Science, Tech, Math › Science How Rust and Corrosion Work Share Flipboard Email Print PhotoStock-Israel / Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated January 22, 2020 Rust is the common name for iron oxide. The most familiar form of rust is the reddish coating that forms flakes on iron and steel (Fe2O3), but rust also comes in other colors including yellow, brown, orange, and even green! The different colors reflect various chemical compositions of rust. Rust specifically refers to oxides on iron or iron alloys, such as steel. Oxidation of other metals has other names. There is tarnish on silver and verdigris on copper, for example. Key Takeaways: How Rust Works Rust is the common name of the chemical called iron oxide. Technically, it's iron oxide hydrate, because pure iron oxide isn't rust. Rust forms when iron or its alloys are exposed to moist air. The oxygen and water in air react with the metal to form the hydrated oxide. The familiar red form of rust is (Fe2O3), but iron has other oxidation states, so it can form other colors of rust. The Chemical Reaction That Forms Rust Although rust is considered the result of an oxidation reaction, it's worth noting not all iron oxides are rust. Rust forms when oxygen reacts with iron, but simply putting iron and oxygen together isn't sufficient. Although about 21% of air consists of oxygen, rusting doesn't occur in dry air. It occurs in moist air and in water. Rust requires three chemicals in order to form: iron, oxygen, and water. iron + water + oxygen → hydrated iron(III) oxide This is an example of an electrochemical reaction and corrosion. Two distinct electrochemical reactions occur: There is anodic dissolution or oxidation of iron going into aqueous (water) solution: 2Fe → 2Fe2+ + 4e- Cathodic reduction of oxygen that is dissolved into water also occurs: O2 + 2H2O + 4e- → 4OH- The iron ion and the hydroxide ion react to form iron hydroxide: 2Fe2+ + 4OH- → 2Fe(OH)2 The iron oxide reacts with oxygen to yield red rust, Fe2O3.H2O Because of the electrochemical nature of the reaction, dissolved electrolytes in water aid the reaction. Rust occurs more quickly in saltwater than in pure water, for example. Keep in mind oxygen gas (O2) is not the only source of oxygen in air or water. Carbon dioxide (CO2) also contains oxygen. Carbon dioxide and water react to form weak carbonic acid. Carbonic acid is a better electrolyte than pure water. As the acid attacks the iron, water breaks into hydrogen and oxygen. Free oxygen and dissolved iron form iron oxide, releasing electrons, which can flow to another part of the metal. Once rusting starts, it continues to corrode the metal. Preventing Rust Rust is brittle, fragile, progressive, and weakens iron and steel. To protect iron and its alloys from rust, the surface needs to be separated from air and water. Coatings can be applied to iron. Stainless steel contains chromium, which forms an oxide, much like how iron forms rust. The difference is the chromium oxide does not flake away, so it forms a protective layer on the steel. Additional References Gräfen, H.; Horn, E. M.; Schlecker, H.; Schindler, H. (2000). "Corrosion." Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.b01_08 Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. Academic Press. ISBN 0-12-352651-5. Waldman, J. (2015). Rust - The Longest War. Simon & Schuster. New York. ISBN 978-1-4516-9159-7. View Article Sources “10 Interesting Things about Air.” NASA: GLOBAL CLIMATE CHANGE: Vital Signs of the Planet, NASA, 12 Sept. 2016. Cite this Article Format mla apa chicago Your Citation Helmenstine, Anne Marie, Ph.D. "How Rust and Corrosion Work." ThoughtCo, Feb. 16, 2021, thoughtco.com/how-rust-works-608461. Helmenstine, Anne Marie, Ph.D. (2021, February 16). How Rust and Corrosion Work. Retrieved from https://www.thoughtco.com/how-rust-works-608461 Helmenstine, Anne Marie, Ph.D. "How Rust and Corrosion Work." ThoughtCo. https://www.thoughtco.com/how-rust-works-608461 (accessed May 6, 2021). copy citation Watch Now: What Are Types of Chemical Reactions?