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Anodizing etch chemical

(-----) 2006

I've had a go at anodising a component but experienced difficulty when etching with sodium hydroxide. The component surface quickly went black and needed scrubbing off. I was expecting it to brighten the surface. What went wrong? Can anyone recommend 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 never brightens 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, leaving the surface gray-black. A lot depends on the alloy, 1xxx has little contamination from other metals in it, 2xxx and 7xxx have a lot.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


My "goal" in etching was to remove the layer of Aluminum oxide on the components surface in order that I could then 'rebuild' an artificially 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) anodize, 4) rinse, 5) dye
The time durations quoted for an etch in Sodium Hydroxide are generally about 15 mins, this component was black and sooty within 1. I think I'll try a nitric acid etch.


Steve Lander
- Lynn, Massachusetts


Hi again Steve. Although caustic does dissolve aluminum oxide, it also dissolves aluminum, and gives it a matte surface (hence the term 'etch'). But 15 minutes is way too much immersion time even for etching; anything over 2 minutes is a lot. Nitric acid is not an etchant for aluminum; however, depending on the aluminum alloy, nitric acid can be a good desmutter, removing the black coloration caused by the copper being left behind when the aluminum is etched away. A much shorter etch, followed by the nitric acid dip, might get you there.

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 considering getting a copy of Aluminum How-To, and also making sure that you are not subject to environmental regulations. If it's strictly a hobby and you never sell anything, you'll probably be okay, even though you may be in violation of your sewer permit. Presumably you realize how dangerous chemicals like strong caustic and nitric acid are, and that protective gear, good ventilation, and secure storage are a must. I think anodizing is a dangerous hobby, but to each his own I guess. Best of luck.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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