| Self-assembled films formed by chemisorption provide a convenient technique to modify surfaces. The highly ordered and packed films can prevent solution from transferring to the metal surface so that it can protect the metal from corrosion effectively. The efficacy of an organic compound as a successful inhibitor mainly depends on its ability to get adsorbed on the metal surface. The driving force for the spontaneous formation includes three different kinds of interactions: the strong bonding between the head group of the organic molecules and the substrate, the intermolecular Van der Waals interactions among the alkyl chains, the group-specific interactions such as H-bonding, dipolar and other interactions. It is the above interactions that make the molecules adsorb on the surface spontaneously to form highly ordered, packed and stable films. When the electrode surface was modified with the films, the physical and chemical properties can be improved remarkably.Corrosion science and its prevention technique is an important research field which has a great effect on the development of the national economy. For the engineering metal, such as copper, aluminium, iron and steel, because they are quite active and easily corroded due to some environmental factors when exposed in air, they have caused much loss to commercial production and national economy. So, it is necessary and urgent to investigate into its corrosion and protection properties. The corrosion process of iron in acid medium is a familiar phonemonon in modern commercial production, so it will have practical significance to study the iron corrosion in acid solutions.In this paper, self-assembled technique was used to fabricate the thin organic films on iron surface. Electrochemical techniques including electrochemical impedance spectroscopy(EIS) and polarization curves were used to study the charge transfer resistance, double layer capacitance, inhibition efficiency and other electrochemical parameters. The surface coverage, configuration and the effect of temperature, immersion time, concentration of the inhibitor, the substituting group, the chain length of the alkyl etc. on the inhibition ability were also investigated. Suitable equivalent circuit was designed and used to fit the Nyquist spectra.In addition, some surface analysis techniques such as XPS, FT-IR, FS, SEM were used to characterize the films. Microcalorimetry method was utilized to study the self-assembled process on iron surface for the first time and the adsorption mechanism was discussed from the power-time curve. The microscopic properties including the bonding mode, adsorption configuration and other parameters were also studied by quantum chemical calculation and molecular simulation, which can make it possible to provide some valuable related information for the selection of the inhibitors and control of the experimental conditions. The main contents of this paper are as the following:1. Study of the N, S-containing heterocyclic compounds self-assembled films on iron surface2-mercaptobenzothiazole (in the following MBT) and benzothiazole(BT), two kinds of N and S-containing heterocyclic compounds, were assembled on iron surface to form films. FT-IR and XPS results indicated that MBT molecules were really adsorbed on the iron surface after several hours of immersion time. EIS results indicated that the presence of the films could protect iron from corrosion effectively; however, the immersion time, concentration and the mercapto group have great effect on the inhibition ability. With increasing the immersion time, the charge transfer resistance increased and accordingly the inhibition ability was enhanced. Whereas too long immersion time will lead to the desorption of some molecules and the decreasing of the inhibition ability. From the polarization curves it can be seen that the anodic current decreased significantly and there was no obvious decrease for the cathodic current for the films-covered electrodes, on the country, the cathodic current increases slightly. Furthermore, the increasing of the concentration has more effect on the cathodic process than on the anode process, so the total results of the increasing of the concentration make the inhibition ability decrease. In addition, the mercapto group has great effect on the inhibition ability; the maximum inhibition efficiency of MBT was 69.1%, which is much higher than that of BT, 34.5%.Microcalorimetry method was utilized to study the self-assembled process on iron surface and the adsorption mechanism was discussed from the power-time curve. The two-step mechanism was confirmed to be: a very fast step at first, which takes a few minutes; then a slow step, which will last several hours. From the power-time curve, some adsorption properties such as adsorption intensity, adsorption rate, mechanism and adsorption heat were studied. Good agreement with EIS results was obtained. Primary results from microcalorimetry were obtained; however, some more detailed work should be done.Quantum chemical calculations showed that the adsorption process had happened during the self-assembling process of MBT molecules on iron. MBT was chemically adsorbed on iron surface via the exocyclic sulphur and nitrogen atoms. The possibility of the