The corrosion behavior of P110 casing steel in simulated concrete liquid and simulated annulus fluid was investigated to reveal the corrosion pattern and protective properties of corrosion products in the two environments. Potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), Mott–Schottky tests, and electrochemical noise (EN) tests were used to study the corrosion behavior of P110 casing steel in simulated concrete liquid and simulated annulus fluid saturated with CO₂. Scanning electron microscopy (SEM) combined with Energy-Dispersive Spectrometer (EDS) mapping was used to characterize the corrosion morphology and elemental distribution of P110 casing steel. The results show that the corrosion resistance of P110 casing steel deteriorates with the increasing immersion days in the simulated annulus fluid, the impedance decreases gradually, and the corrosion-product film shows a loose and porous structure. In the simulated concrete liquid, under the condition of containing a low concentration of Cl⁻, the protection of the corrosion products gradually increases with the extension of immersion days. With the increasing concentration of Cl⁻ and the extension of immersion days, the electrochemical noise resistance and charge transfer resistance of P110 steel decrease gradually, and the protective property of the corrosion-product film decreases, which is capable of forming steady pitting corrosion.

Traditional corrosion inhibitors are constrained in their sustainable development due to high costs, significant environmental toxicity, poor degradability, and inadequate dispersibility. Although environmentally friendly carbon dot-based corrosion inhibitors hold potential as alternatives, their precise and controllable synthesis, as well as high-yield production, remain challenging due to existing technical bottlenecks. This study employs a "synthesis-modification" stepwise approach. Initially, a basic nitrogen-doped carbon dot corrosion inhibitor (N-CDS1) was synthesized. Subsequently, the target product, N-CDS2, was obtained through functionalization with Schiff base groups. By decoupling the complexity of the synthesis pathway, the challenge of precise structural control of carbon dots was overcome and achieving a yield of 62.5% for N-CDS2 in the second step. The structural and corrosion inhibition properties of N-CDS2 were characterized using a combination of advanced analytical techniques, including FT-IR, TEM, electrochemical testing, and SEM. The characterization results confirmed that N-CDS2 retains its carbon dot structure after functionalization and incorporates Schiff base functional groups. At a concentration of 90 mg/L, N-CDS2 demonstrated a corrosion inhibition efficiency of 98.56% for N80 steel in 1 M HCl at room temperature, significantly outperforming N-CDS1 (78.15% efficiency), which lacks Schiff base modification. The corrosion inhibition mechanism of N-CDS2 involves both anodic and cathodic suppression, with thermodynamic analysis indicating a mixed adsorption behavior that follows the Langmuir isotherm model. This study not only advances the practical application of carbon dot materials in the field of corrosion protection but also provides an innovative solution for the green transformation of industrial metal protection technologies.