Corrosion Behavior Of 2205 Duplex Stainless Steel In Concentrated Seawater Under Vacuum Conditions

Due to the severe global water shortage,desalination techniques have been developed rapidly to produce freshwater from seawater.Type 2205 DSS has become one of the most important materials for the maior components of thermal desalination plants due to the excellent propreties.Owing to the high thermal efficiency and low costs,low temperature multi-effect desalination（LT-MED）is one of the most attractive desalination processes.As for the major component of LT-MED plant,evaporators work in a complex corrosion environment of high temperature（<75°C）,high Cl^-concentration,negative pressure,low dissolved oxygen（DO）and even boiling state in the operation processes.The corrosion problem of SS components is the key factor threatening the long-term safe operation of the plants.However,the corrosion behaviors of metallic materials remain unknown in the vacuum pressure environments as appeared in the LT-MED plants.Therefore,the pitting corrosion behavior of 2205 DSS in simulated environment of LT-MED evaporator was investigated mainly by electrochemical measurement techniques（e.g.,electrochemical impedance spectroscopy,polarization curves and potentiostatic polarization）and surface analysis methods（e.g.,electron scanning microscope,transmission electron microscope,X-ray photoelectron spectroscopy and X-ray diffraction）.The corrosion mechanism of 2205 DSS in the 72°C 2-times concentrated seawater under vacuum boiling condition was discussed.The pitting induction and propagation behaviors during the long-term immersion were monitored by the corrosion potential and electrochemical impedance spectroscopy,and the pitting mechanisms were analyzed for the vacuum boiling state and major alloying elements.At the same time,the beneficial effect of rare earth conversion coating treatment on corrosion resistance of stainless steel was preliminarily explored in view of the sensitization problem encountered in the manufacturing process of SS equipment.Negative pressure has a great influence on the stable pitting corrosion and repassivation behavior of 2205 DSS in the concentrated seawater.The dissolved oxygen concentration(C_DO)decreases linearly with the pressure in the degassing process.The solution will enter a boiling state under the vacuum pressure lower than about 30.4 k Pa.Due to the decrement of C_DO,the pressure reduction noticeably decelerates the cathodic reaction process,and the dominant ingredients may change from the oxides Fe₂O₃ and Cr₂O₃,hydroxide Cr（OH）₃ and oxyhydroxide Fe OOH to the last two species in the passive films.Consequently,the protectiveness and polarization resistance of passive film decrease and the pitting potential shifts negatively.When the solution turns into violent boiling state under very low pressures,the mechanical effects of numerous bubble collapse make the passive film much less protective with more defects and incorporated Cl^-ions.This leads to significant negative shift of pitting potential.At the same time,the violent boiling markedly accelerates the mass transport processes of dissolved oxygen in the proximity of electrode surface and the metallic cations produced by anodic dissolution in the pits.These cause the disappearance of oxygen-limiting diffusion in the cathodic polarization process and favor the pit repassivation.During the anodic polarization,the 2205 DSS specimens show a higher occurrence rate of metastable pits under violent boiling condition in the concentrated seawater.The metastable pits mainly initiate at theγ/αboundraies or/and in theα-phase,but rarely inγ-phase.Both the reduction of pressure and increase of applied anodic potential noticeably accelerate the pitting initiation rate.In the pressure range without boiling,the reduction of vacuum pressure has no obvious influence on the growth of stable pits.However,when the vacuum pressure is very low,the boiling of solution will retard the stable pitting propagation,and the stable pits show relatively smooth bottom without secondary pit due to the stirring effect of boiling bubbles.The corrosion evolution processes of 2205 DSS in the concentrated seawater under intermittent vacuum boiling and atmosphere conditios presents can be roughly divided into pitting induction stage,pitting transition stage and stable pitting stage.The presence of vacuum boiling results in lower corrosion potential,and the passive films on the specimen surface are thinner and more inhomogeneous with lower polarization resistance（R_p）values due to the effect of low C_DO and bubble collapse.Therefore,the stable pitting corrosion initiates much earlier under intermittent vacuum boiling condition than the atmosphere condition.The electrochemical impedance spectrum（EIS）results show that the passive films behave as capacitive protection layers and the specimens have high R_p values before the initiation of stable pitting corrosion under both conditions.Compared to the atmosphere condition,the time constants are separated clearly under the intermittent vacuum boiling condition,rather than overlapped together in the low frequencies during the stable pitting stage.The passive film resistance at vacuum boiling state is lower than that at the air state,but the transfer process resistance is just the opposite.The stable pitting corrosion of2205 DSS under the two conditions are controlled by the charge transfer processes.After 13360 h of immersion,the deepest pits are about 41 and 6μm under atmosphere and intermittent vacuum boiling conditions,respectively.The vacuum boiling may retard the pitting corrosion mainly through inhibiting the cathodic reaction process and occlusion cell effect.2205 DSS,316L SS and 446 SS display excellent pitting resistance in the concentrated seawater under ontinuous vacuum boiling condition,with the stable pitting induction time of about 2040,1800 and 4460 h respectively.After about 6800hours of immersion,only very shallow corrosion pits were formed on the specimen surfaces,but the corrosion extent of the 316L SS sample was relatively the most serious,while the 446 SS was the least.This may be attributed to the fact that 2205DSS and 446 SS contain more Cr and Mo elements than 316LSS.Dense and uniform Ce chemical conversion layer mainly composed of Ce O₂forms on the sensitized 2205 DSS surfaces in a mixed solution of citric acid,hydrogen peroxide and cerium nitrate solution.This chemical treatment results in an obvious increase of corrosion potential and pitting potential of sensitized 2205 DSS.This indicates that the Ce conversion layer can significantly improve the pitting resisitance of sensitized 2205 DSS in the simulated LT-MED environment.