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Electrolytes for decorative chromium plating based on trivalent chromium salts are known since several decades. As the use of conventional, hexavalent chromium based plating baths is more and more restricted by governmental regulations, these electrolytes gain ground in electroplating industry. However, compared to hexavalent chromium electrolytes, trivalent chromium electrolytes cannot fully meet the requirements with regard to appearance of the electrodeposited chromium, and there is little knowledge about the influencing factors on the shade of color. In this paper, chromium plated from a solution of chromium(III) sulfate was characterized by colorimetry, SEM and AFM and compared to a sample plated from a chromic acid electrolyte in order to reveal correlations between visual appearance and surface morphology. A relation between an increase of grain size and a color shift from blueish to yellowish was observed. Unlike in hexavalent based systems, grain size, roughness and color depend on layer thickness as the grain growth mechanism appears to be different. A model based on the theory of light scattering at rough surfaces is provided that links roughness and reflection behavior of the chromium surface.

The optical appearance of electroplated chromium layers is a crucial factor for decorative applications. Currently in the decorative chromium plating industry the so far used hexavalent electrolytes are being replaced progressively by solutions of trivalent chromium. However, chromium deposited from trivalent baths tends to have a yellowish color hue at thicknesses beyond 100 nm, which is undesired for most applications. The shift in color is related to a change in surface morphology due to the globular growth of the chromium nuclei. By utilizing pulsed current with on-times in the range of seconds, the grain growth is suppressed and the formation of fresh nuclei is favored. As a result, the average grain size of the layer can be decreased significantly. Compact chromium layers with small grains and improved color values are formed. A blueish appearance and high brightness were maintained up to thicknesses of more than 200 nm. Based on the results a combination of constant and pulsed current is suggested, yielding similar visual appearance as in the case of pulsed current only, but reaching the targeted film thickness much faster. Graphic abstract