Performing electrolytic colouring on Aluminium series 6000

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wickked

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Hello,

I am looking to perform an electrolytic colouring on Aluminium series 6061. I need to reach a matte black finish using this process.

The steps that I follow in the process are the following:

Degreasing

NaOH (10 g/L) + Na₂CO₃ (15 g/L)

60 °C / 10 min.

Chemical etch

Solution containing 15 % H₂SO₄ and 5 % HNO₃.

45 °C / 10 min.

Polishing

Solution containing 60% H3PO4 and 5% HNO3.

85 °C / 2 min.

Sulfuric acid anodization

Concentration: 200 g/L

Temperature: 22°C.

Current density : 1.2 A/dm².

Anodization length : 60 minutes (to obtain a 20 microns layer).

Electrolytic coloring

I have tried different bath composition : (using either a stainless steel electrode or a Titanium with MMO one)

  • Copper sulfate / 40°C / 20 Volts (AC) / 55 minutes. Result: light brown but the oxide layer went from 24 microns to 4 microns during the process!
  • Cobalt acetate / 40°C / 16 Volts (AC) / 55 minutes. Result: Non-uniform brown color.
From what I have read in the literature, the problem might be that the pore diameter of the Aluminium piece are not large enough to perform electrolytic colouring.

Therefore, I tried to perform a second anodization (using phosphoric acid to widen the pores of the Aluminium piece) after the sulfuric acid-anodization.

The solution I used for the phosphoric acid anodization is the following : Phosphoric acid (80 g/L) and sulfuric acid (10 g/L). However, given the low conductivity of the phosphoric acid I wasn’t able to perform the anodization.

I also tried to widen the pores of the Aluminium piece by heating the piece in a 5% solution of phosphoric acid for two minutes. The thickness of the oxide layer went from 28 microns to 18 microns. After that I tried to perform en electrolytic coloring using the following conditions :

Nickel sulfate / 40°C / 25 Volts (AC) / 12 minutes.

But I did not get any coloration at all.

Has anyone experienced similar issues when trying to perform electrolytic colouring on an aluminium piece series 6000 that had been subjected to a sulfuric acid anodization process before?

Could anyone who have some experience using phosphoric acid anodization on aluminium series 6000 give me some good advices on how to perform the process to be able to make the electrolytic reaction work afterwards?

Also, does anyone have some suggestion regarding the best conditions/metal to use in the electrolytic reaction in order to reach a matte black finish on the Aluminium piece?

Thank you very much for your help!!

Merry Christmas!

John
 
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