Corrosion Behavior of 2507 Duplex Stainless Steel in Acidic and High Chlorine Environments
Thermal evaporation technology is widely used in wastewater treatment.With the continuous evaporation and concentration of wastewater,the Cl-content in the solution increases exponentially,resulting in rapid corrosion and failure of the materials used for evaporation equipment.2507 duplex stainless steel is often utilized due to its high content of Cr and Mo elements.This material combines the advantages of both ferrite and austenitic stainless steels,resulting in excellent resistance to pitting corrosion.To investigate the corrosion of 2507 duplex stainless steel in acidic and highly chlorinated environments,dynamic potential polarization,electrochemical impedance,Mott-Schottky curves,and dynamic immersion experiments were used.The electrochemical tests were performed in an acidic solution(pH 3)containing fluorine(F-300 mg/L)and varying concentrations of Cl-(20,60,90,120,150 g/L)at a temperature of 60 ℃.The stainless steel was subjected to an immersion test in a corrosive solution with 120 g/L of Cl-,300 mg/L of F-,and pH 3 at 80 ℃ for 35 d.Dynamic stirring was employed at a linear velocity of 2 m/s,and the solution was renewed every 7 d.Three parallel samples were taken,and the properties of the specimens were characterized using metallographic microscopy,scanning electron microscopy(SEM),energy spectrometry(EDS),and 3D microscopy.The study investigated the corrosive effect of chloride ion concentration on 2507 duplex stainless steel through electrochemical tests,and immersion corrosion tests were conducted to examine the corrosion conditions of stainless steel in a highly chlorinated environment.The results indicate that in various electrochemical tests,the corrosion potential Ecorr in the polarization curve shifts towards the negative direction,and the corrosion current density Icorr gradually increases with the rise in chloride ion solubility.Thermodynamically,the more negative the corrosion potential Ecorr,the higher the tendency of the material to corrode;kinetically,the larger the corrosion current density Icorr,the poorer the corrosion resistance of the material.The higher the pitting potential Eb,the stronger the corrosion resistance of the material.The test results indicate that an increase in the chloride ion concentration accelerates the corrosion of the material.In addition,the impedance spectrum shows a decrease in the capacitive arc radius as the concentration of chloride ions increases.A larger capacitive arc radius indicates greater resistance to charge transfer at the interface between the metal and solution,which in turn suggests stronger corrosion resistance of the material.The polarization resistance Rp reflects the state of the surface passivation film and is crucial in evaluating the corrosion resistance.The Rp value decreases as the concentration of chloride ions increases.The Mott-Schottky curve shows an increase in both donor density ND and acceptor density NA with an increase in Cl-concentration,indicating an increase in point defects within the passivation film and a reduction in the corrosion resistance of the material.The metallographic analysis of stainless steel after 35 d of immersion in the corrosive solution revealed that pitting corrosion primarily occurs in the boundary region of the α-phase and γ-phase or in the y-phase zone.This suggests that in the duplex stainless steel,the α-phase is more corrosion-resistant than the γ-phase.The average corrosion rate of the material was 2.51 μm/a,as measured by the weightlessness method.The SEM morphology revealed a small number of etching holes on the surface of the specimen,with an outer diameter of approximately 70-100 μm.The average pitting depth of the alloy after corrosion was 20.493 μm,and the maximum pitting rate was 0.275 mm/a as measured by 3D microscope.These results indicate mild corrosion and reflect the good corrosion resistance of stainless steel in acidic and highly chlorinated environments.Due to the lack of corrosion data on the materials used in perchlorinated wastewater evaporation equipment,the corrosion of 2507 duplex stainless steel can provide data support for material selection.
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