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Electrochemical Crevice Corrosion Behaviors of Low-Pressure Steam TurbineMaterials in the Simulated Boiler Water Added Chloride and Sulfate Ions
Li-Bin Niu1,+1, Hodaka Kato1,+2, Kunio Shiokawa2, Kenji Nakamura3,Mitsuo Yamashita3 and Yoshihiro Sakai3
1Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan2Fuji Electric Co., Ltd., Hino 191-8502, Japan3Fuji Electric Co., Ltd., Kawasaki 210-9530, Japan
Using a rotor material, 3.5NiCrMoV steel, and a blade material, 13Cr steel, for low-pressure (LP) steam turbines of thermal power plants,electrochemical crevice corrosion tests were conducted in the simulated AVT (All Volatile Treatment) boiler water added chloride and sulfateions. The crevice corrosion behaviors as well as the films formed on the specimen surfaces inside crevices were investigated. The 3.5NiCrMoVsteel and the 13Cr steel in the test water showed crevice corrosions in a general type and a pitting type, respectively. For both the two steels,however, passive films formed on the specimen surfaces inside crevices. It was found that the passive film formed on 3.5NiCrMoV steel wascomposed mainly of Fe3O4, while that on 13Cr steel was composed mainly of Cr-oxides and partly of inward Fe-oxides. Especially, it wasconfirmed that CrOOH and CrO3 were concentrated in the outermost surface of the passive film formed on 13Cr steel.[doi:10.2320/matertrans.M2013202]
(Received May 30, 2013; Accepted September 25, 2013; Published November 9, 2013)
Keywords: steam turbine material, crevice corrosion, boiler water, passive film, repassivation, potentiostatic polarization
1. Introduction
Since the serious accident of the Fukushima Dai-ichinuclear power plant due to the Great East Japan Earthquakeoccurred on March 11, 2011, the movement of de-nuclearpower generation has been growing. In such a situation, theoperating conditions in thermal power plants become moresevere, the reliability and safety of the equipment materialsbecome important issues of concern. In thermal power plants,typical impurity ions such as chloride ions (Cl¹), sulfate ions(SO4
2¹) and sodium ions (Na+) present in steam are usuallycarried over into the steam turbine with the steam. These ionsconcentrate and accumulate inside crevice regions of the low-pressure (LP) steam turbine in the phase transition zone(PTZ) due to the alternative dry and wet phenomenon and thecrevice effect.13) Thereby, the LP steam turbine materialsparticularly inside the crevices will be subjected to pittingcorrosion, stress corrosion cracking (SCC), corrosion fatiguecracking and so on.49) Therefore, to inhibit these corrosionsinside crevices of the materials and to clarify the corrosionmechanisms become important issues in prolonging the lifeof the steam turbine. Effects of the impurity ions (Cl¹, SO4
2¹
and Na+) in steam on the corrosions of turbine materialshave been widely investigated up to now.1017) As one of theresults, it has been made clear that Na+ has little effect onthe corrosions of turbine materials, while Cl¹ and SO4
2¹ havesignificant impacts on the corrosions.1214,17) However, thereare still many unknown things about the crevice corrosionbehaviors of the steam turbine materials in the actualenvironment with mixed corrosive chemical species. Thedetailed knowledge and data on the characteristics of films/oxides formed particularly on the material surfaces insidecrevices have not been obtained sufficiently.
To evaluate the ease of the growth of crevice corrosion,as an electrochemical method the repassivation potential forcrevice corrosion (ER.CREV) measurement has been used.1820)
ER.CREV is a critical potential value of growing crevicecorrosion, the crevice corrosion will progress above theER.CREV, if not it will be repassivated or stop. The methodof determining the ER.CREV of stainless steels has beenstandardized as JIS G 0592.20) In the ER.CREV measurement,the crevice corrosion should be grown as the maximumpenetration depth inside crevice becomes deeper than acritical depth of the growing crevice corrosion. On the otherhand, if the maximum penetration depth does not exceedthe critical depth, as for repassivating crevice corrosion thecritical potential value is the breakdown potential of passivefilm (EZ.CREV).18,21,22) In this work, the crevice corrosionbehaviors of typical LP steam turbine materials wereevaluated mainly by the EZ.CREV measurements and thepotentiostatic polarizations beyond EZ.CREV in the simulatedboiler water added chloride and sulfate ions. Furthermore,characteristics of the films formed and the corrosion aspectsin the crevices were analyzed in detail.
