Study On Crevice Corrosion Behavior And Mechanism Of U75V Rail

Rapid development of railway industry makes safety increasingly important.Corrosion are one of the important factors initiating rail track degradation from the beginning.Direct corrosion cost due to railway corrosion in China was 18.88 billion RMB,so corrosion definitely deserves more attention.U75V rail steel is the main rail type used in railway tracks,and crevice corrosion occurs between the rail foot and its fastening system during service.The combined effect of complex external stress and crevice corrosion on the track can easily cause local damage to the rail foot,posing safety hazards and resulting in serious accidents.Few studies on rail corrosion have been reported,and investigations on crevice corrosion mechanism of rail foot has not been reported so far.Therefore,it is of great significance and engineering value to study the crevice corrosion mechanism of U75V rail steel.In this work,electrochemical measurements,corrosion in-situ monitoring,surface analysis and corrosion simulation were used to study the crevice behavior of U75V rail foot,the mechanism of crevice corrosion,and the influencing factors of crevice corrosion.The main results are as follows:(1)The effect of the corrosion behavior of the main ions of the haze atmosphere was studied in the crevice corrosion simulation solution.The results show that in simulation solution,the rail steel shows slight uniform corrosion in the early stage and gully corrosion pit in the later stage of corrosion,while the SO₄^2-accelerates the overall corrosion of the rail steel surface in the early stage of corrosion,and inhibits the local corrosion on the surface in the later stage of corrosion.The NO₃^-can inhibit the active dissolution of the anode and greatly improve the cathodic reaction rate,which can inhibit the corrosion of the rail steel obviously.The NH₄⁺can promote the corrosion of the rail steel,and the corrosion pits are produced on the surface of the rail steel in the early stage of corrosion,and become more denser in the later stage of corrosion.Moreover,Fe₃C appears on the surface of the steel immersed in the simulated liquid of deoxygenation,indicating that the Fe₃C is prone to accumulate inside crevice during crevice corrosion of the U75V steel.(2)An in-situ variable crevice width crevice corrosion monitoring setup was designed to study the crevice corrosion behavior and mechanism of U75V rails.It is demonstrated that cathodic reaction on crevice exterior was inhibited by increasing gap size leading to decrease of exterior OCP thus decrease of potential difference between exterior and interior.In addition,increase and then decrease of gap size led to decrease and then increase of potential difference between exterior and interior,but not completely reversible.Dissolution of crevice interior was under ohmic-control and accelerated by increasing applied potential and/or gap size.A balance of metal dissolution and ions diffusion could be obtained under certain potential and at certain gap size,resulting in stabilized dissolution rate.Anodic dissolution rate was highest at crevice mouth and lowest near but not at deepest site.Initiation of crevice corrosion was observed,closely related to pearlite nodules with relatively larger interlamellar spacing and different lamellar directions compared to others around.Preferentially dissolved ferrite lamellas with larger interlamellar spacing were more likely to develop into localized attack.(3)The COMSOL finite element calculation simulation software was used to calculate the corrosion potential,anode current density and stress distribution of U75V rail foot with crevice corrosion defects under plastic deformation caused by external stress.The results showed.The corrosion potential of the substrate was reduced by stress,and the decrease range was great at the defect.When the corrosion defects were coupled with the corrosion and stress,the anode current density increased greatly,and the corrosion defect depth increased by about 10 times,indicating that the corrosion rate of the corrosion defects in the crevice interior was greatly accelerated under the stress.