OK, so I tried some electro-etch last night, as Chilla suggested. Used beeswax for the resist, a small battery charger for voltage, and dilute HCl as electrolyte. When the knife was the anode (positive terminal), I got a very nice, quick etch, I assume the reaction is something like Fe -> Fe(+3) + 3e- (or some other oxidation state of Fe); the cathode bubbled, I suppose according to 2H+ + 2e- ->
H2(g).
Now, I have heard that if the polarity is reversed, the metal can be imprinted (not etched) with a black mark. I tried this, and after quite a long time could only see the faintest hint of a mark. Is some different electrolyte required? What is the reaction occurring? (Is it just a plating reaction? That would suggest a different electrolyte is required...)
Thanks,
Kelly