Stainless steel and garden slugs killing each other...

[Konkedout]

I live in the area of Puget Sound Washington.  Good for apples, other garden plants, and slugs.  Slugs do a lot of damage in our garden.

A few years ago, I learned that 9V would dissuade a slug.  Experimenting with an old laptop power supply, I found that 18V would kill them right away with all of their mucus oozing out.

So I designed a pulser which powers a pair of wires spaced about 1/2 inch  (1.27 cm) apart like railroad tracks, laying on top of the garden soil.  It delivers a pulse once every 15 seconds or so with a 1/8 duty cycle.  The idea is to give the slug a chance to get completely across the pair of wires before zapping it.   The pulser delivers 18V which is current limited by a series connected 82 ohm 0.5W carbon composition resistor.  (I happened to have a bunch of those; it is good for the purpose.)

I am powering all of this with a 40W solar panel, a charging module from Amazon, and a 12V 7 A-H sealed lead acid battery also from Amazon.

I initially tried tinned copper AWG 20 hookup wire for the "railroad tracks".  Those wires were corroding badly.  Galvanized steel wire (same diameter) conducts well enough but was also corroding.  So I tried Chinese-made 304 stainless steel also from Amazon.  The DC resistance is acceptable.  This all kills slugs and generally works, BUT.....

Once in a while the stainless steel wire breaks for no apparent reason.  Today my wife mentioned to me that it broke right where a slug died.  I have googled the question of whether the slug mucus is corrosive but I cannot see that that is the case.  The current is limited to (really much less than the calculated 220 mA.)  First the resistance of those 82 ohm resistors tends to run closer to 100 ohms, and I have additional overcurrent protection which shuts it down when I short the wires for example.  So a maximum current of 50 - 100 mA is more realistic.

I would be happy to share my schematic and/or KiCad files for the pulser but that is not the issue.

I wonder what the heck is causing my wire to break?  I might try ordering some (American made but maybe not?) stainless steel wire from McMaster as maybe the Chinese stainless steel wire is low quality.  I do not want to buy gold plated wire!!!  Any ideas?

[IanB]

I think your problem is galvanic corrosion.

DC current, metal, and damp environments will cause nearly all metals to corrode except for noble metals like platinum. Stainless steel will resist corrosion in electrically neutral circumstances, but it will not survive electric current.

A possible solution is to use an AC supply instead of DC. The alternating current will repeatedly "do" and "undo" the corrosion, causing the effect to balance out. But now your electrical supply will become more complicated, and I do not know if it will affect the slugs in the same way. That is something you would have to find out.

[Ian.M]

+1 for IanB's explanation.  Another approach to balancing the corrosion that wont reduce the slug-killing effectiveness would be to reverse the polarity after every pulse.  This is likely to be less effective at reducing corrosion than using AC as the metal ions travel further from the wires.

[JohanH]

Get some hot-galvanized (zink) steel wire from farmer shops. There is specially made wire for electric fencing for cattle. It will last much longer, but will also eventually suffer because you have it on the ground.
There is also some nylon varieties with a steel core. I don't know how it affects longevity.

Btw, they use pulses 3 kV to 12 kV, with energy 1 to 12 J (just looked it up). Different for different animals. If I remember correctly from when I was a kid, the pulses were every few seconds or so. You could get really stunned if you had good ground contact. It could really kick you down. With rubber soles you could barely touch it, it still stung. Every kid learned you shouldn't pee on the wire!

[Coordonnée_chromatique]

You should avoid the zink oxyde in a garden IMO, you should use a less toxic sacrificial anode.

[voltsandjolts]

Titanium isn't a noble metal but it has impressive galvanic corrosion resistance, and titanium wire is fairly cheap. If that doesn't work then, yeh, AC supply.

