Carbon steel is a mixture of iron and carbon, with small amounts of other metals. Adding carbon strengthens the metal, but too much makes it brittle. Carbon black steel has a natural anti-corrosion property due to the oxide layer created during manufacturing. It is often used for gas or water piping and can be welded easily. Cookware called black steel is not manufactured that way, but is seasoned with oil to prevent rust.
Steel is an alloy, or mixture, of iron with carbon and small amounts of other metals. Pure iron can be cast and shaped, but tends to be soft. Adding carbon will strengthen the metal, and most carbon steels contain one to two percent carbon. Carbon black steel is created during the manufacturing process when high temperatures create a thin layer of oxidized iron on the outer surface.
Carbon chemically bonds with iron in steel alloys, creating a material that is much harder than pure iron. As the carbon content increases, the material becomes harder but more brittle, or prone to cracking under stress or load. Steel containing more than 2% carbon is considered cast iron; this can be used for piping and non-structural materials, but is considered too brittle for structural steel.
Iron reacts easily with oxygen in the air, creating iron oxide or rust that will cause parts to fail, so it is often coated to prevent surface rust. An advantage of carbon black steel is the natural anti-corrosion property of the black iron oxide coating, as the oxide acts as a barrier to trap oxygen from the underlying iron. The thin oxide layer is created at high temperatures, forming a durable layer that requires no further treatment or coating.
For applications where corrosion protection is critical, carbon steel can be painted or galvanized. The steel is galvanized by acid washing, then immersion of the steel in a bath of molten zinc. Zinc forms a layer on the outside of the steel and will corrode first when exposed to air or moisture. Zinc plating will improve the life of steel parts, but is not used on black carbon steel, as the oxide layer already protects the steel.
Carbon black steel is often used for gas or water piping because it is inexpensive and can be welded by common methods. Some long-distance pipelines have used black carbon steel pipes, because the pipe can be connected in the field and does not rust quickly. This steel can be used in climates or ground conditions that can accelerate corrosion, but additional protection such as anodes may be required. Anodes are buried zinc rods that corrode first when connected to the steel pipe.
The cookware may be called black steel, but the dark color is a product of an oil treatment called seasoning, rather than manufacturing. Carbon steel is often coated in oil to prevent rust, and steel cookware can be oiled and heated to absorb oil molecules. The steel will darken and acquire some anti-rust properties, but seasoning is a temporary coating and must be repeated periodically.
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