Indian Jornal of Chemical Technology Vol. 12, January 2005, pp.98-104 Heterocyclic anils as corrosion inhibitors of mild steel in formic and acetic acid solutions M A Quraishi* & H K Sharma Corrosion Research Laboratory, Department of Applied Chemistry Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202 002, India Received 28 January 2004; revised received 4 November 2004; accepted 16 November 2004 Effects of four corrosion inhibitors namely, 4-(N, N-dimethylamino) benzylidine imino-3-propyl-5-mercapto-1,2,4- triazole(DIPMT), 4-benzylidine imino-3-propyl-5-mercapto-1,2,4-triazole (BIPMT), 4-salicylidene imino-3-propyl-5- mercapto-1,2,4-triazole (SIPMT) and 4-cinnamalidene imino-3-propyl-5-mercapto-1,2,4-triazole (CIPMT) against corrosion of mild steel in aqueous solutions of 20% formic acid and 20% acetic acid using weight loss and potentiodynamic polarization and impedance methods were studied. All tested inhibitors showed good inhibition efficiency (IE). The IE was found to vary with concentration, temperature, immersion time, and concentration of acid solution. The potentiodynamic polarization results revealed that all the compounds were mixed type inhibitors. The adsorption of these compounds on the mild steel surface exposed to both acids has been found to obey Langmuir adsorption isotherm. Keywords: Mild steel, heterocyclic anils, Langmuir adsorption isotherm, potentiodynamic polarization, impedance. IPC Code: C23F 11/08 Corrosion behaviour of iron and mild steel in organic acid solutions has attracted the attention of several investigators 1-3 . Corrosion studies on metals in organic acid solutions are scarce in comparison with similar studies in mineral acids 4-7 . Mild steel is used in fabrication of reaction vessels, storage tanks by industries, which either manufacture or use organic acids as reactant. Organic acids rank among the most important chemicals in industry today. The reactive carboxyl group –COOH makes them a basic building block for many compounds such as drugs, pharma- ceuticals, plastics and fibers. In the present investigation, a few heterocyclic anils namely, 4-(N, N-dimethylamino) benzylidine imino-3-propyl-5-mercapto-1, 2, 4-triazole(DIPMT), 4-benzylidine imino-3-propyl-5-mercapto-1,2,4-tria- zole (BIPMT), 4-salicylidene imino-3-propyl-5- mercapto-1,2,4-triazole (SIPMT) and 4-cinnamalidene imino-3-propyl-5-mercapto-1,2,4-triazole (CIPMT) have been synthesized and studied for their inhibiting properties on mild steel in 20% formic and acetic acid. These inhibitors are selected on the basis of the following facts: (i) these inhibitors can be synthesized conveniently from relatively cheap raw materials, (ii) the presence of aromatic rings, the electronegative nitrogen, the easily polarisable sulphur and readily solubilizing hydroxy group in aqueous acid medium is likely to induce greater adsorption of these compounds on the metal surface leading to the effective inhibition. Experimental Procedure Materials Colled rolled mild steel strips [composition: C = 0.14, Mn = 0.35, Si = 0.17, S = 0.025, P = 0.03 (wt. %) and the remainder Fe] were used. For weight loss measurements, strips having dimensions 2 ×2 ×0.25 cm were used, while mild steel specimens with an exposed area of 1 cm 2 were used for potentiodynamic polarization and impedance studies. 20% formic and 20% acetic acid solutions were used for all the experiments. Double distilled water and analytical reagent- grade formic and acetic acid were used for preparing __________ ∗ For correspondence (E -mail: maquraishi@redffmail.com; Fax: +91-571-2700528)