2. Experimental Procedure
2.1 Material and specimenThe materials used in this work are 3.5NiCrMoV and 13Cr
steels, which have being used as rotor and blade materialsin LP steam turbines of thermal power plants respectively.The chemical compositions of the steels are listed in Table 1.Two types of large and small plate with sizes of 40mm ©15mm © 2mm and 20mm © 15mm © 2mm were cut fromvirgin steam turbine materials. The plate surfaces wereemery-polished up to #800 and then degreased with acetone.As an initial study, to obtain the basic data on crevicecorrosion behaviors of the two materials the crevicedspecimens of the same steel were used in the present work,even though in the actual steam turbine the crevices are
+1Corresponding author, E-mail: [email protected]+2Graduate Student, Shinshu University. Present address: KobelcoResearch Institute, Inc., Kobe 651-0073, Japan
Materials Transactions, Vol. 54, No. 12 (2013) pp. 2225 to 2232©2013 The Japan Institute of Metals and Materials
formed between the rotor and the blade. As shown in Fig. 1,the creviced specimen was assembled with one plate of eachsize in the same steel, and was tighten with the bolt, nut andwasher made of Teflon.
2.2 Test waterIn thermal power plants various water treatments such
as AVT (All Volatile Treatment), NWT (Neutral WaterTreatment), CWT (Combined Water Treatment) and OT(Oxygen Treatment) are usually performed to preventcorrosions and to restrain scaling in the boiler equipment.For example, in Japan, the boiler equipments are generallyfilled with a kind of AVT water based on JIS B 8223.23,24) Inthis work, the simulated AVT water (pH 9.5, DO < 7µg/L)was used. Table 2 shows the quality of the test water. Thetest water was prepared from ion-exchanged pure water byinjecting 25% ammonia solution to adjust pH to 9.5, byadding 10mg/L hydrazine (N2H4) and continuously bubblingwith N2 gas into the water before and during test to removedissolved oxygen (DO). Moreover, referencing to the actualenvironment,1) 100mg/L Cl¹ and 50mg/L SO4
2¹ of thecorrosive chemical species were added to the test water. TheCl¹ and SO4
2¹ were added as powdered NaCl and Na2SO4,respectively. The temperature of the test water was 363K.
2.3 Electrochemical corrosion testThe following electrochemical crevice corrosion tests
were conducted on the two steels, using a potentiostat(Hokuto-Denko, HZ-3000) with three electrodes: the work-ing electrode of a creviced specimen, the counter electrodeof platinum and the reference electrode of the saturatedKClAg/AgCl. Figure 2 shows the schematic illustration ofthe electrochemical corrosion test apparatus.
Since both of 13Cr steel and 3.5NiCrMoV steel shouldpossibly be repassivated in the present test water with littleDO, the breakdown potentials of passive film (EZ.CREV) forrepassivating crevice corrosion were measured referencingto the method of determining the repassivation potential forcrevice corrosion of stainless steels.20) Figure 3 shows thetest procedure. The creviced specimen was anodicallypolarized at a rate of 20mV/min up to 200 µA of currentvalue (Step1), the current value was then kept for two hours(Step2). After that, the potential was lowered at a rate of10mV/min until the anode current reaches 50 µA (Step3). Atthe potential at which the current reached the 50 µA,
Table 1 Chemical composition of the steels used.
(mass%)
C Si Mn P S Cr Ni Mo V Cu
3.5NiCrMoV 0.24 0.06 0.3 0.009 0.011 1.8 3.88 0.45 0.12 0.06
13Cr 0.21 0.32 0.62 ® ® 13.36 0.46 ® ® ®
®: non data.
2
15
40
15
2 2 (mm)
20
Bolt, Nut, Washer(made of Teflon)
Fig. 1 Assemblage and geometry of a creviced specimen.
Table 2 Quality of the test water for electrochemical corrosion test.
AVT Water(All volatile treatment)
Cl¹
(mg/L)SO4
2¹
(mg/L)
Electricconductivity
(S/m)
Temperature(K)
DO < 7 µg/L*1100 50 0.047 363
pH 9.5 « 0.1*2
*1Controlled by N2H4 10mg/L addition and also continuously N2
bubbling.*2Adjusted with NH3 solution.
N2 Gas
Cooling towerPC recorder
Saltbridge
Test water
Potentiostat
Heater
(KCl-Ag/AgCl)Pt electrode Specimen Ref. electrode
Fig. 2 Schematic illustration of the electrochemical corrosion testapparatus.
