Investigation On Corrosion Inhibition And Mechanism Of Triazole Compounds For Copper In Synthetic Seawater

Many strategies have been developed for the protection of metal in seawater, such as cathodic protection, using corrosion inhibitor, coating, changing the composition of metal, the conversion film technology and so on. Corrosion inhibitor, with many features including small dosage in use, high efficiency, few side effects, wide applications and no additional equipment, has attracted more and more attentions in various fields. In this thesis, five triazole compounds, namely diniconazole, uniconazole, triadimefon, hexaconazole and myclobutanil, are investigated as corrosion inhibitors for copper in synthetic seawater (3.5% NaCl solution). These compounds have been widely used as fungicides and plant growth regulators in agriculture, whereas no report on these compounds as corrosion inhibitors for metals has been published. Due to the predominance in their chemical structures, such as containing unsaturated bonds or various heteroatoms, these compounds can adsorb on to the metal surface spontaneously, and the corrosion reactions could be greatly suppressed. The studied compounds are degradable, low toxic, environment-friendly and easily available, compared with benzotriazole (BTA) which is often employed to protect copper. Therefore, this research has a great importance for both theoretical study and practical application.Determination of weight loss method, polarization curves, electrochemical impedance spectroscopy (EIS), scanning electronic microscope (SEM), X-ray energy dispersive spectroscopy (EDS) and quantum chemical calculations were performed to explore the inhibition performance and the mechanism of these triazole compounds under different concentration, pH, temperature and immersion time conditions. The main results obtained are as follows:All the triazole compounds at different concentrations show excellent inhibition properties for the corrosion of copper. The inhibition efficiencies increase with the increasing concentration of the inhibitors. The good inhibition performance attributes to the protective adsorption film of inhibitor molecules formed on copper/solution interface, which has greatly suppressed the reduction of hydrogen and oxygen in cathodic process and the dissolution of copper in anodic process. This weak chemisorption between these compounds and copper are accordance with Langmuir adsorption isotherm. As a result, the corrosion current densities (icorr) are significantly decreased. The positive shift of the corrosion potentials (Ecorr) suggests that these compounds behave as mixed-type inhibitors with anodic predominance.These compounds also exhibit good inhibition performance under different pH conditions. The formation of the adsorption film cuts off the contact of copper with the corrosive media and greatly reduces the cathodic and anodic processes of the electrochemical corrosion reaction. Furthermore, the corrosion inhibition of these compounds in near neutral and alkaline conditions is better than that in acidic conditions.With the temperature increasing, the formation and the stability of the corrosion product layer and the inhibitor adsorption film are seriously affected, which directly leads to an acceleration of the corrosion rate. Meanwhile, the cathodic process is initially controlled by the diffusion rate of oxygen at lower temperatures, and subsequently controlled by the reduction rate of oxygen as the temperature goes up. However, compared with the blank at each temperature, the corrosion rate is obviously decreased in the presence of the inhibitors. Hence, these compounds show good inhibition for the corrosion of copper at each temperature.All the inhibition efficiencies of the triazole compounds during the 30 days of immersion are extremely high, which confirms a good persistence of inhibition for the corrosion of copper. In early days of the immersion, an adsorption film is rapidly formed on copper surface and the inhibition efficiency increases sharply. Several days later, the adsorption reaches the saturate amount and the inhibition efficiency has a maximum value. In the later period, owing to the degradation of the inhibitors, the inhibition efficiency decreases slightly. However, on the 30th day, the inhibition efficiencies still retain high values.The chemisorption sites of these triazole compounds were studied via quantum chemical calculation, which provided a theoretical basis for the adsorption and the performance differences of the five inhibitors.