The demand for alternatives for chromic acid based electrolytes in chromium electroplating increases rapidly as the use of hexavalent chromium is restricted in serval countries [1]. In chromium plating for decorative applications, electrolyte formulations based on trivalent chromium are already known since several decades [2]. Especially solutions of chromium(III)-sulfate are promising candidates considering process management and quality of the deposits [3, 4]. However, chromium coatings plated from commercial chromium(III)-electrolytes cannot meet the requirements for decorative purposes completely yet. Especially deposition rate, corrosion resistance and color come short compared to layers made from chromic acid baths. In order to control these parameters, a profound knowledge of the deposition mechanism is crucial.

Electrochemical quartz crystal microbalance (EQCM) was used to study the mass-charge balance of the cathodic processes in chromium(III)-sulfate based plating baths. Current efficiency and deposition rate were determined as a function of time, additive concentration and current density. The addition of benzoic sulfimide (BSI) facilitates the reduction to chromium metal. An optimum current efficiency of 8.1 % was observed at 2.5 A dm^‑2 [5]. pH changes near the cathode surface during electroplating were evaluated in-situ by combining a pH electrode with a metal mesh and correlated with the results obtained by EQCM. Furthermore, the dependence of the color of the deposits on surface morphology was observed by SEM, AFM and colorimetry. The mechanism of metal growth appears to be different from chromic acid based systems since the grain size of the chromium depends on layer thickness.

[1] ECHA: Authorisation list, EC no. 215-607-8.

[2] Gianelos, L.: Plat Surf Finish 66 (1979) 5, 56-60.

[3] Handy, S. L.; Oduoza, C. F.; Pearson, T.: Trans IMF 84 (2006) 6, 300-308.

[4] Katirci, R.: Surf Eng 31 (2015) 6, 465-471.

[5] Leimbach, M.; Tschaar, C.; Schmidt, U.; Bund, A.: Electrochim Acta 270C (2018), 104-109.