Colour Anodizing of Aluminum Alloys Outdated)

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The Colour Anodizing of Aluminum Alloys.

There is nothing really mystical about anodizing. The anodizing process

develops a spongy translucent oxide over the aluminum. This oxide layer is

permeable initially and will absorb liquid pigments into its structure.Immersion in boiling liquid will seal the outer surface of the oxide, trapping

any pigment within its structure permanently.

EQUIPMENT

1. An acid proof tank, to be used at room temperatures, large enough to give

about 6 inches clearance all around the work piece. Notch two opposite top

edges to locate a work support bar. (Maybe a big square tupperware

container?)

2. A cathode cut from pure aluminum sheet to cover one side of the tank then

pass across the bottom and up the other side (on the inside). Bend the sheet

out over the top edge of the tank to hold in position and leave a tag for

electrical connection. The thickness of the sheet is not important.

3. Use enough diluted Sulphuric Acid to three quarter fill the tank. An acid

concentration of 10 percent is satisfactory but can vary widely. Many people

use 1/3 battery acid, 1/3 distilled water.

4. A 12 to 15 volt DC power supply capable of 15 amps per sq. foot of

workpiece surface area. (if there is no control on the voltage then there will

be a heavy current inrush as the power is first turned on and as the oxide

coating starts to form.

JOB PREPARATION TIPS

1. Material with a matte finish anodizes better than one with a mirror finish.

i.e. don’t polish the surfaces to be anodized.

2. Because of its porosity you do not get a satisfactory anodized finish on cast

aluminum which has been machined. An as-cast aluminum surface will not

have a lusterous finish when anodized. (looks poor)



3. Don’t bother trying to anodize riveted or bolted assemblies or parts which

are loosely screwed together. (the acid continues to weep from the joints no

matter what you do and ruins the finished appearance. Anodize components

separated).

4. ALL work supports and fixings in the acid bath must be aluminum.

Preferably pure aluminum.

DEGREASING

(The anodized finish is prevented from forming by any traces of grease)

“Tricleanium” at 160 deg Fahrenheit will mildly etch the surface. Leave the

workpiece in it until water does not run into globules when raised above the

surface. (Tricleanium is an alkali cleaner available in Australia – find

something similar). The job may take on a blue hue but this will disappear assoon as current starts to flow. Wash under the tap and hang in the acid bath

while wet. (wear rubber gloves for this, you must not touch the degreased

surface).

ANODIZING

With the power turned on myriad small bubbles will rise off the cathode and

workpiece. Agitate the job every 5 min. to make sure the bubbles don’t hold

on (to a fixed position on the surface of the work). If the work is to be dyed

black leave it in the anodizing tank for 70 mins. For other colors 20 to 30

mins. After this time the job will have taken on a slightly milky appearance.

Other work can be added to or removed from the tank at any time as the

voltage remains constant. If dying is not required take the job out of the acid,

without touching it, and wash it VERY well under the tap. Hang it in boiling

water for 15 mins. to close up the spongy oxide. This makes it more durable.

Sealing Salts (Nickel Acetate is usually used) are sold which can be added to

the boiling water to assist the sealing process and make any dye more UV

resistant. The concentration of Aluminum Sulphate in the bath will slowly

build up. Replace the acid solution from time to time. A damaged anodizedfinish can be removed by VERY brief immersion in a strong caustic soda

solution. Once having sealed up the oxide film it is not possible to increase the

film thickness by reimmersing it in the acid bath because of the high electrical

resistance of the oxide layer.



DYEING

1. Dyes for cotton fabric are satisfactory but don’t add mordants or

anything to the dye, e.g. salt, vinegar etc.

2. Vegetable dyes are reputedly better than chemical ones. (Black dyes,

like tulips, are harder to achieve.

3. Most colors can be used at 1 oz. of dye per 24 ozs. of water. Black needs

to be much stronger, about double or thereabouts.

4. When a sufficient thickness of oxide is built up take the work out of the

acid bath and WITHOUT TOUCHING IT wash really well under the

tap. Place into the cold dye bath without allowing it to dry out.

5. Raise temperature of dye bath to boiling point and allow it to boil for

ten minutes.

6. Remove workpiece and hang it in clean boiling water for 15 minutes.

Remove, cool, then wash it under the tap.

7. The job may be coated with lanoline (wool grease) to make it more

durable. (a trick used commercially).

MASKING

Bare areas of aluminium may be masked with 3 coats of nitro cellulose laquer

(model aircraft dope) or one coat of “Freundorfer” “Freko Positivlack”.

Remove both with lacquer thinners after completion of dyeing.

SUMMARY

The work is cleaned of any grease and must remain free of all grease toprevent blemished surface finishes. A spongy oxide layer is built up in an acid

sulphate bath. The workpiece is washed then placed in the cold dye for a

period to absorb the dye. The workpiece is then boiled to harden and seal the

oxide layer. The workpiece is then washed and dried and may be coated with

lanoline to protect the finish.

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Colour Anodizing of Aluminum Alloys Outdated)