Thiourea derivatives as efficient inhibitors for the corrosion of cold rolled steel in citric acid solution: experimental and computational studies

The prediction of the corrosion inhibition properties of organic compounds from experimental and computational results is a highly interesting field. Correct understanding of substituents or electronic effects can significantly contribute to the design of efficient inhibitors. In this research article, the corrosion inhibitory performance of thiourea (TU), 1,3-dimethylthiourea (DMTU), N-ethylthiourea (ETU), and 1-p-tolylthiourea (p-TTU) in 1.0 M citric acid (H3C6H5O7) solution for cold rolled steel (CRS) was fully investigated by experimental measurements and theoretical calculations. The results show that the inhibition performance of p-TTU exhibits the best inhibition property among these four thiourea derivatives with a maximum inhibition efficiency of 97.36%. The inhibition ability follows the order of p-TTU > DMTU > ETU > TU. Potentiodynamic polarization curves indicate that all these thiourea derivatives are mixed-type inhibitors, where TU, ETU and DMTU mainly inhibit the cathodic reaction, but p-TTU effectively retards both cathodic and anodic reactions. Nyquist mainly appears a depressed capacitive loop, and the charge transfer resistance increases significantly with the addition of thiourea derivatives. The adsorption of four thiourea derivatives on CRS surface conforms to Langmuir adsorption isotherm. The micrographs from SEM and metallurgical microscopy further support the protection effectiveness of thiourea derivatives on the steel surfaces. Quantum chemical calculations and molecular dynamics simulations can well elucidate the relationship between molecular structure and inhibition efficiency for the studied thiourea derivatives.