Study On CO₂ Corrosion Behavior Of X90 Pipeline Steel

This paper designed an orthogonal experiment with four factors and five levelsbased on orthogonal array,studied the corrosion behavior of X90 pipeline steel and its relativity with water cut of crude oil,temperature,CO₂ partial pressure and flow velocity.According to the results of orthogonal experiment,singal factor test was designed.Original designed reaction kettle was used to submulate the corrosion environment of the oilfield produced water in order to study the CO₂ corrosion behavior.Electrochemical impedance spectroscopy（SEM）and X-ray diffraction technique（XRD）were used to study the morphology and phase composition of corrosion product film.Electrochemical measurement technology was used to study the electrochemical performance of CO₂ corrosion product film of X90 pipeline steel.The results of orthogonal experiment showed that the relativity between CO₂ corrosion rate and four corrosion factors was water cut of crude oil,temperature,CO₂ partial pressure and flow velocity,which is in descending order.The results of singal factor test showed that CO₂ corrosion rate of X90 pipeline steel increased with water cut of crude oil,temperature and CO₂ partial pressure increased.Corrosion rate decreased with CO₂ partial pressure up to2.5MPa.Corrosion rate decreased at first then decreased with the flow velocity increased.As the corrosion rate was high,the corrosion is very serious according to the standard NACE.The xrd results showed that the corrosion product film of X90 pipeline steel was FeCO₃.CO₂partial pressure infected the composition of product film,Ca and Mg in the scale can replace Fe of the product（FeCO₃）,and the composition changed into（Ca,Mg,Fe）CO₃,when CO₂partial pressure was more than 0.5MPa.According to the SEM images of corrosion product film,the film had a multilayer structure;the outer film was loose and porous.Due to the less firm binding,the film was easily stripped from the martrix under the action of corrosive medium.The product film electrochemical test findings showed that anodic reaction was controlled by activation,cathodic tafel slop was greated than anodic tafel slope,so the electrode reaction was controlled by cathodic process.With the water cut of crude oil or temperature increased,the corrosion potential and the radius of the capacitance arc decreased,the polarization resistance decreased,and the corrosion current density increased,so the corrosion rate increased.With the CO₂ partial pressure increased,the corrosion potential decreased at first then increased,and the corrosion vurrent density increased at first then decreased.When the flow velocity increased,the corrosion potential then decreased,and the corrosion current density decreased at first then decreased,which was consistent with the results of weightless test.The electrochemical impedance showed the characteristics of two capacitance loops with characteristics of Warburg impedance in the low frequency range.CO₂partial pressure infected the covering area of corrosion product film observably.With CO₂partial pressure increased,the covering area of the film decreased.