The Corrosion Inhibition Of Thiourea And Quaternary Ammonium Salt Surfactant On Carbon Steel Surface

Iron and steels have the characteristics of low cost,high yield,high strength and so on,which are widely applied in transportation,civil engineering,modern industry.However,due to its strong chemical activity,corrosion will occur under the natural condition,which led innumerable losses for economic development.Thus,the corroded steel needed proper surface treatment,in which pickling treatment,as a low-cost and easy-to-operate method,can effectively remove the rust on the metal surface,and was often used in the pre-treatment process of other treatment methods as well.However,iron and steel will be corroded by pickling agent in the process of pickling.The addition of corrosion inhibitor can effectively reduce the phenomenon of“excessive corrosion”in the pickling process.Thiourea had been proved to be an effective pickling additive to inhibit metal corrosion.In recent years,a large number of new organic corrosion inhibitors prepared with thiourea as precursors had been proved to be effective corrosion inhibitors for pickling.Moreover,the compound inhibitor had better protection effect and economic benefit than the single inhibitor.In this paper,the corrosion inhibition performance of Q235 steel immersing in 0.05mol·L^-1 H₂SO₄ aqueous solution with thiourea and quaternary ammonium salt cationic surfactants containing different hydrophilic groups:dodecyl trimethyl ammonium bromide(DTAB),dodecyl dimethyl benzyl ammonium bromide(DDBAB),dodecyl pyridinium chloride(DPC)alone were investigated by corrosion inhibition techniques(static weight loss method,electrochemical impedance spectroscopy,potentiodynamic polarization curve,atomic force microscope,scanning electron microscope,energy dispersive X-ray spectroscopy,X-ray photoelectron spectroscopy).Then the corrosion inhibition performance for Q235 steel of the compound inhibitor formed by adding DTAB,DDBAB and DPC into thiourea inhibitor according to different volume ratio and dosage was measured and compared with that of single inhibitor.Finally,theoretical analysis(quantum chemical calculation,density of membrane structure analysis,fractional free volume(FFV),molecular dynamics(MD)simulation)was employed to explore the inhibition mechanism of single inhibitor and compound inhibitor on the surface of carbon steel.The main experimental results are as follows:(1)The static weight loss method and electrochemical experiments showed that the optimum volume ratio of thiourea with DTAB,DDBAB and DPC was 5:5,7:3 and 5:5,respectively,and the optimum dosage was 1×10^-3 mol·L^-1.Under the same concentration(1×10^-3 mol·L^-1),the inhibition efficiency of the seven inhibitors was in the following order:DDBAB-Thiourea>DPC-Thiourea>DTAB-Thiourea>DDBAB>DPC>DTAB>Thiourea and all the seven inhibitors were mixed-inhibitors dominated by anodic suppression.Moreover,the effects of various H₂SO₄ concentrations on the inhibition performance of the compound inhibitor were investigated.The results showed that the compound inhibitors were less affected by the H₂SO₄ concentration.The higher the H₂SO₄concentration became,the more obvious the protective effects of the compound inhibitor on carbon steel were.The adsorption isotherm showed that the adsorptions of corrosion inhibitors were consistent with Langmuir isothermal adsorption and it was attributed to the result of the simultaneous action of physical adsorption and chemical adsorption.(2)The surface analysis showed that the metal surface treated with corrosion inhibitor was smoother than that without corrosion inhibitor,and the surface of carbon steel immersed with DDBAB-Thiourea inhibitor was the smoothest.Meanwhile,the element changes on the surface of carbon steel before and after immersion showed that the seven corrosion inhibitors can be well adsorbed on the metal surface.(3)Quantum chemical calculations showed that DDBAB molecules had smaller orbital energy gap,and it was easier to supply/receive electrons with iron to form chemical bonds and adsorb reactions than the other three molecules(Thiourea,DTAB and DPC).The corresponding Fukui index also showed that all the four inhibitors had active sites that react with metals.(4)MD simulation showed that the lower concentration of thiourea inhibitor gave rise to the lower coverage of protective film formed on metal surface due to its small molecular volume.For surfactants,the hydrophilic groups were preferentially adsorbed on the metal surface,while the flexible hydrophobic carbon chain will form a hydrophobic layer near metal surface.However,when the content of surfactant was unsaturated,the hydrophobic carbon chain will repel each other due to the steric hindrance effect,which will produce more pores and make the corrosion particles diffuse into the pore structure.After compounding,the content of surfactant was lower,which led to the reduction of steric hindrance.Thus,the hydrophobic carbon chain can also be adsorbed on the metal solid surface.Meanwhile,thiourea molecules with a small volume can be adsorbed on the uncovered part of the metal surface,which made the protective film denser and improved the acid resistance of the metal.In this paper,it was proved that the compound corrosion inhibitor(DTAB-Thiourea,DDBAB-Thiourea,DPC-Thiourea)had better corrosion inhibition effect on carbon steel in H₂SO₄ environment through corrosion inhibition measurement and theoretical analysis.The compounding of corrosion inhibitors not only reduced the dosage and cost to a great extent,but also reduced the environmental pollution.In addition,it also provided new ideas for the development and research of new compound corrosion inhibitors.