Home arrow Knowledgebase arrow Iron and steel rust
Iron and steel rust

Iron and steel, the most commonly used metals, corrode in many media including most outdoor atmospheres. Usually they are selected not for their corrosion resistance but for such properties as strength, ease of fabrication, and cost. These differences show up in the rate of metal lost due to rusting. All steels and low-alloy steels rust in moist atmospheres. In some circumstances, the addition of 0.3% copper to carbon steel can reduce the rate of rusting by one quarter 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 melt down iron and mix in small amounts of other elements. There are different types of steel based on what they mix in and how much. Each type of steel has different advantages, such as stength, durability, chemical reactivity, and many other qualities. Typical elements that are added to iron to make steel include Carbon, Manganese, Silicon, Chromium, and Molybdenum. But most of the atoms of steel  re still iron, because they only usually add in a few percent of other elements.

Image of steel rust

Many elements like to go through chemical reactions with other elements. These are react with oxygen to form a chemical compound that is a combination of that material and oxygen. When iron combines with oxygen, it forms iron oxide, or rust. Iron oxide is a larger molecule than iron, so if iron oxidizes, it often puffs up and may even flake. This is because the rust requires more physical space than the original iron. We can’t keep oxygen away from the iron in our cars since much of the atmosphere is oxygen. Rust weakens the iron and can make parts break. Steel has the same problem, but some of those elements that are added during the steel production can help prevent rust or at least slow it down. For example, "stainless steel" is designed so that it will not rust, but it is more expensive that other types of steel.

Some things cause steel or iron to rust faster than others. Water will cause iron and steel to rust. Dissimilar metals rust faster than single metals because of electrochemical reactions, so steel rusts faster than iron, and joints between dissimilar metals rust very quickly. Salt water will cause rust faster than water because 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 than iron. The puffing causes cracks and voids, which expose more bare metal to the environment. So the rusting of iron can progress and is only limited by destruction of all solid iron. Other metals oxidize, but the oxides of some other metals are no larger than the metal themselves, so they don't puff up or flake. For example, aluminum doesn't puff up when it oxidizes. This helps make aluminum oxide a good protective coating, rather than the start of rapid degeneration.

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 is a good protection for steel. Like aluminum oxide, black oxide molecules are the same size as iron molecules, so black oxide does not grow or flake. Black oxide is true gun bluing and the oxide found on some drill bits. Black oxide is also seen on iron and steel that has been hot-worked.

Steel can be coated with black oxide by a careful regimen of rusting the right amount 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 attractive and durable results after several treatments.

Rapid rust of steel needs water, low pH (preferably with HCl (available at hardware stores as Muriatic acid [caution: very corrosive liquid and vapor), and oxygen. Moderate heat (about 60-80 C) also helps.

Once rust is formed, there is mobile oxygen in the metal, and the oxygen can move deeper into the metal causing further rust. Techniques to kill rust are described below. All rely on dissolving rust or converting rust into black oxide and then coating the metal with something that inhibits the formation of more rust.

Stainless steel does not rust

Stainless steel remains stainless, or does not rust, because of the interaction between its alloying elements and the environment. Stainless steel contains iron, chromium, manganese, silicon, carbon and, in many cases, significant amounts of nickel and molybdenum. These elements react with oxygen from water and air to form a very thin, stable film that consists of such corrosion products as metal oxides and hydroxides. Chromium plays a dominant role in reacting with oxygen to form this corrosion product film. In fact, all stainless steels by definition contain at least 10 percent chromium.

The presence of the stable film prevents additional corrosion by acting as a barrier that limits oxygen and water access to the underlying metal surface. Because the film forms so readily and tightly, even only a few atomic layers reduce the rate of corrosion to very low levels. The fact that the film is much thinner than the wavelength of light makes it difficult to see without the aid of modern instruments. Thus, although the steel is corroded on the atomic level, it appears stainless. Common inexpensive steel, in contrast, reacts with oxygen from water to form a relatively unstable iron oxide/hydroxide film that continues to grow with time and exposure to water and air. As such, this film, otherwise known as rust, achieves sufficient thickness to make it easily observable soon after exposure to water and air.

So stainless steel does not rust, because it is sufficiently reactive to protect itself from further attack by forming a passive corrosion product layer. (Other important metals such as titanium and aluminum also rely on passive film formation for their corrosion resistance.) Because of its durability and aesthetic appeal, stainless steel is used in a wide variety of products, ranging from eating utensils to bank vaults to kitchen sinks.