What causes this?

Looks like your etch found some weak spots to exploit.

Is it like that all over? Or just along the bottom?

Was the original metal some type of casting? In that case, it could be porosity in the original casting uncovered/opened up by the etching process.

BTW: The sun dials you make are way cool. Do you sell them on a web site? Dave

What I remember from chemistry lectures long ago is that the components of an alloy separate as the cast ingot cools, giving crystals of the more stable composition surrounded by a less organized mass of somewhat different proportions. Rolling the metal into sheets spreads out the crystals and depending on their size (cooling rate) may make the alloy vary with location at visible scales, like granite rock.

The junction of two different alloys has an electrical potential that is the difference between their attraction for electrons, like a thermocouple or battery. In a corrosive environment these slightly different alloys act like weak batteries, so the more active material will dissolve faster than the less active one. This is part of why rust forms isolated deep pits instead of cutting in evenly.

Notice the uncertainty:

Another part: HTH JSW

Thank you.

I have a few on Etsy:

The original metal is a 2" washer!

I have a stack of pictures of this phenomenon but in all other instances there was an intervening resist. I thought I solved the problem at one point by having the cathode on top so the hydrogen bubbles would not have to go past the work piece (anode). I am now wondering if the distance between the electrodes is critical. I did another piece on the same day which I did not photograph. Because of the shape of it the distance between the electrodes was very small (1/8"). The piece was full of holes such as those shown in the picture. This however, was a negative etch so the holes were all under the resist. In fact you could tell the result was going to be bad when I spray-washed the piece after it came out of the bath - the resist starting coming off in spots exactly where the holes appeared underneath.

Previously I attributed it to foul biting but seeing it without an intervening resist makes me re-evaluate.

FWIW I never see this when doing brass which is a gasless process.

I am going to run a trial with several electrode distances to see if I am right.

I have read a lot about cavitation but it is still black magic to me:-)

Michael Koblic, Campbell River, BC

Are those truly pits, or are they projections? I could imagine air bubbles (or hydrogen) forming on the surface which would prevent as much access from the etchant.

Either tapping the edge every so often, or applying ultrasonics from a cleaner might break them up fairly quickly.

Good Luck, DoN.

Are you agitating the solution?

I read both several times. Interesting. I can sort of see the pitting type of corrosion more than the crevice type. I am not sure that it is the whole explanation. Some of the considerations that puzzle me are:

1) Why would a weak local battery not be swamped by a 2.5A current passing through the whole system? 2) Why are the pits, if they develop as described in the article, so deep and apparently so uniform in shape (I enlarged the photo - most of them seem comma shaped)? 3) Why does one not see similar phenomenon with brass? There I am removing copper from zinc. One would have thought that local batteries would if anything be "stronger".

There are other considerations derived from the previous experience of similar pits developing under a resist. However, I am not even sure this is the same phenomenon. The pits sure look similar.

I am going to run a few test and see what gives.

Michael Koblic, Campbell River, BC

The undergrad chemistry courses skimmed lightly over a very wide field of industrial applications. They told us we weren't really chemists yet, but when (if) we got a job in industry we knew enough theory to understand the explanations, many of which were proprietary trade secrets. Electrochemistry is particularly secretive about the details that make one battery better than another. It's still black art, or FM as we say in electronics.

I did check other references. Do you really want to dive into the theory of redox potential?

jsw

Non-uniform alloy, just like the other poster said. You can also get this with higher carbon steels when removing rust with phosphoric acid or the like. Preferential etching at certain grain boundaries. Get some lower carbon material and more uniform. Non-metallic inclusions can also get etched out and leave pits, too. Without polishing, etching and viewing a sample under a microscope, it's just speculation, though.

Stan

On Thu, 27 Oct 2011 21:35:27 -0500, Richard wrote: [...]

No, I am not.

[...]

Not especially. Not beyond what I read in the books I have.

I spent this PM making components for a test set up (time I should really be spending otherwise, but until I have this sorted it is hard to move forward). I hope this will provide some insight and confirm/refute my theory.

I should mention that I have never seen similar pitting on steel when using a non-electrolytic process (e.g. Ferric Chloride).

Michael Koblic, Campbell River, BC

They are pits and quite deep at that. The main reason I have the cathode on top now is to allow for the hydrogen to escape without coming into contact with the work-piece.

I am going to try a few things tomorrow and see what pans out.

Michael Koblic, Campbell River, BC

I would buy that if I ever saw similar pits using Ferric Chloride. I never do.

I would go back to using FeCl3 but the depth of etch is so much better with this process. As often as not I get a clean etch. It is the unpredictability that irks me.

Michael Koblic, Campbell River, BC

How consistent is your metal supply? If you buy mild (low-carbon) steel and process it, you will probably get fairly consitant results. If you use a lot of "found" or "mystery" metal, then you don't know what alloy you have, and have to expect variable results.

And buying large washers from the local hardware store leaves you open for almost any flavor of alloy -- often from the same bin. :-)

Good Luck, DoN.

I don't know about their alloy but the thicknesses can vary considerably within the bin.

Half a foot of snow in October??? Support Global Warming!

In truth everyone has a valid point although I am not sure that it explains the problem entirely.

I am about to give up using washers, at least the 2" variety for the very reason you state: The thickness varies quite a bit, the quality varies also. Their availability has decreased: I used to be able to go to Fastenal and rummage in their bin and pick the ones that were useful. Fastenal no longer stock them. You can order them but then you have to take what they send you.

Anyway, to confuse matters further: Here is the first trial I did today:

Make of it what you will. The next thing is to try increase the distance between the plates and/or try to run the whole assembly on its side so the bubbles have a free exit.

I included a picture of a piece that ended up like pig's breakfast just to show that whatever it is that causes the pits it blasts its way through 4 coats of resist if the conditions are right.

That is always assuming that the cause of the pitting in the different pieces is the same.

Michael Koblic, Campbell River, BC

Our temperatures have been sub-normal practically whole year. Either it is not warming or it is not global:-)

One thing is for sure: Putting coffee filter between the electrodes does not work!

Michael Koblic, Campbell River, BC

Um, Mikey, try using one -without- the old grounds in it next time.

-- Inside every older person is a younger person wondering WTF happened.

You are probably right. This would make my original idea of using toilet paper even less acceptable...