Introduction Steel corrosion is particularly severe in marine environments with a high concentration of chloride ions, which causes huge economic losses and also hinders engineering construction in coastal areas. Coating treatment is one of the common methods to protect steel from corrosion. Conventional thicker organic coatings are prone to local breakage, reduced mechanical interlocking force between steel and concrete, and being easy to deteriorate. Since the anions and cations of layered double hydroxides (LDH) both are tunable, this designability gives them a variety of potentials for metal corrosion protection, such as physical barrier, chlorine fixation, corrosion inhibition and chemical self-repair. In this paper, Mg/Al-NO3?-LDH and Zn/Al-NO3?-LDH films were grown in-situ on the surface of steel Q235 by an electrodeposition-hydrothermal method, and their adaptability in a simulated high-alkaline concrete environment and corrosion inhibition performance in NaCl solution were investigated.Methods The in-situ growth of LDH film on steel surface includes electrodeposition and hydrothermal treatment. In the electrodeposition, a three-electrode system was used, with a steel sheet as a working electrode, an Ag/AgCl electrode as a reference electrode, and a platinum sheet as a counter electrode. The electrolyte solution consisted of 50 mL of 0.045 mol/L Mg(NO3)2·6H2O and 0.015 mol/L Al(NO3)3·9H2O, and the steel sheet was immersed in the electrolyte solution to stabilize it for 500 s, and then a potential of ?1.5 V was applied for 300 s. The hydrothermal treatment used a hydrothermal solution consisting of 50 mL of 0.06 mol/L Mg(NO3)2·6H2O and 0.02 mol/L Al(NO3)3·9H2O. NH3·H2O was added dropwise to adjust the pH value of the solution to 9. Subsequently, the electrodeposited steel sheet was vertically placed into a Teflon?-lined autoclave containing the hydrothermal solution, and reacted in a homogeneous reactor at 90 ℃ for 18 h. After the reaction was completed and cooled down to room temperature, the sheet with the film was removed and named M1. Similarly, the obtained sample was named Z1 when replacing Mg(NO3)2·6H2O with Zn(NO3)2·6H2O. To examine the adaptability of the two samples in a high-alkaline concrete environment, the samples M1 and Z1 were immersed in saturated Ca(OH)2 solution for 24 h and then taken out, resulting in samples named M2 and Z2, respectively.The crystal structures of the different samples were characterized using a model D8 Advance X-ray diffractometer (Bruker Co., Germany, λ=0.154 06 nm). Sample’s functional groups and chemical bonds were detected using a model Spotlight 200 Fourier transform infrared spectrometer (PerkinElmer Co., USA). The surface morphology and chemical composition of the samples were determined by a model APREO S high-resolution scanning electron microscope with an X-ray energy spectrometer (Thermo scientific Co., The Netherlands). The impedance and corrosion resistance of the four samples in 3.5% (mass fraction) NaCl solution were analyzed via electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization in a ParStat 4000 electrochemical workstation (AMETEK Co., USA).Results and discussion The electrodeposition and hydrothermal method was employed to in-situ grow a dense and uniform Mg/Al-NO3?-LDH film and a lamellar porous Zn/Al-NO3?-LDH film on the surface of steel Q235. After immersing these two LDH films in a saturated Ca(OH)2 solution for 1 d, the lamellar structure of Mg/Al-LDH remains unchanged, and the interlayer anions are partially exchanged from NO3? to OH? due to the slightly higher affinity of OH? in the solution. Also, the lamellar structure of Zn/Al-LDH undergoes a chemical reaction due to the presence of a large amount of Ca2+ and OH?, resulting in the generated portion of Ca/Al-NO3?/OH?-LDH and amorphous hydroxides remaining on the carbon steel surface. The results above indicate that Zn/Al-LDH is difficult to be stabilized in highly alkaline concrete environments since its constituents, i.e., Zn(OH)2 and Al(OH)3, both are amphoteric hydroxides. In contrast, Mg(OH)2 is an alkaline hydroxide, allowing Mg/Al-LDH to maintain a structural stability in high alkaline environments.The EIS results show that in a 3.5% (in mass fraction) NaCl solution, the impedance of the Zn/Al-NO3?-LDH film is only 3.18 Ω/cm2, whereas the Mg/Al-NO3?-LDH film exhibits a greater impedance of 2 722 Ω/cm2, which is attributed to the dense structure of the Mg/Al-NO3?-LDH film. The results of dynamic potentiodynamic polarization tests indicate that the corrosion protection efficiency of the Zn/Al-NO3?-LDH film and the Mg/Al-NO3?-LDH film for carbon steel is 63.86% and 97.43%, respectively. Clearly, the Mg/Al-NO3?-LDH film is more effective in protecting carbon steel against chloride ion erosion due to its superior physical barrier and the double protection effect of "fixing chlorine".Conclusions Zn/Al-NO3?-LDH and Mg/Al-NO3?-LDH films with a high crystallinity were grown in-situ on the surface of steel Q235 by an electrodeposition-hydrothermal method for corrosion protection. The microscopy showed that the Zn/Al-NO3?-LDH films were lamellar and porous, and the Mg/Al-NO3?-LDH films were dense and uniform. The electrochemical results indicated that in 3.5% (in mass fraction) NaCl solution, the corrosion protection efficiency of Zn/Al-NO3?-LDH film on carbon steel substrate was 63.86%, while the Mg/Al-NO3?-LDH film had a higher impedance and a lower corrosion current density, and its corrosion protection efficiency was 97.43%, effectively protecting the carbon steel against chloride ion erosion. In a high-alkaline saturated Ca(OH)2 solution, the Zn/Al-NO3?-LDH film exhibited an instability, leading to the formation of partially Ca/Al-NO3?/OH?-LDH and amorphous hydroxides. In contrast, the Mg/Al-NO3?-LDH film remained stable, and the anion exchange resulted in the formation of Mg/Al-NO3?/OH?-LDH film.