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.