Corrosion Inhibition Effect Of Phthalocyanine Compounds And Goldthread Extractive On Mild Steel And Its Quantum Chemistry Research

The inhibition effect of metal-free phthalocyanine (H₂Pc), manganese phthalocyanine (MgPc), copper phthalocyanine (CuPc) and copper phthalocyanine tetrasulfric tetrasodium salt (CuPc·S₄·Na₄) on mild steel in 1mol/l HCl in the concentration range of 1.0×10-5mol/l to 1.0×10-3mol/l was investigated by weight loss experiment, electrochemical test, Scanning Electron Microscope with Energy Diversity Spectrometer (SEM/EDS) and quantum chemical method. The result of weight loss experiment shows that the inhibition efficiency of these four compounds decreased in the order of CuPc.S₄.Na₄>CuPc> H₂Pc > MgPc; The highest inhibition efficiency was obtained as 93% under the presence of CuPc·S₄·Na₄ at a 10-3mol/L concentration, the weight loss data of this compound was fitted well with Flory-Huggins isotherm and El-Award model, which indicated its characters of multi-centers adsorption and larger molecule connect surface. The potentiodynamic polarization curves of mild steel in hydrochloric acid containing these compounds showed that cathodic processes and anodic process of steel corrosion were suppressed by ht einhibitor. The Nyquist plots of impedance expressed mainly as a capacitive loop with different compounds and concentrations, and differed with different compound s in some details. The charge transfer resistance Rct increased with the increasing of concentration with different compounds. The SEM/EDS analysis indicated that there are more lightly corroded and oxidative steel surface for the specimens after immersion in acid solution containing 1.0×10-3mol/l phthalocyanine compounds than that in blank, indicated the protection of phthalocyanine adsorption layer on steel sueface. The optimized geometry of these compounds that obtained by ab inito method showed a plan or approximate plan surface, and the Mulliken population analysis indicated the negative charges were mainly located on the Nitrogen atoms, which facilitated to the adsorption of the molecules. The quantum study results showed that the inhibition efficiency of these phthalocyanines increased with decrease of molecule's LUMO energy, which was different from the micro-cyclic compounds, this was mainly because of the symmetric structure and highly delocalized macro-cyclic conjugated system, still the substitution of the electronic-withdrawing sulfonic groups which decreased the HOMO/LUMO energy, reduced the distribution of HOMO and enhanced that of LUMO on the phthalocyanine ring. The optimization of adsorption system indicated the spontaneous character of the adsorption of phthalocyanines on ferric surface, and the system energy decent during the adsorption was discussed with experimental datas. The credibility of quantum parameters calculated by different methods was contrasted and it indicated that the DFT method was the excellence method to do calculation for larger molecular. Also the ZINDO semi-empirical method did better in geometry optimism calculation. The experiments of this article shows that CuPc.S4.Na4 had good inhibition ability and it also summarized the relation between adsorption and inhibition with quantum chemical parameters, this result enriches the research area of macro-circle compounds and provides the basis and direction for developing new inhibitors from these compounds. The character of dyes and pigments for these compounds limited the application of phthalocyanine compounds. In order to obtain more practically useful inhibitor, the solid goldthread extractive was abstract from natural plant coptis chinensis and its inhibition efficiency on corrosion of mild steel in 1mol/L HCl was investigated. The weight loss results showed that extractive was an excellent corrosion inhibitor for mild steel in 1mol/L HCl at 1.000g/L. Potentiodynamic curves suggested that extractive suppressed both cathodic and anodic processes were suppressed, and the corrosion current density decreased. The Complex diagrams for mild steel in 1mol/L HCl containing extractive with different concentrations showed one capacitive semi-circle and the charge tran...