Stainless steel and garden slugs killing each other...

[Zero999]

Why 20kHz?

I don't know about slugs, but data from humans and other mammals shows that high frequencies are less likely to be lethal than mains frequencies and DC, because the nerves typically don't respond to higher frequencies. 20kHz just causes heating due to I²R losses, which might be enough to cook a slug, but are probably less likely to electrocute it, as DC does.

In humans, mains frequencies are typically the most lethal, compared to much higher frequencies or DC. Again, with slugs it's probably different, but I doubt 20kHz is the way to go.

If mains isn't available (obviously use a step down isolation transformer if it is: 12V will probably do), then DC with an inverter will do.

Why not use an H-bridge, that way a low supply voltage can be used? An H-bridge can be made to self-oscillate by adding RC circuits.

This circuit oscillates at about 50Hz. It's a basic astable multivibrator, but with extra transistors added to pull the output up.
https://en.wikipedia.org/wiki/Multivibrator?useskin=vector#Astable

[T3sl4co1l]

Since most metals will be oxidized, I would be concerned about introducing metallic or organo-metallic compounds into the soil.  eg., copper and zinc are considered anti-microbial.  Note that they also kill moss.  Lead is used as an anode in the acidic anodization of aluminum but I wouldn't try it here.

Corroded aluminum is pretty non-toxic, in and of itself, though various alloys do contain a little copper, zinc, iron... The majority is Al, Si and Mg though, all pretty innocuous.

Probably worth noting that most all of these metal oxides are insoluble, and tend to remain in place in the soil.  So, even if Cu, Zn, Ni, etc. are present in toxic levels, with respect to one or another thing in the soil, it's not like the whole area will be contaminated, in just a few months, or decades hence.  Plenty of things grow in or around galvanized metal, lead cladding, copper sheet, what have you.  Not that their heavy metal content in turn might be something you'd want to eat, but at least to say it's not fatal for the direct things growing there.

I found that a stainless steel sacrificial anode corrodes almost as fast as one of mild steel.  This is important!-->  It also releases Hexavalent Chromium which is a hazardous compound, especially if it ends up in a garden.  The Nickel is probably bad also.

Doubtful, though I don't know offhand if the amount will be actually zero, or at what rate if not.  If there's metal to corrode, that has the lower oxidation potential than Cr(III) to (VI) does, and Cr(OH)3 is pretty damn insoluble/nonconductive so makes a poor target for further oxidation.  Cr(VI) can likewise be reduced by free metal.  To reduce, chromate has to be in solution; precipitated chromate could persist, but then, it's likely not too soluble in general, making it less of a hazard (lead chromate is surprisingly not very hazardous, despite what it's made from... not that I would bathe in the stuff..!).

Alas, I never tried the experiment, back when I had chemicals and a place to do it...  Dissolved stainless, yes, but not anode-ized it, in a neutral pH.

Believe I've electrolyzed with it in alkaline, with no yellowing of the solution seen.

Anyway, the resulting crud, predominantly (hydrated or as hydroxides) Fe2O3 and Cr2O3, extraordinarily insoluble, neutral, more or less inert in soil; and NiO, also quite insoluble, but a known toxin you don't want huge amounts of.  (I mean, nickel occurs in soils all over, if not very much; its mere presence isn't a problem, it's the amount that matters.  It is toxic in large amounts, except to a few extreme species -- New Caledonia has some *wild* plants on it, evolved to its ultramafic soils rich in heavy metals.)

But yeah, also just to make this clear: Cr(III) is essentially inert in the environment, it's present in various rocks, it acts much like Fe(III) and Al(III), indeed their ions substitute in various minerals, and reactions.  Even Cr(III) salts have shockingly low toxicity (comparable to table salt IIRC), not that I would recommend spreading it on your food...  It's specifically when it's been oxidized to Cr(VI) that it's carcinogenic, and rather devious at that, when it gets into groundwater say.  There are ions that can sequester it (like PbCrO4 being very insoluble), but it's not held around by clays for example (those trap positive ions like K+, not negative ions like CrO4(2-)), which is why it can be a groundwater hazard.

Aluminum and titanium form a very tough oxide layer that eventually gets thick enough to become an insulator.  BTW, with titanium, you could also get some nifty colors.

Not necessarily.  Titanium I think does just anodize at most pHs?  Aluminum may just corrode.  The anodize process specifically uses acid, though I'm not sure offhand how much that's a matter of just getting good conductivity (could sodium sulfate or nitrate be used instead?), or to optimize the deposit (is it less strong, more flaky, or not as porous, in other conditions?).  Aluminum definitely corrodes in strong acid and base -- it's amphoteric, and most stable (insoluble) at neutral pH.  But that's not to say, non-corrodable: it may powder up and flake off: "rust", instead of forming a tight protective oxide.

Depends on alloy too; some are particularly corrosion prone, 2000 series (copper alloyed) for example.  The most common ones (say, 5000, 6000, 300 serieses) use Mg and Si for strength, which are pretty well-behaved with respect to corrosion.

The main thing is, with voltage applied constantly, there's a pressure towards acid and base at the anode and cathode respectively, perhaps causing anodization (but then no current flows to the snail either) but also dissolution/corrosion, including at the cathode.  So I wouldn't use aluminum here.

Also not to mention, flesh-goop left on the wires will tend to give off all sorts of things -- acids as proteins and sugars and various other things are broken down and oxidized, bases like amines as amino acids break down and putrefaction occurs (if it stays moist long enough for bacteria to move in, I guess?).  Hard to be inert to all those things, without simply insulating the wire in the process!


For my part, I'd suggest trying a low voltage, and AC (at modest frequency, 10s to 100s Hz say; should still be tingling to most nervous systems), more as a discouragement than a fatal punishment.  But I don't have any data to suggest how effective or reliable that would be; you'd have to try it and see.  And maybe you'd prefer the culling effect of executioner over a mere deterrent.

Tim

[Konkedout]

Some time after posting the above...I realized that (to test it on a captured test subject) I need only to assemble the output stage with the two complementary BJTs, diodes, capacitors and resistors, and drive it with my (mal)function generator.

This will make a lot more sense for a "One and done" test.

[Zero999]

Some time after posting the above...I realized that (to test it on a captured test subject) I need only to assemble the output stage with the two complementary BJTs, diodes, capacitors and resistors, and drive it with my (mal)function generator.

This will make a lot more sense for a "One and done" test.

Indeed, your signal generator will probably drive Q1 & Q2 via the capacitors, which will need to be made larger if you're going to test lower frequencies.

[peter-h]

I use this product
https://www.amazon.co.uk/gp/product/B0BYSB6K7M
which works very well It does suffer from corrosion where some organism ended up on it but with a 9V PP3 supply it is ok. Snails and slugs don't get across unless they are very small

[IanB]

I use this product which works very well It does suffer from corrosion where some organism ended up on it but with a 9V PP3 supply it is ok.

Thickness is one of the ways of mitigating corrosion. In engineering, it is common to estimate corrosion rates over the lifetime of a component, and then add sufficient extra thickness to compensate.