Electrical – Copper pipe identification using electrical current

Grounding tails are available (thanks @batsplatsterson), but you could also buy some copper wire; either on a reel or by the foot, and make your own.

As a quick rule of thumb, you should use the same size grounding conductor, as the largest ungrounded (hot) conductor used in that circuit. So you're probably looking at using 14, or 12 AWG wire for switches.

You'll want to use either bare copper, or green insulated wire. Solid or stranded makes no difference, as long as it's the proper size. Some will argue one way or the other about connecting solid to stranded, stranded to stranded, solid to solid, stranded to screw terminals, solid to screw terminals, etc. In reality, if done properly, it really makes no difference. Follow the manufacturer's documentation on all the equipment you're using, and you should have no problems.

As for the actual procedure of grounding the switches and box.

1. Connect a short length of grounding wire to the ground terminal of each switch/device in the box.
2. Connect a short length of grounding wire to the metal box, using a screw in the threaded hole in the back of the box.
3. Using an adequate connector, connect together the grounding wire from the box, the switches/devices, and all other grounding conductors in the box.

What you call corrosion is oxidization this won't hurt the ampacity of the wire, I would want to inspect the system because this is usually caused from being overloaded or getting wet being the causes I have found in the past, a loose connection can also cause over heating but in those cases there is usually some pitting from arcing at the contact point but I don't see any evidence of that. If the insulation is still in good shape the wire should be fine at its listed capacity.

Added to cover ok comment / question: I have a thermal camera that I check panels with, when I find "Hot" temp wise wiring it is usually below 90c. I have seen number 10 wiring loaded well beyond the standard limit for residential in motor controll centers closer to 60 amps with no insulation damage on thhn. When the wire gets hot enough to deform the insulation it is hotter than 90c. At first with a severe overload the wire heats up the insulation starts to smoke and melt but it usually won't catch fire unless there is a splice and at the splice it may get hot enough to catch fire but the wire is "glowing hot" . When wiring gets this hot it usually shorts in the cable hot to neutral or ground at a bend or this is what I have found on several occasions. When the wiring is overloaded severely I think the Inter action with the thermo plastic accelerates the oxide layer as I have seen equipment that was close to 30 years old with bright shiny commer a inch or 2 back under the insulation. High humidity will also cause the oxide, I have seen this in multiple areas and work environments. I just re read your original post and it would be helpful if we could see the condition of the insulation (I was thinking that was a photo of your wire). If the insulation is deformed at all it should be replaced as a best practice, since you don't want to replace it it should be checked with a megger (high voltage resistance meter) at 500v checking between hot and ground, hot and neutral, neutral to ground (both ends disconnected) and the reading should be in the gig ohm range. You don't want to find out the hard way that your neutral to ground has shorted and someone touches the metal frame of a lamp, appliance whatever and gets a bad shock, or worse.