[hanakp]

You could try cathodic protection:

https://en.wikipedia.org/wiki/Cathodic_protection

Passive sacrifical anode would be easiest, since your device is solar/battery powered. It's a bit tricky connecting the metals together though, the joint must be waterproof, otherwise it will corrode first and it stops working. For active protection, you could add negative DC voltage to both rails in respect to ground (that's how analog telephone lines, pipelines etc. are protected).

[ejeffrey]

A sacrificial anode is only going to provide a about 1 volt of potential, depending on the metals used.  That's enough to prevent corrosion from other electrochemical potentials but since you are applying 18V you have to make sure it's connected to the most positive point in the circuit such as the solar panel + terminal.

[AndyC_772]

Is a slug conductive enough that you could detect it by applying a much lower voltage, and only applying a high voltage when there's one across the wires? A low voltage, short duration 'detect' pulse wouldn't have nearly as strong a corrosion inducing effect.

[magic]

I must admit that I don't like slugs, and I like your idea

Maybe mount the wires on a vertical fence, so the corpses fall off faster. Having them laying on the wires cannot possibly do any good in the long run.

[daniel444]

i have been using a 0.9mm titanium wire to boil my daily water for a few years now , i bought it surplus , i got lots of it , i would go with 0.9 or 1mm titanium  wire

its 0.7 ohms per meter for the 0.9mm wire

[BrokenYugo]

You should avoid the zink oxyde in a garden IMO, you should use a less toxic sacrificial anode.

Nevermind the zinc, you can assume anything hot dip galvanized is lead containing if it doesn't say otherwise.

This honestly sounds like a better way to put various metals in the garden than kill slugs, have you tried any sulfur based products? Nematodes? Permethrin or relatives thereof?

[tszaboo]

I had a lot of slugs in my garden about 4 year ago. They lived under the wooden decking. Three years ago we renovated it, dug up the soil and since then its the fraction of the number. 1/20th is a good estimate.
For your wires, reversing the polarity every 2nd cycle could lengthen the life time, maybe.

[Randy222]

Maybe gold plated wire?
I forget the site, but there's a good metallurgy site/forum where you can get all the info on metal and how they react with other environmentals. The slug carries a PH level and that's most likely 90% of the battle.

[Konkedout]

Thank you all for the comments!

One or two posts mentioned galvanic corrosion, and that is my best guess as to what is going on.

The wires need to lay on the soil so that slugs might crawl across.  Slugs are most likely to be marauding when the ground is wet, and there is (maybe a few mA?) flowing between the two wires through the wet soil even when no slugs are present.  But that current flow is likely to be spread out.  I still think there is something about the slug mucus....I think the problem happens when the dead slug has been laying there for hours.

I googled titanium wire; it looks like it is roughly $2 per foot.  I am probably using > 100 feet of SS wire right now.  I don't think I want to try titanium or gold plated wire.

As for chemical solutions; nah we do not use chemicals in the garden.   And as science figures out more and more (such as the microplastics in our bodies) I am not eager to change that practice.   I like to cook in cast iron rather than Teflon for the same reason.

After posting, I was thinking that yeah; alternating the current flow might be what I need to do.  Right now I have two of my circuit boards in different locations.  One of them uses all four of its sequentially pulsed outputs.  The other I think uses two of them.  But for all of that, there is one sort of "common ground" which goes out to one wire of every pair of "railroad tracks."  It  is not so truly a common ground because I think the charging module regulates the negative side so that the negative terminal of the solar panel is not at the same potential as the negative terminal at the battery and at my slug wires or circuit boards.   But this does not matter so long as I do not short those points.

I am thinking to try a complete re-design to output AC instead of DC.  This will be significantly more complicated....but that is life.  I would like to use a frequency > 20 KHz so there is no chance of human-audible beeping, and also to reduce the size of coupling capacitors.  But I would need to confirm that 18V or 20V peak 20 KHz will kill the slug.  Maybe the DC current which causes galvanic corrosion is the same thing that kills the slug.  But testing this will not be as easy as poking a slug with the 18V output of an old laptop power supply.  I need to design, build, and test something - even if it is crude - just to see whether 20 KHz can kill a slug in the same way that DC does.