former is larger than the latter because of the steric effect. Under the experimental condition, although both forms of thiol and thione are likely to coexist, however, the thione form is more stable.2 Study of the conjugated macrocycle-containing compounds self-assembled films on iron surface5,10,15,20-tetraphenylporphyrin(TPP) and 5,10,15,20-tetra-(4-chlorophenyl) porphyrin(TClPP) were synthesized and formed adlayers on iron surface. The products were characterized by 1H NMR and FT-IR. The surface properties of the parphyrin adlayers on iron electrode were characterized by FT-IR, FS and SEM. The FT-IR spectra of the adlayers on the iron surface are in accordance with the bulk sample. Because the two inhibitors we synthesized are fluorescent, the fluorescence spectrum was used to detect the emission peaks. Two strong emission peaks at 658nm and 660nm, which were consistent with the peaks obtained by powder sample, were detected, respectively.The corrosion inhibition ability was studied by electrochemical impedance spectroscopy(EIS) and polarization curves in 0.5mol dm"3 H2SO4 solution. Results indicated that the presence of the porphyrin adlayers was able to protect iron from corrosion effectively and longer immersion time would lead to better inhibition performance. It is also found that the presence of -Cl substituting group has great effect on the bonding intensity between porphyrin molecules and iron. Because the -Cl substituting group of the TC1PP molecule is a strong electron-withdrawing group, the four meso-chlorine atoms will decrease the electron cloud density of the macrocycle, which will weaken the ability of donating electron of the macrocycle to the unoccupied d orbits of iron. As a result, the coverage of the molecules will decrease. This conclusion was consistent with our electrochemical experimental results. The inhibition efficiency(IE) calculated from polarization curves was in good agreement with the coverage(θ) of the films calculated from EIS. Although IE and 0 have different physical meanings, both of them can reflect inhibitive effect of the films effectively.Besides, quantum chemical calculation and molecular simulation were applied to optimize the structure of the two molecules and provide some microcosmic information. Calculated results indicated that the middle macrocycle of the TPP molecules assumed a near flat orientation with respect to the iron surface. The centrosymmetrical four benzene rings are tilted to the plane by 44.8°and 58.8°respectively because of the steric hindrance effect of the hydrogen atoms of the four thiazole rings. All the nitrogen atoms are adsorbed on the pseudo-4-fold hollow sites of the Fe(l 10) surface. The pseudo-4-fold hollow site was a stable adsorption site for Fe (110) surface and similar to the 3-fold hollow site on Cu(111) and Au(111) surface. The interaction energy between a TPP molecule and iron is about -294.34kcal mol-1. The HOMO for TPP is -505.67 kJ mol-1, which is larger than that for TC1PP, -543.69 kJ mol-1. The difference of HOMO and LUMO for TC1PP is a small larger than that for TPP, which means that after being substituted by chlorine atoms, the reaction activity of the molecule will decrease. The conclusion was in good agreement with the EIS results.3. Study of the alkylamine self-assembled films and alkylamine/alkylthiol mixed films on iron surface Butylamine, octylamine, decylamine, tetradecylamine were assembled on iron surface to form films. The films were characterized by SEM and FT-IR techniques. Electrochemical impedance spectroscopy, as a main electrochemical technique, was used to study the protection ability of films. Results indicated the barrier properties of these alkylamine self-assembled films depended on the self-assembled time and chain length of the adsorbates. With increasing the self-assembled time, the barrier property increases; longer chain will lead to an increasing thickness and a stronger intermolecular Van der Waals interaction, which can also improve the inhibition ability. However, an exceptional phenomenon for TDA self-assembled films was found. The films were thicker than the DA films, whereas the protection ability decreased in a small degree on the contrary.Molecular simulation was used in this work to discuss the adsorption mechanism and the adsorption structure of the molecules on iron surface. The results were consistent with those obtained from EIS and could explain the EIS results well. Although the alkyl chains of TDA are highly ordered on iron surface, they have bended to some extent because the long chains are soft. It will make it possible for water and the corrosive ions to penetrate into the film and reach the underlying iron substrate. Consequently, slow corrosion occurred at the iron surface, which will further lead to the desorption of the molecules from the iron surface, resulting in loose packing of the film and decreasing of its protection efficiency.When the TDA films were modified with DT molecules to form mixed films, because the DT molecules can penetrate into the defects of the films and bond to iron, the coverage of the adsorbed molecules increases and the ability of the corrosion inhibition of the mixed films improved remarkably. |
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