Synergistic inhibition of St37 steel corrosion in 15% H₂SO₄ solution by chitosan and iodide ion additives

The inhibiting ability of chitosan alone and in combination with KI for St37 steel in 15% H₂SO₄ solution has been studied using potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), dynamic electrochemical impedance spectroscopy(DEIS), and weight loss (WL) complemented by surface morphological examination with the aid of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The effect of immersion time on inhibition efficiency has been examined for 15 h and the influence of temperature studied over the temperature range of 25–60 °C. Results obtained from all the applied methods portray chitosan as a moderate inhibitor for St37 steel in the studied acid environment. Addition of KI is found to remarkably enhance the inhibition efficiency of the polymer above 92%. DEIS results show that the adsorbed chitosan-KI film on the metal surface is more stable at longer immersion time and performs more effectively. The inhibition efficiency of chitosan decreases with increasing temperature, while that of the chitosan-iodide combination increases with a rise in temperature reaching an optimum value of 99.72% at 60 °C. PDP results show that both chitosan and chitosan + KI behave as a mixed type inhibitor. A calculated synergism parameter confirms that the improved performance of chitosan is due to a synergistic effect. EDS results confirm the adsorption of inhibitor molecules on the metal surface.