Thanks everyone for your responses!!

[magic]

18V AC into grounded 80Ω load is something that any audio amp can drive with ease. Good enough for a prototype

[Randy222]

18V AC into grounded 80Ω load is something that any audio amp can drive with ease. Good enough for a prototype

Would be neat to learn if any specific frequency frys the slug better than another, using an audio amp and bare wires. Another test might just be using insulated speaker wire with a dummy load at the end, and then change the amp input frequency along with output power to see how that affects the slugs.

[Doctorandus_P]

I would not use an audio amplifier, far to complicated, Also note you would need a stereo audio amplifier, to get a nice symmetrical waveform at the output.

The simplest solution is probably some kind of motor driver H-bridge IC, combined with an oscillator (Maybe a NE555). You could build it with a few NE555's and drive the wires with that too, but you would be limited to it's maximum voltage (Which also happens to be 18V (I thought it was only 15V)).

It's important to make it symmetrical to avoid galvanic corrosion as much as you can. Preferably you also put a capacitor in each line so no DC can pass at all.

[T3sl4co1l]

Titanium isn't a noble metal but it has impressive galvanic corrosion resistance, and titanium wire is fairly cheap. If that doesn't work then, yeh, AC supply.

Titanium doesn't usually corrode anodically, but it anodizes instead.  This may make it difficult (non repeatable?) to maintain function.

Under the correct conditions, it is indeed attacked; bromide is such a case, at least in my experience, though I'm not sure what else.  (Usually it's pretty inert, and is regularly used in electrochemical plating baths for this reason, meanwhile a metal part can still make contact with it somehow.)

The only thing that works here is AC (which will cook rather than chemically melt a slug), or inert anodes like platinum ($$$), MMO (pool chlorinator anode), lead dioxide (good luck forging a ceramic into wire), and a few other less effective and harder to use materials.

Tim

[T3sl4co1l]

I would not use an audio amplifier, far to complicated, Also note you would need a stereo audio amplifier, to get a nice symmetrical waveform at the output.

The simplest solution is probably some kind of motor driver H-bridge IC, combined with an oscillator (Maybe a NE555). You could build it with a few NE555's and drive the wires with that too, but you would be limited to it's maximum voltage (Which also happens to be 18V (I thought it was only 15V)).

It's important to make it symmetrical to avoid galvanic corrosion as much as you can. Preferably you also put a capacitor in each line so no DC can pass at all.

Uh--??

(To unpack that:
1. A linear audio amplifier is already symmetrical. A "half bridge" or "totem pole" design is typical, so, there's a positive swing and a negative swing.  If one were missing, it wouldn't be linear.
2. Switching (class D) amplifiers are quite popular, highly efficient, and readily available these days; with a filter network provided, they are electrically equivalent to (1).
3. If the output were DC biased (offset), it would cook speakers just as quickly as it would corrode the wires here.  Even if the amplifier is such a bad (or archaic) design that a coupling capacitor is required to use it... I mean there you go, coupled and done.  Such designs haven't been in use since the 1960s, and who knows if this is what the above had in mind?
4. A stereo amp does nothing here. Where is the other channel going?
5. If both channels go to each wire, then, that's not stereo, they'd be driven equal and opposite, there's nothing stereo about it, it's mono by definition.  The word commonly used for this is "bridged".  The resulting H-bridge topology doubles the voltage output from a given supply voltage, which can be handy.
6. The whole thing is floating, battery and solar, no ground connection required -- only the wires laying on the ground provides any earth reference, by way of leakage to it.  It's SELV, it's not mains powered, no isolation required.  Even if a strongly-offset amplifier were used, the offset can simply be subtracted out, say with a battery (or again, dropped passively with a coupling capacitor), and any offset between the device and earth is simply determined by the resistor divider (sort of) between wires and earth.

