Chloride-induced corrosion is a primary factor contributing to the degradation of coastal concrete infrastructure. To counteract the corrosion of steel reinforcement, epoxy resin coatings are commonly used. However, ensuring their durability in alkaline environments is a challenge, and these epoxy coatings also alter the mechanical bond between steel and concrete. It is thus crucial to develop thin inorganic conversion coatings. Layered double hydroxides (LDH) have a potential application in corrosion protection. However, steel tends to passivate in alkaline environments, limiting the growth of LDH on its surface. Hong et al achieved the in-situ growth of LDH on steel and elucidated its growth mechanism. Despite its potential in corrosion prevention, the growth of LDH presents a lamellar structure, creating micropores that serve as channels for the invasion of corrosive ions. Hence, developing a dense composite film for effective steel protection remains a challenge. This paper was to investigate two methods for LDH densification and characterize their composition, density, and corrosion resistance.

Steel sheets and rebars were sequentially polished with #240, #500, #1 000 and #2 000 SiC sandpaper to remove surface oxides. After polishing, they were placed in anhydrous ethanol for 5 min under ultrasonic cleaning to eliminate surface impurities, resulting in a smooth and uniform bare steel named as B. Subsequently, steel samples were immersed in solutions containing 0.057 5 mol/L Mg(NO₃)₂·6H₂O, 0.025 mol/L Al(NO₃)₃·9H₂O, and 0.375 mol/L urea dissolved in deionized water (DI). The surface of steel materials was washed with 5% dilute nitric acid and then vertically placed in a reaction vessel. The heating procedure was to increase from room temperature to 120 ℃ for 2 h, and maintain at 120 ℃ for 24 h. The pressure was self-supplied by the reaction vessel during the heating process. After the hydrothermal reaction, the samples were removed, rinsed with deionized water and dried for 3 d, resulting in the samples with only Mg/Al-CO₃^2--LDH membrane growth, named as L₁.

Furthermore, LDH samples were placed in a 100% CO₂ for half a day, then immersed in saturated calcium hydroxide for 2 h. Afterwards, they were taken out, placed in an oven at 50 ℃, and dried for 3 d. The sample represented the first method of calcium carbonate densification after LDH growth, named as L₂. For the second method, LDH samples were immersed in saturated calcium hydroxide for 6 h, followed by exposure to CO₂ for 10 s, and then dried in an oven for 3 d, resulting in the sample denoted as L₃. This aimed to investigate the optimal method for calcium carbonate densification between LDH pores. All the samples obtained the two methods were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and specific surface area analysis (BET). The corrosion resistance of the composite films under different conditions was also determined by electrochemical impedance spectroscopy and potentiodynamic polarization analysis.

Based on the XRD patterns and FTIR spectra, a layered double hydroxide (LDH) film and calcium carbonate composite occur on the steel surface. The results show that the peak intensity of calcium carbonate of the sample L₃ is higher than that of the sample L₂, indicating a higher content of calcium carbonate in the sample L₃. Based on the SEM images, the surface pores and the compactness of calcium carbonate occur, and the thickness and cross-sectional compactness appear. The analysis of internal compactness reveals that the compacted samples maintain their thickness and achieve a compaction in internal pores. From the analysis of pore size and pore volume, the pore size and volume decrease for the samples L₁, L₂, and L₃.

The results by electrochemical impedance spectroscopy and potentiodynamic polarization curves indicate that the sample L₃ has an optimum corrosion resistance. Also, the sample L₃ has a superior corrosion resistance rather than the sample L₂. This can be since the sample L₂ is initially placed in CO₂, resulting in some carbon dioxide remaining in the pores. However, the sample L₃ is firstly immersed in Ca(OH)₂ solution. In the soaking process, Ca(OH)₂ solution effectively penetrates the pores in the film layer. This sample is able to form a large amount of calcium carbonate to compact the LDH film through carbonization, having a protective performance. This method of filling is simpler, more economical, and faster, making it more conducive to industrial applications.

The in-situ growth of MgAl-CO₃^2--LDH films on steel substrates was achieved through two methods, allowing for the compact composite of calcium carbonate within the pores of the LDH film.

1) The thickness of the LDH film hardly changed after the incorporation of calcium carbonate, remaining at approximately 17 μm. The compaction of calcium carbonate was on the surface of the LDH and permeated into the inner pores of the LDH film.

2) All the LDH film layers exhibited a superior corrosion resistance in chloride ion intrusion. The sample subjected to Ca(OH)₂ immersion followed by carbonization had a superior corrosion inhibition performance and an impressive corrosion inhibition efficiency of 95.22%.