DIY optical breadboard

The cake is a lie. This is actually a post about anodising aluminium.

The internet is full of information about anodization/eloxation of aluminium, so I’ll just summarize what I’ve learnt is important:

  1. Current control rather than voltage control
  2. Temperature control
  3. Cleanliness is next to godliness
  4. Work in a well-ventilated area
  5. The alloy is important. I’ve used AlMg3 (5754).

 

Every single paper I’ve come upon agrees on that a current density of at least 1.25A/dm² and an electrolyte temperature of 20-30⁰ C is required for a type II coating (what you want for nicely coloured aluminium), so current control is definitely needed.

Every single paper I’ve come upon agrees on the importance of keeping the electrolyte temperature close to 20 °C, so when depositing electrons worth of in excess of 150 W for 45 min. some kind of cooling is definitely required.

On every single webpage/forum/etc with advice for the amateur anodiser that I’ve come upon, cleaning the aluminium of grease/fingerprints/etc has consistently been high on the list of things to be aware of.

Everyone also seem to think that working in well-ventilated area is a good idea, and while there aren’t really any volatile chemicals on the table, the etching and the anodisation processes produces aerosols of NaOH and H₂SO₄ respectively, and I would not do this without some kind of gas-trap (a second time).

And here are all the pictures:

DSC02549

The anodisation rig in its entirety. The I’ve used 16mm² Al wire (blue and black). The electrolysis vessel is a giant 10x25x25cm chromatography ‘beaker’. It’s hidden behind the plastic box which serves as cooler and another smaller box to trap H₂SO₄-aerosols. Everything is mounted ontop of 4 small magnetic stirrers to keep cooling as good as I can easily manage. One PSU is visible.

DSC02550

The 4 PSUs supplying a total of 10 A. The workpiece is 15x20x0.8 cm which gives a current density of 1.5 A/dm². The anodisation took 45 min. The temperature of the electrolyte (~1M H₂SO₄) reached 31 °C (measured at the end of the process.

DSC02559

A slightly better view of the insides.

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The anodisation vessel.

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The cathode. For environmental reason I’ve used aluminium instead of lead.  However green it’s inconvenient, because the cathode must be removed when current is not running or the H₂SO₄ will eat away at it.

DSC02552

The finished product. It could be slightly darker. I guess 30 °C is too high after all. The dye is Sanodal Deep Black MLW. The anodised aluminium was dyed for 30 min at 35 °C with a dye-concentration of 2.5 g/L. The chemist in me would have preferred something homemade, but I could not find anything anywhere about how to make a black dye.

After dying the aluminum was steam-sealed in an oven with a beaker full of deionised water. (That’s probably not the best way to do it, but leakage and wear-resistance is not an issue for me).


1 Groves, G. New Zealand Institute of Chemistry, Anodising.
2 Aerts, T et al. Surface & Coatings Technology 201 (2007) 7310 – 7317, Influence of the anodizing temperature on the porosity and the mechanical properties of the porous anodic oxide film.
3 ASM Handbook, Volume 5: Surface Engineering, p 482-493, Anodizing

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