Mild steel (MS) is a material of choice for diverse structural and industrial applications owing to its excellent mechanical strength and relatively low cost [1]. Certain requisite industrial activities involve the use of acidic solutions that constitute strong corrosive environment for MS.  Therefore, corrosion of MS causes great economic loss and present huge safety threats to the concerned industries. Addition of corrosion inhibitors to aggressive solution has been identified as an economical and practical means of controlling metal corrosion. The existing data have shown that organic compounds containing O, N, S, and P heteroatoms, and pi-electron or aromatic systems often exhibit good anticorrosion properties [2,3]. Quinoxaline derivatives possess these features and their corrosion inhibition properties have been reported. These compounds form parts of many essential biological and pharmaceutical compounds and fit into the class of environmentally benign inhibitors [3].

In the present study, adsorption and corrosion inhibition properties of six quinoxaline derivatives namely, N-{n-[1-R-5-(quinoxalin-6-yl)-4,5-dihydropyrazol-3-yl]phenyl}methanesulfonamides: R = propanoyl & n = 2,  3, 4 for MS-2-PQPP, MS-3-PQPP, and MS-4-PQPP respectively; and R = methylsulfonyl &  n = 2,  3, 4 for MS-2-PQPMS, MS-3-PQPMS and MS-4-PQPMS were investigated on MS in 1 M HCl solution. The techniques employed in the study include potentiodynamic polarization and electrochemical impedance spectroscopy. Quantum chemical calculations and molecular dynamic simulations studies were used to support experimental results.

All the studied compounds were found to exhibit appreciable inhibition efficiency and they are all mixed type inhibitors. The compounds inhibit MS corrosion by adsorbing on the steel surface and geometrically blocking the active sites for the corrosion reaction. Both MS-3-PQPP and MS-3-PQPMS showed higher inhibition performances, which suggests that the interplay of electronic and steric effects of the carbonyl/methylsulfonyl substituents ensures higher inhibition performance when n = 3 (meta-substitution). The adsorption of the studied compounds obeyed the Langmuir adsorption isotherm model. Theoretical quantum chemical calculations and molecular dynamic simulations corroborate experimental results.

References

(1)        De la Fuente, D.; Diaz, I.; Simancas, J.; Chico, B.; Morcillo, M. Corros. Sci. 2011, 53, 604.

(2)        Mashuga, M.E.; Olasunkanmi, L.O.; Adekunle, A.S.; Yesudass, S.; Kabanda, M.M.; Ebenso, E.E. Materials 2015, 8, 3607.

(3)        Olasunkanmi, L.O.; Obot, I.B.; Kabanda, M.M.; Ebenso, E.E. J. Phys. Chem. C 2015, 119, 16004.