| With the rapid development of the economy,the demand of oil,gas,electricity and transportation in China is increasing.Then the buried pipelines,various high-voltage AC wires and other facilities cause cross-laying in space easily,which causing AC corrosion problem of buried pipelines.In China,X80 steel is widely used in buried pipeline steel,and there are many researches on AC corrosion of X80 steel.However,with the demand of pipeline steel grades,the AC corrosion problem of X100 steel has attracted the attention of many scholars.However,the use of long-distance buried pipelines results in a large number welded joints in the pipeline steel,which is unevenly organized and highly susceptible to corrosion.Research on AC corrosion at pipeline joints of pipeline steels has received little attention from scholars.Based on this,this paper explores the AC corrosion behavior of different structures pipeline steels in Golmud simulated solution by electrochemical methods such as open circuit potential,potentiodynamic polarization curve,potentiostatic polarization curve and electrochemical impedance spectroscopy,soaking experiments and surface topography analysis techniques.The research content and main conclusions of this paper are as follows:(1)By the way of heat-treating X100 steel to simulate the welded joint of X100 steel.Electrochemical testing and immersion experiments are performed on three different microstructures to investigate the AC corrosion behavior of hot rolled,annealed and normalized microstructures.In natural corrosion,the annealed microstructure has the worst corrosion resistance,the hot rolled microstructure is centered,and the normalized structure has the best corrosion resistance.With the increase of ambient temperature,the corrosion behavior of the three microstructures is more obvious.Under the influence of AC interference,the cathodic reaction in the electrochemical test is transformed from the depolarization of oxygen to the hydrogen evolution reaction.The order of corrosion resistance of the three different microstructure is as follows.The corrosion rate of the annealed microstructure is the fastest,the hot rolled structure is centered,and the normalizing microstructure corrosion is the slowest.Compared with natural corrosion,the corrosion of three different microstructure of X100 steel under AC interference is more serious.By analyzing the surface corrosion morphology,it is found that pitting corrosion mostly occurs around the granular bainite of hot-rolled microstructure,and there are many pits on the pearlite in the annealed structure,and there are many large corrosion pits distributed at the boundary of the pearlite and ferrite.The pits of normalized microstructure are relatively mild.There are more elongated and needle-shaped M-A microstructure.Different microstructures of X100 pipeline steels have different corrosion resistance.(2)The soil environment in which the buried pipeline is located has a great influence on its AC corrosion behavior.Use different pH to simulate the alkaline soil environment in Golmud area.The open circuit potential,polarization curve and soaking experiment are used to investigate the AC corrosion behavior of X100 steel in Golmud simulated solution with different pH.In a solution with pH of 7,X100 steel is general corrosion.In the solution of pH 9 and 11,local corrosion occurred in X100 steel.The sample was thoroughly corroded under the interference of an AC current density of 30 A/m2.When the pH of the solution is 7,the corrosion pit of the sample is large and deep due to the erosion of Cl-,and the corrosion phenomena is the most serious.When the pH is 9,the corrosion pit is shallow and the corrosion is mild because of the replacement of OH" and Cl-and the protection of surface corrosion products.When the pH of the Golmud simulated solution is 11,the degree of corrosion is more serious than the pH is 9,because the alkalinity is too harsh.(3)By heat treatment to change the size of micro structure of the X80 steel,the influence of the grain size on the corrosion resistance was investigated.The annealing time of X80 steel was changed.It was found that as the annealing time was prolonged,the ferrite size in the microstructure increased and the pearlite gradually decreased.Electrochemical data shows that the corrosion of X80 is more difficult to occur with the extension of annealing time.In the absence of AC interference,the oxygen-absorbing reaction is dominant,and when AC interference is applied,the hydrogen evolution reaction is dominant.After the application of AC interference,the difference in corrosion potential of different microstructure gradually decreases,and it tends to be consistent at high AC density.Observing the surface morphology after metal corrosion,it was found that the corrosion pit occurred at the grain boundary between pearlite and ferrite. |
