| Duplex stainless steels(DSS)are used in petroleum,chemical and nuclear energy fields due to their excellent overall mechanical properties and corrosion resistance in both ferrite and austenite duplex microstructures.The ferrite in DSS plays an important role in improving the strength,reducing the hot cracking resistance,and improving the stress corrosion resistance and strength.However,its existence will also bring problems such as low-temperature embrittlement(heat aging),medium-high temperature sensitization(?phase precipitation)to DSS,and its performance will be greatly reduced.In the service environment of nuclear power pipelines,DSS is exposed to high temperature and high pressure and contains corrosive media for a long period of time.Inevitably,thermal aging embrittlement occurs.Thermal aging,thermal fatigue and corrosion are the three main methods for failure of stainless steel pipelines,and pitting corrosion is the most important form of damage in corrosion failure except for stress corrosion cracking,although there have been many studies on heat aging mechanisms in domestic and overseas,some useful results have been obtained,but there are few public reports on the effects of thermal aging on the pitting properties of DSS.In this paper,the heat aging treatment of Z3CN20.09 M,SAF2205 and SAF2507 three kinds of DSS was carried out by accelerating thermal aging.Characterized by techniques such as optical microscopy(OM),transmission electron microscopy(TEM),electrochemical potentiometric(PC),and electrochemical impedance spectroscopy(EIS).The effects of thermal aging time(400°C,0-10000h)and heat aging temperature(350°C,450°C,500°C,5500h)on microstructure and pitting corrosion performance of DSS were studied and the mechanism was analyzed.The main results are as follows:1)The OM results showed that the ferrite contents of the three kinds of DSS did not change significantly after heat aging treatment with different parameters,indicating that the heat aging temperature and time had no effect on the two phases ratio and morphology of the DSS.Thermodynamically,the temperature of DSS is in the range of 350°C-500°C.Only ferrite undergoes spinodal decomposition and G phase precipitation,and no other phase transition,while spinodal decomposition and G phase precipitation do not cause changes in ferrite morphology and quantity.2)The potentiodynamic polarization curve test results show that the pitting corrosion potential of the Z3CN20.09 M stainless steel in the 0.5 mol/L NaCl solution and the 0.5 mol/LNaCl+nuclear primary circuit simulation solution gradually decreases as the heat aging time increases,and the passivation region becomes narrower.The results of AC impedance spectroscopy show that as the thermal aging time is extended,the radius of the impedance arc of the sample becomes smaller,indicating that the charge transfer resistance of the passivation film decreases,the corrosion accelerates,and the pitting corrosion resistance decreases.3)With the increase of heat aging temperature,the pitting corrosion potential of Z3CN20.09 M,2205 duplex stainless steel gradually decreased,and the lowest at 500?;while the pitting corrosion potential of 2507 duplex stainless steel first decreases and then slightly increases,with the lowest at 450? and slightly higher than 450? at 500?.The results of the AC impedance spectrum were consistent with the polarization curves.This shows that the corrosion resistance of Z3CN20.09 M,2205 duplex stainless steel decreases with increasing temperature between 350? and 500?,while the corrosion resistance of 2507 duplex stainless steel first decreases and then increases slightly.4)When aged at 350 °C,Z3CN20.09 M stainless steel had the lowest pitting potential,smallest impedance arc radius,lowest charge transfer resistance and worst pitting performance,while 2507 DSS had the best pitting corrosion performance.when aged at 450°C and 500°C,2507 DSS pitting performance is best,Z3CN20.09 M stainless steel is the worst.This is because the 2507 DSS has the highest Cr content and has a higher ferrite content than Z3CN20.09 M stainless steel.The aging at 450°C and 500°C is the most severe,with the most precipitation of?' phase.The aging treatment at 350°C is 5500 hours,thermal aging does not occur significantly and is insufficient to cause pitting performance degradation.The content of Cr and Mo in 2507 stainless steel is more,so the best corrosion resistance,Z3CN20.09 M stainless steel worst.It shows that the pitting corrosion performance of a material is closely related to its alloy composition after heat aging treatment at the same temperature and time. |
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