Luoyang surface treatment blackening

Want to ask why Luoyang's surface treatment is black? There are several reasons:

1. oxidation: some metal materials will form an oxide layer after surface treatment, showing black or dark gray.

2. Corrosion: Some metal materials will be corroded during surface treatment, resulting in black oxide.

3. Heat treatment: When some metal materials are heated at high temperature, surface carbonization will occur, resulting in surface blackening.

4. Precipitates: Some metal materials will form some precipitates on the surface after surface treatment, which may appear black or dark gray.