Tailoring Color and Surface Morphology of Electrodeposited Chromium By Application of Pulsed Current

Abstract

The electrodeposition of chromium layers for decorative purposes plays an important role in the surface finishing industry, especially for applications in sanitary, automotive, plating on plastics and similar fields. As the use of conventional hexavalent chromium based electrolytes is strongly restricted in several countries,1 the demand for alternative plating systems increased during the last years. The most promising alternative is the electrodeposition from trivalent chromium based electrolytes considering environmental safety, quality of the deposits and cost issues.2 Commercially applicable processes are already known since several decades.3 However, trivalent chromium based processes cannot fully meet the requirements on surface appearance of the layers.4 This is crucial for the above mentioned applications as slight differences in color can have a high impact on final implementation of the plated parts. In this work chromium layers plated from different electrolytes were characterized by colorimetry, SEM and AFM. A relation between optical appearance and surface morphology was found and supported by a model based on the theory of light scattering on rough surfaces.5 In case of chromium(III) sulfate based electrolytes, grain size and roughness increase with increasing layer thickness, leading to a color shift from blueish to yellowish. To achieve a blueish appearance while having sufficient resistance against corrosion and wear, a fine-grained surface structure has to be obtained at high layer thickness. By application of pulsed cathodic current, grain growth is suppressed and nucleation is re-initiated at each current pulse, giving a surface grain size of 60 to 100 nm. Up to a chromium thickness of 200 nm, the color value still in the blueish range (b* < 0 in the L*a*b* color system). However, special characteristics of chromium electrodeposition from trivalent solutions have to be considered for choosing suitable pulse parameters in large scale applications. European Chemicals Agency, Authorisation List, EC No. 215-607-8, 231-801-5, 231-889-5, 231-906-6, 232-140-5, 232-143-1, 234-190-3, 236-881-5. Z. X. Zeng, A. M. Liang, and J. Y. Zhang, Recent Pat. Mater. Sci., 2 (1), 50 (2009).S. L. Handy, C. F. Oduoza, and T. Pearson, Trans. IMF, 84 (6), 300 (2006).D. L. Snyder, Plat. Surf. Finish., 90 (11), 34 (2003).M. Leimbach, C. Tschaar, D. Zapf, M. Kurniawan, U. Schmidt, and A. Bund, J. Electrochem. Soc., 166 (6), D205 (2019).