Study On B10 Alloy Seawater Pipeline Film Formation Mechanism And Cathodic Protection Parameters

As an important work unit of marine equipment and engineering facilities such as ships,offshore oil platforms,coastal power plants and desalination plants,seawater cooling system undertakes the important functions of heat exchange,cooling,fire protection and so on.B10copper nickel alloy is an important material for seawater cooling system.Due to the long-term transportation of seawater as cooling medium,it is bound to face severe challenges of seawater corrosion.The welded joint is a non-uniform structure composed of weld,base metal and heat affected zone,which is non-uniform in chemical composition,microstructure,residual stress and so on.Under the actual service conditions,the risk of damage and failure is much higher than that of the uniform part of the pipeline,so the failure mechanism value should be further explored.In this paper,the corrosion rate and corrosion tendency of B10 alloy in natural seawater were calculated and analyzed by weight loss method and in-situ electrochemical test method.The average corrosion rate of B10 alloy is 15μm/a,which has good corrosion resistance.The self corrosion potential fluctuated at the beginning of immersion and remained stable after 90days.High resolution scanning electron microscope was used to observe the surface structure of B10 alloy immersed in seawater for a long time,and X-ray energy dispersive spectrometer was used to test its surface element composition.The corrosion law and mechanism of B10alloy immersed in natural seawater for a long time were explored.The main form of corrosion was intergranular corrosion,and no obvious sign of de composition corrosion was found.Through in-situ electrochemical test of seawater pipeline,the evolution law of corrosion product film formation of B10 pipeline base metal,weld metal and heat affected zone material with time in different flow rate seawater was explored.The corrosion products on the surface of B10 alloy gradually accumulated at the initial stage of corrosion,and the film formed by the products reached a stable state at 20d.The effects of seawater flow rate on the film formation process of corrosion products in various parts of B10 alloy pipeline welded joint were investigated by three-dimensional morphology and SEM morphology observation,EDS energy spectrum analysis and Raman spectrum analysis.The corrosion resistance of the product film of B10 pipeline increases with the increase of seawater flow rate.The higher the flow rate is,the denser the film is.The order of protection effect of B10 pipeline product film from strong to weak under different seawater flow rates is:0 m/s>5 m/s>3 m/s>1 m s.Compared with B10 base metal,weld metal and welding heat affected zone,the corrosion resistance of heat affected zone material is the worst,and the order of corrosion resistance of three materials from strong to weak is:weld metal>base metal>heat affected zone.The results show that the corrosion resistance of the product film in HAZ is most affected by the seawater flow rate,and the sensitivity of the corrosion resistance of the three materials to seawater flow rate is in the order of weld>HAZ>base metal.The galvanic current of weld HAZ couple can reachμA·cm^-2at the initial stage of corrosion.After 3 days,it did not exceed0.1μA·cm^-2and remained stable.The higher the seawater velocity in the pipeline,the higher the galvanic potential after stabilization.The influence of seawater flow rate on the cathodic protection parameters of B10 pipeline was investigated by in-situ electrochemical test of seawater pipeline.The cathodic protection current density of B10 pipeline increases with the increase of seawater flow rate,and the protection effect of Fe based sacrificial anode on B10 pipeline decreases with the distance.The protection effect of the downstream is better than that of the upstream.The protection distance increases with the increase of seawater velocity.