Rust is an iron oxide, a usually red oxide formed by the redox reaction of iron and oxygen in the presence of water or air moisture. Rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3). Given sufficient time, oxygen, and water, any iron mass will eventually convert entirely to rust and disintegrate. Surface rust is flaky and friable, and it provides no protection to the underlying iron, unlike the formation of patina on copper surfaces. Rusting is the common term for corrosion of iron and its alloys, such as steel.
Rust is another name for iron oxide, which occurs when iron or an alloy that contains iron, like steel, is exposed to oxygen and moisture for a long period of time. Over time, the oxygen combines with the metal at an atomic level, forming a new compound called an oxide and weakening the bonds of the metal itself. Although some people refer to rust generally as "oxidation", that term is much more general and describes a vast number of processes involving the loss of electrons or increased oxidation state, as part of a reaction. The best-known of these reactions involve oxygen, hence the name "oxidation". The terms "rust" and "rusting" only mean oxidation of iron and its resulting products.
While standing on the rusty bridge look south and along the west bank you will see a rusty color along the bank. No, it’s not a chemical spill of some biohazardous material. You are looking at a seep a hydrological feature found all over the globe.
What looks like rust is, in fact, rust. It’s groundwater that flows through the soil, leaching out some of its constituent minerals and then seeping into the creek here. The groundwater that is seeping out here has passed by some iron along the way. Once the minerals have been exposed to oxygen it turns to this rust color.
Most iron is found within sedimentary rocks, iron-rich means that the sedimentary rocks contain at least 15% iron although most contain iron with less than 15%. The majority of these rock were deposited in three different geological time periods; known as eras: The Precambrian (3.8 billion years ago to 570 million years ago), the early Paleozoic (570 to 410 million years ago), and the middle to late Mesozoic (205 to 66 million years ago).
The Cenozoic Era in Nebraska is divided into the Paleogene and Neogene Periods. The Paleogene Period contains the Paleocene, Eocene, and Oligocene Epochs (65 to 23 million years ago). The Neogene Period contains the Miocene, Pliocene, Pleistocene, Holocene Epochs (23 million years ago until the present). During the Pleistocene, debris carried by glacial ice (called till) was deposited across eastern Nebraska. Geological evidence suggests large continental ice sheets covered parts of eastern Nebraska at least twice during the Pleistocene.
Sources:
Sund, Robert B.; Bishop, Jeanne (1980). Accent on science. C.E. Merrill. ISBN 9780675075695.
Waldman, J. (2015): Rust - the longest war. Simon & Schuster, New York. ISBN 978-1-4516-9159-7