| In the complex environment containing sand and salt such as oceans and salt lakes,hydraulic machinery is faced with both chloride ion corrosion damage and sand erosion damage.There is often a strong synergistic effect between the two damage modes,resulting in a sharp increase in the damage rate of metal components.In this process,there is a close coupling relationship between liquid-solid two-phase flow,mass transfer of corrosive components,electrochemical reaction of metal surface and particle-wall collision,which poses a challenge to the study of this composite damage mechanism.In this paper,the influence mechanism of sand-salt fluid on the coupling damage of hydraulic machinery erosioncorrosion is studied,which has certain guiding significance for engineering practice.High-entropy alloys containing elements such as Cr,Ni,and Mo are expected to become potential materials for hydraulic machinery serving in high-salinity environments due to their excellent corrosion resistance.In this paper,Ni2FeCrMo0.2 alloy with outstanding corrosion resistance is selected as the research object.The internal flow state of the hydraulic machinery is simulated by a rotating stirring erosion-corrosion device.The macroscopic erosioncorrosion characteristics of the material in the sand-salt fluid environment are tested by weight loss and electrochemical methods.Scanning electron microscopy and laser confocal microscopy are used to obtain the surface morphology of the material.The effects of flow rate,sand factors(particle size,concentration)and Cl-concentration on the erosion-corrosion behavior of metal equipment in sand and salt environment were investigated.The interaction mechanism of sand-fluid-ion-metal wall was analyzed,and the internal relationship and synergistic effect between electrochemical corrosion and sand erosion were revealed.It provides theoretical guidance for the selection and protection of hydraulic machinery in complex environments such as oceans and salt lakes.The experimental results are shown as follows:(1)The change of flow rate has a significant effect on the erosion-corrosion coupling mechanism of Ni2FeCrMo0.2 alloy.At low flow rate,the kinetic energy of sand particles is small,and the erosion effect on the hard metal matrix is weak,but the impact on the soft product film and the mass transfer of corrosive ions lead to the dominant mechanism of erosion-promoting corrosion behavior in the process of material damage.With the increase of flow velocity,the impact kinetic energy and frequency of sand particles on the material gradually increase.Although the corrosion is also promoted by thinning the thickness of the product film,the erosion increases exponentially with the change of flow velocity,so the erosion effect occupies the main part of the total damage at high flow velocity.According to the erosion-corrosion coupling mechanism,it can be predicted that corrosion will preferentially occur in plastic deformation areas or cracks caused by sand erosion,and the erosion of sand will also cause more serious damage to the earliest pitting sites.(2)The change of sand particle size affects the corrosion by producing different degrees of erosion.The impact kinetic energy of sand itself is proportional to the particle size.The sand with small particle size cannot cause serious damage to the metal matrix,but it has a certain erosion effect on the passivation film,resulting in the damage of the local product film and the exposure of the metal matrix,thus accelerating the corrosion.With the increase of sand particle size,the surface plastic deformation and matrix spalling caused by sand erosion are more serious,and the influence of coupling damage on corrosion behavior is also increasing.When the sand particle size is greater than 0.5mm,the number of sand particles at the same concentration decreases,which reduces the impact frequency of sand particles on the surface of the material,thus reducing the synergistic effect of erosion-corrosion,but the strong impact force of sand particles still causes serious extrusion deformation to the material.(3)The erosion-corrosion damage rate of Ni2FeCrMo0.2 alloy increases first and then decreases with the increase of sand concentration.The increase of sand accelerates the impact frequency of the material,and the surface of the material becomes rougher due to the scouring effect,which further promotes the occurrence of corrosion.When the concentration exceeds a certain critical value,the viscous force of the solution becomes larger due to the increase of sand particles,which weakens the impact kinetic energy of sand particles on the surface of the material.A large number of sand particles are easy to rebound after impact on the surface of the material,increasing the number of collisions between sand particles,thus weakening the erosion rate and reducing the promotion of corrosion.(4)The corrosion behavior of Ni2FeCrMo0.2 alloy in salt water environment is mainly controlled by the anodic oxidation process,and pitting corrosion occurs on the surface of the material after repeated passivation dissolution reaction.The increase of Cl-concentration has a promoting and then inhibiting effect on the corrosion behavior of Ni2FeCrMo0.2 alloy.The critical value of Cl-concentration is about 3.5%.When it is lower than the critical value,the increase of Cl-concentration will accelerate the dissolution of passive film to enhance corrosion.However,after exceeding the critical value,too much Cl-will generate a large number of corrosion products to accumulate inside the corrosion pit while accelerating the corrosion reaction,thereby inhibiting corrosion.With the increase of Cl-concentration,the formation of a large number of corrosion products destroys the hardened layer on the metal surface,increases the damage degree of sand particles on the material surface,and the promotion effect of corrosion on erosion is increasing. |
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