Adsorption Properties And Inhibition Of Mild Steel Corrosion In Acid Solution By Some Phenyl-Triazol Derivatives

Although various experimental techniques and theoretical methods have been developed to study the relation between the efficiency and temperature, structural properties of triazole derivatives, little is known about the number of water molecules displaced by one molecule of triazole derivatives at the temperature rage studied. The corresponding theoretical research is still far lags in the practice needs and there are many disputes in the investigation of the corrosion inhibition mechanism. This tempted us to investigate the interactions between the adsorbed molecules and metal surfaces with suitable substitutional adsorption isotherms. Due to the complexity of the steel/inhibitor/solution systems, great effort is still in need to explore the mechanism of the interfacial reaction and to develop environmentally-friendly and economic corrosion inhibitors. So, the thesis deals with synthesis of a series of phenyl-triazole derivatives bearing muti-adsorption centers in the inhibitor molecule and to study the mechanism of corrosion and adsorption behaviors. The inhibiting performance of these phenyl-triazole derivatives were evaluated by weight loss method, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM) and quantum chemical calculations. In order toevaluate compounds as corrosion inhibitors and to design novel inhibitors, much more research works were concentrated on the studies of the function way between the corrosion inhibitors and the metal surface and summarized the relationship between the molecular structure of inhibitor, thermodynamic parameters and their inhibiting efficiency. And then, the desorption behaviors of DTE inhibitor from mild steel electrode upon anodic polarization were investigated by differential polarization curve method. Some results were drawn as follows:The inhibition of the mild steel corrosion by the phenyl-triazole derivatives in acidic solutions was investigated. The steady-state open-circuit potential measurements indicated that the studied compounds of the six phenyl-triazole derivatives are mixed-type inhibitors. The results of the EIS measurements showed that the capacitive loops in the high frequency (HF) range and the inductive loops in the low frequency (LF) range. The capacitive loops were related to charge transfer in corrosion process, whereas, the inductive loops originated from the adsorption relaxation of intermediates, which covered the reaction surface. Two equivalent circuits used to fit the EIS data for all the six phenyl-triazole inhibitors, the influences of the adsorption process were better showed, the fit results for the measured and simulated data were more reasonable. IE% and Rct values were increased and the C_dl decreased with increasing inhibitors concentration. The decrease in C_dl might be attributable to a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer. The results suggested that the change in C_dl values were due to the gradual replacement of water molecules by the adsorption of the organic molecules on the metal surface. Compared with the phenomenon caused by adding the phenyl-triazole derivatives to sulphuric acid medium that double-layer capacitance of the interface will go down when the concentration of inhibitor go up, the double-layer capacitance of the interface have no regular changes when the concentration of inhibitor is relatively high in hydrochloric acid medium. This probably duo to that the inorganic anions existing in hydrochloric acid such as Cl- and there is a competitive adsorption process of Cl- and inhibitor molecules.Quantum chemical calculation was theoretically provided the absorption sites of these phenyl-triazole derivatives. The mechanism of corrosion inhibition of mild steel by the phenyl-triazole compounds in acid solutions was established by the Quantitative Structure-Activity Relationship (QSAR). It is useful to provide some valuable information to design novel inhibitors.Thermodynamic parameters of DOT inhibitor such as adsorption heat (?H_ads⁰), adsorption entropy (?S_ads⁰) indicate that the adsorption of inhibitor molecules is an exothermic process, and the negative values of?S_ads⁰ might be explained as a result: before the adsorption of DOT onto the mild steel surface, DOT molecules were chaotic and freely move in acidic solution, but with the progress in the adsorption, DOT molecules afford an ordered adsorbed onto the mild steel surface.The number of water molecules (X) replaced by a organic molecule of TMP was determined from the substitutional adsorption isotherms applied to the data obtained from the weight loss experiments performed on mild steel specimen in acidic solution in the 298-333 K range. The Dhar-Flory-Huggins and Bockris-Swinkels models were found to provide the better description of the adsorption behavior of TMP at 298 K and 308, 318, 333 K, respectively. It could be found that with increasing temperature of aggressive media the X there was a“Peak-like”value for X = 2.8 at 318 K, and it seemed that K and X varied in the same fashion. The slight change of IE% at higher inhibitor concentration (10^-3 M) in the range of temperature studied suggests that TMP effectively protects the mild steel even at high temperature.The negative values of?G_ads⁰ indicated that the adsorption of these phenyl-triazole molecules was a spontaneous process, and the adsorption processes are belong to chemisorption and physisorption. The SEM micrographs also show that these phenyl-triazole derivatives can protect mild steel from corrosion effectively in acidic solutions. This conclusion is consistent with the results obtained by weight loss method and electrochemistry techniques.