The Influence of Current Density for Zinc Electrodeposition on Color Appearance of Black Trivalent Chromate Conversion Coatings

Abstract:

Chromate conversion coating is an important surface finishing process for electroplated zinc coatings that are widely employed in automotive applications. In addition to providing enhanced corrosion protection, the conversion coating offers a shade of colors to the coated products, both for aesthetic and functional benefits. Due to the stringent requirements on environmental issues, the industry is replacing the conventional hexavalent chromate with a more environmentally friendly trivalent chromate for the production of coatings. This effectively poses the requirement of the fundamental understanding on how the keys processing parameters of trivalent chromate conversion coating may relate to coloring of the coating products. In this work, for the first time, a systematic study is carried out to correlate the electroplating parameters, including the current density and electrolyte’s additives, on the formation of the trivalent chromate conversion coating, and hence the color appearance of the top-coats. Focusing on the black conversion coating, the color and optical properties are analyzed using a colorimeter and an optical spectrometer. The results notably show that, while the additives highly influence the observable shade of blackness, current density affects the optical properties in the visual spectrums. The microstructural and chemical characterization techniques, namely FE-SEM, OM, and XRD, are used to shed some light on the underlying mechanism that controls the color appearance. The understanding developed in this study will impact the design and fabrication of the electrogalvanizing products of desired color and esteemed functional performance.

• Periodical: Key Engineering Materials (Volume 658)
• Pages: 161-166
• DOI: https://doi.org/10.4028/www.scientific.net/KEM.658.161
• Online since: July 2015
• Authors: Nirada Pintuperakovit, Waleed Mohammed, Patama Visuttipitukul, Sittha Sukkasi, Yuttanant Boonyongmaneerat*
• Keywords: Electroplating, Optical Properties, Passivation, Trivalent Chromate, Zinc

References

[1] Tomachuk, C.R., et al., Morphology and corrosion resistance of Cr(III)-based conversion treatments for electrogalvanized steel. Journal of Coatings Technology and Research, (2010).

DOI: 10.1007/s11998-009-9213-1

[2] Dingwerth, B., Trivalent passivates need trivalent post-dips. Surface finishing guidebook, (2011).

DOI: 10.1016/s0026-0576(09)80073-1

[3] Bibber, J., An overview of nonhexavalent chromium conversion coatings—Part II: Zinc. Metal Finishing, 2002. 100(2): pp.98-102.

DOI: 10.1016/s0026-0576(02)80162-3

[4] Zhang, X., et al., Comparison of the morphology and corrosion performance of Cr(VI)- and Cr(III)-based conversion coatings on zinc. Surface and Coatings Technology, 2005. 199(1).

DOI: 10.1016/j.surfcoat.2004.12.002

[5] Gigandet, M.P., J. Faucheu, and M. Tachez, Formation of black chromate conversion coatings on pure and zinc alloy electrolytic deposits: role of the main constituents. Surface and Coatings Technology, 1997. 89(3): pp.285-291.

DOI: 10.1016/s0257-8972(96)03013-7

[6] Frąckowiak, E. and J.M. Skowroński, Passivation of zinc in alkaline solution effected by chromates and CrO3–graphite system. Journal of Power Sources, 1998. 73(2): pp.175-181.

DOI: 10.1016/s0378-7753(97)02799-7

[7] Zaki, N., Trivalent chrome conversion coating for zinc and zinc alloys. Metal Finishing, (2007).

DOI: 10.1016/s0026-0576(07)80361-8

[8] Martyak, N.M., J.E. McCaskie, and L. Harrison, Corrosion behavior of zinc chromate coatings. Metal Finishing, 1996. 94(2): pp.65-67.

DOI: 10.1016/s0026-0576(96)93872-6

[9] Rocco, A.M., et al., Evaluation of chromate passivation and chromate conversion coating on 55% Al–Zn coated steel. Surface and Coatings Technology, (2004).

DOI: 10.1016/s0257-8972(03)00847-8

[10] Long, Z.L., Y.C. Zhou, and L. Xiao, Characterization of black chromate conversion coating on the electrodeposited zinc–iron alloy. Applied Surface Science, (2003).

DOI: 10.1016/s0169-4332(03)00572-5

[11] Tencer, M., Electrical conductivity of chromate conversion coating on electrodeposited zinc. Applied Surface Science, 2006. 252(23).

DOI: 10.1016/j.apsusc.2005.10.039

[12] Campestrini, P., et al., Study of the formation of chromate conversion coatings on Alclad 2024 aluminum alloy using spectroscopic ellipsometry. Thin Solid Films, (2002).

DOI: 10.1016/s0040-6090(02)00253-5

[13] Othonos, A., et al., Photothermal radiometry on nickel (pigmented aluminium oxide) selective solar absorbing surface coatings. Solar Energy Materials and Solar Cells, (1998).

DOI: 10.1016/s0927-0248(97)00232-8

[14] Eppensteiner, F.W. and M.R. Jennkind, Chromate conversion coatings. Metal Finishing, (2007).

DOI: 10.1016/s0026-0576(07)80360-6