I'm an electrical engineer who has been taught transistors and silison and solid state theory and all that. But when I apply the theory to batteries and cathodic protection, my understanding seems to break down. Can someone explain how batteries and cathodic protection work, from the point of view of band diagrams?
I understand that when two metals touch, their outer electrons will not be of the same energy, so the electrons will move from one metal to the other, leaving both metals charged. I also understand that this charge will dissipate at the other surfaces of both metals, such that near the surface, the metals appear far less charged.
Then, if this combination is dunked in an ionic solution (such as lead and lead oxide in sulphuric acid, or zinc and steel in salt water) then ions of opposing charge will each gather at one of the metals, pulled in by the charge (I guess?), then react at the surface, causing .... the charge to build the other way? This part has me confused.
In a battery the metals are not touching, so the charge builds up and a voltage appears across the terminal, and this charge pushes away the ions, slowing the reaction and slowing the build up of charge. If a circuit is connected, then the metals are "touching" via this circuit and charge flows from one metal to the other, allowing the reaction with the disolved ions to proceed.
But then in cathodic protection, the metals are bolted to one another, and both will react with the ions, causing a build up of charge, which can dissipate to the other metal via the interface. I understand that then a big metal boat, with a big cathodic protection, would have a voltage (and current flow) across its hull, as the reaction with the steel of the hull will be spread across the surface of hull where it meets the water. But then why would this potential difference continue above the water, and if and anchor were tossed over to land, and the chain didn't touch the water, why would there be current through to ground? Why does it not all flow back through the bolt connecting the zinc and to the boat?