Rust is a form of corrosion that occurs on certain types of metal. A number of factors must be present for rust to occur. Some environments — those with heavy moisture, poor metal quality and constant electricity — tend to be more corrosive than others. Essentially, rust occurs when iron oxidizes and absorbs water. Understanding the properties of rust can help you avoid losing quality materials to this damage.
Different types of metal have varying rust problems. Stainless steel with additives like nickel and chromium do not tend to oxidize like other metals, such as iron or lower-quality steel. Metals with heavy amounts of iron in them are typically more susceptible to rust. Metals like copper or lead corrode, but do not rust, while gold and platinum do not react with oxygen.
Though iron-based metal has physical changes when it rusts -- the brown-orange corrosive layer looks different -- it scientifically considered a chemical change. A physical change in science, like freezing or evaporating, does not change the substance involved. When iron or steel is exposed to water, it picks up oxygen and a new chemical compound, iron oxide, is formed.
Permanence and Mass
Unlike the physical changes of ice or evaporation, a chemical change is permanent. Rust cannot be reversed, only removed. In addition to permanence, rust also adds mass to the metal, which is why it often extends past the surface of surrounding metal.
Rust and corrosion occur more frequently when metals is exposed to water or salt. A layer of paint or enamel prevents contact and can delay oxidation -- exposure to oxygen. Adding zinc to iron or steel can help rust from forming, as the zinc acts as a "sacrificial lamb" and corrodes first, saving the other metal. "Galvinized" metal has been coated with a thin layer of zinc, which corrodes on the surface, but does not penetrate the metal.