Why the decorative chromium coating electrodeposited from trivalent chromium electrolyte containing formic acid is darker

The color of the decorative chromium coatings electrodeposited from trivalent chromium electrolytes containing formic acid or formate is usually darker than that from conventional hexavalent chromium electrolytes. For the purpose of clarifying the reason for this situation, X-ray diffraction (XRD), selected area electron diffraction (SAED), electron probe microanalyzer (EPMA) and high-resolution transmission electron microscopy (HRTEM) were employed to investigate the phases and fine microstructure of the typical decorative chromium coating produced from a typical trivalent chromium electrolyte containing formic acid. The results show that the coating is amorphous embedded with the nanocrystals of graphite, Cr23C6 and body-centered cubic structure chromium (bcc Cr). The presence of the nanocrystal graphite with strong infrared emission performance is the main reason for the darker color of the coating.