Corrosion Behavior And Mechanism Of A517Gr.Q Grade Marine Steel For Offshore Engineering

The ocean contains all kinds of resources needed for the development of human society,and improving the ability to develop marine resources is of great strategic significance for accelerating the development of the marine economy and building a maritime power.At present,the corrosion of metal materials in the marine environment severely restricts the development of offshore equipment.The corrosion resistance of steel materials used in key parts is far from the advanced level of foreign countries.As a new type of large-sized steel material,A517 Gr.Q grade marine steel has a yield strength of more than 780 MPa and a steel thickness of 177.8 mm.It has broad application prospects in the field of marine engineering,but the corrosion behavior in the marine environment is still unclear.Therefore,this paper explores the corrosion behavior and mechanism of A517 steel in the marine environment by simulating the two corrosive environments of the marine full immersion zone and the splash zone,and provides the necessary theoretical and experimental basis for the practical engineering application selection and the development of corrosion resistant steel materials.In this paper,the corrosion behavior of A517 steel was analyzed by laboratory simulation of the corrosion environment of the whole immersion zone in the ocean.The corrosion behavior of A517 steel was analyzed by weight loss method,SEM,EDS,XRD and electrochemical techniques.The results show that the maximum potential difference of A517 steel in the thickness direction is 13 mV,which is less than the minimum potential difference of 50 mV in galvanic corrosion,indicating that the corrosion sensitivity of the material in the thickness direction is consistent.With the prolonged immersion time,the corrosion weight loss gradually increases,and the average corrosion rate decreases first and then increases and finally stabilizes.The corrosion rate is about 0.127 mm/a.The initial stage of corrosion power is mainly controlled by the ultimate diffusion of dissolved oxygen,and later is the redox charge transfer control of corrosion products.Corrosion firstly propagates in the inclusions such as Al2O3 and MgO,and the corrosion products show obvious delamination.The outer rust layer is mainly ?-FeOOH which is loose and easy to fall off.The inner rust layer is mainly dense and uniform Fe3O4,and the rust layer is also The presence of ?-FeOOH and ?-FeOOH was detected.The presence of a large number of microcracks in the rust layer is beneficial to the interface between the erosive C1-and oxygen into the rust layer and the matrix metal,which weakens the protective effect of the product film and promotes the corrosion.The corrosion behavior of A517 steel was studied by laboratory simulation of the dry and wet alternating environment of marine splash zone.The corrosion behavior of A517 steel was studied by weight loss method,SEM,EDS,XRD,FTIR and electrochemical test.The influence of system humidity on corrosion was also studied.The results show that the corrosion of A517 steel is more serious in the dry and wet alternate environment of the splash zone.In the high frequency dry and wet alternating process of steel surface,there is not only the ultimate diffusion of dissolved oxygen,but also the oxidation of the corrosion product itself.The corrosion weight loss increases with time in both humidity conditions,and the average corrosion rate increases first and then stabilizes.The corrosion rate at 62%RH is about 0.400 mm/a,which is about 3-4 times that of the full immersion area,and the corrosion rate at 83%RH is about 0.540?0.623 mm/a,which is about full immersion.4 to 5 times the area.The corrosion products are all composed of ?-FeOOH,?-FeOOH,?-FeOOH and Fe3O4.In the 62%RH environment,the residence time of the electrolyte membrane is short,the dry-wet alternating phenomenon is more obvious,more corrosion products are formed on the surface of the sample,and more oxidized ?-FeOOH is formed,and the rust layer is porous and porous.For uneven overall corrosion,the corrosion of the material is more serious.In the environment of 83%RH,more dense and uniform Fe3O4 is formed in the product film,which has a good blocking effect on the intrusion of aggressive C1-.Because of the high humidity environment,the electrolyte membrane stays on the surface of the sample.The long time,the corrosion reaction takes a long time,so it shows a higher average corrosion rate.