Inhibition Effect Of Azoles And Quinoline Derivatives On Corrosion Of Mild Steel In Hydrochloric Acid

Due to its excellent mechanical property, simple smelting process and low price, the mild steel has been widely used in many industries such as machinery manufacture, petrochemical engineering, construction industry and national defense, etc. Nevertheless, it has been confirmed that the mild steel is highly susceptible to corrosion in acid media which are often encountered in chemical and relevant industrial processes such as acid cleaning, acid picking, acid descaling and oil well acidizing, which will induce potential problems in industrial equipment safety and consequently lead to massive economic losses. As a matter of fact, one of the most practical and effective methods in corrosion protection of metals is to use inhibitors. While, the inhibition effect for corrosion is largely dependent on the molecular formulation and structure of the inhibitor, In general, the organic compounds with heterocyclic structures containing π-bonds and N, S, O, or P heteroatoms act well as the most effective inhibitors against acid corrosion of steels in acidic media. Among them, the N heterocyclic series corrosion inhibitors are becoming a hot research with its excellent corrosion inhibition performance for mild steel in acid media. Azoles and quinoline derivatives as a kind of N-heterocyclic organic compounds is easy to be synthesized, especially, those with different substituent groups (e.g. methyl, mercapto, amino, phenyl, etc.) of azoles and quinoline derivatives have been studied extensively due to their high inhibitive efficiency for corrosion.In this thesis, three kinds of azoles (ABM, AT, ABT) and a new quinoline derivatives (2-(quinolin-2-yl)quinazolin-4(3H)-one) have been selected and synthesized, respectively, to be studied as corrosion inhibitors. The synthesized compound is characterized by FTIR and(?)HNMR. The mild steel was used as the working electrodes. And the objective of this thesis is to investigate the adsorption and inhibition effect of azoles and quinoline derivatives on the corrosion of mild steel in 1 M HCl solution by means of potentiodynamic polarization measurements, electrochemical impedance spectroscopy (EIS) and surface analysis (SEM, XPS) methods. In addition, the inhibition efficiency and mechanisms of azoles and quinoline derivatives was studied systematically by quantum chemical calculation. Moreover, several quantum chemical calculations have been conducted to correlate the inhibition efficiency and mechanisms with the molecular properties of the different azoles and quinoline derivatives.The main work is as follows:1. Electrochemical measurementsThe influences of azoles and quinoline compounds with different concentrations on corrosion inhibition in 1 M HCl solution were studied by polarization curves and EIS. The results show that they act as good mixed-type inhibitors in acid solutions, and the corrosion of steel is mainly controlled by the charge transfer process. The adsorption of each inhibitor on mild steel surface follows Langmuir adsorption isotherm with both physisorption and chemisorptions modes.2. Surface analysis of mild steelThe mild steel specimens for SEM measurements were immersed in 1 M HCl solution without and with addition of azoles (5.0mM) and quinoline inhibitors (0.185mM), respectively, for 48 hrs at ambient temperature (～25℃). Morphology analysis by SEM indicates the damage of the steel surfaces is significantly abated after adding corrosion inhibitors. In addition, the steel specimens inl M HCl solution with 0.185mM quinoline inhibitor was studied by EDX and XPS. The results show that the quinoline compound molecules can adsorbed on steel surface to form a protective inhibitor film.3. Quantum chemical calculationsQuantum chemical calculations were performed with Gaussian 09 program package. The molecular structures of the studied inhibitors were geometrically optimized through density function theory (DFT) by means of B3LYP function with 6-311G which is in conjunction with double numerical plus d-functions (DND) basis set. Concurrently, for the purpose to ensure the accuracy of the quantum chemical approaches, frequency analysis was involved to ensure that the calculated structure will be the minimum point on potential energy surface (without imaginary frequency). The following quantum chemical parameters were calculated, including energy of the lowest unoccupied molecular orbit (ELUMO), energy of the highest occupied molecular orbit (EHOMO), energy gap (△E= ELUMO-EHOMO), Mulliken charge, dipole moment (μ) and fraction of electrons transferred (△N).All of the parameters obtained by quantum chemical calculation will be used to study the correlation between inhibition efficiency and molecular structures of the inhibitors, which provides a theoretical basis for the adsorption of the inhibitors on the mild steel surface.