Corrosion Behavior Of X70 Pipeline Steel In Simulated Seawater Solution

With the development of offshore oil and gas resources,X70 pipeline steel has been widely used for the transmission of oil and natural gas as subsea pipelines in the marine environment.p H value,Cl^-concentration,and microbial corrosion are important factors that affect the safe operation of pipeline steel during service.Therefore,studies on the corrosion behavior of pipeline steel in seawater environment is of great significance to enhance its safety and serviceability.X70 pipeline steel was used as the research material.The electrochemical corrosion behavior of X70 pipeline steel in simulated seawater solution with different p H and Cl^-concentrations,and the marine microbial acid-produced bacteria-Citrobacter farmer on corrosion behavior of X70 pipeline steel under biofilm,were studied by means of electrochemical impedance spectroscopy（EIS）,linear polarization impedance（LPR）,polarization curve and immersion experiments,combined with surface analysis techniques such as scanning electron microscopy（SEM）and energy spectrum analyzer（EDS）.The conclusions are as follows:The corrosion of X70 pipeline steel in acidic（p H=3）simulated seawater solution is much greater than that in near neutral（p H=7.7）simulated seawater solution.The corrosion weight loss of X70 steel immersed in acidic solution for 7 days is about 16 times as much as that in near neutral simulated seawater solution.Most of the surfaces were uniformly corroded in acidic simulated seawater solution,yet corrosion circles centered on inclusions are locally formed.With the prolongation of immersion time,the radius of capacitive reactance arc and the impedance value|Z|are both increased.Adsorption first occur and then weak diffusion,thus the charge transfer resistance R_ct increases and the corrosion rate is decreased.A relatively thick corrosion product film is formed on the surface of pipeline steel in the near neutral simulated seawater solution.The chloride ions are aggregated on the surface and destroyed the corrosion product film so that tiny corrosion pits form and locally expanse forming large pits.The corrosion rate increases and then tends to be stable.X70 steel is active in the simulated seawater solution.Compared with that in the near neutral seawater solution,the self-corrosion potential E_corrof the steel in the acidic simulated solution moves negatively 137 m V,and self-corrosion current density i_corr is about 21 times of that in the near neutral seawater solution.The corrosion mechanism of X70 pipeline steel is different in near-neutral simulated seawater solution with different Cl^-concentration.The corrosion is more serious when Cl^-concentration is 19 g/L.E_corr of X70 pipeline steel moves negatively with the increase of Cl^-concentration in seawater solution,the i_corr of 15 g/L Cl-concentration is the minimum,and the i_corr of 19 g/L and 33 g/L solution is larger but not significant difference.This is because a relatively thin film of corrosion products is formed on the surface of X70 steel in the simulated seawater solution with Cl^-concentration of 15 g/L and 19 g/L,and it’s a little bit bigger in 19 g/L.Due to the competitive adsorption of Cl^-and oxygen,there are more pitting pits in the 19 g/L solution.When the concentration of Cl^-is 33 g/L,a relatively thick film of corrosion products is formed on the surface.The corrosion is mainly uniform and pitting pits can be seen occasionally.However,the corrosion product films formed on the surface of X70 steel with different Cl^-concentrations makes little contribution to the resistance to pitting,which is very serious even under the condition of low chloride.The corrosion of X70 pipeline steel is accelerated in the presence of acid produced bacteria-Citrobacter farmeri.The radius of capacitive reactance arc and the impedance value|Z|of X70 steel in sterile solution present the dynamic change of first decrease and then increase with the increase of immersing time.The corrosion decreased at the initial stage and then increased steadily in the solution of containing C.farmeri,but it is still far less than that in the sterile solution.The fitting results show that the charge transfer resistance R_ct of X70 steel in sterile solution is about 2～4 times that in C.farmeri solution.The corrosion rate of X70 steel in C.farmeri solution is significantly higher,and the i_corrof X70 steel in C.farmeri solution is 1.27×10^-5 A·cm^-2,which is 2.2 times than that in sterile solution(5.77×10^-6 A·cm^-2).A layer of corrosion product film is formed on the surface of the X70 steel in the sterile solution,and the surface is mainly uniform corrosion.However,a layer of biofilm is formed on the surface of the sample in the containing bacteria solution,which could alleviate the surface corrosion in a certain extent,but more pits are obviously visible,and the local corrosion is more serious.