Corrosion Behavior And Mechanism Of High-strength Ferritic Weathering Steel In Acidic Severe Environment

With the implementation of China’s "The Belt and Road" development strategy,coupled with energy conservation and emission reduction policies,the domain of high-speed railway transportation has witnessed an era of rapid development.Presently,the traditional railway gondola car body steel confronts certain drawbacks such as low strength,limited corrosion resistance,and short service life,rendering it incapable of meeting the performance requisites of steel in intricate and harsh atmospheric environments.Consequently,it is imperative to develop a novel type of weathering steel that possesses outstanding comprehensive properties,such as ease of welding,high strength,and remarkable corrosion resistance,which can be employed in an adverse atmospheric corrosion environment.This research paper lays emphasis on investigating the atmospheric corrosion resistance of railway car body steel in the harsh acidic surroundings of the coastal industry.The service life of low-alloy high-strength weathering steel for railway freight cars is enhanced through the design of microalloying elements and microstructure regulation.In order to address the issues of inclusion-induced corrosion and rust layer growthshedding behavior of weathering steel in the coastal industrial atmospheric environment,the micro-mechanism of inclusion-induced corrosion in the harsh medium environment is studied by means of simulation calculation and experimentation.The mechanism of inclusion modification to significantly reduce pitting sensitivity is elucidated.Furthermore,the distribution and structure of rust layer elements are scrutinized at the nanoscale,and the mechanism of rust layer hindering chloride ion and the formation of a layered structure are clarified.The primary research contents and results are summarized as follows:The controlled rolling and controlled cooling process was determined based on the phase transformation law and thermodynamic calculation,resulting in the preparation of low-carbon,high-strength ferritic weathering steel.Additionally.high-strength ferritic weathering steel containing nano-precipitates was created through austenite recrystallization zone rolling and isothermal processing.The steel possessed a yield strength,tensile strength,and elongation of 635 MPa,750 MPa,and 21.2%,respectively.The microstructure after high-temperature final rolling and controlled cooling consisted of polygonal ferrite,with uniformly dispersed nanoprecipitates on the ferrite matrix.However,the deformation process of the austenite non-recrystallization zone did not occur during the rolling process.Instead,the deformed austenite grains underwent deformation-induced phase transition at the grain boundary,and deformed in the austenite and ferrite two-phase regions to form fine ferrite grains.The deformation of the austenite non-recrystallization zone facilitated strain-induced precipitation.Before the deformation of the two-phase zone,many precipitates were present in the austenite structure.Rolling in the twophase zone led to the promotion of precipitation ripening.The addition of rare earth element Ce enhanced the size of nano-precipitates,and the non-recrystallization zone was hot-rolled.The precipitate size of the steel without rare earth element was about 10 nm,while the Ce element was 0.11%,and the precipitate size gradually increased to approximately 40 nm.A pitting corrosion model induced by chemical corrosion of inclusions was developed,the mechanism of inclusion-induced pitting corrosion was determined,and the micro-mechanism of alloying to reduce pitting sensitivity was proposed.By comparing the pitting corrosion induced by inclusions in Q345B and high-strength ferritic weathering steel,found the high-density lattice distortion in the matrix around Al2O3-MnS inclusions was the key to induce corrosion.The lattice distortion of inclusions after Ti-Ce synergistic modification was low,which improved the chemical stability of the adjacent matrix.However,there was a contradiction between the pitting behavior caused by inclusions and the micro-galvanic effect.The volt potential of CeAlO3-Ce2O2S was significantly higher than that of the surrounding matrix by about 30 mV,but in the immersion test,it showed a higher propensity to dissolve than the matrix.Furthermore,due to the modification of rare earth element Ce,the size of inclusions decreased,and there was no evident lattice distortion zone around Ce2O2S inclusions.Studies have demonstrated that the interaction between the inherent physical and chemical properties of different inclusions and the erosion factors in the environment is the key factor causing pitting corrosion of steel.TiN in Ti-containing ferritic weathering steel possesses excellent pitting corrosion resistance and can be used to modify inclusions.It has been verified that Ti-Ce synergistic addition can precisely modify the non-metallic inclusions in high-strength ferritic weathering steel and improve the pitting corrosion resistance of the material.The inclusion and pitting corrosion data statistics of the surface samples before and after corrosion revealed that the probability of pitting corrosion induced by inclusions in Q345B was 36.4%,while that in Ti-containing steel was 15.2%and in Ti-Ce steel was 9.81%.The mechanism of corrosion resistance of the rust layer in the coastal industrial atmospheric environment has been clarified.The quantitative effects of element concentration,structural defects,and internal structural differences in the rust layer on chloride ion penetration have been studied.The findings demonstrate that the corrosion resistance of the new ferritic weathering steel in the coastal industrial atmospheric environment is superior to that of the weathering steel Q450NQR1 used in the active car body.After 120 hours of corrosion,the corrosion rate is less than 40%compared with Q345B.The final stable rust layer of the new ferritic weathering steel in the simulated coastal industrial atmospheric environment is composed of an outer rust layer primarily composed of γ-FeOOH,an intermediate layer composed of amorphous-nanocrystalline,and an inner rust layer composed of layered amorphous material and amorphous-nanocrystalline mixed structure.The layered structure of the inner rust layer comprises an amorphous structure of Cr/Tirich region,with the Cr element concentration generally above 6.0%.The amorphous region of the Fe-rich and Cr-poor region is mixed with nanocrystals,and the Cr element is below 0.41%.The amorphous substance of the inner rust layer of the corrosion product film impedes Cl-significantly,thereby playing a pivotal role in enhancing the corrosion resistance of steel in chloride ion harsh environments.While the amorphous/nanocrystalline interface of the interlayer and the vacancies at the crack provide a transport path for Cl-,the complete interlayer can effectively hinder HSO3-and SO42-.The effect of the rare earth element Ce on the corrosion resistance of steel fluctuates significantly.The corrosion rate of Cecontaining steel is higher in the early stage,but the corrosion rate decreases after the formation of a stable rust layer in the later stage.The 3D detection of the surface shows that the Ce element can reduce pitting damage.It has been proven through first-principles calculations that the inclusion of Ti can effectively prevent the penetration of corrosive Cl-into the ferritic weathering steel matrix.The study also analyzed the mechanism of the layered rust layer structure on crack propagation and rust layer spalling-regeneration behavior.The research found that the crack propagation in the inner rust layer of the new ferritic weathering steel mostly occurs at the interface of the layered structure,while the cracks in Q345B rust layer mainly propagate along the rust layer/matrix interface,which increases the risk of rust layer falling off.In the case of ferritic weathering steel,the inner rust layer of the layered structure will remain on the surface of the substrate after the outer rust layer falls off,which accelerates the formation and stability of the regenerated rust layer.Micro-alloying elements such as Cr,Ti,and other elements can also promote the formation of the layered structure of the regenerated rust layer,with rare earth elements showing a particularly strong promotion effect.After the mechanically peeled rust layer was re-corroded for 10 days.the surface height difference of the regenerated rust layer was 44 μm for Ti-containing steel sample and 19 μm for Ti-Ce steel sample,while the surface drop of Q345B was 125 μm,indicating that the inclusion of micro-alloying elements can improve the integrity of the rust layer,reduce local corrosion thickening,and minimize the discontinuous cracks and holes in the rust layer.