The Effect Of Scaling On The Corrosion Of P110 Steel And Its Control Measures

Scaling is a common phenomenon in oil field. Barium sulfate scale will be produced when there are two kinds of incompatible components in oil field. Corrosion and scaling are closely related. The serious under-deposit corrosion will occur owing to the accumulation and uneven distribution of the scale when the scale is formed in the metal surface. Usually, the corrosion products scale can be formed that is often said the corrosion product films, such as ferrous sulfide, ferrous carbonate and ferrous hydroxide. Also, the scale can be produced ascribed to the incompatible fluid in the media, such as barium sulfate scale. The influence of the latter on the corrosion of oil pipe steel is mainly explored in this paper.Firstly, the influences of temperature, ion concentration and salt content on scaling were investigated by titration method. Weight loss was used to determine the corrosion rate of P110 steel. Secondly, the samples with uniform thickness and the similar scale layer were obtained from the P110 steel after being soaked in the simulating that in the high-barium oil field. The electrochemical responses of P110 steel surface with and without scale at different temperatures and salt contents were analyzed by using linear polarization and electrochemical impedance spectroscopy. The galvanic corrosion caused between P110 steel surface with and without scale was also studied in simulated water. The correlation between the corrosion behavior and the corresponding scale layer of P110 was studied by scanning electron microscope(SEM), energy dispersive system(EDS)and X-ray diffraction(XRD). Finally, a barium sulfate scale inhibitor used in oil field was developed by the orthogonal experiment and the corresponding effectiveness was evaluated by dissolution method.The static scaling method and weight loss were used to obtain the correlation between the scaling and the corrosion rate of P110 in the high-barium oil field. The results showed that as the temperature increased, the scale rate and the scaling of barium sulfate decreased in solution, however the corrosion rate of P110 steel increased, the scale had no protection to the metal matrix; with the increase of scale ion concentration, the scale increased but the corresponding corrosion rate of P110 steel decreased, the scale had a protection to the metal matrix; When the salt content increased, the scale rate and the scaling of barium sulfate in solution decreased, but the decrease was not very large, the corrosion rate of P110 steel was higher, also the scale had no protection to the metal matrix. The formation of the scale layer at the low temperature was denser than that at the high temperature. Barium sulfate scale was detected by X-ray diffraction.Electrochemical tests were used to detect the differences of electrochemical responses between the surface of P110 with and without scale. The results demonstrated that a Warburg impedance at the low frequency was found at the Nyquist of the sample with scale, however, the capacitive loops at high frequency range was very small, implying a serious corrosion under the scale. At different temperatures, the electrode with scale is negative electrode, and the one without scale is positive electrode at the initial stage after connecting the electrode with and without the scale of P110 into galvanic couple. As prolonging the immersion time, the current direction was inversed, the electrode with scale became positive electrode being corroded. At a higher temperature, the potential difference between the two electrodes was much larger which accelerated corrosion reactions. The higher salt content promoted Cl-migration, and was beneficial to reduce the pH value in occlusion cavity, where the auto-catalyzing acidification effect was more likely to occur and aggravated the corrosion under the scale.A barium sulfate inhibitor was obtained with ethylene diamine tetraacetic acid(EDTA)disodium salt as agent, assisted by nitrilotriacetic acid, citric acid and sodium polyacrylate. The best descaling rate of barium sulfate scale can reach 50.77% when temperature was 70? and descaling time 24h.