Synthesis and cyclic voltammetric studies of azo dye compounds derived from 1,5-dihydroxynaphthalene and their application as corrosion inhibitors for carbon steel in hydrochloric acid solution

New bis-azo dyes derived from 1,5-dihydroxynaphthalene were synthesized and characterized by elemental analysis, Fourier-transform infrared (FTIR) and proton nuclear magnetic resonance spectroscopy (¹H NMR). These compounds were examined using differential pulse polarography (DPP) and cyclic voltammetry (CV) in Britton-Robinson buffer solutions with pH values ranging from 2 to 12. When the two NN centers of the examined azo compounds were cleaved to form the amine group, the bis-azo group was reduced with the loss of eight electrons, resulting in an irreversible diffusion-controlled cathodic peak. The reduction mechanism was postulated in view of the data obtained was found to be H⁺, e, e, H⁺. The application of these azo compounds as corrosion inhibitors for carbon steel in hydrochloric acid solution was investigated and the inhibition efficiency was found to be increasing with both time and concentration, reaching 97 % after 24 hours in the presence of 10⁻³ M for the three azo compounds. The Langmuir adsorption isotherm indicates that the suppression of corrosion is caused by the inhibitor molecules donating electrons to the empty d-orbitals of the surface of the carbon steel (chemical adsorption), as the calculated Gibbs free energy (∆G_ads) is found to be around −40 kJ/mol. The influence of temperature on the parameters of corrosion was examined, and the thermodynamic parameters of corrosion were computed and examined. The results showed that increasing temperature causes increasing of the inhibition efficiency of the tested bis-azo dyes. The order of inhibition efficiency followed their donating affinity which increases in the order p-OCH₃ > p-CH₃ > m-CH₃. Also, the results showed that these compounds causes decrease in entropy, enthalpy and activation energy due to their chemical interaction with the metal surface.