Mechanism of Cr(III)-based oxide-hydroxide film electrodeposition in an acidic solution

The electrodeposition of chromium oxide-hydroxide films from Cr(III) electrolytes has recently become a significant focus in electrochemistry, holding promise for applications in packaging industry and corrosion protection, as an environmentally friendly alternative for Cr(VI). However, the complex behavior of Cr(III) complexes has resulted in a scarcity of precise electrochemical investigations in this field. In this study, we employ in-situ techniques such as Electrochemical Quartz Crystal Microbalance (EQCM) and Electrochemical Scanning Tunneling Microscopy (EC-STM) in conjunction with cyclic voltammetry (CV) to elucidate the mechanisms governing the formation of chromium oxide-hydroxide films through electrodeposition in an acidic environment. CV studies explore the electrochemical reactions involved in the electrodeposition process, showing that hydrogen evolution is the dominant reaction. The resulting interfacial pH changes trigger the hydrolysis of hydrated chromium(III) cations, leading to oligomerization and the precipitation of a chromium hydroxide compound. In-situ monitoring of mass changes during film formation process using EQCM reveals that there is a critical pH range for oxide-hydroxide film formation, outside of which no deposition/precipitation occurs. EC-STM results confirms the formation of a film on the cathode surface, and its sensitivity to the interfacial pH. At a sufficiently high local pH, under our experimental conditions happening at potentials more negative than -0.95 V, the film alters into a state that becomes stable and insoluble.