Optimizing corrosion resistance: how pH shapes the inhibition mechanism of ZnAl-NO₃ ^- LDH on mild steel

Abstract Mild steel is extensively used in various industrial applications but is susceptible to corrosion in aggressive environments. This study investigates the efficacy of layered double hydroxides (LDHs) as corrosion inhibitors for mild steel, specifically focusing on zinc-aluminium LDH intercalated with nitrate synthesized at different pH values (pH 7, pH 8, and pH 10). The variation in pH significantly influences the composition and subsequent inhibition behavior of the LDH. Characterization of the synthesized LDHs was performed using Powder X-ray Diffraction (PXRD) and Fourier Transform Infrared Spectroscopy (FTIR). Additionally, Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), and CHNS elemental analysis were utilized to assess surface morphology and inhibitor loading. Results demonstrated that the ZnAl LDH synthesized at pH 8 exhibited a well-defined structure and the highest inhibitor loading. Corrosion inhibition studies were conducted on mild steel immersed in a neutral 3.5 wt% NaCl solution, utilizing Electrochemical Impedance Spectroscopy and Potentiodynamic Polarization. Notably, 0.1 g/L of the pH 8 ZnAl LDH achieved an impressive inhibition efficiency of 95.18 %, as indicated by the potentiodynamic polarization results. The LDH demonstrated both anodic and cathodic inhibition effects, with the corrosion inhibition mechanism attributed to the controlled release of nitrate ions, which form a passive layer on the steel surface, and the entrapment of chlorides within the LDH structure, thereby reducing the concentration of harmful chlorides in the environment.