Rust in Iron and SteelIron and steel, the most commonly used metals, corrode in many mediaincluding most outdoor atmospheres. Usually they are selected not for theircorrosion resistance but for such properties as strength, ease of fabrication, andcost. These differences show up in the rate of metal lost due to rusting. All steelsand low-alloy steels rust in moist atmospheres. In some circumstances, theaddition of 0.3% copper to carbon steel can reduce the rate of rusting by onequarter or even by one half.Iron is a pure element. Every atom in a chunk of iron is an iron atom. Iron is nice,but it’s not very strong. Steel is even better than iron. To make steel, they meltdown iron and mix in small amounts of other elements. There are differenttypes of steel based on what they mix in and how much. Each type of steel hasdifferent advantages, such as stength, durability, chemical reactivity, and manyother qualities. Typical elements that are added to iron to make steel includeCarbon, Manganese, Silicon, Chromium, and Molybdenum. But most of theatoms of steel re still iron, because they only usually add in a few percent ofother elements (Rust in screw)
Many elements like to go through chemical reactions with other elements.These are react with oxygen to form a chemical compound that is a combinationof that material and oxygen. When iron combines with oxygen, it forms ironoxide, or rust. Iron oxide is a larger molecule than iron, so if iron oxidizes, itoften puffs up and may even flake. This is because the rust requires morephysical space than the original iron. We can’t keep oxygen away from the ironin our cars since much of the atmosphere is oxygen. Rust weakens the iron andcan make parts break. Steel has the same problem, but some of those elementsthat are added during the steel production can help prevent rust or at least slowit down. For example, "stainless steel" is designed so that it will not rust, but it ismore expensive that other types of steel.Some things cause steel or iron to rust faster than others. Water will cause ironand steel to rust. Dissimilar metals rust faster than single metals because ofelectrochemical reactions, so steel rusts faster than iron, and joints betweendissimilar metals rust very quickly. Salt water will cause rust faster than waterbecause salt water is a better electrical conductor. Like most chemical reactions,heat also speeds rust.If steel starts to rust, it will puff up because iron oxide is a larger molecule thaniron. The puffing causes cracks and voids, which expose more bare metal to theenvironment. So the rusting of iron can progress and is only limited bydestruction of all solid iron. Other metals oxidize, but the oxides of some othermetals are no larger than the metal themselves, so they dont puff up or flake.For example, aluminum doesnt puff up when it oxidizes. This helps makealuminum oxide a good protective coating, rather than the start of rapiddegeneration.Rust is really Fe2O3, a reddish form of iron oxide. Iron has another oxide, Fe3O4,which is sometimes called black oxide, black rust, or hammerscale. Black oxide isa good protection for steel. Like aluminum oxide, black oxide molecules are thesame size as iron molecules, so black oxide does not grow or flake. Black oxide istrue gun bluing and the oxide found on some drill bits. Black oxide is also seenon iron and steel that has been hot-worked.
Steel can be coated with black oxide by a careful regimen of rusting the rightamount and boiling the rusted metal in water to convert it. This is how non-caustic gun bluing is done, and although it is tedious, it produces very attractiveand durable results after several treatments. Rapid rust of steel needs water, low pH (preferably with HCl (available athardware stores as Muriatic acid [caution: very corrosive liquid and vapor), andoxygen. Moderate heat (about 60-80 C) also helps.sOnce rust is formed, there is mobile oxygen in the metal, and the oxygen canmove deeper into the metal causing further rust. Techniques to kill rust aredescribed below. All rely on dissolving rust or converting rust into black oxideand then coating the metal with something that inhibits the formation of moreStainless steel does not rust:Stainless steel remains stainless, or does not rust, because of the interactionbetween its alloying elements and the environment. Stainless steel containsiron, chromium, manganese, silicon, carbon and, in many cases, significantamounts of nickel and molybdenum. These elements react with oxygen fromwater and air to form a very thin, stable film that consists of such corrosionproducts as metal oxides and hydroxides. Chromium plays a dominant role inreacting with oxygen to form this corrosion product film. In fact, all stainlesssteels by definition contain at least 10 percent chromium.The presence of the stable film prevents additional corrosion by acting as abarrier that limits oxygen and water access to the underlying metal surface.Because the film forms so readily and tightly, even only a few atomic layersreduce the rate of corrosion to very low levels. The fact that the film is muchthinner than the wavelength of light makes it difficult to see without the aid ofmodern instruments. Thus, although the steel is corroded on the atomic level, itappears stainless. Common inexpensive steel, in contrast, reacts with oxygenfrom water to form a relatively unstable iron oxide/hydroxide film thatcontinues to grow with time and exposure to water and air. As such, this film,
otherwise known as rust, achieves sufficient thickness to make it easilyobservable soon after exposure to water and air.So stainless steel does not rust, because it is sufficiently reactive to protect itselffrom further attack by forming a passive corrosion product layer. (Otherimportant metals such as titanium and aluminum also rely on passive filmformation for their corrosion resistance.) Because of its durability and aestheticappeal, stainless steel is used in a wide variety of products, ranging from eatingutensils to bank vaults to kitchen sinks. Reference:http://www.clihouston.com/knowledge-base/iron-and-steel-rust.html