Comparative studies of three isatin-thiosemicarbazones against corrosion of AA6060 aluminum alloy exposed to acidic NaCl medium

Abstract

Three isatin-thiosemicarbazone derivatives with different substituents (R) attached at C5 of the indolin-2-one ring, TSC-ISA-R (R = -H, -OCH3 and -OCF3), were synthesized and evaluated as medium-term corrosion inhibitors for AA6060 aluminum alloy exposed to acidic chloride medium. The inhibitory behavior and performance of TSC-ISA-R were investigated in relation to the molecular modification by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), potentiodynamic polarization (PDP) analysis and weight loss measurement. Quantum chemical calculation and molecular dynamics (MD) simulation were further performed to study the inhibition behavior of the derivatives towards the corrosion of AA6060 alloy. Surface morphology analysis showed that TSC-ISA-OCH3 significantly mitigated the corrosion of the alloy while TSC-ISA-OCF3 had little inhibition effect. The inhibition efficiencies observed from weight loss measurements for TSC-ISA-H, TSC-ISA-OCH3 and TSC-ISA-OCF3 at 3 × 10–2 mM were 27.03%, 79.73% and 12.16%, respectively. PDP analysis indicated that both TSC-ISA-H and TSC-ISA-OCH3 were cathodic type inhibitors and TSC-ISA-OCF3 was a mixed-type one primarily hindering the cathodic reactions of AA6060 alloy. The results of EDS and XPS revealed that TSC-ISA-R molecules were adsorbed on the alloy surface, and the adsorption was closely correlated with the substituent modification. The theoretical results obtained from quantum chemical calculations and MD simulations for the protonated TSC-ISA-R correlated well with their ranking of the experimental inhibition efficiencies. The substituent effects of -R were proposed to significantly affect the electron densities on para-amino N1 and exocyclic imino-N2 atoms of TSC-ISA-R capturing H+ in the medium, and thereby probably underlay their differences in the inhibition behavior and performance.