Pote
ntia
lC
urre
nt
Time
200 µA
20 mV/min
10 mV/min
10 mVEZ.CREV
2 h50 µA 2 h
Step1 Step2 Step3 Step4
Fig. 3 Schematic illustration showing the test procedure for EZ.CREV
measurement.
L.-B. Niu et al.2226
potentiostatic polarization of up to two hours was performed.As the rise of anodic current was observed here, further10mV of the potential was lowered and at this potential thepotentiostatic polarization of up to two hours was performed(Step4). Step4 was performed repeatedly, the EZ.CREV wasdetermined as the potential at which the anodic currentupward trend was no longer observed during potentiostaticpolarization for two hours.
Using the creviced specimens of the two steels, potentio-static polarizations were performed for 20 h at the potentialswhich were slightly nobler (+30mV in the present experi-ments) than the breakdown potentials of passive film(EZ.CREV) for repassivating crevice corrosion obtained above.Time variations of current for the creviced specimens in thepotentiostatic polarizations were recorded.
2.4 Observation and analysis of specimen surfaceAfter the electrochemical crevice corrosion tests, the
specimen surface inside crevice was observed with a digitalcamera and an optical microscope (OM) or a scanningelectron microscope (SEM). The film formed on the innersurface of the crevice was analyzed in detail by X-raydiffraction (XRD) and X-ray photoelectron spectroscopy(XPS). XRD analysis was performed using a CuK¡ radiationat scanning range 2ª between 20 and 70°. The acquired XRDpattern was compared with available XRD patterns ofreference materials to identify the phases formed on thesurface. XPS analysis was undertaken using monochromaticAlK¡ radiations at 1486.92 and 156.094 eV at constantanalyzer pass energy. The organic matters on specimensurfaces were first removed with Ar+ sputtering. To calculatethe composition of the formed films, calibration of XPSspectra was performed with reference to the peak area andthe intensity factor · of the element: ·(Fe2p3/2) = 10.82,·(Cr2p3/2) = 7.69, ·(Ni2p3/2) = 14.61, and ·(O1s) = 2.93.The composition of the films was estimated through waveform analysis with reference to standard XPS peak data.25)
3. Results
3.1 Breakdown potential of passive filmThe current and potential curves in the breakdown
potential of passive film (EZ.CREV) measurements of thecreviced specimens are plotted in Fig. 4. It was confirmedthat the creviced specimens of both the 3.5NiCrMoV steeland the 13Cr steel in the present test water exhibitedrepassivating behaviors, and their EZ.CREV were ¹566mV(vs. Ag/AgCl) and ¹298mV (vs. Ag/AgCl), respectively.13Cr steel showed a smaller anodic dissolution rate insidethe crevice as it exhibited a nobler EZ.CREV.
Figures 5 and 6 show the appearances and the opticalmicrographs of the specimen surfaces inside crevice after theEZ.CREV measurements. It was observed that black film andwhite film formed on the specimen surfaces inside crevice of3.5NiCrMoV steel and 13Cr steel, respectively. On the otherhand, it was also confirmed that crevice corrosions occurredon both the two steels. As shown in Fig. 6, a generalcorrosion type was observed on the specimen surface insidecrevice of 3.5NiCrMoV steel, while pitting corrosion wasobserved on that of 13Cr steel.
3.2 Potentiostatic polarization behavior and character-istics of the films formed
Potentiostatic polarizations were conducted for 20 h(7.2 © 104 s) on the creviced specimens in the test water.For the specimens of 3.5NiCrMoV and 13Cr steels, themeasurements were carried out at the potentials of ¹536mV(vs. Ag/AgCl) and ¹268mV (vs. Ag/AgCl), respectively,which were slightly nobler than their EZ.CREV (EZ.CREV
+30mV). Figure 7 shows the time variations of current inthe creviced specimens in potentiostatic polarization meas-urements. For both the two steels, larger anodic currentflowed immediately after the start of the test. The anodiccurrent of both the two steels continued to decrease overtime, and it decreased to near 0 µA in the first two hours(7.2 © 103 s) after the start of test and became stable in thevicinity of 0 µA eventually. In addition, just after the startof test much larger anodic current flowed in 13Cr steel.However, as shown in Fig. 7, it dropped faster than that in3.5NiCrMoV steel.
