Why does the top electrode of a layden ar corrode? - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: February 23, 2018, 06:32:12 pm
content: I made a layden jar out of a glass bottle with salt water/copper wire and aluminum foil around the outside, the aluminum was exposed to the air no tape around it. To my surprise over the course of a summer the copper wire and steel alligator clip that was acting as the top conductor had completely turned green and the alligator clip completely fell apart when I touched it. Does this mean it's a polarized electrolytic like capacitor and will store charge better with plus in the top conductor? I'm guessing the water side is the plus and Al foil side is the negative? Where did the charge come from? Is it my choice of metals? Would copper foil and aluminum wire do the reverse or does that have to do with glass and water? Not surprisingly I couldn't measure any charge with a multimeter and charging it didn't work nor did hooking it up to one of those ebay high voltage sparker devices cause any difference in output. The thing did blow up all the LEDs with in 1' of it. Why are these LEDs glowing blue?DOH

author: Zero999
author_rank: Super Contributor
timestamp: February 23, 2018, 08:22:51 pm
content: The corrosion is a separate phenomenon to the jar having capacitance. It's because the dissimilar metals formed an galvanic cell, with the electrolyte being the moisture in the atmosphere and possibly salt, as it would have been impossible for it not to have being contaminated with that, when it was put together. Galvanic corrosion can be a problem, when ever an object is made from two dissimilar metals, which are in electrical contact with one another.https://en.wikipedia.org/wiki/Galvanic_corrosion

author: John Heath
author_rank: Frequent Contributor
timestamp: February 24, 2018, 05:51:56 pm
content: I was repairing and old time temp LED sign. Each dot pixel was 8 LEDs in series. At 1.5 volts per LED plus a 5 volt margine for load resistor the power supply voltage was 15 volts to drive 8 LEDs plus resistor in series. The big problem with this sign was rust from being outside for so long. The plot thickens. The LEDs closest to the 15 volts were the ones that were the most rusted. Rust likes a positive voltage. Now that I think of it telephone lines are negative relative to ground. If millions of dollars went into telephone lines they would want to do their best to not have them rust out. They make the current loop voltage is negative relative to ground. If the layden jar were left with positive charge on the top I could see it rusting out a little faster than expected.

author: Zero999
author_rank: Super Contributor
timestamp: February 24, 2018, 06:37:54 pm
content: Quote from: John Heath on February 24, 2018, 05:51:56 pmYes, you're right, telephone lines are positive earth for that reason. I also believe car electrical systems are negative earth, to protect the engine and chassis from corrosion, which is much more difficult to repair/replace than the cables.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: February 24, 2018, 06:55:34 pm
content: The salt will quickly cause cuprous oxide to form on copper and oxidize (rust) steel or iron parts. Added to that you have the galvanic reaction of the voltage difference in the 2 electrodes, which will accelerate the oxidation.A laden jar capacitor can charge to a high voltage if you use air instead of salt water, however you can make a homemade capacitor using baking soda and aluminum foil. I found this video demonstrating how to do this, however, there is no instruction into the electrical characteristics of the device. The capacitance of the homemade device can be measured with a decent capacitance meter.Hope this helps...

author: duak
author_rank: Super Contributor
timestamp: February 24, 2018, 08:05:58 pm
content: I understand unplated copper is a poor choice for HV wire. I don't know why but I'll bet the HV attracts all sorts of particulate in the vicinity and if there are any chlorine or sulfur compounds present then chemical corrosion will start much faster. Automotive ignition wires are either stainless steel or carbon impregnated fiber. I've got some HV wire with silicone insulation for an injet printer that's definitely not copper.Also, I think the device in question is a Leyden (or Leiden) Jar named after Leiden in the Netherlands.Cheers,

author: amyk
author_rank: Super Contributor
timestamp: February 24, 2018, 09:23:59 pm
content: Quote from: John Heath on February 24, 2018, 05:51:56 pmIf you have a potential across a conductive solution, the negatively charged electrons move from negative to positive but the positively charged ions from positive to negative --- and that's what causes the loss of material from the electrode, just like electroplating.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 25, 2018, 01:07:32 am
content: " the ions move" This brings to mind Van De Graph generators. I built one from a kit. They had the contact to the Van De Graph belt a stripped wire with the smaller ends facing the belt nut not touching. Clearly they made a mistake but in my capable hands it was quickly fixed so that the wire ends made contact with the belt. It did it not work when finished. I am not proud to say this but in my defense I was desperate. I broke the universal guy rule and read the instruction manual. It said in large print do not have the copper wire touch the belt. How did they know I did that ? In any event the wire was pulled back so it would not touch the belt and the Van De graph generator started to work. Why? It took a few days to sort it out. " the ions move" A current will take the path of least resistance. The path of least resistance for an electron is copper. However if you are a charged ion the path of least resistance is air not copper. A lesson I will not forget having learned it the hard way.

author: Beamin
author_rank: Super Contributor
timestamp: March 04, 2018, 11:00:13 am
content: Quote from: Hero999 on February 23, 2018, 08:22:51 pmFor that to work the metal have to be touching. In this case the metals are inches apart separated by glass and air. It has more to do with the electrodes on car batteries which are both made from the same metal. Sea equipment will have sacrificial electrodes actually bolted to the object. But if they are not touching that's different. I have a feeling if I wrapped the outside in copper the same thing would happen.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2018, 11:20:05 am
content: Quote from: Beamin on March 04, 2018, 11:00:13 amForget the layden jar, the aluminium foil and glass for the moment. That's just a distraction.The galvanic action is due to the copper wire being in contact with the steel alligator clip, with the electrolyte being the moisture in the air and traces of salt on the conductors. You'll get the same result if you suspended the copper wire connected to the alligator clip, above a glass of water.

author: Beamin
author_rank: Super Contributor
timestamp: March 09, 2018, 08:59:43 pm
content: Quote from: Hero999 on March 04, 2018, 11:20:05 amI made three and the other two did not have aligator clips or anything attached and corroded worse. Probably due to the charge not being spread out. Those were never charged or used just sat in the corner collecting dust.