DERIVATIVES OF INDOLE AND ANILINE AS ORGANIC CORROSION INHIBITORS OF LOW CARBON STEEL

Abstract

Inorganic corrosion inhibitors are frequently used in protecting metals from acidic solutions often employed in industrial processes. However, their applications have been limited by high cost of production and toxicity.Thus, the search for alternatives which are affordable and with negligible environmental effects. Functional electronegative elements and π- electron conjugated double bonds which are important features in corrosion inhibition process are contained in heterocyclic compounds such as indole and aniline. However, the corrosion inhibition properties of indole and aniline have not been adequately investigated. Therefore, this study was designed to investigate the corrosion inhibition efficiencies and adsorption mechanism of some derivatives of indole and aniline on low carbon steel. Derivatives of indole: 6-benzyloxyindole and 3-methylindole and aniline:2-nitroaniline, 3-nitroaniline and 4-nitroaniline were employed for the corrosion inhibition. The corrosion rates and inhibition efficiencies of these derivatives were determined by weight loss and potentiodynamic polarisation methods. Images of low carbon steel in the absence of inhibitor and acid, low carbon steel in the presence of acid only and low carbon steel in the presence of acid and inhibitors of concentration 10 x 10-5 M were recorded with the aid of Scanning Electron Microscopy (SEM).The concentration range of inhibitors employed was 2 x 10-5 M to 10 x 10-5 M in 1.0 M HCl at 303K - 333K. The derivatives of indole and aniline were optimised using density functional theory at the B3LYP/6-31G* level. The experimental inhibition efficiencies of the inhibitors were compared to their theoretical inhibition efficiencies. Data generated were analysed by using multiple regressions at α0.05. The corrosion rates from weight loss (mg cm-2h-1 x 10-3) and potentiodynamic polarisation (mmpy) methods followed the order: 6-benzyloxyindole (0.55, 34.62) < 3-methylindole (0.98, 69.93) < 2-nitroaniline (3.43, 242.14) < 3-nitroaniline (3.49, 244.98) < 4-nitroaniline (3.55, 248.53).Inhibition efficiencies of the inhibitors increased with decrease in temperature, with all the inhibitors having their highest inhibition efficiencies at 303 K and at the concentration of 10 x 10-5 M. The inhibition efficiencies obtained from weight loss and potentiodynamic polarisation methods followed the order: 6-benzyloxyindole (92.29% and 94.11%) > 3-methylindole (86.32% and 88.11%) > 2-nitroaniline (52.24% and 58.82%) > 3-nitroaniline (51.37% and 58.34%) > 4-nitroaniline (50.50% and 57.74%). Potentiodynamic polarisation curves revealed that the organic inhibitors acted as mixed-type inhibitors. The SEM images revealed that the corrosion reaction did not take place homogeneously over the surface of low carbon steel in 1 M HCl solution. However, the surface of the low carbon steel was protected by these organic inhibitors. The activation energies (12.21 – 55.40 kJ) and the Gibb’s free energy values (-4.51 − -8.80 kJ) obtained from the weight loss method supported a physical adsorption mechanism. The adsorption was found to obey Langmuir adsorption isotherm. A good fit (R2 = 0.9957) between the experimental and theoretical inhibition efficiencies was observed. The 6-benzyloxyindole and 3-methylindole inhibited the corrosion of low carbon steel in hydrochloric acid and the process was through physical adsorption mechanism.Hence, these indole derivatives could serve as alternatives to inorganic corrosion inhibitors.