Well, the rate of oxidation (and thus metal removal) is proportional to the electric field (in volts/meter, for example). The metal is all at the same potential, and so is the cathode. However, the shape of the electric field as the potential falls from anode to cathode is determined primarily by the shape of the anode. A sharp corner or spike on the surface will concentrate the electric field, causing a large potential drop in a short distance from the surface, and give a high rate of oxidation (that's why electropolishing works to level a surface). Inside a pit the potential is almost constant so the electric field is very low and oxidation is very slow so high spots get eaten away and low spots left alone. Think how much easier it is to draw a spark from a sharp point, compared to a smooth sphere - the potential drop is the same but it happens over a much shorter distance at the sharp tip. in this case, though, I think the problem is something else or otherwise the corners of the edge of the strip would have been eaten away. Most likely defects or inclusions in the metal, or variations in hardness or composition. If that's the case there isn't anything you can do except get new metal, or maybe try annealing it first. If it is due to the shapes of anode and cathode or the buildup of nonuniform concentration gradients as the etch proceeds, vigorous stirring really will help. Just make up a lance with lots of < 1/32" holes, bend the tube into a loop and put it at the bottom of your tank, and apply 20 to maybe 60 psi of air.
----- Regards, Carl Ijames wrote in message news: firstname.lastname@example.org...
I have never tried to etch a piece of metal that was not flat.
Here is a face of a polar sundial prepped the usual way, using my go-to resist with an extra coat "to make sure". The main difference was a cylindrical cathode parked in the centre of the semicircle.
The result sucks:
..and the next 4 pics.
What sucks more is the fact that I cannot explain why. Are ions attracted to corners?
Michael Koblic, Campbell River, BC