Effects Of Solidification Cooling Rate And Mo/Si Content On Electrochemical Properties Of 2507 Super Duplex Stainless Steel

There are two main sources of Cl^-in the coal chemical equipment system:First,the raw coal contains a large amount of Cl element,which will eventually enter the water and becomes Cl^-;Second,the water used by the coal chemical industry contains a certain amount of Cl^-.The equipment and pipelines are prone to corrosion when the Cl^-content of the system is too high,so it is necessary to conduct in-depth research on the corrosion problem in the special environment of coal chemical industry.2507 super duplex stainless steel has ultra-low carbon,high nickel,molybdenum and nitrogen and other elements,which has higher strength,plastic toughness,and excellent resistance to Cl^-corrosion,stress corrosion and erosion corrosion than ordinary duplex stainless steel.At the same time,2507 super duplex stainless steel also has the characteristics of good welding performance,easy processing and obvious cost advantages.Its application area is continuously expanding,especially suitable for various harsh corrosive environments（such as circulating cooling water system containing Cl^-,wastewater storage emission system,etc.）In the actual production process of 2507 super duplex stainless steel,the solidification and cooling rates of the slabs are different,which will have a certain impact on the two comparison examples.And its high alloying characteristics and the different diffusion rates of the alloying elements in the two phases will cause 2507super duplex stainless steel to produce Cr-rich and Mo-rich intermetallic precipitation phases during cooling,causing Cr-depleted and Mo-depleted local areas have an impact on the mechanical properties and corrosion resistance of the material.In this paper,2507 super duplex stainless steel with different Mo and Si contents was selected as the research object in different parts of the continuous casting slab.With the help of metallographic microscope,scanning electron microscope,ferrite instrument and other equipment,the solidification and cooling of the slab was studied and analyzed effects of changes in velocity and Mo and Si content on the two-phase microstructure and evolution of precipitates in 2507 super duplex stainless steel.Using potentiostatic polarization method and AC impedance spectroscopy analysis method combined with metallographic observation to mastered the effects of different solidification cooling rates and different Mo and Si element contents on the electrochemical corrosion behavior and corrosion resistance of 2507 super duplex stainless steel.The main results obtained are as follows:（1）Through Thermo-Calc and JMat-Pro thermodynamic software,the simulated calculation predicted that the increase of Mo and Si elements would increase the initial precipitation temperature and the maximum precipitation amount of the phase;When the solidification and cooling rate was greater than 100℃/min or equal to 100℃/min,no precipitation phase was generated in the tissues.When the solidification rate is less than 100℃/min,σphase and CHI phase occur in the tissues.（2）The observation of microstructure showed that with the increase of solidification and cooling rate,the content of precipitated phase in the microstructure decreased.Increasing the contents of Mo and Si elements will promote the precipitation of phase,and the change of Mo and Si elements does not change the effect of cooling rate on the precipitation of phase.（3）In 2507 super duplex stainless steel,theσphase first precipitates along the grain boundaries of ferriteαand austeniteγin a point shape,and grows toward the interior of ferriteα.Through the analysis of the enriched elements of each phase in the structure and the change of the comparative example,it was found that the precipitation of theσphase was performed according to theα→σ+γ₂ eutectoid reaction method.（4）As the solidification cooling rate increases,the content of theσphase in the structure decreases,and the uniformity of the microstructure becomes better.The results of the kinetic potential polarization curve show that as the solidification cooling rate increases,the pitting potential E_b of the sample increases,and the self-corrosion current density I_corr decreases.As the spectral radian increases,the log∣Z∣endpoint of the Bode plot also increases.That is,as the solidification cooling rate increases,the corrosion resistance of the sample increases.（5）When the solidification cooling rate and the Si content in the structure are the same,as the Mo content is increased from 3.4%to 3.7%,the passivation interval of the dynamic potential polarization curve becomes wider,the passivation ability of the sample is enhanced,and the pitting potential E_b As the value increases,the self-corrosion current density I_corr decreases;the radian of the AC impedance spectrum increases,and the log∣Z∣end value of the Bode diagram also increases.That is,the Mo content in the structure increases,and the corrosion resistance of the sample increases.（6）With the same solidification cooling rate and the same Mo content in the structure,as the Si content increases from 0.30%to 0.46%,the polarization curve of the sample shifts to the left,the blunt current density increases,and the pitting potential E_b increases.The corrosion current density I_corr decreases;the arc of the AC impedance spectrum increases,and the log∣Z∣endpoint of the Bode diagram also increases.That is,the Si content in the structure increases,and the corrosion resistance of the sample is enhanced.