Corrosion Inhibitor for Mild Steel in Mixed HCl and H₂SO₄ Medium: A Study of 1,10-Phenanthroline Derivatives

A phenanthroline derivative, [1,2,5]-oxadiazolo-[3',4':5,6]-pyrazino-[2,3-f]-[1,10]-phenanthroline (OPP), was synthesized and evaluated for corrosion inhibition on mild steel in a mixed acid solution of 1 mol/L HCl and 0.5 mol/L H₂SO₄. OPP achieves 99.53% efficiency at 20 °C and sustains above 86% up to 60 °C. The polarization curve shows that OPP reduces current densities at less negative potentials, with E _corr shifting less than 85 mV versus the control, indicating primary anodic protection as a mixed inhibitor. OPP shows a 10-fold increase in charge transfer resistance compared to the control as measured through EIS, demonstrating that it creates a substantial challenge to electron loss from Fe. SEM and AFM confirm that OPP alleviates the damage of acid on the steel surface. OPP adheres to the Langmuir model and chemisorption mode. XPS and EDS register OPP-iron complex formations and uniform nitrogen distribution, respectively, indicating successful OPP adsorption. OPP retains around 98 and 85% efficiency after 10 cycles at 20 and 60 °C, respectively, indicating good stability even at high temperatures. Quantum chemical results further suggest strong OPP-steel interactions, with low potential for localized corrosion and optimal adsorption geometry for robust bonding (-10.59 eV adsorption energy).