Inhibition Effect And Mechanism Of Pyrimidine Derivatives On The Corrosion Of Cold Rolled Steel In HCl And H₂SO₃Solutions

N-heterocyclic compounds are considered to be the most effective corrosion inhibitors for steel in acid solution. As an important kind of N-heterocyclic compound, pyrimidine derivatives whose molecules possess the pyrimidine ring with two N heteroatoms could adsorb on the steel surface and then exhibit the inhibitive performance. The aim of this PhD thesis is to accumulate new important data of the inhibition effect of a series of pyrimidine derivatives on the corrosion of steel in HCl and H2SO4solutions, to reveal the effects of inhibitor concentration, temperature, immersion time and acid concentration on inhibition action, to elucidate the effect of substitutive group on inhibiting action, to study the adsorption of inhibitor on metal surface in the view angle of molecular, and to propose the reasonable inhibitive mechanism. In this thesis, pyrimidine (PM),2-chloropyrimidine (CP),2-bromopyrimidine (BP),2-hydroxypyrimidine (HP),2-mercaptopyrimidine (MP)2-ammopyrimidine (AP),2,4-diaminopyrimidine (DAP)4-amino-6-hydroxy-2-mercaptopyrimidine (AHMP),4,6-dihydroxy-2-mercaptopyrimidine (DHMP),4-phenylpyrimidine (4-PPM) and5-phenylpyrimidine (5-PPM) are selected to be the subjects based on the consideration of their molecular structures. The adsorption and inhibition effect of11pyrimidine derivatives on the corrosion of cold rolled steel (CRS) in HCl and H2SO4solution was studied using weight loss, electrochemical methods (open circuit potential-time curves, potentiodynamic polarization curves, electrochemical impedance spectroscopy) and instrumental analysis (UV spectra, scanning electron microscope, atomic force microscope) methods. In addition, the inhibitive mechanism of pyrimidine derivatives was studied systematically by molecular simulation calculation (quantum chemical calculation, molecular dynamics simulation). The protonated forms of pyrimidine derivatives were considered simultaneously in the process of molecular simulation calculation. The comparison of adsorptive reactivity between neutral and protonated molecules is discussed in detail. The molecular structures are optimized, and the theoretical parameters including the frontier molecular orbital energy, Mulliken atomic charge, dipole moment (μ), Fukui function, global hardness, global softness and electrons transferred from the inhibitor to metallic surface were calculated, and then the correlation between the molecular structure and inhibition effect of these pyrimidine compounds was discussed based on these parameters. The adsorption mode of inhibitor molecule on Fe (001) surface was studied by molecular dynamics (MD) simulations, and both adsorption energy and binding energy were obtained. The results are summarized as follows:1. Pyrimidine acts as a moderate inhibitor for the corrosion of CRS, the inhibition action of pyrimidine derivatives is improved and better than pyrimidine owing to substitution group to the pyrimidine ring. The inhibition efficiency of various kinds of substituted group follows the order:-C6H5>-SH>-NH2>-Br>-OH>-Cl. The inhibition efficiency increases with the number of substitution group. The forth position of phenyl group has higher performance than the fifth position of phenyl group.2. The adsorption of PM, CP, BP, HP, MP, AP, DAP, DHMP, AHMP,5-PPM or4-PPM obeys Langmuir adsorption isotherm, and the inhibition efficiency follows the order:HC1> H2SO4. Inhibition efficiency increases with the inhibitor concentration, while decreases with the temperature and acid concentration. Prolonging immersion time results in increasing inhibition efficiency from1to6h, and then almost remains constant from6h to156h.3. PM, CP, BP, HP, MP, AP, DAP, DHMP, AHMP,5-PPM and4-PPM act as mixed-type inhibitors, and the electrochemical inhibition is caused by geometric blocking effect. EIS spectra exhibit individual capacitive loop in1.0mol/L HC1, while a large capacitive loop at high frequencies followed by a small inductive loop at low frequency values in0.5mol/L H2SO4. The presence of pyrimidine derivatives in HC1and H2SO4solutions increases charge transfer resistance (Rt) while decreases the double layer capacitance (Cdl). SEM and AFM clearly shows that the addition of pyrimidine derivatives could drastically retard the corrosion of steel in HC1and H2SO4media.4. The large protonated affiliation energy (PA) values confirm that the studied pyrimidine derivatives are easily protonated in acid mdia. The electron densitis of both HOMO and LUMO are localized principally on the whole molecular. MD simulations reveal that neutral and protonated molecules adsorb on the Fe (001) surface in the nearly flat manner, and the sequence of either adsorption energy (Eads) or binding energy (Ebin) is in accordance with that of inhibition efficiency.