FEM Modeling And Simulation Of Electrochemical Corrosion System Of Typical Steel Materials

Metal corrosion in the electrolyte is an electrochemical process involving multi-phase and multi-factor coupling.The secondary effect of the process affects the kinetic process of corrosion in real-time,making the study of electrochemical corrosion systems extremely complicated.In recent decades,researchers have gradually applied mathematical modeling and numerical simulation methods to the field of metal electrochemical corrosion.To simulate the final corrosion evolution law and predict the corrosion rate,this deterministic numerical model starts from the basic corrosion mechanism and uses kinetic equations and thermodynamic parameters to establish a corrosion model.To deeply study the mechanism of different influencing factors of the metal electrochemical corrosion system and reveal the coupling mechanism of the corrosion kinetic process under different factors,this work selected the two most common electrochemical corrosion systems for finite element modeling simulation and experimental research,that is,atmospheric corrosion of carbon steel and pitting corrosion of stainless steel.The former is the most common steel corrosion type in the most widely used environment.The latter is a representative of localized corrosion caused by microstructural heterogeneity in passive metals.By discussing and solving the commonality of the two corrosion types(multi-phase,multi-kinetic process,corrosion interface movement,and the influence of corrosion product deposition)and their uniqueness(electrolyte film in carbon steel atmospheric corrosion,corrosion product density,inhomogeneity of droplet environment and the heterogeneity of microstructure in stainless steel pitting corrosion),combined with the experimental method,the finite element model of carbon steel atmospheric corrosion under thin electrolyte film and droplet and MnS-induced stainless steel pitting corrosion in brine solution was established,not only analyzed the dynamic process of the two corrosion systems and the coupling mechanism of each influencing factor but also provided quantitative explanations for a large number of empirical laws.The main research content and contributions are as follows:(1)Based on the electrochemical principles of carbon steel corrosion under thin electrolyte film and the material’s electrochemical data obtained from experiments,a finite element model was established to study atmospheric corrosion of Q345 carbon steel under thin electrolyte film,taking into account factors such as the dynamic deposition process of corrosion products,oxygen transport at the gasliquid interface,and the instantaneous change in oxygen reduction limit diffusion current density.The model included the influence of corrosion products and electrolyte film thickness.The quantitative relationship between the corrosion rate of carbon steel under thin electrolyte film,corrosion product deposition,and film thickness was obtained through model calculation.The simulation also confirmed that the corrosion rate change of carbon steel under electrolyte film is closely related to the control steps of the corrosion process,namely oxygen diffusion and electrode surface discharge process.The generation of corrosion products not only impedes material transport but also impedes the electrode surface discharge process.The model also tracked the changes in the proportion of the two during the corrosion development process.Finally,the simulation results are verified by thin electrolyte film corrosion experiments on carbon steel.The model provided necessary supplements to the coupling mechanism of thin electrolyte film thickness and corrosion product porosity on atmospheric corrosion of carbon steel and also provided a reference for establishing an atmospheric corrosion prediction model.(2)Based on the electrolyte film corrosion model and the Evans droplet corrosion experimental mechanism,a finite element model for atmospheric corrosion of carbon steel under droplets was developed by considering the effect of droplet shape on oxygen diffusion,the movement of the corrosion interface,and the dynamic deposition process of different corrosion product densities.The model reproduces the kinetic process of carbon steel droplet corrosion,reveals the mechanism of local cathode and anode formation in droplet corrosion,and provides a quantitative explanation for the classic Evans droplet experiment.The simulation results show that the looser corrosion products and lower pH value in the central region lead to higher current density in the dissolution of carbon steel,resulting in the formation of local anodes in the center region of the droplet and local cathodes in the edge region.With the development of corrosion,the boundary point of cathode and anode moves to the central area.When the corrosion product is in medium porosity,the local corrosion effect of droplets is the strongest.In addition,the difference in composition of droplet corrosion products and the inhomogeneity of porosity distribution is confirmed by experimental means.(3)A comprehensive finite element dynamic model for stainless steel pitting corrosion was established to address the issue of pitting corrosion caused by MnS inclusions in 304 stainless steel.The model takes into account the local electrochemical behavior of the three phases in the metal electrode(stainless steel matrix,passive film,and MnS inclusions),substance transport and homogeneous reactions in the liquid phase,and the deposition of corrosion products on the electrode surface.By tracking the movement of the corrosion boundary,the propagation path of trench corrosion was simulated.The results show that for the small-sized MnS inclusion phase(such as radius 1 μm or less),due to the microgalvanic effect between the inclusion phase and the cathode passive film,MnS is preferentially corroded as the anode.Then the matrix and MnS dissolved together,and finally,the small MnS particles dissolved entirely and disappeared,leaving only open pits and repassivation occurred.However,when the radius of MnS is above 3～5 μm and the area ratio of cathode and anode is slightly smaller(less than 10:1),as the corrosion progresses,the cathode-anode area ratio evolves.The mixed potential continues to move negatively beyond the self-corrosion potential of MnS The polarity reversal of MnS in the three-phase micro galvanic corrosion system,from the anode phase to the cathode phase,further promotes the active dissolution of the surrounding matrix phase,forming a groove corrosion form.At the same time,the remaining part of the MnS inclusion phase this phenomenon is highly consistent with the observed phenomenon in the experiment.(4)On the basis of the above model,by further changing the size of the MnS inclusion phase,the ratio of the cathode to anode area,the morphology of the inclusion phase and its embedded form on the surface,the mechanism of each factor on the process of MnS-induced micro galvanic corrosion was revealed.The research results show that when the area of the passive area and MnS is relatively small(less than 10:1),the matrix is more inclined to repassivation;when the area ratio of the two is large(more than 99:1)and the radius of MnS is less than 5 μm,the trench corrosion may develop into steady-state pitting corrosion However,when the area ratio is large(more than 99:1)and the radius of MnS is 10～15 μm or more,the corrosion potential is higher and the pH value of the solution in the pit is smaller,which greatly increases the probability of steady-state pitting corrosion.In addition,considering the accelerated dissolution effect of pH change in the pit on MnS and matrix phase,a steady-state pitting corrosion model of stainless steel induced by MnS inclusion phase was established,and the simulation results were consistent with the typical steady-state pitting pit morphology.