| Seawater pumped storage power is a new type of pumped storage power generation to ensure the economic,reliable and safe operation of power system.It can also improve the economic benefits of power industry.The construction of seawater pumped storage power station has broad development prospects.Station use seawater directly as the medium and the operation conditions of station are harsh.Therefore,metal structures will suffer from severe normal corrosion and localized corrosion in high-pressure flowing seawater.Metal materials have different corrosion resistance in seawater.And localized corrosion is the main form of corrosion that affects the strength and service life of metal materials.Pitting corrosion,crevice corrosion,stress corrosion,corrosion fatigue,and so on will accelerate the early damage and failure of metal,which will even cause major accidents and affect the stable operation of power station.Materials failure caused by localized corrosion increases the engineering cost and maintenance cost,leading to great economic losses.Therefore,it is of great significance to study the metal materials selection and anti-corrosion of metal structure in seawater pumped storage power station.Based on the above background,the corrosion laws and corrosion mechanisms of carbon steel,low alloy steel,stainless steel,copper alloy and titanium under simulated corrosion environments of station were studied in this paper.The main results and conclusions are as follows:（1）The corrosion behavior of carbon steel,low alloy steel and stainless steel in seawater was studied by electrochemical testing technology.Temperature affects the performance of the passive film of stainless steel.With the increase of temperature,the passive film appeared to be dissolved,and the performance of passive film decreased.However,the passive film formed on stainless steel had excellent corrosion resistance so that the corrosion rate of stainless steel was very low.The mechanism model of pitting corrosion of carbon steel and stainless steel under the influence of temperature in seawater was proposed.That is,the corrosion of metal is related to the state of surface oxide film（passive film）.When the temperature rises to a certain temperature,the surface oxide film becomes active and begins to dissolve.When the temperature continues to rise,the oxide film is in the state of local dissolution,and there is a tendency of pitting at this time.When the temperature rises to a certain value TC,the oxide film on the metal surface completely dissolves and disappears,and the metal is in a uniform corrosion state.The TC of stainless steel is much higher than that of carbon steel and low alloy steel.At room temperature,carbon steel is in active dissolution state.The oxide film on carbon steel surface is not completely dissolved,and there is a tendency of pitting corrosion at lower temperature.904L austenitic stainless steel and2205 duplex stainless steel have a wide passivation range in seawater.Potentiostatic polarization results showed that stainless steel can maintain a passivation state in a certain potential range.When the potential exceeded a certain value,the performance of passivation film will be greatly reduced.（2）The effect of hydrostatic pressure on Q235 carbon steel and 2205 duplex stainless steel was studied by electrochemical testing and SEM.With the increase of pressure,the corrosion potential of Q235 carbon steel in the whole immersion cycle shifted negatively.The anodic dissolution accelerated,but the corrosion current density of carbon steel had no significant difference.The pitting tendency of carbon steel was evaluated by calculating the value of Eb100-Ecorr.It was found that the pressure would affect the pitting behavior at the initial immersion stage.Potentiostatic polarization and SEM results showed that the pitting tendency of carbon steel increased with the increase of pressure.The results of cyclic polarization curves also showed that pitting corrosion occurred in the later stage of 4 MPa seawater immersion.The performance of passive film on 2205 duplex stainless steel decreased with the increase of pressure.（3）The corrosion behaviors of carbon steel,low alloy steel and stainless steel in simulated power station with hydrostatic pressure of 4MPa were studied by electrochemical test,SEM,XRD,EDS,XPS,FTIR and Raman spectroscopy.The corrosion product film of Q235 carbon steel,X70 pipeline steel and Q345 low alloy played a certain protective role on metal.Cracks appeared on the surface of rust layer later and pitting corrosion occurred at the interface of the metal and corrosion products.The results of cyclic polarization curve also show that pitting corrosion occurred on the surface of carbon steel.The corrosion behavior of stainless steel in high pressure environment was related to the performance of passivation film.The performance of passivation film was depended on the formation rate and dissolution rate of passivation film.With the increase of soaking time,the passivation film performance of 904L stainless steel increased slightly and it was basically stable.The passivation film thickness and performance of 2205 duplex stainless steel increased with the increase of pressure action time.In the later stage,the passivation film was in dissolution state and its performance was reduced.In short,stainless steel was still in passivation state,having no pits.（4）The corrosion behavior of carbon steel,low alloy steel and stainless steel in the flowing environment of simulated power station hydrostatic pressure of 4MPa was studied by electrochemical testing,SEM,XRD,EDS,XPS,FTIR and Raman spectroscopy.The results showed that the corrosion rate of carbon steel or low alloy steel is aggravated in the flowing seawater.The flowing seawater had physical stripping effect on the corrosion products formed in the early stage.The anodic dissolution process of the continuous fresh metal surface accelerated and the corrosion rate increased.In the early stage of immersion,pitting corrosion was induced at the interface of metal and corrosion product.Moreover,the rust layer cracks occurred more quickly in the flowing seawater,which also accelerated the occurrence of pitting corrosion.The result of cyclic polarization curve confirmed that pitting corrosion occurred earlier.Under the flowing environment,more intensive pitting pits appeared on the surface of the metal substrate,and the local corrosion was more serious.The flow velocity also affected the formation of passive film on the stainless steel surface.The flowing seawater has physical scouring and thinning effect on the surface of passive film.Compared with the static environment,the performance of passive film decreased.Although the flowing seawater can promote the formation of passive film at a certain flow rate,the passive film of stainless steel was in a state of continuous dissolution at this experimental flow rate.And the performance of passive film decreases with the increase of water flow action time.（5）The corrosion behavior of copper alloy and pure titanium at simulated heat exchanger temperature was studied by electrochemical test,SEM and XRD.The corrosion current density of copper or copper alloy at simulated heat exchanger temperature was not high.Local corrosion of copper and tin brass occurred at simulated heat exchanger temperature.And dezincification corrosion of tin brass occurred.The results showed that the corrosion of Cu-Ni alloy was mainly uniform at the simulated heat exchanger temperature.The oxide film structure will become loose with the prolongation of immersion time.Cl-affected the performance of the oxide film on the surface of Cu-Ni alloy,which makes it turn into the weak protective green in the later stage.Titanium TA2 has excellent passivation performance and no corrosion at simulated heat exchanger temperature.（6）In this paper,the corrosion behavior of alternative metal materials under simulated operation conditions of seawater pumped storage power station is studied.And according to the previous literature research,the idea of material selection for metal structure of seawater pumped storage power station is provided.X70 pipeline steel is recommended for the water delivery pressure pipeline of the power station.TA2is recommended for the heat exchanger of the seawater cooling system.Q235 carbon steel is recommended for the gate leaf of the inlet/outlet gate.Stainless steel or copper alloy is recommended for the trash rack.Reinforced concrete is recommended for the tailrace tunnel of the power station.Stainless steel is recommended for the small seawater pipeline,and duplex stainless steel is recommended for the structures in harsh environment. |
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