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It is a surface treatment performed in a warm bath with blackening salt - chemical passivation of the surface:

  • a very thin (raughly of 1 μm) black layer of ferrous-ferric oxide is formed
  • the oxide layer is relatively durable and well wettable for oil - the process ends with the preservation of parts
  • good protection against corrosion and abrasion resistance
  • suitable materials for this surface treatment are iron castings, non-alloy and low-alloy steels