Evadne Ruggerip age 12 of Price Utah for her question:

What causes iron rust?

Iron rusts by a slow kind of burning. This will happen when the iron is exposed to moist air. Certain gases in the air then react with the iron and change it into rust. oxygen gas in the air is necessary to start the rusting. A small mount of carbon dioxide gas is also necessary. But these gases cannot cause the iron to rust unless moisture is also present.

The rusting breaks up the hard iron into smalls reddish‑brown flakes. The rusted material is usually a mixture of several chemicals. The main one is hydrate ferric oxide. There may also be ferrous oxide and iron carbonates in the rust. These iron compounds form as‑the metal Iron burns  or oxidizes$ with the gases and moisture in the air.

In times a solid iron bar will rust entirely away into reddish flakes and rusty powder. Iron bridges and steel cables are covered with special coatings to prevent them from rusting. Outdoor iron and steel must be sealed off from air and moisture or they would soon rot away.