Corrosion protection of steel bars in alkaline concrete environments poses a common challenge in marine engineering. One approach to mitigate steel bar corrosion is the addition of corrosion inhibitors to the concrete. In alkaline environments, the passivation of rebars occurs through anodic passivation coupled with the cathodic oxygen reduction reaction (ORR). The catalysis of ORR can expedite anode passivation. To investigate the corrosion inhibition of steel bars in alkaline environments, meso-tetra(4-carboxyphenyl)porphine (TCPP), known for its ORR catalytic properties, is selected. TCPP forms adsorption films on the surface of steel bars, facilitating the formation of passivation films. TCPP primarily adsorbs onto active sites on the surface of the passivation film, where lattice iron ions have leached. The adsorbed TCPP accelerates the formation of the passivation film through ORR catalysis, inhibiting the development of passivation film defects and enhancing the integrity and protection of the passivation film. The most significant effect is observed when the concentration of TCPP is 0.5 mmol/L. The physical adsorption of TCPP is primarily determined by the negative charge centers, namely the carboxyl group O and the pyrrole N. However, due to steric hindrance caused by the unrestricted rotation of the carboxyl benzene, the pyrrole N does not play a dominant role in chemical adsorption. Instead, the active site for chemical adsorption is the carboxyl group O. The adsorption process significantly reduces the diffusion coefficient of TCPP molecules, providing a robust and stable adsorption binding. Phthalocyanine molecules without carboxyl benzene groups adopt a planar structure, allowing them to form stable adsorption configurations on the iron surface through flat adsorption. This observation provides guidance for the design of novel metal phthalocyanine molecules. Specifically, the development of metal phthalocyanine molecules with modifying groups that are coplanar with the phthalocyanine ring and possess restricted rotation can achieve flat adsorption, improve coverage rate, and enhance adsorption configuration stability.
To enhance the stability of epoxy (EP) coatings on steel bars while improving their active and passive corrosion protection properties, the functional composite coatings were designed and prepared. Intercalated zirconium phosphate nanofillers (LCZrP) loaded with corrosion inhibitors were firstly synthesized and designed, and then were integrated into silane coatings (SC), which were preapplied to the bar substrate to enhance the integrity and interfacial adhesion properties. Lastly, an epoxy layer was applied, resulting in the functional composite coatings (LCZrP/SC/EP). The chemical composition and surface morphology were performed using scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS). The corrosion resistance of the coatings was evaluated using electrochemical impedance spectroscopy (EIS), complemented by adhesion tests to analyze the service stability of the coatings. The findings revealed that the silane interlayer notably improved the interfacial bonding at the junction where the epoxy resin interacts with the steel bar, thereby enhancing the corrosion resistance. Notably, the coating presented the optimal corrosion resistance when the nanofiller content was 0.04 wt.%. The impedance modulus at 0.01 Hz (|Z| 0.01Hz ) for the LCZrP/SC/EP coated coating was 5.05 × 10 10 Ω·cm 2 , which was two orders of magnitude greater than that of the EP coatings and the EP coating with silane interlayer (SC/EP) by one order of magnitude. Following 30 days of immersion, its |Z| 0 . 01Hz remained higher than the EP coating as well as the SC/EP coatings. The integral LCZrP/SC/EP coatings exhibit excellent long-lasting corrosion protection, owing to the self-responsive characteristics of LCZrP, the interfacial adhesion of the LCZrP/SC interlayer, and the enhanced barrier effect of the EP coatings. This combination effectively integrates active and passive protection strategies, thereby preventing metal corrosion. The above solution ensures that the coatings provide durable and reliable protection against corrosion over extended periods.
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