Inhibition of corrosion of iron in HCl solution by semicarbazides and thiosemicarbazides

Electrochemical studies of the inhibition and the effect of a series of semicarbazides and thiosemicarbazides on the corrosion of iron in 1 M HCl were performed. The inhibitors studied were: semicarbazide, 4-phenyl semicarbazide, thiosemicarbazide, 4-methyl-3-thiosemicarbazide, 4,4-dimethyl-3-thiosemicarbazide, 4-ethyl-3-thiosemicarbazide, and 4-allyl-3-thiosemicarbazide. The percentage inhibition increased with increase in the concentration of the inhibitors. The presence of sulfur in the thiosemicarbazide instead of the oxygen in the semicarbazide causes inhibition to drop at all concentrations. 4-phenyl semicarbazide, with the phenyl group, exhibits a lower inhibition at all concentrations than does the semicarbazide. 4-ethyl-3-thiosemicarbazide showed a higher corrosion inhibition than 4-allyl-3-thiosemicarbazide at concentrations below 1x10(-4) M. However, at higher concentrations, these two inhibitors had similar percentage inhibitions, which reached about 77% at a concentration of 1x10(-2) M. 4,4-dimethyl-3-thiosemicarbazide was a better inhibitor than 4-methyl-3-thiosemicarbazide. Furthermore, an increase in the length of the substitutent group leads to an increase in the inhibition efficiency at the same concentrations as evident from the actions of 4-methyl-3-thiosemicarbazide and 4-ethyl-3-thiosemicarbazide. 4-ethyl-3-thiosemicarbazide was found to be the best inhibitor at all concentrations. The degree of inhibition for all the inhibitors closely followed the adsorption isotherm of the inhibition on the metal surface.