Anodising Aluminium

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Some three years ago I took to wondering if it would not be practical for the home workshop enthusiast to renew those faded or damaged anodised parts which contribute so much to the good looks of a completed project and or new parts.
Since practical advice seems to be rather difficult to come by, I read a few books, followed some experimentation, achieved much discovery of an artful process, and Success !!!

The process itself, though chemically complex, is rather simple.
First it involves the transformation of the surface aluminium oxide to
aluminium hydroxide (anodise), then to a hydroxide monohydrate.
An interesting property of hydroxide is its ability to absorb dyes into the microscopic porosity's of its surface. After impregnating, or dyed with a colour medium, the surface is then "sealed" into a monohydrate and the surface becomes very hard and resistant to wear.

Method And Materials

The process requires the use of either chromic or sulphuric acid in the
anodising electrolysis bath. I have limited my use to the sulphuric process
because of the ready availability of battery acid and the ability of the
process to absorb a wide variety of dye .The acid used is "1270 SElead
acid(, battery electrolyte obtainable wherever you buy new car batteries.
Cut this 50/50 with distilled water to obtain the anodising solution.


NOTE Always wear eye protection and rubber gloves when working with acid.

CAUTION Always add the acid to the water never the reverse.


Distilled water is recommended because the use of ordinary tap water invariably contains some minerals which will cause smutty deposits on the work and generally not contribute to consistent results. The acid anodising solution needs to be stored and used in a suitably sized plastic, or glass, open ended container.
A lead strip cathode plate(s) is required. The lead plate(s) should be about twice the surface area of the largest workpiece (anode) to be anodised.
I use a variable DC power supply (2 to 30 V surplus unit) which I find ideal but any charged car battery will do the job. I find that most small parts require around 10 V to maintain the required current density through the bath; so 12 V should suffice for most work.
An ammeter reading from 0 to 3 AMPS (for 20 square inch, maximum) Is a must, as well as a heavy duty rheostat in series with the supply and the anodising tank.
The resistance of the rheostat will, of course, vary with the size of the work
contemplated, but it can be calculated from the required
current density of 145 to 175 milliamperes (ma) per square inch of the anode
workpiece. The workpiece surface area must be calculated In order to set the anodising current. And, a surplus wirewound rheostat (variable resistor) of a few hundred ohms will do.

Let's assume that we want to anodise a propeller spinner, or flywheel, the surface of which, is calculated to be 2.5 sq. inches . The anodising current density required will be:

Minimum: 0.145 X 2.5 = 0.362 Amps
Maximum: 0.175 X 2.5 = 0.437 Amps

So, the current must be between 362 and 437 ma. In anodising this part I would
maintain my adjustment at 4oo ma.
The part to be anodised must be chemically clean. No effort must be spared buffing and cleaning prior to complete degreasing in hot water using a strong detergent. At this stage handle the part with rubber gloves or not at all.
Use only the aluminium contact strip fixed to the part. Rinse well and you're ready to anodise.

 

The anodising tank is set up as follows:


Process Notes

A good anodised coating thickness will be built up after 40 minutes at the
calculated current density. Keep a running check on the current reading as this will tend to vary during the process.
Too low a current setting will result in a surface that will have difficulty
absorbing the colour dye.
Too high a current setting will result in overheating the tank solution and a porous finish which will leach out the dye during fixing.
A good anodised surface will have a slightly milky appearance when ready for fixing; or colour dyed and fixed.
Copper , brass. or iron will contaminate the tank and degrade the process.
Use only lead or aluminium contact strips. I use wooden clothes pegs to set my workpiece height in the tank.
Few, if any, parts you make will be made from pure aluminium. Most will be made from aluminium alloys which contain varying proportions of copper, manganese, silicon, and sometimes, other elements in the mix. These alloys have an effect on the ultimate colour shade obtainable with a given dye and process. If colour shade repeatability is required, the same alloy, process times, and temperatures must be carefully duplicated. Previously anodised parts must first be strip-cleaned in a strong Akali to remove all traces of prior anodising oxides.

COLOUR DYES

Organic dyes are usually used because of their great variety and depth of colours. Industrial dyes can be obtained, but only in too large a quantity for our purposes. Coloured artist Inks are generally suitable, as are food colour
Dyes with varying results. A yellow dye gives the "gold" anodised look because of the translucence of the anodic coating and the metallic reflection.
Some writing Inks are also suitable such as Skip or Carter, which gives a great "black".
The trick is to find a colour with a pigment size small enough to enter the microscopic anodic oxide coating and be sealed there. Experimentation and patience are both recommended.

COLOUR PROCESS

Dyes may be used hot, usually 150 F, or at room temperature. The dye and the effect required will determine the choice. I usually use mine at room temperature and an immersion time of between one minute to 15 minutes, depending on the depth of shade required. Agitation is required. A "coarse" dye will just accumulate on the surface and will wash off during fixing.

FIXING PROCESS

Fixing is done in plain old H2O (near boiling) for about 20 minutes.
Preferably, use distilled water to avoid those nasty mineral deposits
on our nice parts - temperature: 200 F.
A certain amount of dye will leach out into the water before the surface seals. It is best to avoid actively boiling water since this agitation will accelerate the colour loss.
Chemical additives for the fixing bath are available, but I haven't found any to recommend.
To keep colour loss at a minimum I have found that rotating the part in
steam for ten minutes before total immersion does a considerable job in reducing leaching probably by closing the pores and sealing the dye before washing it out.
The finished part is buffed with a clean cloth to remove any smutty deposits.
A little wax brings out the colour.


Reproduced from an article by Ed Cox in Strictly IC Aug/Sept 1988....USA Publication
(No Copyright infringement intended)