Effect of molecular structure on the efficiency of amides and thiosemicarbazones used for corrosion inhibition of mild steel in hydrochloric acid

The effect of changing functional groups of some amides and thiosemicarbazone derivatives on their inhibition efficiency has been reported with a view to establishing a relationship between inhibitor efficiency and molecular structure, using the weight loss and hydrogen evolution techniques. The compounds used for this study are urea (U), thiourea (TU), acetamide (A), thioacetamide (TA), semicarbazide (SC), thiosemicarbazide (TSC), methoxybenzaldehyde thiosemicarbazone (MBTSC), 2-acetylpyridine-(4-phenyl)thiosemicarbazone (2AP4PTSC), 2-acetylpyridine-(4-methyl)thiosemicarbazone (2AP4MTSC), benzoin thiosemicarbazone (BZOTSC) and benzil thiosemicarbazone (BZITSC). All the compounds inhibited corrosion to varying degrees. It was found that the molecules which include a thiocarbonyl group, e.g. TU, TA and TSC, have higher inhibition efficiency than the corresponding compounds which do not, e.g. U, A and SC. The results (at 30 degrees C and 40 degrees C) indicate that the order of efficiency of the thiocompounds in solution and the extent of their tendency to adsorb on mild steel surfaces are as follows: TSC > TU > TA, whereas for the thiosemicarbazone derivatives, the order is BZOTSC > BZITSC > MBTSC > 2AP4MTSC < 2AP4PTSC. Physical adsorption mechanism has been proposed for all the inhibitors except MBTSC, BZITSC and BZOTSC, which are chemically adsorbed. The mechanism of corrosion inhibition considers the total molecular structure of the inhibitor, and the nature and spatial relationship of the different functional groups. (C) 1999 Elsevier Science S.A. All rights reserved.