Anodizing Aluminum with Stainless Cathode

Usually, the recommendation is to use aluminum cathodes for anodizing aluminum. In fact, often the recommendation is made to use aluminum or even to use the exact same alloy:

But at the same time, various other sources suggest that lead, aluminum, titanium, stainless steel, and even regular steel make suitable cathodes....sometimes depending on the anodizing bath.

Anyway, I wanted to see if I can use the sodium bisulfate bath with a stainless cathode. Some concern when using stainless cathodes for anodizing or electroplating is the release of hexavalent chromium. The chromium ions will discolor the solution, if they are present - though any discoloration could come from other ions, as well.

Experiment 1:

300 mL cold filtered water

30 g NaHSO4

0.5A current setpoint. Anode area about 4-5 in^2

Cathode: Stainless steel bucket ~36 in^2

10:59 : start. 31.6V 0.49A

11:05 : 27.1V

11:40 : 21.5V

12:00 : stop. 20.1V rinse.dye.dry

Takes the dye well.

The solution is still clear. There is some sort of deposit on the cathode. Dark gray. Can't be scratched easily.

Experiment 2:

continue with previous bath.

New anode.

1A current density:

12:07 : start

12:15 : 1.0A 22.7V

12:49 : 20.7V

13:07 : stop. 20.6V. rinse, dye, dry

There is some pitting in the anode. It does take dye well, though.

The solution has turned quite warm, so the increased current density may be only indirectly responsible for the pitting. It could be the temperature.

The solution is now greenish blue. The area ~1 in above the water level has been etched in some way. Above that the container is unchanged.

The deposit has increased in blackness. At the bottom of the container it is easily scratched off, but towards the water level it is very tenacious.

It looks like something leached out of either the stainless steel or the aluminum to cause the black coating and green color of the bath. It is not clear what, but I did not get the clear solution I was hoping for. Maybe with temperature control this leaching and the pitting of the aluminum could be controlled.

The second sample takes on a little more dye to yield a darker color (hard to see on pics).

A scratch test shows the first sample to be much harder than the second one, even though the higher current density seems to have yielded a ticker layer into which to absorb the dye. This is consistent with earlier results that yield a harder surface with colder plating baths....

The dye uptake, pitting at higher temps and hardness testing match the results I got using aluminum cathodes. Stainless Steel can be used as an anode.

Upon exposure to the atmosphere, the black coating has started to turn yellowish brown:

That yellowish brown deposit looks a lot like the hard anodized aluminum surface. If it is aluminum oxide of some sort, I should be able to clean it off with some sodium hydroxide. A test shows that the sodium hydroxide is incabable of removing the deposit. It did, however remove the yellow discoloration and now everything is black. This could be iron oxide. To test that, I am going to put vinegar into the bucket. Vinegar has shown itself very effective at removing black oxide (and brown rust) coatings from other steel objects in the past. The black coating starts vanishing in a few seconds and is gone in 2-3 minutes.

So it looks like we may have found a way to grow black oxide on stainless steel. This may be useful in future projects, though it's not clear why the black oxide would turn to brown oxide by itself.