Study On Erosion-corrosion Coupling Damage Mechanism Of Carbon Steel Used In Offshore Engineering

Because seawater is usually mixed with a large number of solid particles,ships and marine structures face severe erosion corrosion risks in flowing seawater.And the metal damage caused by the coupling effect of erosion and corrosion is usually much higher than the sum of the metal damage caused by pure electrochemical corrosion and pure mechanical erosion.Therefore,the research on the coupling damage mechanism of erosion and corrosion of carbon steel used in marine engineering is of great significance.In this paper,the influence of seawater flow rate and impact angles on erosion corrosion was studied by means of numerical simulation,weight loss measurement,electrochemical test and surface analysis.The proportion of pure corrosion component,pure erosion component,erosion accelerated corrosion component and corrosion accelerated erosion component in the total damage of carbon steel were quantified to analyze the interaction mechanism between erosion and corrosion.The main work of this paper is summarized as follows:(1)Through numerical simulation,it was found that the influence of sand on water flow could be ignored because of the low sediment concentration.So there was no significant difference between the flow field distribution under the condition of sand and sand-free.Under different flow rates,the flow field was radially distributed on the surface of the specimen.And the flow rate and wall shear force increased from the central position to the periphery.With the increase of impact angle,the static area of flow field gradually moved down from the upper part of the nozzle axis to the center of the sample.(2)In sand-free natural seawater,the corrosion products on the surface of EH 32 carbon steel at different flow rates were Fe3O4 and γ-Fe OOH.With the increase of flow rate,the Fe3O4 rust layer on the outside would gradually become dense,hindering the mass transfer process of oxygen in the solution.As a result,the pure corrosion rate of EH 32 carbon steel in seawater would not raise significantly with the increase of the flow rate.However,with the increase of the flow rate,the corrosion morphology of the sample surface would gradually develop from "flow mark" to pitting.The pure corrosion rate of EH 32 carbon steel was less affected by the impact angle.And the CFD numerical simulation could accurately predict the corrosion behavior at different impact angles.(3)In the natural seawater with sand,the cathodic protection was applied to the carbon steel to remove the corrosion component.It was found that the mechanical damage caused by the seawater containing sand to the EH 32 carbon steel was weak when the flow rate was in the range of 1-5 m/s.But the mechanical damage would increase sharply when the flow rate reached8 m/s.When the impact angle was 45°,the synergistic effect of normal stress and shear stress on the sample surface was the strongest,which made the mechanical damage of EH 32 carbon steel the most serious.(4)By studying the erosion and corrosion coupling damage behavior of EH 32 carbon steel in natural seawater with sand,it was found that under the impact of sand the corrosion product film was difficult to form on the surface of EH 32 carbon steel,leading to the corrosion rate increasing sharply.The density of corrosion pitting increased with higher flow rate.But as corrosion products accumulated in the corrosion pitting,the pitting would not deepen significantly with the increase of the flow rate.The synergistic effect of erosion and corrosion was the main cause of metal damage under different impact angles.When the impact angle was 45°,the synergistic effect was the strongest,which made the total weight loss rate of erosion corrosion of EH 32 carbon steel reach the peak.