Effect of the substitution of an oxygen atom by sulphur in a pyridazinic molecule towards inhibition of corrosion of steel in 0.5 M H2SO4 medium

The inhibition of the corrosion of steel in 0.5 M H2SO4 by new pyridazine compounds is carried out by weight loss measurements and electrochemical methods. The studies clearly show that when we substitutes an atom of oxygen in 2-[4-(2-chlorobenzyl)-3-methyl-6-oxopyridazin-1(6H)-yl]acetohydrazide (P-1) by an atom of sulphur to obtain 2-[4-(2-chlorobenzyl)-3-methyl-6-thiooxopyridazin-1(6H)yl]acetohydrazide (P-2), the inhibition efficiency increases from 15 to 95% at 10(-4) M. The inhibition efficiency increases with the increase of inhibitor concentration to reach 99% at 2.5 x 10(-4) M of P-2. Potentiodynamic polarisation studies show that pyridazine derivatives are cathodic-type inhibitors in 0.5 M H2SO4 and these compounds act on the cathodic reaction without changing the mechanism of the hydrogen evolution reaction. The inhibition efficiency of P2 is temperature-dependent in the range from 298 to 343 K, the associated activation energy have been determined. P-2 is adsorbed on the steel surface according to a Temkin isotherm adsorption model. Significant correlations are obtained between inhibition efficiency with the calculated chemical indexes, indicating that variation of inhibition with the inhibitors structure may be explained in terms of electronic properties. (C) 2004 Elsevier B.V. All rights reserved.