Study Of The Inhibition Mechanism And Synergistic Effect Of Corrosion Inhibitors In Sweet System

Carbon dioxide (CO₂) is a naturally occurring constituent in oil andgas production and is found in amounts varying from trace levels to asmuch as50%. Dissolved CO₂in the produced brine is very corrosive tocarbon steel pipelines. Therefore CO₂corrosion has a very importanteconomic impact in this industry. The problem has caused theapplication of many corrosion control methods and research around theworld. One of the most cost-effective methods is the injection of organicinhibitors into oil wells, gas wells and pipelines. However, there are fewstudied about the corrosion inhibition mechanism, especially for thesynergistic mechanism of corrosion inhibitors in sweet system. In orderto make deeper studies on the corrosion inhibition mechanism andsynergistic inhibition effect, several inhibitors were synthesized andcharacterized by infrared spectroscopy in the paper. Their inhibitionperformance was investigated by weight-loss method andelectrochemical techniques. Based on these results, as well as X-rayphotoelectron spectroscopy analysis and quantum chemical calculationsresults, the inhibition mechanism and synergistic mechanism were proposed to explain the inhibition behavior of the inhibitors in sweetsystem.1. The electrochemical behavior of imidazoline derivates (OIMQ)and/or thiourea (TU) was studied by using polarization curves and EIS.The results indicated that a synergistic effect existed between TU andOIMQ. The optimum proportion of them was1:1(wt%) when the totalusing concentration was10mg/L. OIMQ was an anodic-type inhibitorand caused by negative catalytic effect; while TU was a mixed-typeinhibitor and caused by geometrical blocking effect. For TU+OIMQcombination, it was also a mixed-type inhibitor and caused bygeometrical blocking effect, and the inhibitor molecules could desorbfrom the metal surface when the potential applied on the metal was overthe adsorption potential. EIS results showed that the Warburg impedanceemerged on the Nyquist plots, which can be attributed to formation of abi-layer inhibitor film on the metal surface. This result was inaccordance with the XPS result reported by previous studies, that TUmolecules mainly exist on the bottom of the inhibitor film; however,OIMQ molecules mainly exist on the top of the inhibitor film.2. The synergistic inhibition effect of OIMA and SB inCO²-saturated solution was found by weight-loss method, polarizationcurves and EIS, which was not reported before. The results showed thatOIMA could prevent the mild steel from CO²corrosion to some extent and its inhibition can be strengthened by the combination use of SB. Thesynergistic inhibition effect of OIMA and SB is obvious. Based on theEIS, XPS and PZC measurement results, an adsorption model wasproposed to elucidate the synergistic effect between OIMA and SB.When10mg/L OIMA and10mg/L SB were added into theCO₂-saturated solution, N3atom in OIMA ring was combined withhydrogen ion to form an ammonium ion and the positive charged OIMAmolecules physically adsorbed onto the negative charged surface of theelectrode in the test solution. The chemical adsorption might take placesimultaneously which involving the charge sharing or transfer fromOIMA to the metal surface. Consequently, parts of the negative chargeson the metal surface were neutralized and the metal surface had extrapositive charge. And then, SB, mainly in the form of phCOO^-, will bemoved toward to the metal surface by coulomb force and then adsorb onit and make the inhibitor film more compact and thicker. EISmeasurements showed that OIMA adsorbed first on the metal surfaceand followed by the adsorption of SB after20min later when OIMA andSB were added into solution.3. A new inhibitor for CO₂corrosion-modified polyamide (MPA)was synthesized and its structure was characterized by IR spectra.Weight-loss measurement and polarization curves were used toinvestigate its inhibition. The results showed that MPA belongs to a mixed-type inhibitor and controls anodic process principally, whichcould inhibit N80steel from CO₂corrosion efficiently and the inhibitionefficiency improved with the increase of MPA concentration. In viewthat the MPA had many active centers in molecules, Fukui function wascalculated by quantum chemical calculation to find the effectiveadsorption centers of MPA. The results showed that S atoms of MPAcould donated electrons to form chemical bonds with iron atoms on themetal surface and also accepted the negative charge from the metalsurface to form back-donation bonds. N33and N34could formback-donation bonds too. As a result, MPA could adsorb on the metalsurface firmly and protect metal efficiently.4. In order to study the effect of benzene ring number and locationon the inhibition efficiency of imidazoline, three kinds of imidazolinewith benzene ring(s) were synthesized and characterized by IR spectra.Weight loss method and potentiodynamic polarization method werecarried out to evaluate their inhibition. The results showed that the threeimidazoline derivates can inhibit CO₂corrosion effectively and PIMQprovides the highest inhibition efficiency among them. They mainlyrestrain the anodic dissolution and belong to a kind of anodic-typeinhibitors. By thermodynamic calculations, we found that the adsorptionof these derivates on the mild steel surface accords with Langmuiradsorption isothermal equation and belongs to chemical adsorption; the adsorption process is endothermic process, increasing temperature is ofbenefit of improving corrosion inhibition; the adsorption process is anentropy increasing process. Quantum-chemical calculation found thatthe energy gap of PIMQ was lowest. The difference of inhibitionefficiency among the three inhibitors was mainly caused by differencesin EHOMO.5. EIS and RDE were employed to study the inhibition efficiency oftwo imidazolines with different hydrophilic group, OIMA and OIMO,under flow conditions. The results showed that OIMA and OIMO caninhibit Q235steel from CO₂corrosion efficiently under flow conditionsand OIMA had better performance. Under flow condition, Warburgimpedance emerges in Nyquist plots, indicating that the electrodeprocess is controlled by concentration diffusion. And another phaseangle maximum at high frequency region appears in Bode plots underflow conditions (except low concentration of OIMO at low rotationspeed). R_ctincreases with the increasing of rotation speed, and thendecreases. Quantum chemistry calculation results showed that the energygap of OIMA was lower than OIMO. The corrosion inhibition of PIMAwith TU, SP and OP was studied respectively, the results showed that thesynergistic inhibition effect of PIMA and TU is obvious, but PIMA+OPand PIMA+SP have antagonistic effect. A neuronic network with4-9-1structure was established, by which the inhibition efficiency of imidazoline and its derivates under flow condition can be predictedaccurately.