The Study Of Corrosion Mechanism Of Green Corrosion Inhibitors On Iron And Copper Electrodes

Iron and copper are the most important engineering materials, and have been widely used in catalysis, materials, surface science and industry. However, during the application process, the interaction between metal and environment made it possible to occur corrosion, causing great losses to the national economy. In recent years, a large number of organic corrosion inhibitors have been developed. However, most of these inhibitors are expensive and toxic, leading to the limitation of their applications.On the other hand, a majority of studies have been concentrated on the function from the macroscopic point. However, little work focused on microscopic point, which influenced the development of high-efficient inhibitors.The surface-enhanced vibrational spectroscopy including surface-enhanced Raman scattering spectroscopy(SERS) and surface-enhanced infrared absorption spectroscopy(SEIRAS), is regarded as the effective technique to investigate the interaction between inhibitors and metallic surface at the molecular level, especially benefiting to understand of the inhibition mechanism on the metal surface. SERS has the high spatial resolution and sensitivity, which is capable of directly detecting information on monomolecule layer. SEIRAS in attenuated total reflection(ATR)configuration plays an important role in the basic and applied research in electrochemistry due to its high sensitivity, simple surface selection rule and no substrate limitation. Both SERS and SEIRAS techniques are important in detecting of self-assembly monolayers(SAMs), which are widely used to study the interfacial adsorption, molecular orientation and change of absorption configuration. By combining surface-enhanced vibrational spectroscopy with traditional electrochemical measurements, the inhibition mechanism from the macroscopic and microscopic points have been obtained, which is helpful to explain the structure-activity relationships and to develop more efficient and environment-friendly corrosion inhibitors.Generally speaking, the influential factors of the inhibition mechanism betweenthe inhibitors and metals are functional groups of the inhibitors, type of metal, and environment, et al. As a result, the following research work has been carried out,which is focused on the inhibition mechanism on iron or copper dependent on the metallic type,functional group and environment:1. Inhibition mechanism of 2-mercaptobenzothiazole(MBT) on copper electrodeIn order to solve the controversy of the inhibition mechanism of MBT on copper and to gain the information about inhibition efficiency and adsorption model on different metals, structure-activity relationship of MBT on Cu electrode have been studied by using Tafel, EIS, DFT, XPS and ATR-SEIRAS techniques. The situation of the electron transfer has been clarified by in situ ATR-SEIRAS, as well as the activity of the adsorption site for the inhibitor. MBT molecules adsorbed on the Cu surface vertically through exocyclic S atom in a thiolate form at a potential negative of 0V(vs saturated calomel electrode(SCE)); At the potential positive of 0 V, MBT molecules interact with the Cu surface via exocyclic S and endocyclic N atoms, resulting in the formation of a compact protecting layer(Cu(MBT)n) to prevent Cu dissolution.Moreover, the polarisation and impedance measurements indicated that inhibition efficiency can arrive at 91%. By comparing with the previous research of MBT on Fe and Ni, the differences in the numbers of d orbital lead to the diverse adsorption configurations, which resulted in the different inhibition efficiencies. The potential-dependent adsorption and reaction process have been clarified owing to the advantages of the ATR-SEIRAS, which promoted to understand of the structure-activity relationship between corrosion inhibitor and metals.2. Inhibition mechanism of 2-quinoxaline carboxylate sodium(2-QC) on iron electrode surfaceQuinoxaline and its derivatives are regarded as a new, green and efficient inhibitors. As far as we know, the study of inhibition mechanism of 2-QC on iron has not been reported. The influences of concentration, time and temperature on the inhibition efficiency were studied via Tafel and EIS measurements. The inhibition efficiency would arrive at 99.8% under the optimal self-assembly condition, in whichthe concentration is 10 m M and a self-assembly time is 12 h. The inhibition efficiency can be improved as improving the self-assembly temperature, indicating the 2-QC chemisorbed on the iron surface. Subsequently, SERS, SEIRAS and density functional theory(DFT) were introduced to study the inhibition mechanism under the optical self-assembly condition. 2-QC strongly bonded to the iron electrode surface via the lone pair electrons of the two O atoms of the carboxylate group in a bidentate configuration with a vertical orientation at 0.2V; whereas at-0.8V, only one oxygen atom of the carboxylate and one of endocyclic N atom adsorbed on the iron electrode surface to form a unidentate configuration with titled orientation. Nevertheless, the molecules did not remain on the iron electrode surface at the more negative potentials.The results revealed that the inhibition mechanism of 2-QC on iron at the molecular level. The potential dependent adsorption action was also illustrated, which laid the solid foundation of the study of inhibition mechanism and the exploration of the new type inhibitors.3. Inhibition mechanism of quinoxaline(QX) on iron electrode surfaceIn order to acquire the impact of functional groups and p H on the structure-activity relationship of inhibitors, QX was selected to act as a corrosion inhibitor for iron. By comparing the inhibition efficiency and adsorption configuration of QX with 2-QC, it is easy to understand the impact of structure of inhibitors on inhibition mechanism. Moreover, the inhibition mechanism of QX in different media(0.1 M KCl and 0.05 M HCl) were studied to explore the p H effect.Electrochemical measurements showed that the inhibition efficiency of QX is lower than the 2-QC in the same medium(0.1 M KCl), indicating the fact that the more adsorption sites, the higher inhibition efficiency. DFT calculation showed that the oxygen atoms of the 2-QC have more negative charge. This means that O atoms can easily adsorb on the iron surface, and thus improve the inhibition efficiency.In neutral and acidic media, spectroelectrochemical techniques were introduced to explore the inhibition process of the protonation and the deprotonation of the QX.DFT calculation also can the provide theoretical support. In the neutral medium, theinhibition efficiency was about 60% and QX acted as a mixed-type corrosion inhibitor.While the inhibition efficiency was about 80% in the acidic medium, and QX served as the anodic-type inhibitor. In the natural medium, SERS spectra explained that the QX would adsorb on the iron surface through the N atoms in a titled way. While the QX would exist in the HQX formation in the acidic environment. The SERS result revealed that HQX would lie on iron surface to protect the iron from corrosion. This may be due to the large steric hindrance.To sum up, the studies of inhibition mechanism of MBT on copper, 2-QC and QX on iron metal surface have revealed the impact of the nature of metal, the molecular structure and corrosive media on the inhibition mechanism. Meanwhile, the new study method on the inhibition mechanism from the microscopic view has been provided,which laid the theoretical foundation for the development of a new generation of metal corrosion inhibitors.