Atomic Level Explanation of Active Cathodic Protection

Question

I was trying to understand why active current cathodic protection doesn't work for cars and I realized I don't really understand what's going on with cathodic protection at an atomic level.

I understand that oxidation happens when an atom gives up an electron and switches to a higher oxidation state as a result -- usually as a result of interaction with a more oxidative (? wanting electrons) compound. But I don't understand how the cathodic protection works at an atomic level -- could someone please explain.

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Just so I don't get the same answers which don't really go deep enough let me include my thought process. I think it's wrong somewhere but don't really know where.

So a super naive view of how cathodic protection might work is that it simply adds more electrons to the material at risk of corrosion so it doesn't net lose electrons. However, I keep seeing claims that it can only work if it creates a net flow of electrons from the protected cathode to some anode which doesn't fit.

Slightly less naive view. The electrostatic force is hugely powerful so you can't really add enough extra electrons to a material to compensate for the electrons stolen (but stay atomically nearby) by the oxidating agent without removing the now negatively charged oxidating compound. So what happens is that the oxidizer steals electrons but is then pulled away toward the annode by the net current allowing more electrons to flow in and restore the oxidation state now that the now negatively charged oxidating agent has been pulled toward the anode.

That seems more promising, but I'm still confused. Rust happens pretty slowly so so even without a good electrolyte connecting annode and cathode why doesn't the same process just happen via ions in the air? I mean electrolysis creates gaseous oxygen at far quicker rates than steel rusts.

Answer 1

As a piece of metal dipped in water chemically corrodes by dissolving into ions, electrons are released and accumulate in the metal up to a certain voltage point (measured relative to a reference electrode nearby made of carbon) which is characteristic for every different reactive metal. If we now force a flow of electron current into that piece of metal from a power supply at a voltage greater than that which has accumulated in the metal, then we can force the chemical dissolution reactions occurring at the wetted surface of the metal to run backwards, halting the creation of metal ions and setting into motion other chemical reactions at the surface of the metal (usually involving the breaking apart of water molecules into hydrogen and oxygen gas) which occur instead.

As long as we force the dissolution reactions to halt in this way, the metal part will never corrode. Entire ships moored in salt water can be left there indefinitely without rusting by submerging carbon rods the size of telephone poles nearby and connecting the rods to the positive terminal of a very large, low voltage power supply while the ship's hull is connected to the negative terminal. Hydrogen fizzes off the negatively-biased hull and oxygen fizzes off the positively-charged carbon electrode. This is active cathodic protection.