According to Wikipedia, rust is stated as this,

“Rust is a general term for describing iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture. There are also other forms of rust, such as the result of the reaction of iron and chloride in an environment deprived of oxygen, such as rebar used in underwater concrete pillars, which generates green rust. Several forms of rust are distinguishable visually and by spectroscopy, and form under different circumstances.[1] Rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxideFeO(OH)·Fe(OH)3”

According to NACE, formerly known as the National Association of Corrosion Engineers, rust causes billions of dollars in damages each year. Not only is it unsightly, untreated corrosion weakens metals and causes permanent damage.