As a rule, I try to avoid (ignore) posts that aren't contributing [much] to a thread; but this one seems so exceptional, in how irrelevant it is to the present thread, and, seemingly, out-and-out mistaken, that I feel it necessary to point out the disconnects, and misinformation.)

Tim

[BlownUpCapacitor]

I would like to use a frequency > 20 KHz so there is no chance of human-audible beeping, and also to reduce the size of coupling capacitors.  But I would need to confirm that 18V or 20V peak 20 KHz will kill the slug.  Maybe the DC current which causes galvanic corrosion is the same thing that kills the slug.  But testing this will not be as easy as poking a slug with the 18V output of an old laptop power supply.  I need to design, build, and test something - even if it is crude - just to see whether 20 KHz can kill a slug in the same way that DC does.

I'm not a slug expert, but I know that frequencies above 20kHz have limited effects on the human body. That's why you can touch a Tesla coil and not get an electrical shock because the frequency is so high your nervous system doesn't react to it. I don't know if a slug's nervous system is faster or slower than the human nervous system.

So what I think would work fine is just using an H bridge inverter to make a low frequency switching at say 5Hz so it's not very audible, but can still stop a slug. The H-bridge will also allow high currents to flow if needed.

[daniel444]

stainless is vulnerable to pitting corrosion , where it gets corrosion all in one little spot rather than evenly 

[duak]

I also live in the Pacific Northwest and like the idea of dealing with slugs electrically - electrocide?  Co-incidentally,  I use electrolysis to remove rust from tools and car parts and picked up a few bits and bobs.

The sacrificial anode (positive) electrode is oxidized by the reaction with water.  Usually, a sheet of mild steel is used because it's cheap but it dissolves far faster than the rust on the cathode.  A graphite anode isn't affected but I've never seen flexible carbon rods.  (Carbon fiber?)

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.

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.

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.

AC is probably the easiest and safest way to minimize the corrosion problem.  I should give it a try myself.

Cheers,

[Ian.M]

I suspect a large thick section scrap aluminum casting would be  satisfactory as a sacrificial anode for quite a few years e.g. a gearbox housing or aluminum cylinder head or engine block.  It needs to be totally free from paint, oil or grease, and any baked on dark residue should be removed abrasively.   Aluminum is a normal component of many soils and is only problematic in acid soils - see https://agsource.com/wp-content/uploads/2024/04/F-6278509012-Agro-Soil-Aluminum-Tech-Bulletin_HR-2.pdf so if you have acid soil, pack round the aluminum with plenty of crushed chalk or limestone to keep it safe for nearby plants.

Drill and tap or through bolt using zinc plated hardware to attach a heavy gauge insulated wire, preferably 7 strand 10 AWG or similar, with a preferably crimped on ring terminal.  Use a light smear of Noalox on the inside of the crimp and the ring terminal when  assembling the connection.  Clean up all excess Noalox, abrade and degrease the aluminium round  the connection and coat with a thick layer of hot tar or polyurethane sealant or marine sealant for min. 1" round the fastener, over the ring terminal and up the wire so no dissimilar metals are exposed to avoid local corrosion. 

Bury below the frost line - about a foot down should do in the Puget sound area unless in a notably exposed location or at altitude.

[Konkedout]

I think I need to build something in order to run a test.  Fortunately if I build this I ought to be able to bring a slug (on a small wood plank?) to my lab and not vice versa.

If I make this comparator oscillator run at about 20 KHz, Q1 and Q2 should produce a ~ 0-40V square wave with a 40V supply.  Then AC coupling through C8 = a 1 uF or larger capacitor of some sort ought to give me a 20V peak square wave with negligible DC current. 

This design is preliminary/based upon the through hole parts which I have in my lab.  Most of the parts I have are SMT but those are too difficult to use without a pcb, and I don't want to get a pcb for one 1-minute test.  This schematic is sort of rough; I can adjust values somewhat on the bench to get it to work as I want....