A. The oxygen of the air combines with the surface of the iron, and produces oxide of iron, which is generally called rust.
This rust is a species of combustion.
Q. Why does hot iron scale and peel off, when struck with a hammer?
A. The oxygen of the air very readily unites with the surface of the hot iron, and forms a metallic oxide (or rust) which scales off when struck with a hammer.
Q. Does iron rust in dry air?
A. No; iron undergoes no change in dry air.
Q. Why do stoves and fire-irons become rusty, in rooms which are not occupied?
A. Because the air is damp; and moist air oxidizes (or rusts) iron and steel.
Q. In what part of the year is it most difficult to keep stoves and fire-irons bright?