Metallurgical characterization of electrodeposited hard chromium from a trivalent inorganic electrolyte

Le Barbenchon Roman - La Rochelle Université (France)

Since 2010, hexavalent chromium compounds like chromium trioxide CrO3 were the subject of a series of stringent regulatory restrictions to protect the human health and the environment, particularly in European Union. As hard chromium coatings are deposited from hexavalent chromium baths, their use is subject to authorization by European REACH regulation since September 2017 [1]. More recently, the European CHemicals Agency (ECHA) was mandated to evaluate the possible evolution of authorization to restriction for the use of hexavalent chromium. Final decision is expected by the end of 2026, but it is sure that manufacturers will have to take into account a lot of precautions towards their operators and to continue to progress on the substitutions.
Several research were performed so as to find alternative solutions but today no one satisfies all the required properties covering all existing applications [2,3]. One of these alternatives is the electrodeposition from a trivalent chromium bath using organic complexing compounds [4]. One of the drawbacks of this process is the high content of incorporated carbon that affects the mechanical properties of the Cr coating during thermal treatments. Recently an inorganic Cr III bath has been developed by IRT M2P, which reduces the incorporation of carbon [5].
The objective of our work is to understand the relationships between electrodeposition parameters, metallurgical states and functional properties of electrodeposits obtained from this new inorganic electrolyte. The microstructure was studied by using different experimental tools: XRD, SEM, EBSD, … while contamination was measured by micro-X-ray fluorescence and by hot extraction analyses with a focus on light elements (H, O, N, C). Our results show a strong dependence of deposition kinetics, morphology, microstructure and chemical composition with the deposition parameters (temperature and current density). In many conditions, a “new” metastable chromium phase, that was never reported in electrodeposition process is observed [6,7]. The formation of this metastable Cr phase seems to be strongly dependent on the chemistry of the deposition bath. The morphology of deposits also evolves from nodular at lower deposition temperatures to angular at higher temperatures. A thermal approach (DSC, TGA micro-GC, TTh) was also implemented to better understand the microstructure of the deposits and the link with the incorporated light elements.