Effect Of Rare Earth Cerium To The Corrosion Behavior Of X70 Pipeline In Western Soils Of China

On the basis of the"West-to-East Natural Gas Transmission Project", X70 rare earth steels which have different composition of Ce are smelted. The corrosion behaviors of X70 rare earth steel in representative western soils, Baotou soil and Ku'erle soil, have been studied through burying specimens in situ and performing experiments in simulated environments, and at the same time, the effect of different chloride ion content to X70 and X70 rare earth steel is researched.First, microstructure of four steels is observed by SEM. The results show that, X70 steel is composed by ferrite and pearlite structure; The surface of rare earth is shown typical cellular microstructure and it is existing grain boundaries, the result from energy spectrum analysis shows that it is mainly about rare earth complex. In addition, the SEM observation shows that, the ferrite grain was refined after adding rare earth into X70 steel, but when the content of rare earth reached a certain degree, the grain size have no change obviously.The results of burying specimens in the Bao tao soils whose moisture content is 10% for 45 days revealed that general corrosion had taken place on all of the specimens; the soils are in different temperature of 15℃, 30℃and 45℃. The corrosion rates of steels are all increased with prolonging of the burying time and the temperature increase. The corrosion rates of X70 rare earth steel were lower than X70 steel through measuring EIS in different times, the corrosion resistance of X70 steel whose rare earth content is 0.048% is highest. From the result of X-ray diffraction, both the corrosion products of X70 steel and X70 rare earth steel are iron oxides (FeO, Fe2O3, FeOOH, Fe3O4).The experiments for four kinds of steels in simulated Ku'erle environments were carried out, which included immersion and electrochemical measurements in Ku'erle soil saturated with water and in simulated soil solution. The macroscopic corrosion morphology shows pitting have been spotted in the surface of X70 steel and X70 rare earth steel, the corrosion degree of X70 steel is highest, three other steels are all have different degrees corrosion, but their corrosiveness are lower than X70 steel; all the steels'corrosion products are including iron oxides. The polarization curve and EIS results show that, the electrochemical's reaction resistance of X70steel increases with prolonging of experiment's time, which lead to corrosion degrees decreased, this result matches to the result of weight loss method. It was also found from the experiments of steels immersed in different content of chloride ion solutions that, the corrosion rate of X70 steel increases with the increase of chloride ion concentration; in the same time, the corrosion rate of X70 rare earth steel are lower than that of X70 steel. The macroscopic and microscopic corrosion morphology show that, the pitting take place on the surface of samples; with the increase of chloride ion concentration, the quantity of pits increase and the areas are larger than before. It shows that the increase of chloride ion concentration result in potentials move to negative direction, the ability of ion enter into substrate of metal increased, the passive film are easier to be destroyed, so the corrosion rate speeds up, which induced the pitting accurance futher.At last, three corrosion behavior of burying in Bao tou soil, immersing in Ku'erle soil simulated solution and immersing in different content of chloride ion solutions was compared. The corrosion regularities of pipeline steels have strong similarity. It is sufficiently indicate that adding rare earth into pipeline steels can give their corrosion resistance a certain improvement, the rare earth content of 0.048% of X70Re steel was up to the highest corrosion resistance effectiveness.