Research On CO₂ Corrosion Behavior Of Submarine Wet Gas Pipelines Under Different Wetting States

Natural gas with water transported in gas-liquid two-phase could reduce the cost of developing natural gas.However,it may lead to internal corrosion of pipelines because of water and corrosive gas?CO₂?in transported medium,and even the safety of pipelines.Therefore,it is necessary to research on mechanism of CO₂ corrosion in offshore wet-gas flowlines.This research benefits to develop reliable and reasonable safety assessment for pipeline engineering.In this paper,based on corrosion characteristics of offshore flowlines and CO₂ corrosion mechanism,an experimental set-up and the experimental scheme for simulating CO₂ corrosion of offshore wet-gas flowlines was determined.The electrochemical testing and scanning electron microscopy?SEM?,energy spectrum analysis?EDS?and X-ray diffraction?XRD?were used to analyze the effects of temperature,pH,wetting condition on CO₂ corrosion of X65 steel.Finally,the influence of surface wetting on CO₂ corrosion was discussed.The main research works and results of this paper are as follows:?1?Based on the difference of surface wetting of the inner wall of flowlines at different clock positions of the pipe section,a device was assembled to evaluate CO₂ corrosion resistance of pipe materials under different wetting condition.With this device,an experimental scheme for evaluating corrosion resistance of pipe materials under various wetting conditions was proposed.?2?When temperature increased,the corrosion current density icorr of X65 steel in full immersion and intermittent immersion increased,followed by a reduce in corrosion rates.The corrosion rate reached the maximum at 55?.In contrast,icorr of steels exposed to moist CO₂ atmosphere reduced with the increase of temperature.CO₂ corrosion product film was mainly FeCO3.When temperature increased,the slurry-like product film was denser under full immersion;the crystal grain size of the crystal film became larger in moist CO₂ atmosphere;the protection of the product film containing fine grains under intermittent immersion was enhanced.The change of corrosion rate with temperature related to the difference in dissolution behavior of the matrix and the deposition of corrosion products.?3?The increase of pH of the solution can inhibit CO₂ corrosion under full immersion and intermittent immersion,but has limited influence on the corrosion behavior of X65 steel in moist CO₂ atmosphere.At the same time,as solution pH increased,FeCO3 crystals appear on the initial corrosion product film under full immersion and intermittent immersion,and the thickeness of product layer increased.The pH of the solution increased,and the concentration of the reactants involved in the cathodic reaction changed,which in turn affected the cathode reaction control mechanism and the composition of the corrosion product film.?4?With the extension of time,icorr of X65 steel gradually increased but the growth rate slowed down and the film resistance also began to increase under full immersion and intermittent immersion.icorr of X65 steel gradually decreased in moist CO₂ atmosphere,and the film resistance increased significantly and the growth rate increased.The initial product film became more thick under full immersed,and FeCO3 grains precipitated on the initial layer.The local differences in protective product film appeared under intermittent immersion.There were two layers in crystal corrosion product in moist CO₂ atmosphere,and the outer layer composed by fine FeCO3 grains improved protection of the product film.?5?The corrosion rate of carbon steel in gas phase was the smallest,and the one in intermittent immersion was slightly larger,and the maximum related to full-immersed one.Therefore,carbon steel corrosion in moist CO₂ atmosphere was difficult.The difference in corrosion rate and corrosion product film was because of mass transfer process of the corrosive medium and the deposition process of the corrosion product.The initial active dissolution in the liquid film was faster,but FeCO3 crystal deposition in the liquid film was much faster than the liquid phase,so final corrosion rate in moist CO₂ atmosphere was much lower than that under full immersion.The local non-uniform corrosion of X65 steel under intermittent immersion was related to thickness variation of drops on the substrate.