Effect Of Microstructure On Corrosion Behavior Of E550 Ship Plate Steel Fully Immersed In Sea Water

With the development of Marine economy,ships are widely used,and the steel used in ocean engineering is used in the complex Marine environment for a long time,which is subjected to the corrosion of seawater.In this thesis,the effects of different microstructures and corrosion time on the electrochemical corrosion behavior and mechanism of E550 steel were studied by laboratory simulated seawater immersion test and electrochemical test.The macro corrosion product morphology and micro corrosion product morphology of E550 steel under different full immersion time were observed and analyzed by metallographic microscope,ultra depth of field 3D microscope,scanning electron microscope（SEM）and energy dispersive spectrometer（EDS）.The corrosion rate of E550 steel under different conditions was determined by weight loss method.The main research results and conclusions are as follows:（1）The microstructure of E550 high strength ship plate steel was controlled by annealing at 900℃for different time under laboratory conditions.The experimental results show that the microstructure of E550 steel after annealing heat treatment is mainly composed of ferrite（F）+pearlite（P）.With the increase of holding time,the percentage of ferrite decreases and the grain size increases.（2）In the simulated seawater immersion test,the corrosion products on the surface of E550 steel（（B+F）steel）without annealing treatment at the initial stage of corrosion were relatively uniform,while the corrosion products on the surface of annealed E550steel（（F+P）steel）were unevenly distributed and relatively loose,mainly in spherical and powder shape.At the later stage of corrosion,the corrosion products on the inner side of the sample are dense,and the corrosion products on the surface have many holes,which are mainly massive.According to the analysis,the corrosion products of E550steel in seawater are mainly Fe₃O₄,Fe₂O₃ and Fe OOH.（3）In the simulated seawater immersion test,the effect of microstructure on the corrosion rate of E550 steel increases first and then decreases,and with the extension of corrosion time,the difference of corrosion rate gradually decreases,the corrosion rate decreases as the proportion of pearlite decreases.At the initial stage of corrosion,the corrosion rate of E550 steel（（B+F）steel）without annealing treatment was lower and the corrosion resistance was better;With the increase of corrosion time,the corrosion rate of annealed E550 steel（（F+P）steel）is smaller,and the higher the proportion of ferrite,the better the corrosion resistance.（4）In the laboratory simulated seawater immersion test,the corrosion mechanism of E550 steel is mainly pitting corrosion and local corrosion.With the increase of corrosion time,the corrosion mechanism is crevice corrosion and overall corrosion.（5）In the electrochemical experiment,E550 steel with different microstructure has similar electrochemical corrosion behavior in simulated seawater solution.Under the same microstructure,the impedance value is the largest,the corrosion current density is the smallest,and the corrosion resistance is the best at 0 days of full immersion corrosion;The impedance value is the smallest,the corrosion current density is the largest,and the corrosion resistance is the worst after 7 days of full immersion corrosion.At the same full immersion time,at the initial stage of corrosion,the impedance value of E550 steel without annealing treatment（（B+F）steel）is larger,the corrosion current density is smaller,and the corrosion resistance is better.With the increase of corrosion time,the impedance value of E550 steel after annealing treatment（（F+P）steel）increases,the corrosion current decreases,and the corrosion resistance becomes better.