ANTI-CORROSIVE PROPERTIES OF 1,4-BENZOTHIAZINE DERIVATIVES ON MILD STEEL CORROSION IN 1 M HCL SOLUTION: EXPERIMENTAL AND THEORETICAL STUDIES

Abstract

The corrosion inhibitory effect of two benzothiazine derivatives, namely2-(3-oxo-2,3-dihydro[1,4]- benzothiazin-4-yl)acetic acid (P1) and2-(2-benzylidene-3-oxo-2,3-dihydro[1,4]-benzothiazin- 4-yl)acetic acid (P2)for mild steel in 1M HCl has been studied using electrochemical impedance spectroscopy (EIS), Tafel polarization curves and weight loss measurements. It was found that the inhibition efficiency of the two investigated inhibitors increases with increase in concentration of inhibitors. P1 and P2 show corrosion inhibition efficiency of 92 and 90%, respectively at 10−3M and 308 K. Impedance experimental data revealed a frequency distribution of the capacitance, simulated as constant phase element. Potentiostatic polarization study showed that P1 and P2 are mixed-type inhibitors in 1M HCl. The results obtained from electrochemical and weight loss studies were in reasonable agreement. The adsorption of P1 and P2 on steel surface obeys Langmuir's adsorption isotherm. The correlation between inhibition efficiency and molecular structure of the inhibitors is investigated by determination of chemical indexes, which were performed using density functional theory (DFT) at (B3LYP/6-31G) (d,p) level.