Synthesis, Measurement And Investigation On Mechanism Of Novel Heterocyclic Compounds As Corrosion Inhibitors In Acid Soultion Of Mild Steel

Corrosion inhibitors hold the important status in the metal protection project, and they are playing the more and more vital role in the national economy construction. According to the present anticorrosion of thermal energy equipments and anticorrosion situation in other industry domain, it is an effective and economic anticorrosion method to use corrosion inhibitor during acid pickling. Although corrosion inhibitor have now made a big progress in molecular designs, synthesis and crafts, compound effect and application so on and so forth, the development of corresponding theory is still far lags in the practice and there are many disputes in the investigation of the corrosion inhibiting mechanism. So, it becomes an interesting subject for chemists to study the corrosion inhibiting mechanism and develop or perfect the theory through various means and techniques. In this paper, three series including twelve kinds of newly heterocyclic compounds were selected or synthesized to be studied as corrosion inhibitors of mild steel in 1mol/L HCl using weight loss experiment, electrochemical test, scanning electron micrographs (SEM) and quantum chemical calculations. And then, we studied further their corrosion inhibiting mechanism on mild steel, discussed theoretically the function way between the corrosion inhibitors and the metal surface and summarized the relationship between the molecular structure and their inhibiting efficiency.The three series novel heterocyclic compounds involved in this paper included: purines, Schiffbase-triazoles and thiaurea-triazoles. Different processing methods were adopted according to different characteristics of different compounds and some results were drawn as follows:The purines were proved to have positive inhibiting effect and high inhibiting efficiency through weight loss experiment, electrochemical test and scanning electron micrographs. According that the purines are small planar molecules, the quantum chemical calculations were performed and the adsorption features were informed based on the experimental phenomena and results. A new carbon surface model made up of 108 iron atoms were established and used for the first time at inhibiting area home and abroad, from which the adsorption curves were obtained combined the adsorption features of purines. It was suggested the adsorption mechanism of the purines was physical adsorption through closing up to the metal surface in planar orientation and the adsorption was arrived from theπ-πsuperconjugate interaction between the plane of the purines and the metal surface. The establishment and use of adsorption curve model caused that the study on the mechanism of corrosion inhibitors was no longer limited merely to the analysis and extrapolation of the corrosion inhibitor molecules themselves, but conduct the research in the entire corrosion system foundation. The most important is that a preliminary model was theoretically provided to screen corrosion inhibitors in certain areas.The experimental results showed that Schiffbase-triazoles act as good corrosion inhibitors and have high inhibiting efficiency, wherein the inhibition efficiency of compound CMTT arrived at 97% at 10^-3mol/L. In view that the Schiffbase-triazoles had many active centers in molecules and had both flexibility and rigidity in the molecular structures, Fuqui function were calculated by quantum chemical calculation to judge the effective adsorption centers of the molecules. The adsorption of Schiffbase-triazoles on the metal surface was confirmed chemical adsorption and the acting mechanism was drawn as follows: the sulfur atoms of Schiffbase-triazoles donated electrons to form chemical bonds with iron atoms of the metal surface, and the -C=N- double bonds accepted the negative charge from the metal surface to form back-donation bonds. The investigation of novel Schiffbase-triazoles as corrosion inhibitors not only provided the new highly effective non-toxic acid pickling corrosion inhibitors, moreover the union of -C=N- double bonds and the heterocyclic rings enhanced the inhibiting efficiency, but also provided an effective way to increase the solubility toward the poor solubility of many heterocyclic compounds and opened an avenue for the application of more compounds in inhibiting area.The thiaurea-triazoles also have good inhibiting effect and high inhibiting efficiency, especially the TBU compound whose inhibition efficiency arrived at 98% at 10^-3mol/L. Depending that the sulfur atoms played a predominant role between the interaction of thiaurea-triazoles molecules and the metal surface, they were connected into the surface of mild steel and then the entire system were optimized through quantum chemical calculation. The mechanism of this kind of compound was confirmed theoretically that the thiaurea-triazoles adsorbed onto the metal surface by the coordinating bonds between the sulfur atoms and the surface iron atoms, and at the same time, the conjugated system based on benzene ring in the molecules covered the surface of mild steel throughπ-πsuperconjugate interaction, which led to a type of adsorption with both physical adsorption and chemical adsorption. What's more, it was found that the adsorption and inhibiting efficiency of these compounds were greatly influenced by the steric effect of the small subsistent in the molecules, which is rarely reported at present.It is worthy to be noted that, during the measurement and fitting of the electrochemical impedance spectroscopy for all the inhibitors, adsorption process were found to greatly influenced the EIS when the inhibiting efficiency increased. Two types of equivalent circuit were used in this paper, the influences of the adsorption process were better showed, and the fitting results were more reasonable.The work that the study on novel heterocyclic compounds as corrosion inhibitors afforded highly effective and non-toxic acid pickling inhibitors with potential application prospect and proposed solutions to some questions existing in present inhibiting area, more importantly, the utility of new model and method in the analysis of inhibiting mechanism provided one kind of new theoretical model for corrosion inhibitor screening, provided certain scientific basis for the design and synthesis new corrosion inhibitors and promoted the development and consummation of the corrosion inhibitor theory.