Carbon Steel Corrosion Inhibitor And Copper-based Self-assembled Film Studies

Copper and steel are primary metallic materials and are widely used in transportation, construction, machinery, chemicals, electronics, national defense industry, and so on. But they are susceptible to corrosion in the environment due to their active chemical nature. They would be corroded markedly especially in harsh conditions, such as acid solutions, aqueous solutions containing Cl-, SO42-and dissolved oxygen. Thus, the corrosion of copper and steel has become an important subject for the corrosion researchers. The use of inhibitor is an effective method in common corrosion protection technology. With increased environmental awareness, the development and investigation for highly efficient and environmentally friendly corrosion inhibitors has been an important issue t issue in the field of corrosion science.Self-assembled monolayers (SAMs) are well ordered and dense monolayers formed by the spontaneous adsorption of active molecules via chemical bonds on the solid/liquid or solid/gas interface, which are belong to the thermodynamic stability system. Because the preparation method for self-assembled monolayers is simple, independent of substrate shape, and the film formed is compact, it has become one of the film technologies with broad application prospects. Efficient inhibitor molecules assembled on the metal surface could widen the scope of application of the inhibitor. Furthermore, self-assembly technology could adjust the physical and chemical properties of the assembled film by designing molecule and optimizing the assembly process. This offers the possibility to study physical properties of inhibitors in two-dimensional and even three-dimensional field for exploring their inhibition mechanism, and provides a new avenue to develop and research new-type corrosion inhibitors.In this paper, the work is divided into two parts:The first, we selected an organic heterocyclic compound and an organic phosphonate as inhibitors, and investigated their inhibition performance on the corrosion of mild steel in sulfuric acid and sodium chloride solution, respectively. The corresponding inhibition mechanisms were discussed based on the modern characterization techniques and quantum chemical calculations.The second, some heterocyclic compounds with multi-center adsorption sites were assembled on copper surface. The electrochemical methods were used to investigate the inhibition effects of self-assembly monolayers on the copper in different corrosive media and influence of assembly conditions on the SAMs quality. The relevant adsorption and corrosion inhibition mechanism of SAMs have also been explored.1. Inhibition effect of2,5-dimercapto-1,3,4-thaidizole on the corrosion of mild steel in sulphuric acid solutionThe effect of2,5-dimercapto-1,3,4-thaidizole (DMTD) on the corrosion of mild steel (MS) in sulphuric acid solution was investigated using weight loss and various electrochemical techniques. Scanning electron microscope (SEM) and Fourier transform infrared (FTIR) spectroscopy were used to determine characteristic of the inhibitor adsorbed on the carbon steel. The results of Tafel curves indicated the DMTD acted as mixed-type inhibitor for corrosion of carbon steel, but did not change the mechanism of the cathodic hydrogen evolution. Linear polarization resistance and electrochemical impedance spectroscopy (EIS) measurements revealed that the addition of DMTD changed the electrical double layer structure of the carbon steel surface and increased the polarization resistance of the corrosion reaction. Relevant thermodynamic and kinetic parameters obtained by the weight loss measurement showed that the adsorption of DMTD molecules on the carbon steel surface is a spontaneous and exothermic process and obey the Langmuir isothermal adsorption model. Capacitance-potential curve tests indicated carbon steel surface in sulfuric acid carried positive charges before and after adsorption of DMTD molecules. According to the results of experiments and quantum chemical parameters, the adsorption mechanism of DMTD on carbon steel surface could be a mixed adsorption process through both the electrostatic attraction and formation of covalent bond as driving forces.2. Inhibition effect of a composite phosphonate on the corrosion of mild steel in sodium chloride solutionsTafel curves and electrochemical impedance spectroscopy were used to investigate the inhibition effect of sodium tripolyphosphate and sodium1-phenylvinyl phosphate on the carbon steel corrosion in3%sodium chloride solution. Tafel curves tests revealed sodium tripolyphosphate acted as an anodic inhibitor, but sodium1-phenylvinyl phosphate was a mixed-type inhibitor with strong inhibition performance on both the anodic and cathodic corrosion reaction. Compared with single inhibitor, composite inhibitors performed a better corrosion inhibition on carbon steel. At the same time, the electrochemical tests revealed that a competitive adsorption would occur between the two kinds of inhibitors in composite phosphonate system, which is disadvantageous for the formation of protective film.3. Potassium5-mercapto-3-phenyl-1,3,4-thiadiazole-2-thione self-assembled monolayer against copper corrosion in sodium chloride solutionPotassium5-mercapto-3-phenyl-1,3,4-thiadiazole-2-thione (MPTT) was self-assembled on copper surface. The influence of assembly conditions on the SAM inhibition properties was investigated by Tafel curves and EIS measurements, and the results indicated that high MPTT concentration, suitable assembly time, and elevated temperature could enhance the inhibitive effect of SAM on copper corrosion under experimental conditions. The self-assembly process includes initial fast adsorption and following structural rearrangement stage. Suitable prolonging assembly time and elevating assembly temperature could promote the rearrangement process of MPTT molecules, and decrease defect density of the SAM on copper surface. It was also found that the adsorption of MPTT obeys the Langmuir adsorption isotherm, and is a spontaneous and composite adsorption process. According to quantum chemical calculations, infrared spectroscopy and contact angle measurement, MPTT molecules may be adsorbed on the copper through the electronegative atoms S and N as adsorption centers, and substituent benzene ring was arranged outside the adsorption layer through π-π interactions, thus forming a compact and hydrophobic protective film.4. Inhibition of copper corrosion in hydrochloric acid solution by the self-assembled monolayers of trithiocyanuric acidThe trithiocyanuric acid (TTCA) molecules were prepared on the etched copper surface in ethanol solution by using self-assembly technique. The FTIR confirmed that the TTCA molecules were adsorbed on the copper surface, and the contact angle and SEM measurements revealed the self-assembly film has a layered structure with hydrophobic nature. The inhibition effect of self-assembly film on the copper corrosion was investigated by electrochemical methods and the results revealed that it was related to the assembly concentration and assembly time. The optimal condition for assembly process is immersion in2.5mM TTCA ethanol solution for12h. Tafel curves tests showed that the self-assembled monolayers acted as a mixed type inhibitor against both cathodic and anodic corrosion reactions, and the highest inhibition efficiency reached91.2%. EIS measurement indicated TTCA molecules adsorbed on copper surface decreased the total capacitance of the metal/solution interface, and increased the charge-transfer resistance of the corrosion reaction. Quantum chemical calculations showed that the adsorption of TTCA molecules may be dominated by physical interaction, and the electronegative atoms of S and N may participate in bonding with the metal surface.5. Corrosion inhibition of copper in sodium chloride solution by self-assembled monolayers of trithiocyanuric acidThe trithiocyanuric acid (TTCA) molecules were assembled on the non-etched copper surface in aqueous solution at different pH values by using immersion method. The assembly conditions were optimized by using electrochemical impedance spectroscopy measurements, and the results revealed that the TTCA films could provide better protection performance against copper after3h of assembly in0.1mM TTCA acidic aqueous solution. Meantime, the inhibition performance of the films was investigated by changing the concentration of chloride ions and the corrosive immersion time of TTCA-modified electrodes in corrosive media, and the results revealed that the TTCA films possess good stability and durability in sodium chloride solution. According to electrochemical experiments, FTIR and contact angle measurements, the TTCA film assembled in acidic solution could be mixed films involving TTCA molecules and [Cu-TTCA] complex, which could act as a barrier to hinder aggressive species from attacking the underlying copper substrate and provide good protection for copper in sodium chloride solution.