called NIGROSINE obtainable from laboratory suppliers. Approximately 1/4 teaspoonful
in 1/2 litre of water. Nigrosine does not seem to fully dissolve in water but this makes no difference to its dyeing properties. This dye only works well if it is used hot
The dyeing process:
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Dyes other than Nigrosine can be used cold. Suitable containers for the mixed dyes are wide
mouthed coffee jars (labels removed and re-labelled with the dye name) Aldi stores own brand
coffee jars are good for this purpose
When the anodising process is completed, the work should be washed under water and not
touched, hold by the hook and hang the work by the hook in the dye. The work should be left in
the dye for about 30 minutes, occasionally moving to agitate the dye.
In the case of the black dye, Nigrosine, the jar should be warmed in a water bath heated to near
boiling point (a pan of simmering water with the dye container in the water).
Sealing the Anodised layer:
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The anodised layer, now dyed, is still porous and needs sealing to permanently retain the dye.
This is achieved by placing straight from the dye bath into boiling water for about 30 minutes.
The water bath method works well.
Health and Safety:
The Sulphuric acid and the Sodium hydroxide are both corrosive liquids that should be handled with
care. Eye protection and rubber gloves should be worn when handling the chemicals and carrying out
the anodising process. All containers should be clearly labelled with the contents.
Sulphuric acid
CORROSIVE
Sodium hydroxide
CORROSIVE
Causes severe burns.
In case of contact with eyes, rinse immediately with plenty of
water and seek medical advice.
Take off immediately all contaminated clothing.
After contact with skin, wash immediately with plenty of water.
Wear suitable protective clothing.
Wear suitable gloves.
Wear suitable eye/face protection.
Causes severe burns.
In case of contact with eyes, rinse immediately with plenty of
water and seek medical advice.
Take off immediately all contaminated clothing.
After contact with skin, wash immediately with plenty of water.
Wear suitable protective clothing.
Wear suitable gloves.
Wear suitable eye/face protection.
The use of boiling water for sealing the anodised layer presents obvious hazards and should be used
in a safe area.
Alan Mawson. LCC Adviser. April 2004