What is the black stuff that comes off when steel is forged or forged?

Oxide scale: ferrous oxide, ferric oxide, ferric oxide.

Oxide scale is a corrosion product formed by the oxidation of steel at high temperatures. It is composed of ferrous oxide, ferric oxide, and ferric oxide. From the inside to the outside: ferrous oxide, ferric oxide, ferric oxide. Among them, ferrous oxide has a loose structure and has a weak protective effect, while ferric oxide and ferric oxide have a dense structure and have better protective properties.

The oxide layer is brittle and has no extensibility. It is easy to crack and detach under mechanical action and thermal processing. Iron oxide and ferrous oxide generate iron hydroxide under the action of water, causing the oxide scale to expand, crack, or even fall off. On the original oxide scale, there are always cracks that reach as deep as the matrix. When the electrolyte pours into the crack, the iron and oxide scale form a primary battery. Oxide scale is the cathode, and iron acts as the anode to accelerate corrosion. Therefore, the larger the area of oxide scale, the faster the corrosion rate of the steel matrix and the more serious the corrosion.

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