What Rust Is
In order to understand how rust works and spreads, you first have to understand what rust is. "Rust" is the common name for what is scientifically known as iron oxide, a form of corrosion that occurs when iron (or one of its alloys, such as steel) reacts with oxygen and there is water (or heavy air moisture) present.
Other metals have oxidation processes as well, but they do so differently and the result is not commonly considered rust. Copper corrosion is green (and accounts for the color of the Statue of Liberty) while aluminum corrosion spreads extremely slowly.
The Molecular Process of Spreading
The process of metal corrosion is an electrochemical process. It happens on a molecular level as electrons transfer from iron molecules to the surrounding oxygen molecules, changing the makeup of the iron and turning it into rust. This is happening to iron all the time. In fact, it is impossible to find a piece of iron without at least some oxide present within it. However, the rate of rusting is usually slight and slow but is accelerated by water, especially if the water has a high concentration of electrolytes (substances in the water that help electrons move).This is why the presence of salt causes rust to spread more quickly.
Rust does not spread through contact like a biological infection. Instead, the process of iron oxidization occurs independently based on the conditions surrounding a particular piece of metal. This means that if one part of the piece is exposed to water, oxygen, and electrolytes but the rust of the piece is kept clean and dry, the protected metal will not rest at the rate of the wet metal.
Iron alloys will have different corrosion rates based on their makeup.
How Prevention Works
Steel is commonly protected from rusting through a process known as galvanization. In this process, the steel will be dipped with a coating of zinc, which protects the steel by reacting with water molecules. If the zinc coating on a piece of galvanized steel is scratched or scraped away, the exposed area will be vulnerable to rusting.