Correlation Between Inclusions And Corrosion Behaviour Of Low-alloy Steels In Marine Environment

In the harsh marine environment,low-alloy steels are susceptible to localised corrosion in the presence of aggressive anions,such as Cl– and SO42–.Owing to the existence of high lattice distortion region or micro-crevice,the steel matrix/inclusion interface was susceptible to localised corrosion.In this thesis,the correlation between inclusions and corrosion behaviour of low-alloy steels in marine environment was investigated,by means of field emission scanning electron microscopy/energy-dispersive spectrometry(FE-SEM/EDS),electron backscatter diffraction(EBSD),scanning vibration electrode technique(SVET),Confocal Raman microscopy system(CRM),3D optical microscope(OM)and electrochemical workstation.The main conclusions can be drawn as follows.The microstructure of the three types of EH36 steel were composed of ferrite and pearlite,and the pearlite volume fraction and grain size of EH36-B steel were lower than EH36-A and EH36-C steels.Besides,the results of electrochemical tests shown the EH36-B steel has a better corrosion resistance than EH36-A and EH36-C steels.The complex inclusions in EH36-B steel were composed of Ca S,calcium aluminate and a small amount of Mg O·Al2O3 spinel.The various Ca O/Al2O3 ratio of calcium aluminate in complex inclusions led to different ability to induce localised corrosion,thus the inclusions were categorised as ‘active’ or ‘inactive’.The residual stress at the active inclusion/matrix interface was higher than the yield strength of the steel,therefore a high lattice distortion region was induced by the local plastic deformation.In this region,initial localised corrosion was preferentially triggered,attributed to the high electrochemistry activity.Moreover,the occluded cell accelerated the localised corrosion propagation,and finally the dissolution of the inclusions and the matrix resulted in a stable pit.With the addition of Ti and Cu elements,the number density and average diameter of inclusions in the simulated coarse-grained heat-affected zone(CGHAZ)of Ti-bearing and Cu-bearing steel were higher than that of base steel.The complex inclusions formed in Cu-bearing steel were(Ti,Al,Mn)-Ox-Mn S,which was similar to that in base steel.Thus,in Cu-bearing steel and base steel,localised corrosion was initiated by the dissolution of Mn S.The main inclusions in Ti-bearing steels were modified into Ti N-Al2O3/Ti N,and the localised corrosion was initiated by the dissolution of high lattice distortion region at inclusion/matrix interface.The corrosion rate increased in the following order: Base steel ≈ Cu-bearing steel < Ti-bearing steel.The higher interface density in Ti-bearing steel provided more initiation sites for localised corrosion,hence the corrosion rate of Ti-bearing steel was obviously higher than that of base steel.Owing to the existence of Cu-enriched rust layer,the anodic reaction was inhibited,which contributed to the enhanced corrosion resistance of Cu-bearing steel.