Victory!

Looks great. details?

It is really a follow-up to my previous thread "What causes that?"

Doing the same piece previously I ended up with a lot of small holes in one area and there was a debate as to what causes them. I went through some tests which are also documented on my flickr site. To cut the long story short I believe that hydrogen bubbles which are formed on the cathode should be kept away from the anode (the workpiece). I believe that what one was seeing was a cavitation effect of these bubbles.

I run the process with the electrodes side-by-side and about 3 inches apart and this seems to have done the trick.

When I finish with the current work (Christmas rush!) I shall see if this also solves the problem of foul biting I have had with some pieces: I believe that the cavitating bubbles blasted little holes in the resist.

Michael Koblic, Campbell River, BC

Congrats on the progress. I still think stirring will help, if you ever find some time to try it. The farther apart the electrodes are the less important their relative shapes and the fewer problems with non-uniform current density. For electropolishing 304 and 316 SS I like to basically line the tank so there is cathode everywhere the anode "looks", just use a large enough tank so the distance to the cathode is greater than feature sizes on the anode and preferably larger than the anode itself. For workpieces in the 1" cube or smaller range I make the anode-cathode distance

3-4" at least, and if I'm manually holding the piece I stir it around some to vary the geometry as it polishes. As for your question on why bubbles form on the resist, remember, you are constantly producing gas at both electrodes so you are keeping the electrolyte supersaturated with dissolved gas. Any nucleation site will result in bubble formation, even if there is no current flow there. This is one reason stirring can help, it keeps knocking the bubbles off the active anode surfaces. The bubbles act like little insulators so while a bubble is stuck in one spot that spot is not being electrochemically etched. There is an upper limit, however, because right at the active surface you get a thin film that is more viscous than the bulk solution due to the dissolving metal, and this film helps level the current density and gives uniform etching (the higher viscosity slows the etching and it's more viscous at the bottom of a pit where the film isn't readily rinsed off so the bottoms of pits etch slower; the tips of high spots stick up out of the film and the sharp tip concentrates the electric field, both of which increase the rate of etching to knock the high spot down to the average surface). You need this film but you also need to rinse it away to maintain maximum current density, so some stirring helps both ways. Go extra crazy stirring and you can totally sweep this film away and back come the pits. I played with this some with citric acid electrolyte and electropolishing SS, and it took some heroic stirring to cause problems so I mention it mostly for completeness sake, I doubt you will see it.

----- Regards, Carl Ijames wrote in message news: snipped-for-privacy@4ax.com...

On Wed, 9 Nov 2011 13:32:59 -0800 (PST), Rex wrote:

It is really a follow-up to my previous thread "What causes that?"

Doing the same piece previously I ended up with a lot of small holes in one area and there was a debate as to what causes them. I went through some tests which are also documented on my flickr site. To cut the long story short I believe that hydrogen bubbles which are formed on the cathode should be kept away from the anode (the workpiece). I believe that what one was seeing was a cavitation effect of these bubbles.

I run the process with the electrodes side-by-side and about 3 inches apart and this seems to have done the trick.

When I finish with the current work (Christmas rush!) I shall see if this also solves the problem of foul biting I have had with some pieces: I believe that the cavitating bubbles blasted little holes in the resist.

Michael Koblic, Campbell River, BC

That's pretty much what I got now.

I found no evidence of gas being produced on the anode in my set-up. At one stage I even isolated the electrodes and tried to collect the gases. However, if you change the electrodes (e.g. both cathode and anode stainless) there is gas production at the anode. I assume it is chlorine but there is much less of it than hydrogen on the cathode.

I think the bubbles on the anode are the same ones that develop elsewhere in the tank e.g. on its walls - the bath gets heated and dissolved air comes out like in a tea kettle (the last run was consuming almost 30W).

I will argue that the situation is opposite: The bubbles burst and produce microcavities.

That is true. I found the process to be wonderful at removing machine marks.

I found stainless to be quite a different beast from mild in this process.

Michael Koblic, Campbell River, BC

Going back to my days of etching circuit boards....

Aquariums make good containers. Can be free or really cheap if you find the right yard sale or craigslist posting.

Heat can help. Aquarium heaters are handy for that.

One method of stirring is to run an aquarium bubbler-stone under the workpiece. Aquarium pumps/filters can also be used. You may see a theme developing here....

Don't try to use (or let someone else use) an aquarium used for plating or etching for fish...

You may want to experiment with different electrolytes. Washing soda (sodium carbonate) or baking soda (sodium bicarbonate) instead of salt, for instance.

(...)

The etchant will disintegrate the stone.

AMHIKT, go ahead. :)

--Winston