Along with the exploitation of ocean oil and gas resources, the corrosion phenomenon of submarine pipeline has drawn greater attention since it serves as the most significant way for marine resources transport. Due to the harsh marine environment, the reasons for the external corrosion damage of submarine pipeline are complicated. The frequent run-time submarine pipeline corrosion accidents in recent years expose the deficiency of the domestic researches concerning the reasons of the external corrosion and the responses of the accidents. Therefore, this study aims to analyze the reasons for external corrosion of submarine pipeline and propose the corresponding solutions, through experiments related to materials science, marine environment, chemistry and chemical engineering and the actual situation of engineering.Based on the previous researches both at home and abroad, the statistical induction regarding characteristics and rules of submarine pipeline external corrosion failures is conducted. Then, a further in-depth research is carried out:firstly, the related parameters and the corrosion test results are explored by selecting a pipeline which has undergone an external corrosion accident. Secondly, the conjectures and hypothesis about accident causes are put forward on the basis of laboratory tank test. Thirdly, the electrochemical properties of pipe materials are studied by the designed experiment taking hypothesis as the starting point. On the one hand, the influence of factors such as temperature, acidity and salinity on the corrosion-resistance performance of pipeline material is investigated by employing the weight-loss method, surface state observation, steady-state polarization and electrode potential measurement etc. On the other hand, the performance of pitting corrosion resistance, the phenomenon of cathode screen and the acceleration of the oxygen concentration cell of pipeline material are examined and the relative improvements concerning such influential factors are also provided.The main conclusions of this paper are summarized as follows:(1) Pipeline materials X65 steel performs well in corrosion resistance aspect under the condition of different temperatures, salinity and pH values. Meanwhile, no pitting tendency of such material is found according to the surface state observation and steady-state polarization;(2) In the solution of 3.5% NaCl, the corrosion potential shows a negative move, the corrosion rate accelerates with the rising temperature; in the solutions of different salinity, the corrosion rate reduces with the increase of Cl- concentration; in the solution of different pH values, the corrosion rate significantly accelerates in both weak acid and weak alkaline solutions;(3) The surface energy of heat-shrinkable sleeve and waterproof cap concerning the PE material at pipeline repaired joint is low, tending to separate and thus causing water accident; The cathode screen of PE material is very obvious, so that even under the protection of cathode current, The electrode potential of the material in the Influent area closes to the corrosion potential, and the CP system can not play a protective effect. Therefore, this study considering about the problems about PE material in the pipeline repaired joint, put forward a new kind of unshielded coating so that to avoid external corrosion of subsea pipeline caused by the phenomenon of cathode screen.(4) Under the effect of oxygen concentration cell, the hypoxic area of X65 steel serve as the anode, and corrosion current of it will increase significantly,3 times higher than normal, Therefore, this study conducts research from the perspective of material composition, then obtains the effects of different components on the oxygen concentration of corrosion. |