Letter 41048

Anodising etch chemical  

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I've had a go at anodising a component but erxperienced difficulty when etching with sodium hydroxide. The component surface quickly went black and needed scrubbing off. I was expecting it to brigten the surface. What went wrong? Can anyone reccomend a concentration and time duration to use next time or an alternative chemical.

Steve Lander
scientist/hobbyist - Lynn, Massachusetts

What was your goal in etching the part, Steve? An etch does not brighten the surface, it dulls it. The black coloration is due to the fact that the etch dissolves aluminum but leaves behind silicon, copper, and the other materials in the alloy.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

My "goal" in etching was to remove the layer of Alumiun oxide on the components surface in order that I could then 'rebuild' an artifically thick layer of AlOx, by electrolysis, that could be dyed.

From what I've researched the process commonly used to anodize is 1)degrease, 2)etch, 3)anodise, 4)rinse, 5)dye
The time durations quoted for an etch in Sodium Hydroxide are generally about 15mins, this component was black and sooty within 1. I think I'll try a nitric acid etch.

Regards,

Steve Lander
- Lynn, Massachusetts

Although caustic does dissolve aluminum oxide, it also dissolves aluminum, Steve. I think 15 minutes is way too much immersion time; anything over 2 minutes is a lot. In the process it etches the aluminum, giving it a matte surface. Nitric acid is not an etchant for aluminum; depending on the aluminum alloy, nitric acid can be a good desmutter, removing the black coloration caused by the copper left behind when the aluminum is etched away.

I think I would describe the process commonly used to anodize (when bright dipping is not needed) as 1) degrease, 2) non-etch caustic clean, 3) caustic etch, 4) desmut, 5) anodize, 6) dye, 7) seal -- with rinses between each step.

I would suggest starting by getting a copy of Aluminum How-To, and also making sure that you are not subject to environmental regulations. Presumably you realize how dangerous chemicals like strong caustic and nitric acid are.

Ted Mooney finishing.com Brick, New Jersey

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