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 Post subject: DIY Aluminium Anodising
PostPosted: Sat Dec 29, 2012 1:07 pm 
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Joined: Tue Dec 04, 2012 7:19 pm
Posts: 27
Location: South Africa
A lot of parts and pieces on our treasured rides have bits of aluminium many would want to anodise either with natural sheen or more popular in colour like I did with my DIY built adjustable coilover shocks.
Aluminium anodising is actually a very simple process everyone can do with the very basic and simplest of equipment at home.

That said, I am not a boffin on anodising and will never claim to be, but over the past two years I learned the hard and expensive way until I got excellent results and want to share with the community. Hopefully it may be helpful to someone.
A lot of info is available on anodising on the internet, but mostly it just the general concept being described or shown and the little secrets like temperatures and recipes or not really shared openly….

Here is a diagram of basic process anodising and I apologize for the poor quality thereof but I could not upload the format I created it on and had to take a picture of the laptop screen :oops:

and here is the basic setup of the power supply to anodise, again, picture taken off screen as above....

For starters, what you need is the following;

1. Power supply;

Preferably aluminium anodising is done at about 18VDC but I personally use 14.5VDC which gives me good results and less heat to eliminate in the anodising electrolyte.
Car batteries (12VDC) can be used successfully and I did so myself before I got my homebuilt power supply sorted out.
Connect positive terminate to the aluminium part (anode) and the negative to the cathode (lead or aluminium plate).

Current required is approx 0.016A per centimetre square of area, e.g.: 230cm sq part = 3.7A

Here is my home built power supply which can be set at 6VDC, 14VDC and 18VDC with adjustment between setting.

2. Anodising electrolytes (Bath mixtures)

Actually you need different electrolytes for the process and here are they.

a) Etching Bath:

Firstly you need to etch the ally part and for this bath you need a caustic soda mix. Mix about 1kg caustic soda (Sodium Hydroxide) flakes to about 6 liters of water. This bath must be heated to about 40C degrees for best results.
To heat pour in a stainless steel pot or container and heat on stove plate or if you have a submergible electric heating element, (stainless steel element tubing, not aluminium!) simply drop this in the bath until correct temperature is reached. Use only plastic containers for the caustic soda mix.

Warning: when aluminium is submerged into the caustic soda bath, the reaction causes a highly flammable HYDROGEN gas, hence, do not smoke doing this :o , and wear a nose mask because the fume are a bit nasty.

This bath can be used to remove old anodising – simply drop anodised part in and within seconds it will start dissolving the old anodising.

Here is a picture of a caustic bath in action with temperature shown

b) De-smutting Bath:

More commonly called desmut and it is used to clean the aluminium before the anodising can be done. After the ally part is removed from the caustic bath and rinsed, some dark spots or stains etc will be noticed and it are remnants of some other metals used in ally and the desmut bath will clean this off leaving the ally perfectly clean.
This bath is a 20% Nitric acid to water ratio and used at room temperature.
Warning: always add acid to the water and wear eye hand protection. Use de-ionised or distilled water for best results.

Here is a desmut bath in action - note cleaned ally just above fluid level

c) Anodising Bath:

Temperature is critical on this bath and has to be within the 16 – 22C degree range for best results. Any temperature over that starts sealing the anodic layer formed and no anodising forming.
To cool the electrolyte freeze some plastic bottles solid filed with water – use bottle sizes suitable for your bath size. I usually pop some iced bottles in the anodising bath before I start etching to lower the fluid temperature to about 15C and when temperature gets to about 19C when anodising, dropping an iced bottle back into the bath to prevent temp to creep into the 20C range.

To mix the anodising electrolyte, simply mix a 10 – 12% Sulfuric acid to water ratio mix. Use de-ionised or distilled water for best results. I have used filtered tap water for all baths with great success when no distilled or de-ionised water is available.
Warning: Warning: always add acid to the water and wear eye hand protection.
Check "Anodising Process" chart st top of page for period needed to anodise.

Here is my anodising electrolyte cooled with iced bottle before anodising starts

and here the anodising is under way with some testers fitted to show temperature and voltage used. I found that by adjusting just the voltage to the desired level (14 - 14.5VDC in my case) the amperage drawn will will be "controlled" by the submerged area in the bath, IOW, if you have a small power supply, do not overload it on the current - use a tester and check regularly. My setup's transformer and rectifiers are rated at 350 amps :twisted:

Caution: to be sure you have electrical contact when hanging the parts in the electrolyte, sand the aluminium wire hangers with sand paper to remove surface oxidation that isolate the wire from making contact with the copper tube carrier (can use ally tube as well) to which the positive is clamped - I always give the copper tube a once over with sandpaper to make sure contact is complete.
If the part has not taken an anodising coating the problem is usually either bad contact between the ally wire hangers and copper carrier tube, or the electrolyte heated above 22C deg and start sealing the surface or voltage dropped to low and this is usually the case when working with a car battery not fully charged. You will find in this case the "anodising" layer or dye just wipes off. In this case start all over beginning with etching.

Dye baths for colour:

Commercial dyes specifically made for aluminium anodising is best. Some persons uses fabric dyes but this is a hit and miss affair and the results not as permanent or bright as commercial ally dyes and not worth the effort.

Here is the dye at correct temperature for good results from my experience

Common fabric dye

Commercial aluminium dye

For DIY use it is cheaper to just boil seal the anodising - natural or colour finish. Some commercial sealing salts (liquid) are available but expensive and usual even boiled when using the sealing salts...
I just boil away after the colour is applied in a pot on little gas burner
When parts are boiled sufficiently, remove, cool in water and wipe dry. Spray some CRC or similar on parts and rub down. If wanted to, you can now polish/buff the parts for further deep glossy sheen.

Final results

Anodising finish is only as good as its surface its applied on. It is best to rather polish all parts to perfect sheen / shine before starting the anodising, first cleaning off the polishing compounds, etching etc. The samples shown below are anodised on raw aluminium for demonstrative purposes hence the finish very rough.

Anodising Setting Up ODT.odt [23.09 KiB]
Downloaded 132 times
Anodising Process ODT.odt [16.42 KiB]
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