So I understand that this corrodes to keep the hull of the boat from corrodong but why? And what causes the corrosion in the first place?
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It’s like putting a piece of bad food in a trap so bugs don’t go after the food you want to eat.
The metal is designed to attract electrons and get eaten more easily and quickly than the metal you care about (engine/drive parts). Since the metal is cheap and easy to replace, you just put a sacrificial anode on it and let the bugs eat away, replacing it every so often.
Naturally occuring water like sea water or even fresh water has other things dissolved in it. These minerals turn water into a weak conductor or electrolyte solution. This allows the water to create an electrical current. So essentially the metal hull acts like a battery.
Electrons will be pulled one way along the current. Unfortunately that means electrons are pulled towards the dissolved stuff in the water and bond with it. This either pulls the atom away from the steel or changes now it behaves with the other atoms around it weakening the steel.
The sacrificial anode works by forcing the current to flow towards it like an outlet in your home. This way the anode is the only thing giving up atoms. And since the anode isn’t a part of the structure it can simply be discarded and replaced as needed.
Put two different metals in contact with each other via an electrolyte and you get what’s called a galvanic effect. That makes one metal slowly corrode.
Sea water is a pretty good electrolyte for this since it is electrically conductive to an extent.
Metals have different galvanic potential: https://www.mfcp.com/technical-info/galvanic-corrosion
If you put metals with similar galvanic potential together, they will corrode very slowly or not at all. The moment you have metals with different potentials, you get the corrosion.
The thing is that sometimes, you want the different metals because of their other properties. Steel is very good for structural purposes, but you may need copper for something else, and so on.
The anode is put there to have a metal with a galvanic potential that makes it corrode first. It will corrode preferentially over the other metals in the boat (or other device).
There’s a magnesium anode in the water heater in your home for that reason too. Magnesium is basically the first thing that will corrode, so it’s great as a sacrificial anode.
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