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Why Does Iron Get Rusty?

When you leave a piece of iron for a few days in a place where it’s damp or wet, a covering of rust forms over it just as if someone had come along and painted that reddish color on it. What is rust? Why does it form on iron and steel?

Rust is iron oxide. It is created when iron burns by uniting with oxygen dissolved in water. This means that unless there is moisture in the air or water is actually present, the oxygen is not dissolved in water, and rust cannot begin to form.

When a drop of rain falls on a bright iron surface, the drop will remain clear for a short time. But the iron and the oxygen in the water soon begin to unite and form iron oxide, or rust, inside the drop.

The drop will turn a reddish color and the rust is suspended in the water. When the drop evaporates, the rust will remain and form a reddish coat on the iron itself.

Once started, rust will spread even in dry air. This is because the rough spot of rust helps whatever moisture there is in the air to condense; it attracts the moisture and holds it.

That’s why it’s easier to prevent rust from starting than to prevent it from spreading once it has started. Since iron and steel articles often have to be stored for long periods of time, the problem of preventing rust is an important one.

Sometimes such articles are coated with special paint or a plastic film. But what would you do if you had to keep the insides of battleships rust-free when these ships are not in use?

Our Government has solved it by using dehumidifiers. These are machines that draw moist air out of the compartments of the ship and replace it with dry air, so rust never gets a chance to start.

Read: What Is Humidity?