Appearances of the specimen surfaces inside crevice after20 h potentiostatic polarization measurements are shown inFig. 8. Similar to the results of EZ.CREV measurements, blackfilm and white film formed on the specimen surfaces insidecrevice of 3.5NiCrMoV steel and 13Cr steel, respectively.Figure 9 gives SEM micrographs showing the aspect ofspecimen surfaces inside crevice after 20 h potentiostaticpolarization measurements. 3.5NiCrMoV steel showed ageneral corrosion type inside the crevice. Especially, it can beclearly observed that the black film observed with the nakedeye exhibits crystallinity. On the other hand, pitting wasobserved on the thin film formed on the specimen surfaceinside crevice of 13Cr steel.
The black film formed on the specimen surface insidecrevice of 3.5NiCrMoV steel and the white film formed onthat of 13Cr steel were analyzed in detail with XRD andXPS. Figure 10 shows the XRD pattern with a SEMphotograph of the black film inside crevice of 3.5NiCrMoVsteel. It was found that the black film which showedcrystalline state was a magnetite (Fe3O4) film with an inversespinel structure. On the other hand, as shown in Fig. 10,Ni-oxides were not detected. Figure 11 shows the XPS
200
0
-400
-450
-500
-550
-600
Cur
rent
, I/
μA
Pote
ntia
l, E
/mV
(vs
. Ag/
AgC
l)
200
0
200
-700
-400
-100
Time, t/h
EZ.CREV = -566 mV
3.5NiCrMoV steel
Potential
Current
0 11
EZ.CREV = -298 mV
13Cr steelPotential
Current
140
Fig. 4 The current and potential curves in EZ.CREV measurement of thesteels in the test water added 100mg/L Cl¹ and 50mg/L SO4
2¹.
Electrochemical Crevice Corrosion Behaviors of Low-Pressure Steam Turbine Materials 2227
spectra of the outermost surface of the white film formedon the specimen surface inside crevice of 13Cr steel.Figure 12 gives the depth profile of the film. It is revealedthat the passive film was composed mainly of Cr-oxidesand partly of inward Fe-oxides. Furthermore, it is made clearthat 82.2 at% CrOOH and 17.8 at% CrO3 were concentratedin the outermost surface of the white film observed on 13Crsteel.
4. Discussion
In both of the EZ.CREV measurement and the potentiostaticpolarization test, 3.5NiCrMoV steel and 13Cr steel showedcrevice corrosions in a general type and a pitting type,respectively. It is considered that the crevice corrosionof 3.5NiCrMoV steel was mainly dominated by anodicdissolutions through the black film (Fe3O4 film) formed on
50 μm100 μm
(a) 3.5NiCrMoV steel (b) 13Cr steel
Fig. 6 Optical micrographs showing the specimen surfaces inside crevice after EZ.CREV measurement.
(a) 3.5NiCrMoV steel (b) 13Cr steel
Solution level Solution level
Fig. 5 Appearance of the specimen surfaces inside crevice after EZ.CREV measurement.
0
400
800
1200
1600
100 101 102 103 104
0
50
100
150
200
250
102 103
13Cr steel at -268mV
3.5NiCrMoV steel at -536mV
3.5NiCrMoV steel at -536mV
13Cr steel at -268mV
Time, t/s
Cur
rent
, I/
A
Fig. 7 Time variations of current for the creviced specimens inpotentiostatic polarization measurements.
L.-B. Niu et al.2228
the specimen surfaces inside crevice, while that of 13Cr steelwas dominated by pitting occurred on the white film (Cr-enriched film) due to the concentration of Cl¹. However,not only 13Cr steel but also 3.5NiCrMoV steel exhibitedrepassivating behaviors in the electrochemical crevicecorrosion tests. It is presumed that in the present test waterwith little dissolved oxygen (DO), the black film formed onthe specimen surfaces of 3.5NiCrMoV steel worked as thepassive film as the same as the white film formed on 13Crsteel.
Figure 13 gives schematic illustration of the crevicecorrosions of the two steels in the simulated AVT water withmixed impurity ions. For 3.5NiCrMoV steel, as shown inFig. 13(a), metallic elements (M) of the base material elutedas metal ions (Mn+) within the crevice, and hydrogen ions(H+) were generated by the hydrolysis of Mn+. The reactionsare shown in eqs. (1) and (2), respectively.
