Surface treatment of bolts

Electroplating terms: Galvanizing, yellow chromate passivation (also known as colored passivation), with a film thickness of 4–5 micrometers.

After chromium plating, a yellow coating is formed

Yellow chromated = treatment with yellow chromate. Four types of chromate treatments are known, named after their color: 1) Colorless and blue chromate treatments: The blue chromate layer is at most 80 nm thick; it is pale blue in color, and depending on its thickness, it can exhibit rainbow colors such as gold, slightly red, light blue, green, or yellow due to light refraction. A very thin layer of chromate has almost no color of its own, and it is classified as a colorless chromate treatment. Chromate treatment solutions can be composed of hexavalent and trivalent chromates and their mixtures, as well as a supporting electrolyte and inorganic acids. 2) Yellow chromate treatment: The yellow chromate treatment layer is about 0.25–1.1 thick, appearing golden yellow with the typical strong purple-green iridescent color. The chromate treatment solution is essentially composed of water-soluble hexavalent chromates, a supporting electrolyte, and inorganic acids. Yellow is caused not only by trivalent chromium produced through reduction during the reactions in the cambium, but also by a large proportion of hexavalent complexes present (80–220 mg/m2). The chromate treatment solution is used at room temperature. According to DIN 50021 SS, in a salt spray chamber, the total corrosion resistance of fully chromated yellow coatings before the appearance of initial corrosion products is 100–200 hours. According to DIN 50961, Chapter 10, Table 3, the minimum requirement for Method Group C is 72 hours for workpieces located in the chamber, and 96 hours for workpieces located on the rack. 3) Olive-colored chromate treatment: A typical olive-colored chromate treatment layer has a thickness of up to 1.5 pm, and ranges in color from olive green to olive brown. The chromate treatment solution is essentially composed of water-soluble hexavalent chromates ； It supports electrolytes and inorganic acids, especially those composed of phosphates or phosphoric acid, and may also contain formates. A large amount of chromium (VI) (300–400 mg/m³) is incorporated into the layer. The chromate treatment solution is used at room temperature. 4) Black chromate treatment: The black chromate treatment layer is essentially a yellow or olive-colored chromate treatment to which colloidal silver is added as a pigment. The chromate treatment solution has a composition roughly identical to that of the yellow or olive-colored chromate treatment solution, and it also contains silver ions. If the composition of the chromate treatment solution is appropriate, oxides of iron, nickel, or cobalt are mixed as black pigments into the chromate layer on zinc alloy layers such as Zn/Fe, Zn/Ni, or Zn/Co, making silver unnecessary in this case. Depending on whether the base is yellow or olive-colored, a large amount of chromium (VI) at 80–400 mg/m2 is incorporated into the chromate layer. Chromate treatment: The solution is used at room temperature.