The effects of formate ion concentration on the complexation of Cr(III) in aqueous Cr(III) electrolytes from Cr_2(SO_4)_3 salts

The effects of formate ion concentration on the complexation of Cr(III) in aqueous Cr(III) electrolytes from Cr_2(SO_4)_3 salts

van Lottum, Anouk (TU Delft Mechanical, Maritime and Materials Engineering)

Mol, J.M.C. (mentor)
Ankora, M.V.E. (graduation committee)
Dey, P. (graduation committee)
Taheri, P. (graduation committee)

Delft University of Technology

Mechanical Engineering

2021-04-09

Trivalent chromium (Cr(III)) is one of the most promising non-toxic replacements for hexavalent chromium (Cr(VI)) coatings in the steel packaging industry. The application of a chromium layer to packaging steel is essential for providing a protective layer on the steel packaging’s external surface area, which prevents corrosion during its use. However, the deposition process of Cr-metal from Cr(III) solution cannot produce a deposit thicker than 10 μm with sufficient corrosion and wear resistance without the application of a complexing agent. This study provides both an experimental research as an comparative computational model. Experiments are set up to analyse the effect of the formate ion HCOO(-) concentration as a complexing agent in Cr_2(SO_4)_3 electrolytes, which results are compared to the output of the used computational model. The results provide insights into the initial composition of species in plating electrolytes, which is essential for the determination of the deposition mechanism from Cr(III) electrolytes to improve the electroplating process. Ligand exchange and bonding with HCOO(-) in chromium complexes is studied with Ultraviolet-visible light (UV-VIS) spectroscopy and Attenuated Total Reflectance Fourier InfraRed spectroscopy (ATR-FTIR) at various concentrations of HCOO(-). Additionally, computational modelling is performed with Density functional theory (DFT) to predict the spontaneous character of various ligand substitutions, and simulations of spectroscopic spectra are performed as a reference to mimic the experimentally observed data. It was found that ligand exchange with HCOO(-) in Cr_2(SO_4)_3 solutions occurs spontaneously within two days at elevated temperatures at the analysed concentrations. The experimental data shows, there is a transition point in the complex formation between the ratio of [Cr(3+)]:[ HCOO(-)] is respectively [1]:[1.61] and [1]:[3.32]. This indicates that the complex formation decreases past the ratio of [Cr(3+)]:[ HCOO(-) ] = c.a.[1]:[3] and that the concentration at which the most chromium-formate complexes can be observed is within the range of [Cr(3+)]:[ HCOO(-) ] = [1]:[1.61] to [Cr(3+)]:[ HCOO(-) ]= [1]:[3.32]. The formation of chromium-formate complexes is beneficial for the deposition process, as the addition of formate increases the amount of chromium deposited on the steel surface. Furthermore, it is found that the presence of SO_4(-2), originating from the chromium salt, is beneficial for the ligand exchange of HCOO(-). Also, the bonding of formate to chromium is found to be as monodentate binding.

Trivalent chromium
formate
complexation
UV-Vis
ATR-FTIR
Density Functional Theory
concentration

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Student theses

master thesis

© 2021 Anouk van Lottum