Investigation Of Inhibition Behavior Of Sophorolipids For X65Steel Corrosion In Oilfield Produced Water Saturated With CO₂

Carbon dioxide in oilfield produced water often causes severe corrosion ofcarbon steel. The use of inhibitors has been identified as a very practical and mosteconomical method of combating CO2corrosion. Most inhibitors are chemicalsynthetics. Although their inhibition efficiency is superb, their high toxicity cannotbe overlooked. Sophorolipids, produced by Torulopsis, is a sort of environmentallyfriendly biosurfactant with minimal toxicity and excellent surface loweringproperties even at elevated temperatures and high salt concentrations. Sophorolipidsconsists of a large number of carbonyl, hydroxyl, etc, and the affinity of polargroups promotes the binding of sophorolipids molecules to the metal surface.Therefore sophorolipids can be adsorbed on the metal surface, and long hydrophobicchains have mainly shielding effect. So sophorolipids maybe can be effectivealternatives of synthetic inhibitors. In order to broaden the applications areas ofsophorolipids, this paper aims at the investigation of the effectiveness and inhibitionmechanism of sophorolipids for X65carbon steel against CO2corrosion.The methods and results are as follows:The surface properties of sophorolipids are measured in the oilfield producedwater saturated with carbon dioxide. Sophorolipids structure is analyzed by nuclearmagnetic resonance spectrometer. The inhibiting effect of sophorolipids oncorrosion of mild steel in simulated oilfield produced water saturated with CO2isinvestigated by weight loss measurement, polarization, electrochemical impedancespectroscopy and SEM.1. Results of surface properties measurements show that: The CMC value ofsophorolipids is about30~40mg/L, the surface tension from68.8mN/m inblank solution reduces to40mN/m in sophorolipids solution at25℃.2. Nuclear magnetic resonance spectrum shows that: Sophorolipids does containcarbon-carbon double bond, numerous hydroxyl groups, and carbonyl groups and so on. Those groups are contributed to the adsorption of sophorolipids onthe metal surface.3. Results of weight loss measurements show that: Sophorolipids attain goodinhibition effect on X65steel, and inhibition efficiency increases withincreasing of sophorolipid concentration at25℃,40℃and60℃. When in thepresence of50mg/L sophorolipids, the inhibition efficiency reaches90%.Sophorolipids doesn’t have inhibited CO2corrosion effectively with furtherincrease of temperature.4. Results of polarization curve show that: At25℃,40℃and60℃, the values ofcorrosion potential shift towards positive direction and the shift increases withthe increasing of sophorolipids concentration. The curves are displaying apreferential suppression of the anodic reaction. When the concentration ofsophorolipids is greater than50mg/L, anodic desorption demonstrates exists inanode polarization curve. Rate of sophorolipids absorption is more quickly,when concentration of sophorolipids is greater. Anodic desorption phenomenonof sophorolipids is not obvious in anode polarization curve at80℃.5. Results of electrochemical impedance spectroscopy show that: When theconcentration of sophorolipids is greater than50mg/L, a tough measureabsorbed film is formed on X65steel to decrease the extent of anodic andcathodic reaction after immersing in the sophorolipids solution for0.75h at25℃,40℃,60℃and80℃. With addition of70mg/L, the absorbed film canprotect X65steel from corrosion in168h at25℃,40℃and60℃. the absorbedfilm doesn’t have inhibited CO2corrosion effectively in168h at80℃.6. Results of surface morphology show that: The X65steel surface was stronglydamaged in the absence of inhibitor at25℃,40℃and60℃, and corrosion ismost severe at40℃. The X65steel surface exposed to70mg/L sophorolipidswas prevented from corrosion. Sophorolipids doesn’t exhibited an excellentinhibition effect at80℃.