Steel is not a pure metal, but actually an alloy mixture of iron and carbon. Iron comes from the earth as a natural ore. This creates problems in construction from the amount of carbon in the ore. To make steel, iron is melted to reduce the carbon; at which point other elements are added. What all of this means is that iron is susceptible to rust and corrosion.
When iron comes into contact with water, oxygen, other oxidants or acids, it rusts. If salt is present in the air from seawater or icy roads, the iron rusts at an accelerated pace. Oxygen and water forms an oxide coating of ferrous oxide. Ferrous oxide rust is usually permanent, weak and unsightly.
Another degrader of iron and its alloys is carbon dioxide. The oxygen in carbon dioxide forms iron hydroxide. Hydroxide molecules flake off from the outer layer, exposing the next layer of iron to the oxidizing elements. As the new iron becomes exposed, reacts and flakes off, the corrosion continues until the iron is consumed.
The oxides formed by corrosion take up more space than the original metal. This expansion in volume eventually generates at a tremendous growing force, causing irreparable damage to iron structures.