Black oxide, which is also called blackening, is a type of chemical conversion coating process mainly applied to ferrous materials like steel or iron. The metal reacts chemically with a specific alkaline oxidising solution to produce an extremely thin layer of iron oxide(Fe₃O₄) film. This film(about 0.5-1.5μm)is uniformly black and tightly bonded to the metal substrate.
Unlike iron-based materials that naturally oxidize and rust in air, black oxidation can serve as a surface treatment method. The core reason lies in the fact that in an alkaline environment, a dense black oxide film is formed through the combined effects of high temperature and oxidants, tightly bonded to the iron substrate.
