| The riser is the passage connecting the offshore drilling platform and the subsea wellhead,traditional riser material using high strength steel.Aluminum alloy materials have the advantages of light weight,high specific strength,good corrosion resistance,and easy processing.They play an important role in oil drilling,ocean drilling,polar drilling,and deep scientific drilling in the mainland.At present,a few foreign businesses have made aluminum alloy risers,and have achieved good application results.The riser is subjected to the corrosion of seawater in the ocean and is subjected to the fatigue caused by vortex-induced vibration,so the riser is an important and vulnerable part of offshore drilling.There is little research on corrosion fatigue performance of aluminum alloy riser materials in China,so it is necessary to study it and provide data support for industrial production.This paper simulates the working environment of aluminum alloy risers in different sea areas and different depths by changing the three factors of temperature,pH and salinity,and selects representative parameters to test the 7075 and 2024 aluminum alloy riser materials through orthogonal test method.The potentiodynamic polarization curve was Tafel-fitted to the polarization curve to obtain the corresponding corrosion current density and corrosion potential,and the average annual corrosion depth was calculated to compare the corrosion rates and corrosion sensitivities of the two materials.The combination of the parameters that have the greatest influence on the corrosion resistance of the material.The 7075 and 2024 aluminum alloys were pre-corrosion in this environment,and the fatigue properties were tested and compared.The fractures were observed and analyzed by scanning electron microscopy to understand the corrosion mechanism of the two materials.The results show that the corrosion resistance of 7075 and 2024 aluminum alloys increases with the increase of three parameters in the simulated seawater environment with a temperature of 20°C to 50°C,a pH of 7 to 9 and a salinity of 3% to 4%.The overall trend of corrosion performance is that the corrosion rate increases and the degree of corrosion increases.The temperature has a great influence on the corrosion rate of two kinds of aluminum alloy,and the PH value has a great influence on the corrosion degree of two kinds of aluminum alloy.2024 aluminum alloy is superior to 7075 aluminum alloy in corrosion resistance.The reason for the difference in the corrosion resistance of the two materials is the difference in composition.In 2024,the content of copper is relatively high,which increases the overall potential of the material and improves its corrosion resistance.The 7075 contains higher elements of magnesium and zinc,making the overall potential of the material descend,resulting in 7075 corrosion resistance less than 2024.The corrosion morphology was observed by scanning electron microscopy after the pre-corrosion of 7075 aluminum alloy and 2024 aluminum alloy.The main corrosion types of 7075 aluminum alloy are uniform corrosion,exfoliation corrosion and pitting corrosion;the main corrosion types of 2024 aluminum alloy are uniform corrosion and pitting corrosion.Observed from the corrosion morphology,2024 aluminum alloy corrosion resistance is better than 7075 aluminum alloy.The fatigue properties of 7075 aluminum alloy and 2024 aluminum alloy were tested after pre-corrosion in simulated seawater.The experimental results show that the fatigue properties of the two kinds of aluminum alloy materials decreased and the fatigue life of the 7075 and 2024 aluminum alloys were 8463661 and 9637086 times respectively after the pre-corrosion.The fatigue property of 2024 aluminum alloy is better than that of 7075 aluminum alloy after pre-corrosion.Under the scanning electron microscope,multiple fatigue cracks originate from the fatigue source area of the fracture after pre-corrosion.The reason is that the surface of the material after pre-corrosion is flawed,and the stress concentration at the defect is under the cyclic loading.Crack propagation eventually results in fatigue fracture.According to the overall analysis of the experimental results,the corrosion resistance and fatigue resistance of 2024 aluminum alloy is better than that of 7075 aluminum alloy,which is more suitable for deepwater drilling aluminum alloy riser materials. |
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