Alkaline chemical oxidation blackening is to heat the qualified spring after production and processing into the solution with concentrated alkali, so that a thin and dense oxide film can be obtained on the surface of the spring, which has the function of anti-corrosion.
   When the spring is in the heated solution, because the alkali corrodes the spring surface, iron ions (ferrous compounds) are caused. Adding oxidant sodium nitrite to the alkali can change the oxidation process and form a dense oxide film (ferric oxide) on the spring surface. This kind of oxide film is mainly composed of magnetic iron oxide, and the formation time of oxide film is 30 ~ 60 minutes. If the time is prolonged, the thickness of the oxide film cannot be increased.