Preparation and Corrosion Inhibition Performance of Nitrogen-doped Carbon Dots Based on Radix Isatidis Dregs

The work aims to prepare a green eco-friendly nitrogen-doped carbon dots (N-CDI) corrosion inhibitor, in order to improve the recovery and utilization of the Radix Isatidis dregs and reduce a series of problems such as environmental pollution and economic losses caused by carbon steel corrosion in the industrial circulating cooling system. Herein, the N-CDI was synthesized by one-step hydrothermal method, with the Radix Isatidis and ethylenediamine as raw materials, and the corrosion inhibition performance of the N-CDI on Q235 carbon steel in 1.0 mol/L hydrochloric acid solution was studied. The structure particle size and fluorescence properties of as-prepared N-CDI were estimated through fourier-transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, X-ray photoelectron (XPS) spectroscopy, transmission electron microscopy (TEM) and fluorescence spectroscopy. The inhibition effect on Q235 carbon steel of as-prepared N-CDI in 1 mol/L HCl solution was appraised through Gravimetric, kinetic potential polarization and electrochemical impedance spectroscopy (EIS) methods, and then the mechanism of corrosion inhibition was explored in depth. N-CDI exhibited efficient inhibition performance. At 303 K, the corrosion inhibition rate of Q235 carbon steel could reach 91.6% when the mass concentration of N-CDI in 1 mol/L HCl solution was only 15 mg/L, and the corrosion inhibition performance was significantly improved compared with that before doping. FTIR and XPS confirmed that N-CDI components contained a large number of —OH, —NH2 and other corrosion inhibition functional groups, which could form adsorptive membrane on the surface of carbon steel through electrostatic gravity, and lone pair electrons could be ligated with the unoccupied d orbitals of Fe form coordination bonds, thus forming chemical adsorption film on the surface of carbon steel. In the range of 303-323 K, the adsorption of N-CDI on the carbon steel surface obeyed Langmuir adsorption isotherm. Gibbs adsorption free energy (ΔGads) was from ‒35.67 kJ/mol to ‒40.08 kJ/mol, so N-CDI was a mixed-type inhibitor. On the surface of carbon steel, it firstly formed an adsorption film for physicochemical adsorption, and gradually changed to chemical adsorption as the dominant with the increase of temperature. After addition of N-CDI, the double capacitance decreased, while the charge transfer resistance increased significantly, but the impedance curves only showed a single semicircle, indicating that the corrosion process of carbon steel at the metal/solution interface was only controlled by the charge transfer process. The charge transfer resistance of N-CDI peaked at 2 h, and the adsorption film formed by the corrosion inhibitor molecules on the carbon steel surface was more dense and complete, but the corrosion inhibition rate dropped from 88.4% to 77.4% after 24 h. In the anodic polarization curve, the more negative the desorption potential corresponding to the inflection point of the Icorr plunge, which also indicated the more obvious corrosion inhibitor desorption phenomenon. SEM micrographs indicated that the corrosion degree of carbon steel after addition of corrosion inhibitor decreased significantly. Finally, this study confirms that the Chinese medicine slag CDs has the effect of inhibiting the corrosion of Q235 carbon steel in acidic medium, and has the advantages of low dosage, high efficiency and good thermal stability. It can effectively treat the Chinese medicine residue without carbon emission, and provide the synthesis idea of biomass as precursor for corrosion inhibitor of CDs, which has a broad application prospect. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.