M ! Mnþ þ ne ð1ÞMnþ þ nH2O ! MðOHÞn þ nHþ ð2Þ
The Mn+ are considered to be Ni2+ and Fe2+ mainly.However, as shown in Fig. 10, it has been confirmed thatthe film formed on the specimen surface inside crevice of3.5NiCrMoV steel was composed mainly of Fe-oxides(Fe3O4). It is considered that this is because most of Ni2+
was eluted into the solution, did not remain in the film. Raoand Singhal26) have reported that Ni was absent in the Cr-enriched passive film formed on a stainless alloy containingnot only 16.5%Cr but also 6%Ni. There are also manyreports on the preferential dissolution of Ni.2729) In addition,it is considered that the Fe3O4 film on the specimen surfaceinside crevice of 3.5NiCrMoV steel were formed by thefollowed Schikorr reaction, eq. (3), or the hydrolysis reactionof Fe2+, eq. (4).
(a) 3.5NiCrMoV steel (b) 13Cr steel
Solution level Solution level
Fig. 8 Appearance of specimen surfaces inside crevice after potentiostatic polarization test for 20 h.
1.5 μm 5.0 μm
(a) 3.5NiCrMoV steel (b) 13Cr steel
Fig. 9 Aspect of specimen surfaces inside crevice after potentiostatic polarization test for 20 h.
Electrochemical Crevice Corrosion Behaviors of Low-Pressure Steam Turbine Materials 2229
CrOOH, CrO3
Fe
Fe
excess of Cr ions
Cl-
pitting
(b) 13Cr steel
little DO Fe3O4
Ni Fe Ni Fe
Fe Ni Fe Ni
(a) 3.5NiCrMoV steel
Fig. 13 Schematic illustration of the crevice corrosions of LP turbinematerials in the simulated AVT water with mixed impurity ions.
0
20
40
60
80
100
0 100 200 300 400
CrOOH
CrO3
Fe met.
Fe(OH)2
Depth z /nm (rel. SiO2)
Com
posi
tion
ratio
(at
%)
Fe met. Fe(OH)2 FeOOH
CrOOH CrO3Cr met.
Fig. 12 Depth profile of the passive film formed inside crevice on 13Crsteel after potentiostatic polarization test in the test water.
20 30 40 50 60 70
Inte
nsity
, a.u
.
2θ , (°)
: Fe3O4
: Fe
1.0µm
Fig. 10 XRD pattern with SEM photograph of the film formed inside crevice on 3.5NiCrMoV steel after potentiostatic polarization test inthe test water.
581 571
Cr2p3
Cr3+
Cr6+
Cr3+ : 82.2%Cr6+ : 17.8%
: O1s: Cr2p3
02004006008001000
Binding energy, Eb/eV
Inte
nsity
, a.u
.
573575577579
(CrOOH)
(CrO3)
Fig. 11 XPS spectra of the outermost surface of the film formed inside crevice on 13Cr steel after potentiostatic polarization test in the testwater.
L.-B. Niu et al.2230
3FeðOHÞ2 ! Fe3O4 þ 2H2Oþ H2 ð3Þ3Fe2þ þ 4H2O ! Fe3O4 þ 8Hþ þ 2e ð4Þ
By the eqs. (2) and (4), the pH inside crevice could sink.Generally, the pH inside crevice is mainly dependent on theconcentrations of Cl¹ as well as the Mn+.30) On the otherhand, it has also been reported that the pH inside crevicedecreased only a little in the potentiostatic polarizations inwhich the specimens showed a repassivating behavior as theanodic currents decreased continuously.31) For the crevicedspecimen of 3.5NiCrMoV steel in the present test water withhigher pH, it is considered that the pH inside crevicedecreased just a little, therefore the Fe3O4 film formed insidecrevice was in the thermodynamic stability region.32) For13Cr steel, Cr-enriched passive film formed on the specimensurface inside crevice. However, as shown in Fig. 13(b),pitting occurred on the passive film due to the concentrationof Cl¹ inside the crevice, and the pitting grew due to thefurther degradation of the environment inside the pits.
