DIY Home Aluminum Anodizing

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  • Some hints and tips on home aluminium anodising Dye

    This pages contains some small comments to help you on your path to home anodising - just some small remarks that I've noticed during my anodizing aluminium experiments.

    • The caustic soda phase is very critical. A minute or so in the bath is required. Leaving it a bit longer gives a very matt finish to the final aluminium part: Which is a Good Thing (TM) if you are anodising an aluminium optical part.
    • The thing to remember with the caustic soda bath is that it is dissolving the aluminium. Leave it too long and it the part gets smaller. This is generally noticed when two closely fitting parts no longer closely fit. Also, after 10 minutes in the bath, tapped holes will no longer hold their correct sized screws.
    • You can remove anodising with a few extra minutes in the caustic soda bath. I find that a mug of caustic soda granules in 2 litres of hot water will strip off the anodising in about 5 minutes. The caustic soda solution ends up looking very yucky. Also see the point above.
    • After the caustic soda bath phase, it is very important to clean the part. Because aluminium is normally in an alloy with lots of other metals, these metals will smut the surface. You can buy de-smutting solution if you like. However, I find a good wash works wonders. After the caustic soda bath, fill the sink with hot water with lots of washing up liquid. With rubber gloves on, scrub the part with a toothbrush until it is an nice even finish. Don't use a "green scrubby thing" or a Brilo pad - this will scratch the surface (which you might want, so hey!). This scrubbing is VERY important to the final finish.
    • Getting a good electrical connection is vital. It is worth spending some time making something out of aluminium that has a threaded section to screw into tapped holes in your aluminium part. If you are anodising several things at once, you'll find the one with the poorest connection doesn't anodise well
    • Don't use steel screws to hold the part in the acid - steel in the acid will dissolve!!!
    • Don't use those pretty coloured aluminium bolts sold for bicycles either. These are anodised... I supposed you could caustic soda them in the bath first and then use them... I haven't tried.
    • Anodised aluminium does not conduct electricity well. If you have used a bit of scrap aluminium to hold a part in the acid bath, clean off the anodising with a file before you use it again.
    • I find about 1 hour in the acid bath gives a good anodized layer.
    • Be very very careful not to touch a freshly anodised part with your fingers. The grease will ruin the final part. You end up with a nice anodised fingerprint on the part.

    • If you use steel bulldog/alligator clips to attach the power to the acid bath, be sure to dip them in the bicarb solution before you put them away, other wise they will slowly dissolve.
    • Boil mercilessly. Boiling for 45 minutes is not going to harm the part. This ensures a good seal on the anodising.
    • It doesn't matter if the part being anodised touchs the plastic sides of the acid bath.
    • It does matter if it touchs the lead cathode.
    • Make the cathode as big as you reasonably can. Fold the lead around on itself several times to get a good surface area.
    • After a few sessions the acid bath will have a lot of muck at the bottom. This doesn't effect the anodising qualities.
    • If you snap a hardened steel tap off in your job, anodise it anyway. When the anodising is finished, you will find the broken end of the tap that was embedded in the aluminium part has dissolved. Don't do this too often otherwise you'll end up with very mucky acid.
    • With larger parts, the acid bath will fizz lots. It is best to do the anodising outside.
    • You are going to use a lot of distilled water. Battery topup water can be purchased in Halfords in gallon containers, but it is about 3 quid. This gets expensive. I've yet to figure out a way of getting hold of lots cheaply. I suppose I could make my own water distiller.
    • Don't keep the caustic soda in the same box/drawer/cupboard as the acid bath.
    • Do put a warning label on the box to say it contains nasty stuff.
    • Don't leave lead cathodes where animals are going to lick them.
    • Shutting your pet cat in the living room during anodising sessions is a)A really good idea b)Really pisses the cat off. This also applies to small children.

    This picture shows my part completed homemade telescope focuser. The black drawtube was dyed with Navy Blue multi-purpose dylon. The blue bits with some old solution of Madonna blue.

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