Anodising in a plastic cup.
Anodising aluminium is not difficult.
Anodising is that ability to induce oxidation on the surface of
aluminium in such a way as to create a hard wearing and
protective layer.
There is also the advantage that it can also be dyed.
I have used this on the valve bases of my modified Katadyn water filters, which although a good filter, is a terribly badly designed base. I redesigned the base and anodised it to keep it free from damage indefinitely.
Clean usually by immersing in a cleaning fluid such as carbon tetrachloride, but good old domestic cleaning will do, if fully rinsed.
To etch the surface, some caustic soda is now applied in solution about 40 gms per litre,
Followed by rinsing under a tap. (If you want, some nitric acid can be used to neutralise the caustic soda.)
Don't put any greasy hands on the surface, - use a piece of wire.
Place the item in cup or bowl or whatever, suspended by a piece of wire, so it makes a good electrical contact with the suspensing wire.
Now pour in the electrolyte of dilute sulphuric acid, - in proportions of one part acid to seven parts water.
I use battery acid, available from motor garages. Check you have a non soluble bottle, such as discarded bottles used for nail varnish remover, although I use a polythene camping bottle
and it has not dissolved yet after five years.
Make the item the positive, by apply the +ve clip of your battery charger to the support ing wire, and using a piece of cleaned alloy or other metal wrapped around the inside of the cup or bowl, as the negative cathode.
To regulate the current, make a guess of the surface area of the
item and aim for about 1 to 1.5 amps per dm2 of surface area.
In real money, that is about 1/10 th amp per square inch.
To regulate this current I work out my current needed, then look
for various car or bike light bulbs and fit the best one which
limits the current to about the intended rate, using a
multimeter.
If you have a 12 volt car battery charger, which can put out 3
amps, then this may be perfectly good for most purposes.
If you have a very small item then a domestic power supply for
radios and such like usually have about half to one amp output.
Choose a lamp about the corrct ampereage, (e.g 13v, 36 watt lamps - 3 amps) so this allows the correct amount of current in Amps to flow.
Brew the item in the live electolyte at about 25 degrees for about half an hour for a full anodised film.
Now choose A B or C :
A Rinse in cold water.
B If you want a sealed surface, then boil for 15 minutes in a mixture of 5gm nickel acetate and 5gm boric acid to 1 litre, then rinse.
C If you prefer a dyed surface, then dip the item in dye solution and bring to the boil or until the desired colour density is reached. Then rinse.
Clean and dry.
You will find the anodised surface is now an insulator capable of insulation of more than 12 volts so any electric connection must need the surface to be scraped to bare metal.
Any minor flexing of thin metal will not cause the anodise to fall off, although cracking will occur and allow corrosion.
For harder anodising, such as plain alloy barrels without liners, as once tested by Yamaha, try using a refrigerated acid bath, possibly try first in a freezer section set on full to see if the test piece is suitably abrasion resistant.
Always remove the anodised item immediately after dicoenecting the current, otherwise the solution will start to disolve all the hard work.
To degrease, it is possible to reverse the current in a solution of 3 to 5 percent sodium hydroxide in water.
Play safe, but have fun.
J.P.
Version 1s. Copyright (C) J.Partridge. 2003.