The breakdown potential of passive film (EZ.CREV) is aparameter to evaluate the stability of passive film for therepassivating crevice corrosion, reflects the balance ofanodic dissolution and repassivation inside the crevice. For3.5NiCrMoV steel, due to the Fe3O4 formation inside crevicewas thermodynamically stable, the specimen in the test watershowed a repassivating behavior. On the other hand, since theFe3O4 film was not as tight as the Cr-enriched film formed on13Cr steel, the anodic dissolutions barely passed through thepassive film and thereby the steel showed lower EZ.CREV.Furthermore, because SO4
2¹ were contained in the test water,it should be another factor to promote the anodic dissolutionsinside crevice of this steel. It has been reported that SO4
2¹
promote the general corrosion of low-alloy steels.15,17) For13Cr steel, even though pitting corrosion occurred insidecrevice due to the concentration of Cl¹, because the anodereactions inside crevice were restrained by the formationof Cr-enriched passive film, it showed higher EZ.CREV.In addition, it has been reported that the SO4
2¹ in theenvironments containing Cl¹ have the ability to mitigatethe localized corrosions including pitting corrosion, crevicecorrosion and so on, of stainless steels.3335) The authors36)
have also found that the breakdown of the passive filmformed inside crevice on 13Cr steel was somewhat restrainedby the SO4
2¹ coexisted with Cl¹ in the test water. On theother hand, in fact the Cl¹ and the SO4
2¹ could accumulatedin the crevice regions of LP steam turbines in higherconcentrations than those in the present test water.1) In suchcases it should be noted that the growing crevice corrosion ispossible to occur.
Because most of the DO in the present test water wereremoved, it is considered that the DO of extremely smallamount in the internal crevice was consumed exhaustively ina very short time, according to the oxygen reduction ofeq. (5). For both the two steels, it is therefore considered thatthe anode and the cathode reactions inside crevice weremainly the ionization of metallic elements of eq. (1) and thereduction reaction of hydrogen ions of eq. (6), respectively.
ðn=4ÞO2 þ ðn=2ÞH2Oþ ne ! nOH� ð5ÞnHþ þ ne ! ðn=2ÞH2 ð6Þ
For a creviced specimen at the potential which is noblerthan the breakdown potential of passive film EZ.CREV, theanode and cathode reactions inside the crevice will beaccelerated. Meanwhile, the hydrolysis of the metal ions ofeqs. (2) or (4) inside the crevice, which generate not onlyH+ but also the metal hydroxides and metal oxides, will bepromoted. In the present potentiostatic polarization measure-ments, as shown in Fig. 7, the anodic current of both the twosteels exhibited larger values just after the start of the testsand then it decreased gradually to near 0 µA. It is consideredthat the anodic dissolutions came to be restrained because thepassive films formed on the specimen surfaces inside crevicewith early anodic dissolutions occurred. For 13Cr steel, theanode reactions inside crevice were mainly of the anodicdissolutions of chromium. Due to the relatively high rate ofthe anodic dissolutions of chromium, much larger anodiccurrent flowed at the beginning of the test. However, becausethe Cr-enriched passive film formed fast inside the crevice,the anodic current decreased faster. For 3.5NiCrMoV steelin the potentiostatic polarization measurement, the anodiccurrent also decreased because the anodic dissolutions wererestrained by the passive film (Fe3O4 film) formation on thespecimen surface inside crevice. It has been reported thatthe magnetite (Fe3O4) film is effective in improving thecorrosion resistance of low-alloy and carbon steels used inpower plants.37,38) In the present potentiostatic polarizationmeasurements, it was confirmed that in both of the two steelseven though at a slightly higher potential than its EZ.CREV,the passive film formed on the surface inside crevice andconsequently the anodic current decreased and became stableeventually.
5. Conclusions
Electrochemical crevice corrosion tests were performedon the low-pressure steam turbine materials in the simulatedAVT water added chloride and sulfate ions. The films formedon the specimen surfaces inside crevice were analyzed indetail. The results obtained are as follows.
(1) The creviced specimens of the rotor material,3.5NiCrMoV steel, and the blade material, 13Cr steel,in the present test water exhibited repassivating crevicecorrosion behaviors. Their breakdown potentials of passivefilm (EZ.CREV) were ¹566mV (vs. Ag/AgCl) and ¹298mV(vs. Ag/AgCl), respectively.
(2) Crevice corrosions of 3.5NiCrMoV and 13Cr steels inthe test water occurred in a general type and a pitting type,respectively. It is suggested that the crevice corrosionsoccurred mainly due to the degradations of the environmentinside crevice, such as the hydrogen ion generation and thechloride ion concentration.
(3) Passive films formed on the specimen surfaces insidecrevice of both the two steels in the test water, and bythis reason the crevice corrosions were restrained. As for3.5NiCrMoV steel, the passive film formed on the specimensurfaces inside crevice was mainly composed of Fe3O4.As for 13Cr steel, the passive film was composed mainlyof Cr-oxides and partly of inward Fe-oxides. Especially,only Cr-oxides concentrated in the outermost surface of thepassive film.
Electrochemical Crevice Corrosion Behaviors of Low-Pressure Steam Turbine Materials 2231
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