[fryguy]

Basically, the reason that things corrode is that there are dissimilar metals, one with more electrons than the other creating a voltage between them. So your part is a battery and the ocean is the "wire". The electrons move from your part (corroding them), to other parts with less electrons. If instead you have a sacrificial piece of zinc, the zinc has more electrons than your part so the electrons will come from the sacrificial piece of zinc instead of your part. Think of it like a lightning rod, but for corrosion instead of lightning. It's more complicated than this, but that's the general idea.