Structure effect of some thiosemicarbazone derivatives on the corrosion inhibition of Fe78B13Si9 glassy alloy in Na2SO4 solution

The corrosion and corrosion inhibition of Fe78B13Si9 glassy alloy by some thiosemicarbazone derivatives in 0.2 M Na2SO4 solution containing 10% MeOH has been studied. Inhibitors efficiency was estimated from electrochemical (polarization and impedance) measurements. Nearly identical results obtained from both techniques and morphological study are consistent. The thiosemicarbazone derivatives gave maximum corrosion inhibition of the alloy in 0.2 M Na2SO4 solution and evidence to act as mixed inhibitors. Interestingly the corrosion of iron-base alloy was almost controlled by charge transfer process but sometimes it was controlled by diffusion. This has been explained in basis of impedance data and literature work. A correlation between the efficiency of corrosion inhibition and their molecular structure was established. A schematic represent model of adsorbed molecule on the alloy surface was suggested. Experimental results fitted the kinetic-thermodynamic isotherms. Negative values of ΔGads0 were obtained for the compounds from potentiodynamic polarization and impedance studies indicated the spontaneous adsorption of inhibitors on the alloy and the value of active suites are approximately equal to unity. Linear free energy relationships (LFER) have been used to correlate the inhibition of benzaldehyde thiosemicarbazone and its p-substituted derivatives with the Hammett substitute constant (σ), the small positive slope (ρ) of the correlation line (ρ = 0.32) shows a weak dependence of the adsorption character of the reaction center on the electron density of the ring.