Steel is a mixture, of iron with carbon and small amounts of other metals. Pure iron can be melted and shaped, but also tends to be to some extent soft. With the addition of carbon, this will help to radically strengthen the metal. Black carbon steel is created during the manufacturing process when high temperatures create a thin layer of oxidized iron on the outer surface. Most carbon steels contain from one to two percent carbon.
Carbon bonds chemically with the iron in steel alloys, which generates a much harder material than pure iron. As the carbon content is increased, the material becomes harder, but at the same time more brittle, or likely to breaking under stress or load. Steel containing carbon in an amount over 2% is considered cast iron, which can be used for piping and non-structural materials, but it is definitely considered too brittle for structural steel.
Rust and Corrosion Protection
Iron reacts with oxygen in the air or moisture, which then goes on to create iron oxide (or rust), that will further cause parts to fail, so it is usually coated to prevent surface rust. One advantage of black carbon steel is the natural anti-corrosion property of the black iron oxide coating, because the oxide acts as a barrier to keep oxygen from the iron underneath. The thin oxide layer is formed at high temperatures, creating a durable layer requiring no further treatment or coating. There is also another method of cold blackening of steel, plus other ferrous metals, which Is possible, whereas the process works at room temperature without the need for a hot solution in order to work.
For applications where corrosion protection is of the essence, carbon steel has to be painted or galvanized. Steel is galvanized by way of an acid wash and then dipping the steel into a molten zinc bath. The zinc simply creates a protective layer on the outside of the steel. Galvanizing also helps to improve the working lifetime of steel parts, but is not used on black carbon steel, because the oxide layer already protects the steel.
Black carbon steel is frequently applied for gas or water utility piping, due to its low cost being able to be welded using common welding methods. Some very long-distance oil pipelines have utilised black carbon steel piping, because the pipe can be easily connected in the field and does not rust quickly. This steel can be employed in climates or ground conditions that may accelerate corrosion, but extra protection such as anodes may be necessary.
Cookware may be called black steel, but the dark colouring is a product of an oil treatment called seasoning, rather than manufacturing. Carbon steel is frequently coated with oil to prevent it from rusting, and steel cookware may be oiled and heated to absorb the oil molecules. The steel will darken and gain some rust-preventive properties, but this kind of seasoning is temporary and must be applied periodically.
Next time you see black carbon steel, you’ll know just how it was created.