Studies Of The Self-Assembled Films Of A Heterocyclic Compound Containing Nitrogen And Schiff Bases On Iron And 20# Steel Surface

This thesis mainly focused on SA films of different types of organic molecules on iron or 20# steel surface. Electrochemical techniques including EIS and Tafel polarization curves were applied to characterize the inhibition effect of the SA films. In addition, XPS, FT-1R, SEM and EDS were also applied to study the formation of the SA films. At last, computational methods, including quantum chemical calculations and molecular dynamic simulations, were utilized to study the adsorption mechanism at the molecular level. The main research results are as following:1. SA films of 2-mercapto-5-methyl-1,3,4-thiadiazole (abbreviated as McMT) were prepared on iron and 20# steel electrode surface. The assembling time varied from 0.5 to 35 hours as experiments needed. Results of the EIS tests show that the effect of assembling time upon the value of inhibition efficiency, which is, the longer the electrode is immersed in the McMT solution, the higher the inhibition efficiency is. It can be concluded from the Tafel polarization curves that the formation of McMT SA films inhibited both the anodic dissolution of iron and cathodic reduction of hydronium for iron and steel electrode. The inhibitive effect becomes stronger as the self-assembling time increases.In order to clarify the formation of the McMT films, XPS, SEM/EDS measurements were carried out. Furthermore, quantum chemical calculations and molecular simulations were applied to investigate the structure of McMT molecules on Fe (1 1 0) surface. The results show that McMT molecules have nearly flat orientation with respect to the iron surface. Furthermore, thiol-formed McMT is dominated between the two tautomers in the film.2. Four kinds of Schiff base, which are N, N'-(ethane-1,2-diylidene)bis(2,6-dimenthyl-benzenamine) (SB-1),N,N'-(ethane-1,2-diylidene)bis(2,6- diisopropylbenzenamine)(SB-2),N,N'-(butane-1,2-diylidene)bis(2,6-dimenthylbenzenamine) (SB-3), N,N'-(butane-2,3-diylidene)bis(2,6-diisopropylbenzenamine) (SB-4), were synthesized. SA films of the four Schiff bases were prepared on iron surface. FT-IR measurements were applied to confirm the formation of the SA films. Peaks located at～1600 cm^-1, which were attributed to the "-C=N-" stretching vibration, were observed from the FT-IR results of all the four Schiff base SA films. Compared to the FT-IR results of the pure Schiff bases, red shifts occured, which may be caused by the stronger coordination bonding between the N atoms and iron.The inhibition abilities of the SA films for iron in 0.5 mol dm^-3 H₂SO₄ solution were studied by using EIS and Tafel polarization curves. It can be concluded from the results that four kinds of Schiff base SA films can protect iron form corrosion effectively in acidic solution. In addition, the incease in the immersion time will improve the coverage and thus contributed to the formation of defect-free SA films. The inhibition efficiency calculated is seen to decrease in the order SB-l>SB-3>SB-2>SB-4.Molecular dynamic simulations were performed to discuss the adsorption mechanism of the four Schiff bases on Fe (1 1 0) surface. Results show that when adsorbed on iron surface, most of the SB-1 molecules have a nearly flat orientation with respect to the Fe (1 1 0), thus formed highly-ordered, compact films. For the other Schiff bases, large substituent groups will make it difficult for the molesules to arrange regularly on the Fe (1 1 0) surface. Results of Radial Distribution Functions (RDF) calculation are cosistent with those obtatined from the electrochemical methods, which were the interactions decrease in the order SB-1>SB-3>SB-2>SB-4.