The authoritative public forum
  for Metal Finishing since 1989
-----

Black copper oxide on aluminium and steel

Hi,

I've been trying to find a way to coat commercially pure aluminium and possibly steel with black copper oxide, preferably chemically as I'll be coating yard square sheets and would rather not have to rewind a welder with busbar...

George Ingram
- Swansea, Carms, Wales
2002

---

I'm not real clear on what you're looking for, but my suggestion would be to have the aluminum sheets copper plated with about .0005" (this will prevent consuming all the copper in the oxidizing stage) and then use a commercially available copper blackening chemical.

Dan Brewer
chemical process supplier - Gurnee, Illinois
2002

---

2002

Black copper oxide (CuO) is also a semiconductor.

I agree with Mr. Brewer's suggestion to first copper plate the aluminum (or steel). However, many or most commercial blackening solutions (especially, room temperature processes) form copper sulfide or copper selenide, which are semiconductors and less resistant to moisture and salt. You can form an adherent, protective black cupric oxide by a) heating the copper in air or b) using a hot (~212 F), oxidizing, alkaline solution. If you use a heat treat oven, make sure that the sheets are separated and prevented from sagging and that a good air/oxygen flow is maintained. A thin layer of copper will simultaneously oxidize and diffuse into the aluminum (possibly resulting in CuO atop CuAlO2 or CuAl2O4, which may or may not be bad); there should be little diffusion into steel.

Here's a site with color photos of copper sheet oxidizing on an electric stove burner. It was black by the time the burner was glowing red. Turn off the heat at this point for good oxide adhesion (the author of the article oxidized 1/2 hour to detach the thick CuO after cooling). (http://www.angelfire.com/ak/egel/solcell.html) <-- Ed. note, May 2011: this was a very interesting page, but unfortunately no longer works.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

---

May 16, 2011

You may want to try this method which I use to Blacken Aluminium castings.
Prepare approx. 100 gm of Sodium Chloride,(table salt)
200 gm copper sulphate ⇦this on eBay or Amazon [affil links] in tray of expired battery acid ⇦this on eBay or Amazon [affil links] (SG less than 1100 = contents one dead car battery) with about 25 gm copper filings.

Submerge the aluminium in the bath long enough to see a distinct layer of copper metal forming on the surface of the aluminium. 15 minutes should be more than enough.

If you want to speed things up a bit, connect the aluminium to the negative terminal of another 12 V battery and a copper electrode to the positive terminal and submerge both in the tank - keep both well apart from each other.

If the anode gasses off vigorously reduce the current by adding 50 to 100 ohms resistance in series with the circuit until the bubbling stops.

Wash the copper metal off to reveal the black finish underneath.

Hedley Rice
- Brisbane, Qld, Australia