Inhibitive Efficiency of some Substituted Thioureas for the Corrosion of Aluminium in Nitric Acid

The inhibitive efficiencies of thiourea and some of its derivatives have been studied in relation to the corrosion of aluminium (Grade 1060) in nitric acid at 25°c, 35°c and 45°c. The efficiency in 20% HNO3 at 25°c decreases in the order: p-chlorophenyl thiourea > m-chlorophenyl thiourea ≥ o-chlorophenyl thiourea ≥ phenyl urea > 1:3 di-phenyl thiourea ≥ 1:3 di-p-chlorophenyl thiourea. ≥ 1:3 di-m-chlorophenyl thiourea > thiourea. The adsorption of the compounds is in agreement with Langmuir's adsorption isotherm equation up t a concentration of 250 ppm, at all the three temperatures studied. The percentage efficiencies of the inhibitors were found to increase with increase in temperature from 25° to 45°c. There is no relation between the relative efficiencies of the inhibitors and their critical current densities or primary passivation potentials. All the inhibitors were found to act predominantly on local cathodes although partially effective on the anodes.