The combination of electrodeposited chromium (III) and PVD as an industrial viable solution for the replacement of electrodeposited chromium (VI)

Abstract

Chromium electrodeposition from hexavalent Cr (VI) aqueous solutions for Cr thin films and coatings has been vastly used for distinct purposes, due to the attractive appearance and outstanding mechanical, wear and corrosion properties of the resulting coatings. Regardless, its toxicity led to the implementation of European legislation to rescript its use, such as the European Norms Regulation on the Registration, Evaluation, Authorisation, and Restriction of Chemicals (REACH) and Restriction of Certain Hazardous Substances (RoHS). Thus, a replacement for Cr VI must be found, however, it would be important that the resulting films keep their characteristics. In the literature chromium electrodeposition from trivalent chromium (Cr III) aqueous solutions has been proposed and tested, however, its mechanical and optical properties are not comparable with the ones of Cr VI-based coatings and an alternative method is then required as Cr III alone does not deliver the necessary requirements. In this work, a combination of Cr III electrodeposition (ECD) in conjugation with a physical vapour deposition (PVD) multilayered coating of Cr, N, and C is explored to provide a coating capable of fulfilling the industry requirements. Subjecting the developed samples to a Neutral Salt Spray Test (ISO 9227:2027), shows that both the Cr III and Cr III + PVD coatings have similar results regarding corrosion resistance. Moreover, impedance electrochemical spectroscopy demonstrates the coating porosity as the critical point opening the door to further improvements. Aesthetically, applying the PVD layers allows for replicating the former shiny and black chromed look and expands to more different and appealing colours. Hence, this innovative alternative, based on the use of trivalent chromium by electro-deposition followed by PVD coatings, has proven potential to substitute the hexavalent chromium ECD process, while being a viable and sustainable alternative and expanding its use through different colours.