Metal parts underwater are subjected to two basic types of corrosion: galvanic corrosion and stray current corrosion. Both can harm your boat, propeller, and motor if not correctly monitored and avoided. What is Corrosion? There is nothing mysterious about corrosion. The process metal goes through in changing is slightly complicated, but not especially complex. To best describe corrosion, let's start with the most common type, rust. We all know rust, but to understand rust, we have to go back to the very beginning. Iron ore has a chemical composion of two iron atoms bonded with three oxygen atoms. As it is mined out of the ground, it's a brownish- red powder useless to us. But by refining, purifying, and smelng, we create iron, which is useful. We can use it as plain iron, or we can process it further and combine it with other elements to get different types of steel. Let's say the iron is made into hinges for your backyard fence. Everyone knows that if you leave iron out in the rain, it rusts. If it rusts long and badly enough, the metal disappears and you're leſt with a pile of brownish-red powder-rust or iron oxide, which has the same composion as iron ore. Here's why. Iron atoms want to return to their normal state as iron ore, iron oxide, or rust. Which are all the same things. That's the state in which iron is most comfortable and most stable. Leſt alone, it won't turn into anything else. And most metals used in manufactured products want to do the same — return to their natural state. Electrochemical Reacons In looking at this chart, you can also see that gold is the least acve metal of all metals, however magnesium is the most acve, therefore the most likely to protect your boat from currents Iron leſt out in the rain results in a specific kind of corrosion. It’s called an electrochemical reacon, meaning there is an electrical change. Here’s how that works: For two iron atoms to really interlock with three oxygen atoms and make iron, they have to share some electrons, which releases a few electrons. Since electricity is just a flow of electrons, those free electrons become a lile bit of electricity when the chemical change takes place. Remember the iron wants to corrode into iron oxide because that is its natural, most stable state. And all it needs for this to take place is oxygen. Water is a supply of oxygen, so iron rusts fastest when it gets wet. You knew that already but now you know why. And that same scenario applies to aluminum and aluminum oxide. Those are the deep, dark secrets of corrosion as they apply to metals. Those are also the basics of an electrochemical reacon, which is known as galvanic corrosion. All galvanic corrosion is an electrical reacon. Not all electrochemical reacons, however, are galvanic corrosion. Galvanic Corrosion Galvanic corrosion is an electrochemical reacon between two or more different metals. The metals must be different because one must be more chemically acve (or less stable) than the others for a reacon to take place. When we talk about galvanic corrosion, we're talking about electrical exchange. All metals have electrical potenal because all atoms have electrons, which have an electrochemical charge.