Corrosion and Electrolysis Mechanisms of Elemental Cobalt in NaCl Solution

To recycle/remove/protect the cobalt from polycrystalline diamond (PCD), hard alloys, and Co catalyst, this study investigates cobalt’s structural features, the inherent properties of the NaCl solution, and the interfacial chemistry at the solid–solution interface. For cobalt, the corrosion dependence on its structure is examined by X-ray diffraction (XRD), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT). In NaCl solution, properties─including dissolved oxygen (DO), electrical conductivity (EC), and viscosity─affect corrosion and electrolysis. Regarding interface science, the corrosion products (Co1.176(OH)2Cl0.348(H2O)0.456, β-Co(OH)2, and Co2(OH)3Cl) are identified through XRD and X-ray photoelectron spectroscopy (XPS). Chloride ions trigger the conversion of the β-Co(OH)2 pathway to the Co1.176(OH)2Cl0.348(H2O)0.456 and Co2(OH)3Cl pathways, a novel phenomenon in solid-solution corrosion. The residual rate (RR) of β-Co(OH)2, designated as RR(β-Co(OH)2), is originally defined to describe the transformation, which is detectable via electrochemical impedance spectroscopy (EIS) and modeled using a newly extended electrochemical element Wpc (based on the Finite-Length Warburg and Generalized Finite Warburg components) along with a novelly extended constant phase element (CPE) derived from the traditional CPE model. Furthermore, a new equivalent circuit model for EIS is developed to represent this conversion process in electrochemistry initially at an unstable solid-solution interface, and the concentration dependence of the effect caused by chloride ions is freshly revealed. This work also reveals other significant trends and mechanisms that depend on the NaCl solution concentration.