I do quite a bit of machining at home and have had a few cracks on small off cuts as practice. I will have to get my act together and go the whole hog soon as i have got all the gear together now. There is a great little book from the "workshop practice series" called "electroplating number 11" by J. Payner. You can get it at carbatec and this outlines the process very well including the amounts and types of chemical needed in the baths (Ch 9).
With my experimenting I have found the best proceedure to be as follows. Finish the part on the lathe/mill and rub it down lightly with steelwool to get a good finish. I then give it about 10 minutes in white spirit followed by a rinse in water. This is followed by ten seconds in 2 molar nitric acid (dilute) then again flushed with water. The part should now be spotless and needs to be if you are going to get a good finish. The next step is to place it into a 2 molar solution of sodium carbonate (soda ash @ bunnings) and sodium hydroxide (caustic soda @ bunnings) and time depends on the amount of frosting you want in the finished product and it also cleans it further at the same time. I have found about 20 sec is right for my eye. It is again washed in water then its ready to anodise.
The bath needs to be acid proof so either an acid proof plastic tub or stainless steel tub must be used. It needs to have a bit of volume (about 10 - 15L) as temperature kills the anodising process. The electrolyte is around 2 molar sulfuric acid (dilute acid) and depending on what you want to do varies in strength from about 12 - 15% (lower value for colour, higher value for hardness and plain mill finish). The part needs to be made the anode (positive terminal). I use lead sheet as the cathode (neg terminal) and this is placed around the outside of the tank. I also use an aquarium pump with one of those bubble scoobie things to aerate the electrolyte and help keep it cool. The power pack is a adjustable 30V/ 20amp continuous special from DSE. You need to work out the time based on the surface area to be annodised (there is a little formula for this) but I have found it is cooked when the metal takes on a yellowish hue. I have also found 15V to be around right.
Once its cooked, rinse with water again then depending on the finish do one of two things. If you want to leave it plain just steam it for 20 minutes and this will seal the pores making the layer permenent. If you want to colour it then its into the dye. This is easier said then done. I have tried all sorts as they are just organic dyes and even had some mild success with cloth dyes but could not make it colour fast. In the end I just ordered some from Ron Newman at Moonlite as I could not find a local distributor. The dyes are thick and need to be heated to about 60 deg C before the part is added. They tend not to dissolve very well and will leave streaks on the finished parts unless continuously stirred to dissolve the solids. Lucky I have access to hot plates with magnetic stirrers at work
. The part is left in the dye for about 20 minutes then rinsed under a tap. It is then placed in the sealing agent ( a nickel salt) at 85 deg C for a further 20 mins and wallah its done. The finish depends on the ali itself. The harder alloys tend to have other metals like copper and magnesium in them. The copper will not soak up the dye and can spoil the finish. 2000 series alloys dont colour well but the 6061 does. If you stuff up its back into the nitric acid to strip the anodised layer off and start again.
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Whilst white spirits, solid sodium hydroxide (VERY CAUSTIC) and solid sodium carbonate can be bought in high enough purity from bunnings, the nitric and sulfuric acids can only be bought from a chemical supplier and they may not sell to the general public as these acids are very concentrated (98% for the sulfuric) and extremely dangerous for those who do not know how to handle them (even those that do ). I got mine in 2 litre containers ( I am a qualified industrial chemist) and diluted them as soon as I got home (15L of both) to prevent any silly accidents with the conc acids. If you are not familiar with the correct proceedure
LEAVE THESE ALONE OR GET SOMEONE WHO KNOWS WHAT THEY ARE DOING TO DO IT FOR YOU AS YOU WILL GET HURT. The sodium hydroxide must also be given respect as although not as bad as the conc acids it will dissolve your body parts into soap and glycerol and can be nasty in the solid and concentrated liquid form.
ALWAYS WEAR PROTECTIVE CLOTHING SUCH AS SAFTEY GOOGLES, GLOVES AND OVERALLS.
It is extremely important to remember to
ADD THE ACID TO WATER IN SMALL AMOUNTS not the other way around. Conc sulfuric will spit and the solution will become quite hot when dissolving it into water the correct way. The nitric will fume and the vapours are not pleasent and will burn your nasal passages and eyes if you are unlucky enough to cop it so do this in a well ventilated area. If you tip water into the conc acids you better have a swimming pool close by because you have about three seconds before the acid that has just exploded out of the container starts to really bite. NEVER ADD WATER TO CONCENTRATED ACID. If you spill it on yourself go straight to a tap and flush the area with water for a good 10 mins and go straight to the doctor. If you get it in your eye flush it well with water and get to a doctor quickly. Any other spills can be nullified by throwing solid sodium carbonate on them then hosing off after 5 minutes.
In their dilute forms (2 molar or 12%) they are not nearly as dangerous but still demand respect. Eye damage will still occur (wear glasses and protective clothing and gloves) and it will sting if it goes into cuts and dry your skin out if left on your skin for too long (sodium hydroxide will feel soapy as it is dissolving the fats on your cells). If you spill the dilute acid on your body get to a tap and flush with water. If you get it in your eye flush with lots of water and go immeadiately to the doctors. If you get the hydroxide on yourself do the same as above. To dispose of the acids dilute with lots of water (@ least 10 x the acid volume) and they will be relatively harmless (dont drink it). The same goes for the sodium hydroxide.
Mark
