Low-frequency pulse plating for tailoring the optical appearance of chromium layers for decorative applications
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.