Exploring novel indenoquinoxaline Schiff bases as corrosion inhibitors for conversion coating on mild steel: ultrasonication-assisted synthesis and electrochemical and computational studies.

Three new organic inhibitors, IQ-SSC, IQ-BSC, and IQ-HSC, based on indenoquinoxaline Schiff bases, were synthesized via a rapid, highly efficient, and ultrasonication-assisted protocol, with excellent yields of 99%. The electrochemical behavior and weight-loss measurements of bare mild steel and coated mild steel with organic inhibitors were evaluated in both 0.5 M H2SO4 and 3.5% NaCl solutions. The electrochemical results indicated that the I corr of the mild steel coated with the novel organic inhibitors decreased by more than half compared with that of the uncoated mild steel. Demonstrating their efficacy as mixed-type inhibitors, 200 mg L-1 of the IQ-SSC, IQ-HSC, and IQ-BSC inhibitors achieved inhibition efficiencies of 89.02%, 92.08%, and 92.33%, respectively. The computational studies exhibited an acceptable fit of the calculated data with the experimental data and concluded that the efficiency of the coated MS was IQ-BSC > IQ-HSC > IQ-SSC. Through a combination of surface and solution analyses with theoretical modeling, we demonstrate that IQ-BSC achieves its excellent inhibition performance by forming a protective organic layer that acts as a barrier against corrosive species on the steel surface. The high nitrogen content and benzene rings within the inhibitor molecules act as the primary active sites, facilitating strong physicochemical adsorption that ensures durable protection of the mild steel surface. Corroborated by potentiodynamic polarization data and highly negative standard Gibbs free energy of the adsorption values () ranging from -34.59 to -35.68 kJ mol-1, the adsorption process was confirmed to be spontaneous and of a mixed physicochemical nature.