Study On Corrosion Of X80 Pipeline Steel Welded Joint And Its Mechanism

With the development of economy and society,the demand of energy is increasing,and the corrosion of transportation pipeline is becoming more and more serious.X80pipeline steel is the highest grade pipeline steel in the world.The pipeline is usually connected by welding,but the welded joint is the weak link in pipeline transportation because of its structure gradient,stress concentration and so on.At present,the research on the corrosion of welded joint is mainly through the methods of sampling and thermal simulation of each zone,which studies the behavior of the welded joint when it corrodes alone,ignoring the effect of electric couple between the zones.There are few studies on the corrosion behavior of pipeline steel in near neutral soil simulation solution,the paper first studied the problem and found that the matrix were active dissolution and no passivation.In ferrite region,the corrosion rate at grain boundary were slower than that in crystal within 4 days of immersion;In granular bainite region,corrosion holes and flocculent corrosion products were found in bainite range.Corrosion tendency and self-corrosion current density of X80 pipeline steel showed a trend of decreasing at first and then increasing.When the immersion time was 2 days,the self-corrosion current density was the lowest and the corrosion resistance was the best.In order to study the influence of each zone on the corrosion process of welded joint,the submerged arc welding joint of X80 pipeline steel was immersed in near neutral soil simulation solution and saturated H₂S water solution as a whole,the difference of corrosion behavior between different regions was studied.The variation of corrosion morphology,corrosion depth,composition of corrosion product film with corrosion time and the difference of welding joint in different areas were studied by means of Scanning electron microscope,energy spectrum analyzer and 3D texture topography acquisition system The variation of corrosion resistance of X80 pipeline steel welded joint and the difference between different zones were studied by electrochemical method.The corrosion behavior of welded joint of X80 pipeline steel in NS4 solution with5%CO₂/N₂ was studied.It was found that the corrosion rate in every zone of the welded joint increased firstly and then decreased during 4 days of corrosion.On the second day of corrosion,the self-corrosion current density was the highest and the corrosion resistance was the worst.Within 4 days of corrosion,the order of corrosion resistance of each zone is heat affected zone>base metal>welded metal.Compared with the base metal of X80 pipeline steel in single immersion,the base metal of X80 pipeline steel in whole immersion has more serious uneven corrosion,worse corrosion resistance,higher corrosion fluctuation and corrosion tendency,it is due to the galvanic effect existing in the welded joint,which promotes the corrosion of the base metal.The corrosion behavior of the welded joint of X80 pipeline steel was studied when the welded joint was as a whole immersed in H₂S saturated solution at 60?.It was found that within 4 days of corrosion,the corrosion product film on the surface of the welded joint has a continuous process of forming and falling off,the thickness of the corrosion product film was the highest at the third day after corrosion,which is related to the large-scale accumulation of the corrosion product;on the 4th day of corrosion,the corrosion products fell off in a large area,which was related to the low adhesion of the corrosion products.During 1-3 days of corrosion,the welded metal was the anode of the battery,the heat affected zone were the cathode;In the corrosion 4 days,the base metal was the anode of the battery and the welded metal were the cathode,because the zone of heat affected zone is much smaller than that of welded metal and base metal,the galvanic couple effect will not cause local corrosion.During 1-3 days of corrosion,the order of corrosion resistance of each zone was heat affected zone>welded metal>base metal,and base metal was the weak area of corrosion.On 4 days of corrosion,the order of corrosion resistance of each zone changed to welded metal>base metal>heat affected zone,the heat affected zone was the weak area of corrosion.