EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Electrodeposited chromium coatings exhibit high hardness, good wear resistance and corrosion resistance . Development of electrodeposited trivalent chromium coating Cr(III) baths becomes a new challenge for replacing the conventional Cr(VI) baths due to the hazardous character of hexavalent chromium salts. Several Cr(III) electrolytes were developed in the past , but the optimization of the electrodeposited coatings is required with a better understanding of their growth mechanisms and microstructure. This work is one of the objectives of the CRONOS 2024 project which focus on the optimization of the metallurgical state and properties of electrodeposited chromium produced from the trivalent chromium bath.
The organic species in the bath required for the electrodeposition and the co-deposition of hydroxides due to local pH variations near the cathode interface may induce high impurities concentrations in the deposits, mainly hydrogen, oxygen and carbon. Hot extraction analysis was carried out to evaluate the chemical composition of the deposits, confirm this contamination. In comparison with Cr(VI) deposits, the Cr(III) deposits are highly contaminated by H, O and C impurities. Several experimental analyses were performed in order to characterize the microstructure of Cr(III) deposits. XRD and TEM analysis revealed that the as-deposited Cr(III) is mostly amorphous. This amorphization is probably linked to the incorporation of high amounts of impurities during the electrodeposition. The origin and the localization of impurities incorporated in the coating are not well explained to date. In order to evaluate the individual contributions of impurities on the amorphization of the deposit, evolution of the impurities content and microstructure versus thermal treatments were studied by using different techniques namely DSC, in situ-TEM, micro-hardness and hot extraction analysis. This multi-scale approach can help to a better understanding of the amorphous structure of the as-deposited Cr(III) coating and it stability.