Effect of degree of hydrolysis of polyvinyl alcohol on the corrosion inhibition of steel: theoretical and experimental studies

Two grades of polyvinyl alcohol (PVA) namely partially hydrolyzed (designated as PVA-I) and fully hydrolyzed (designated as PVA-II) were studied with the aim of determining the influence of PVA degree of hydrolysis on the corrosion inhibition effect of mild steel in HCl medium. The investigation was carried out using weight loss, electrochemical impedance spectroscopy, potentiodynamic polarization (PDP), and linear polarization resistance techniques. Results obtained indicate that the two grades of PVA inhibited the acid-induced corrosion of mild steel. Inhibition efficiency increased with increase in the PVAs concentration but decreased with increase in temperature. Results from all the techniques employed were in close agreement and revealed that corrosion inhibition effect followed the order PVA-I > PVA-II. PDP data show that both grades of PVA act as a mixed-type inhibitor. Impedance spectra show a high frequency capacitive loop related to the charge-transfer process of the metal corrosion and the double-layer behavior. Corrosion inhibition by the PVAs is assumed to occur by virtue of adsorption of PVA molecules on the mild steel surface which can be approximated by Langmuir adsorption isotherm model. Quantum-chemical calculations and molecular dynamic simulation approaches were also employed in the study and they have provided useful insights into the active centers of the two grades of PVA as well as the nature of interaction between the mild steel surface and the PVAs.