Corrosion Behavior Of X70 Pipeline Steel In Marine Environment Splash Zone

With the development of China's economy,traditional land-based oil and gas exploitation has been unable to meet the growing demand for oil and gas consumption,and the development of oil and gas resources has gradually expanded to the ocean.The offshore oil and gas exploitation platforms are far away from the onshore oil processing and consumption center.The safe and stable transportation of oil and gas resources is an important foundation for ensuring the supply of marine oil and gas.The pipelines are an important way to transport oil and gas resources from the ocean to the land,and high-strength pipeline steel is an important pipeline engineering material.However,the complex and severe marine environment with high salt,heat and humidity is prone to corrosion of pipeline steel,which adds great challenges to the safe operation of oil and gas pipelines.The frequent occurrence of marine oil and gas pipeline leakage accidents has become the focus of attention.The splash zone is the area with the highest metal corrosion rate in the marine environment.Therefore,in order to ensure the stable supply of oil and gas resources and reduce the pollution of the marine environment caused by oil and gas leakage,it is of great significance for the integrity management of oil and gas pipelines to conduct research on the corrosion behavior of high-strength pipeline steel in the splash zone of the marine environment.In this paper,we establish a corrosion test platform for simulating the splash zone of the South China Sea marine environment.Taking the environmental parameters of the spray zone with the highest metal corrosion rate in the South China Sea marine environment as a research case,we use dynamic potential polarization and AC impedance electrochemical test methods to test the electrochemical corrosion rate of X70 steel samples under different conditions.Combined with a slow strain rate tensile test device and a scanning electron microscope,this paper tested the stress corrosion cracking behavior of the sample and characterizes the surface corrosion morphology.The effects of temperature and p H changes on the corrosion behavior and their changing law.The research results show that with the increase of the experimental temperature,the corrosion current density and stress corrosion sensitivity of X70 steel samples increase.The corrosion morphology obtained by SEM observation also changes from ductile fracture characteristics to brittle fracture characteristics.The secondary cracks on the side gradually decreased,and the corrosion product film gradually peeled off with the increase of the test temperature,and showed the characteristics of anode dissolution.At lower temperatures,the relative content of ?-Fe OOH in the corrosion product layer is higher,and the relative content of ?-Fe OOH in the corrosion product layer increases with increasing temperature,and the Cl ion aggregation characteristic appears.A mixed control mechanism was developed with anode dissolution and local hydrogen evolution.The corrosion current density and stress corrosion cracking sensitivity of X70 steel specimens showed a rapid decline and then tended to slow down with the increase of p H value.The AC impedance test results were consistent with the change rule of the dynamic potential polarization test;in the environment of p H reduction,hydrogen ions Accelerated electrochemical corrosion has obvious anode dissolution characteristics;as the p H value increases,the relative content of ?-Fe OOH in the corrosion product layer increases,which has a certain protective effect on the substrate,slows down the anode dissolution process,and reduces the electrochemical corrosion rate and stress corrosion sensitivity..