The New Student's Reference Work/Rust
Rust. Neither wrought iron nor steel nor cast iron can be exposed to moist air without rusting. But wrought iron, being nearly pure iron, rusts more readily than either of the others, which contain carbon. When a drop of rain falls on a clean, bright surface of iron, for a short time the drop stays clear, showing the bright surface of the iron through it. But soon a greenish appearance, which is a precipitate, is seen in the drop, and quickly becomes a reddish-brown — the peroxide of iron. This is rust. The rust does not stick to the iron but is hung in the water, and becomes a coating only when the water has evaporated. Iron remains quite free from rust in an atmosphere containing water-vapor, so long as the water-vapor does not condense as liquid water on the surface of the iron. But when rust once forms, the iron will go on rusting in an atmosphere in which a piece of clean iron will not rust, because liquid water will condense on rust when it will not on bright iron. So it is much easier to prevent the first formation of rust than to stop the process. To prevent rust, oil-paint is used, also a zinc coating (galvanizing) and japanning.