Study On The Corrosion Mechanism And Rate Prediction Method Of Gathering Pipeline

Based on the survey of the gathering pipeline corrosion information from domestic to overseas data,through 20 # steel gathering line of failure analysis and indoor electrochemical testing,the main corrosion controlling factors and corrosion mechanism of the failure pipeline were clarified,and the corrosion rate in the working environment was predicted.The results show that the pipeline corrosion failure is mainly caused by dissolved oxygen.The high concentration of chloride iron in the medium and the low concentration of corrosion resistant elements which is distributed unevenly in the material accelerate the failure of the pipeline.Under the two kinds corrosive environments?CO₂ and CO₂+O₂?,electrochemical corrosion of20# steel is controlled by the cathode activating.The presence of trace amounts of dissolved oxygen does not produce significant effect on the corrosion potential.But cathodic reaction is significantly promoted,and the corrosion current density increases.In the initial stage of the reaction,there exist three time constants for 20# steel in the both corrosive environments?CO₂and CO₂+O₂?.With the increase of the corrosion time,the capacitive arc of the high-frequency end increases,and the inductive arc of the low-frequency end becomes smaller.After 48 h testing,a complete corrosion products film has been formed on the sample surface.The inductive arc of the low-frequency end disappears,only the capacitive arc is shown.The protective role of corrosion product film enhances significantly,the value of polarization resistance increases sharply,and the corrosion rate decreases.The prediction results of corrosion rate indicate that,the promoting effect of rising temperature on the electrochemical reaction of steel superior to that on the protection of corrosion product film,and high temperature causes the high corrosion rate.The influence of low CO₂ pressure on the corrosion rate is insignificant,while the existence of dissolved oxygen promotes the cathodec reaction process of 20# steel.The corrosion rate increases with increasing dissolved oxygen content.Under the operating conditions of oil's gathering and transportation,the average corrosion rate of 20# steel is 0.0357mm/a in laboratory experiments,which is consistent with the predicted results.