Experimental and theoretical investigation of corrosion inhibition effect of two piperazine-based ligands on carbon steel in acidic media

In this study, the corrosion of carbon steel in 1.0 M HCl solutions containing either 1,4-bis(2-(2-hydroxyethyliminomethyl)phenyl)piperazine (1) or its reduced form, 1,4-bis(2-(2-hydroxyethylaminomethyl)phenyl)piperazine(2), as possible corrosion inhibitors, were compared to a solution containing no (1) or (2). The various concentrations of inhibitors, 0.2, 1.0, and 2.0 mM, were used to explore the inhibition ability of the synthesized materials against carbon steel corrosion. The morphology and chemical analysis of the as-received and corroded samples were investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). By assessing potentiodynamic data, the icₒᵣᵣ of carbon steel electrode is the highest value among the other inhibitor-containing solutions, demonstrating a minimum corrosion resistance in 1.0 M HCl solution. While, the protection of carbon steel (1) and (2)-containing solution increased by raising the concentration of (1) and (2) from 0.2 to 2.0 mM. The reduced form of (1) showed a superior corrosion resistance for carbon steel compared to inhibitor (1). The corrosion efficiency of carbon steel in 1.0 M HCl attained a maximum value of 66.08% for inhibitor (1) and 88.51% for inhibitor (2) when the concentration of inhibitor reached the highest value. The results obtained from electrochemical impedance spectroscopy, EIS, illustrated that the capacitance of the double layer, Cdₗ, decreased with an increment of inhibitor (1) and (2) concentration, while the charge transfer resistance of carbon steel enhanced as the inhibitor increased. SEM images reveals that the carbon steel had a sever corrosion inhibitor-free solution, while the surface in inhibitor (1) and (2)-containing solution looks more smooth and uniform. The obtained results in EIS and Tafel measurements were clearly consistent with our observation in the SEM images. According proposed inhibition mechanism, the inhibitor adsorption on the carbon steel prevents the hydrogen evolution from the cathodic sites and dissolution of iron in the anodic regions. Adsorption of both inhibitors, (1) and its reduced form (2), was found to obey the Temkin adsorption isotherm. The inhibitory properties of inhibitor molecules have also been studied by theoretical calculations, using Gaussian 09 software program, enabling a comparison of the properties of (1) and (2).