Corrosion Inhibition of Copper by Thioureas and N, O, S-Ligating Ring Compounds

Abstract

Certain N, O, S-ligating ring compounds and thioureas were investigated to understand their role of inhibiting copper corrosion in acetonitrile. For 5 quinones under study including xanthone, xanthene, thioxanthone, acridone and 1,4-naphthoquinone, acridone is the best inhibitor with the corrosion inhibition of 98.98% whereas 1,4-naphthoquinone exhibits the lowest inhibition, due to weak interaction between carbonyl group and copper. With the presence of sulphur to form stronger bond with copper, thioureas have better inhibiting behavior than quinones. For 4 thioureas namely. thiourea, diphenylthiourea, phenylthiourea, and ethylenethiourea; ethylene thiourea derivatives shows the best inhibition with the corrosion inhibition of 99.48% and diphenylthiourea the lowest due to steric effect from phenyl group. When halide ions are present, the efficiency of thioureas in inhibition decreases due to more preferable copper complexation to halides, with the strongest copper-halide bond formation by the free iodide ion, consistent with the results from X-ray crystallography and Cyclic voltammetry. The inhibition ability of dimedone for copper in acetonitrile at 25 °C was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Corrosion resistance was found to be increased with inhibitor concentration up to 93.68% inhibition efficiency at 3.00 mM, indicating that dimedone molecules can cumulatively adsorb on the copper surface and finally form a protective film on copper-solution interface. This is also supported by the decreasing of copper oxidation in cyclic voltammogram. Polarization curves revealed that dimedone is of mixed type inhibitor. The adsorption of dimedone on copper surface obeys the Langmuir isotherm and the adsorption mechanism is of physisorption type. The standard energy of adsorption (AG° ads) values were found in good agreement for both polarization and impedance to be -8.43 and -8.17 kJmol*' respectively. Fourier Transform Infrared spectroscopy (FT-IR) confirmed the interaction of copper with oxygen on dimedone. The mole ratio method suggested that the complexation ratio of copper-dimedone is 1:2. Scanning electron microscopy (SEM) of copper surface after immersion in dimedone solution indicates the presence of a protective layer on the electrode surface. The frontier molecular orbital energy Еномо (highest occupied molecular orbital), ELUMo (lowest unoccupied molecular orbital), and the Mulliken charge distribution obtained from Quantum chemical calculations revealed that the small value of energy gap (AE) for dimedone reflects strong adsorption of the molecules on copper surface. The enhanced corrosion inhibition is possibly due to the compact film structure blocking the electron transfer from the solution to copper surface.