Mechnistic Study On Local Corrosion Induced By Inclusions In Low-alloy Steels For Marine Engineering

Due to excellent mechanical properties and good welding properties,low-alloy steels are widely used in marine engineering construction.In this thesis,based on low-alloy steels by using different deoxidized methods?Al deoxidization and Zr-Ti composite deoxidization?and adding different alloying elements?rare earth?RE?and niobium?Nb??,the local corrosion behavior induced by inclusions in the simulated marine environment were investigated.The methods used in this research were combining first-principle calculations and a variety of analytical instruments,including scanning electron microscope and X-ray energy spectroscopy?SEM/EDS?,transmission electron microscope?TEM?,scanning vibrating electrode technique?SVET?,confocal Raman microscopy?CRM?,and X-ray diffraction?XRD?and electrochemical workstation.The main results are shown below:The microstructure of low-alloy steels is composed of ferrite and pearlite,and Nb microalloying can obviously refines grains.The corrosion current density of RE microalloyed and Zr-Ti composite deoxidized steels in simulated marine environmrnt is smaller compared to other steels,indicating that the corrosion resistance of low-alloy steels can be improved by adding RE and Zr-Ti composite deoxidization.The results of neutral salt spray show that corrosion products play an important role in inhibiting corrosion rate.In the early stage of corrosion,?-FeOOH and Fe₃O₄ are the main component in corrosion products,whereas?-FeOOH is the main component in the later stage of corrosion.Nb microalloying has no obvious effect on the formation of inclusions,while deoxidizing elements are mainly involved in the formation of inclusions in Nb-bearing steels.The polygonal hard Al₂O₃ inclusions are mainly formed in Al deoxidized Nb microalloyed steels,and the inclusions in Zr-Ti composite deoxidized Nb microalloyed steels are composed of?Al,Ti,Zr?-O_x.Due to the higher stability of?Al,Ti,Zr?-O_x and Al₂O₃ inclusions compared to the matrix,local corrosion originates from the interface between inclusions and matrix,which leads to the dissolution of matrix and formation of micro gap.The dissolution of matrix results in the accumulation of Cl^-and other corrosive ions in the micro gap,which leads to autocatalytic acidification effect and further accelerates the development of local corrosion.RE microalloying can obviously refine and spheroidize inclusions,and modify the composition of inclusions.In Al deoxidized RE microalloyed steels,small spherical inclusions are mainly formed,which are composed of REAlO₃ and RE₂O₂S.In Zr-Ti composite deoxidized RE microalloyed steels,two types of inclusions are mainly formed:RE_xZr_yO_z-RE₂O₂S and RE_xZr_yO_z-RE₂O₂S-TiN.Local corrosion originates from the dissolution of the RE₂O?₂S part in the inclusions,which lowers the pH in the pit,further accelerating the dissolution of REAlO?₃/RE_xZr_yO_z parts.TiN is very stable and hardly to dissolve in a corrosive environment,and eventually detached from the substrate.After the complete dissolution/detachment of inclusions,stable pits are formed on the steel surface,further reducing the propogation rate of local corrosion.The in-situ SVET results comfirmed that the dissolution of inclusions induced the electrochemical activity of pitting corrosion,showing a dominant increase of anodic current density in the early stage of corrosion.After the complete dissolution of the inclusions,the anodic current density decreased significantly,indicating the corrosion rate was reduced.In the later stage of corrosion,corrosion products play an important role in inhibiting the propagation of local corrosion.Raman results show that the corrosion products in the cathodic region are mainly composed of?-FeOOH and Fe₃O₄,whereas the corrosion products in the anodic region are mainly composed of?-Fe₂O₃,?-FeOOH and Fe₃O₄.The work functions of inclusions,Fe matrix and Fe₃C in different low-index crystal planes were calculated by first-principle.The higher the work function,the lower the corrosion tendency.The results of corrosion tendency show:TiN>La₂O₂S>LaAlO?₃>Fe>Fe₃C>La₂Zr₂O₇>Al₂O₃.The calculation results are generally consistent with the experimental results.Although TiN shows a high corrosion tendency,it shows high stability in simulated marine environment.