Inhibition of carbon steel corrosion in CO2-saturated 3.5 wt.% NaCl solution by vanillin chitosan oligosaccharide and synergistic effect of KI additive

The chitosan oligosaccharide and vanillin are used as raw materials to synthesize non-toxic and water-soluble chitosan oligosaccharide derivative called vanillin chitosan oligosaccharide as a green corrosion inhibitor. The corrosion inhibition properties of 20# steel in CO₂-saturated solution system at 25°C and 3.5 wt% NaCl were studied. The synergistic effect of potassium iodide (KI) and VCOS on corrosion inhibition was also studied. Various techniques such as weight loss (WL), electrochemical analysis, atomic force microscopy (AFM), scanning electron microscopy (SEM), quantum chemical calculations, and molecular dynamics simulations were used to understand the inhibition properties. The inhibition efficiency of VCOS enhanced remarkably after the addition of KI, reaching an optimum value of 93.1%. EIS results showed that the inhibition of VCOS + KI on metal surface increased with time. Polarization measurements showed that VCOS and KI acted as mixed inhibitors by demonstrating anode dominance. SEM and AFM were used to study the formation of inhibition film on metal surface after 3 days immersion. We concluded that the mixed inhibitor followed Langmuir adsorption isotherm. In addition, quantum chemistry and molecular dynamics simulations were used to verify the relationship between corrosion inhibition efficiency and molecular structure.