Iron oxide, a reddish-brown compound, is normally referred to as rust. It forms when iron and oxygen react in water or in moisture in the air. The reaction of iron and chloride underwater is also referred to as rust. Certain factors speed up the rusting process, such as salt in the water.
TL;DR (Too Long; Didn't Read)
Rusting is a common form of corrosion, which occurs when metal atoms react with their environment. Salt water does not make a metal rust, but it accelerates the rusting process because electrons move more easily in salt water than they do in pure water.
How Metals Rust
Not all metals rust. For example, aluminum doesn’t rust because it has a protective layer of aluminum oxide on its surface. This stops the metal coming into direct contact with water (or moisture in the air) and oxygen. On the other hand, iron rusts because it forms hydrated iron oxide when it comes into contact with water (or moisture in the air) and oxygen.
Rusting cannot occur without both water and oxygen. Water helps iron react with oxygen by breaking up the oxygen molecule. During the initial stages of rusting, iron loses electrons and oxygen gains electrons. Ferrous and ferric ions then react with water to form ferrous hydroxide, ferric hydroxide and hydrogen. The hydroxides lose their water to make even more iron compounds. The sum of all these chemical reactions makes the rust flake, so it falls off the iron and exposes new iron, which can then also begin to rust.
Salt Water vs. Fresh Water
Current flows more easily in salt water than it does in fresh water. This is because salt water, an electrolyte solution, contains more dissolved ions than fresh water, meaning electrons can move more easily. Since rusting is all about the movement of electrons, iron rusts more quickly in salt water than it does in fresh water. Certain metal objects that spend a lot of time submerged in salt water, such as boat engines, rust quickly. However, objects do not have to be completely submerged in salt water for this to happen because increased moisture in the air and salt spray can provide the electrolyte's cation (positive ions) and anions (negative ions).
Preventing Rusting of Metal
Coating iron with a protective layer of zinc stops it from rusting because zinc stops the reaction between iron and oxygen and water. This is known as galvanization. Specially manufactured paint can also stop salt water or salty air from making metal rusty.
About the Author
Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.