| Acidic solutions are widely used for industrial cleaning, pickling, descaling, oil well acidification, etc. In order to diminish metal dissolution and acid consumption, the inhibitors have been developed. Recently, the synergistic effects of the metal corrosion inhibitors have attracted attention as an effective means of improving inhibition, decreasing the required amount of acid, and increasing the variety of inhibitors applied to acidic media.Compared to the corresponding conventional surfactants, Gemini surfactants have the lower critical micelle concentrations (CMC) and the higher solubilization capacity, being able to lower the surface tension of water greatly. The surface active molecules of this new kind of surfactants contain two hydrophilic head groups and two hydrophobic groups per molecule. It has been demonstrated that the cationic Gemini surfactants are a type of effective corrosion inhibitors for steel exposed to acidic media. This work investigates the corrosion inhibition of several cationic Gemini surfactants and their synergistic inhibition with the halide ions to steel in H2SO4 solution of 0.5 mol L-1 and in H3PO4 solution of 1.0 mol L-1. The corrosion inhibition of these cationic surfactants to zinc in H2SO4 solution of 0.5 mol L-1 is also considered.Such thermodynamic functions as AG, AH and AS and kinetics parameters like activation energy, Ea and pre-exponential factor, A of the Gemini surfactants have been obtained from the experimental data of the weight loss. An adsorption mechanism of these cationic Gemini surfactants on steel surface in acidic medium was proposed. The electrochemical mechanism on the corrosion inhibition of these surfactants in acidic solutions under different circumstances was also discussed. Moreover, the structure optimization and frequency for the Gemini surfactant cations were calculated by a quantum chemistry program, from which the adsorption mechanism of the Gemini surfactants on metal surface in acidic medium was explained. The main contents and results of the present work are as follows:1?Such Gemini surfactants as 12-3-12,18-3-18 and 18-6-18 have the outstanding corrosion inhibition effect to steel in 0.5 mol L-1 H2SO4, and they with the halide ions are also the excellent inhibitors for steel corrosion against sulfuric acid. The experimental data of gravimetry was to fit to the Langmuir adsorption model, obtaining the related thermodynamic and kinetic parameters. From this it could be deduced that the complex system of Gemini surfactant with halide ions has a tendency to be adsorbed onto the metal surface site with the manner from monolayer to multilayer, while the single Gemini surfactants (12-3-12) with the manner of two stage monolayer to multilayer. The corrosion rate was considered to be simultaneity controlled by Ea and A based on the kinetic parameter feature. Also, Tafel extrapolation method of high-polarization curves was illustrated in detail, and the polarization curves indicated that the synergistic corrosion inhibition system acts as a mixed-type inhibitor. The analysis of electrochemical impedance spectroscopy (EIS) suggested that the electrode reaction was activation-controlled rather than diffusion-controlled, and the equivalent circuit on EIS was given.2?The feature of the corrosion inhibition of steel in 1.0 mol L-1 H3PO4 solution by the cationic Gemini surfactant 12-2-12 and 12-2-12/halide ion system was similar to that in 0.5 mol L-1 H2SO4 by 12-3-12,18-3-18,18-6-18 and their synergistic system. The potentiodynamic polarization parameters were determined using a modified Tafel extrapolation, and this method was analyzed, too.3?Regarding the n-s-n kind of cationic Gemini surfactants and their synergistic system with halide ions, their adsorption mechanism on the steel surface and the corrosion inhibition mechanism of steel in acidic medium were presented. The chemical adsorption is principal for the single Gemini surfactants on the steel surface, while it decays for its complex system with halide ions and the increase of the physical adsorption at same time. The effect of the synergistic system on the steel surface was considered as an interaction of two kinds of adsorptions.4?The experiments were made by using various bromide and chloride salts in order to understand the synergistic effects between the halide and the Gemini surfactants to clarify whether the corrosion inhibition of the cold rolled steel in 1.0 mol L-1 H3PO4 solution is affected by cations of these halide. The results indicated that any effect of the cations of the halide salts studied on the inhibition efficiency was not found in the present work, suggesting that synergistic function takes place just between cationic surfactant and halide anions. A compared experiment was made on the corrosion of steel in 1.0 mol L-1 phosphoric acid by using respectively single NaBr and KBr/12-2-12 as the inhibitor under same conditions. The inhibition efficiency of the latter was far better than the former.5?The low-polarization and EIS methods were used to investigate the corrosion inhibition of several Gemini surfactants (12-2-12,12-3-12, 18-3-18 and 18-6-18) at varies concentrations to zinc in 0.5 mol L-1 H2SO4 solution. The parameters of low-polarization curve were obtained by a special designed Matlab program according to the three-point method, and the program would have the generality to some extent. The equivalent circuit of EIS was similar to the corrosion of steel in acidic solution with the Gemini surfactants. The results indicated these Gemini surfactants have the good inhibition corrosion effect on zinc in 0.5 mol L-1 H2SO4 solution. The principle for calculation of parameters of low-polarization curve was present.6?The structures of cations of the Gemini surfactants were built and primarily optimized by the software, Chemoffice2005. Then the density functional theory(DFT) at B3LYP/6-31G (d, p) basis set level was used to do full geometric optimization and frequency calculations and to determine the global activity indices such as the highest occupied and the lowest non-occupied orbital energies?omo and.ELUMO as well as the energy gap?E=-ELUMO-EHOMO for the Gemini surfactant cations. Based on the calculated and experimental results, the corrosion inhibition mechanism to metals in acid, and the adsorption phenomenon for the Gemini surfactants were explored. |
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