Surface and interfacial properties of poly(methyldiallylammonium chloride): Effect of hydrophobic pendant and synergism (KI) on corrosion of C1018CS in 15% HCl

Background: In the current investigation, a dicationic surfactant made of poly(methyldiallylammonium chloride) (PMDAAC) was tested for its ability to suppress corrosion of C1018 carbon steel (C1018 CS) in a 15% HCl (acidizing) solution. Methods: Weight loss, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), linear polarization resistance (LPR), scanning electron spectroscopy (SEM), energy dispersive X-ray (EDX), atomic force microscope (AFM), and density functional theory (DFT). Significant findings: Weight loss tests show that PMDAAC has no significant inhibitory potential for the C1018CS/15% HCl system, as seen by its inhibition efficiency (IE) of 76.57% at 50 ppm concentration. However, adding 5 mM KI significantly increases the% IE of PMDAAC from 76.57% to 95.26% at the same concentration. A similar trend in the boost of% IE of PMDAAC from 85.33% to 99.28% and 83.28% to 98.42% was observed through PDP and EIS studies, respectively. PDP study showed that the current density (i_corr) values kept decreasing with increasing PMDAAC concentration. PMDAAC behaves as a mixed- and interface-type inhibitor, being able to retard the progress of both anodic and cathodic reactions. EIS analysis showed that the values of charge transfer resistance (Rct) kept increasing with increasing inhibitor concentration. The adsorption isotherm study suggests that PMDAAC becomes effective by adsorbing on the C1018 CS surface, and its adsorption follows the Langmuir isotherm. A high value of 6.37 × 10⁵ L mol^-1 corresponds to the equilibrium constant for the adsorption process (K_ads), indicating very strong adsorption of PMDAAC onto C1018 CS. PMDAAC was found to chemisorb onto the metal surface, as confirmed by the Gibbs free energy of adsorption (ΔG_ads^∘) found to be – 43.76 kJ mol⁻¹. The adsorption of PMDAAC on the C1018CS surface was confirmed by SEM and EDX studies. The improvement in surface morphology and alteration in elemental alignment in the presence of PMDAAC demonstrate the adsorption mode of corrosion mitigation. Lastly, the interaction of PMDAAC and its constituting components were described using the DFT method.