For rust to occur in a piece of iron (or its alloys, such as steel), it must be in contact with oxygen (which makes up 21% of the air we breathe) and water. This means rust can occur not just after it has been formed and shaped into the screws holding you garden shed together, or the bolts holding a bridge together, but at almost any stage of its manufacturing process, from initial extraction and production to when it is in place as a final structure if its hasn't undergone correct rust prevention or treatment methods.

Rust treatment for surfaces

Due to the conditions necessary for rust to form, metal can only rust on the surface, not from the inside, so if rust is treated, removed or prevented from accumulating in the first place, the metal below is in good condition. Rust treatment is available if rust does occur.
When and where does rust occur?

Rusting during production

During production, there are many opportunities for the occurrence of rust whilst cutting the metal into shape, machining to refine its figure, as well as the milling of the metal. This is because the metal is almost constantly exposed to air and frequently exposed to cutting fluids (water) during the production process.

Transportation rusting

During transportation, for example during the process of sea shipping, land freight, and air freight; rusting and corrosion could occur, as the metal is exposed to oxygen and possibly water, which are the two elements responsible for causing rusting.

Parts/tools/equipment rusting during storage

Rusting could also occur when storing spare parts, tools, or other equipment. If they're not cared for or properly packaged and protected, they could be left exposed long enough to react with the oxygen and water vapour in the air, resulting in corroding and rusting.

Infrastructure rusting when exposed to elements

Large buildings with iron or steel framework, metal bridges and other examples of infrastructure are all exposed to the elements, therefore are at risk of rusting