mott-schottky analysis of passive films on si containing stainless steel alloys

Journal of The Electrochemical Society, 158 (11) C391-C395 (2011) C3910013-4651/2011/158(11)/C391/5/$28.00 © The Electrochemical Society

Mott-Schottky Analysis of Passive Films on Si Containing StainlessSteel AlloysIhsan-ul-Haq Toor∗,z

Department of Mechanical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

Effect of silicon on the defect density of Fe–20Cr–xSi (x = 0, 1, 2) stainless steel alloys was investigated in deaerated pH 8.5 boratebuffer solution at room temperature using Mott-Schottky analysis. Results showed that silicon was incorporated/dissolved in thepassive film of Fe–20Cr–xSi (x = 0, 1, 2) alloys. Mott-Schottky analysis revealed that the addition of silicon decreased the acceptordensity (NA, VCr−3), i.e., increased the Cr+3 content of the passive film. Also the donor densities, shallow donor (ND1, VO+2) anddeep donor (ND2, VCr+6), of the passive films formed were decreased. XPS analysis confirmed the presence of Si in the passivefilm of Si containing alloys. This enriched silicon along with higher Cr+3 concentration of the passive film, dictated its enhancedprotectiveness and stability.© 2011 The Electrochemical Society. [DOI: 10.1149/2.083111jes] All rights reserved.

Manuscript submitted May 24, 2011; revised manuscript received August 25, 2011. Published October 5, 2011.

Stainless steel alloys are widely used in different industrial ap-plications due to their excellent corrosion and mechanical properties.Along with Ni and Cr, many other alloying elements such as Si, Cu,Mn, and W are used depending on the final application requirements.There have been some investigations on the role of silicon on thecorrosion properties of stainless steel alloys and it was reported thatpitting corrosion resistance was increased due to silicon addition inthese alloys.1–9 Different explanations were given for improved corro-sion resistance of Si containing alloys, such as increased passive filmstability 6 and changes in the grain boundaries.4 In a previous study,Toor et al.10 investigated the effect of silicon addition in a specificallydesigned 304Si alloy, on its localized corrosion resistance, repassiva-tion kinetics and stress corrosion cracking and results were comparedwith commercially available stainless steels such as 304, 316 and leanduplex stainless steel (LDSS). It was found that, the presence of siliconin alloy 304Si, improved its localized corrosion resistance, repassiva-tion rate and resistance to stress corrosion cracking as compared torest of the alloys.It is well known that the excellent corrosion resistance of stainless

steels is mainly due to formation of thin oxide film (passive film) ontheir surface in different environments. Different alloying additions inthese alloys affect the structure and composition of this thin oxide filmdifferently, so understanding the role of individual alloying elementson the passive film structure and composition, i.e. corrosion resistanceproperties of stainless steel and Ni base alloys, is an important area todesign and develop new corrosion resistance alloys for different indus-trial applications. Passive films on stainless steels are believed to beCr-enriched (Fe, Cr) oxide/hydroxide,11–14 although there is still somecontroversy as to the detailed structure and composition of the passivefilm. The extreme complexity of the metal/passive film/electrolytesystem of stainless steels makes the deep comprehension of the pas-sive film difficult. It is because of the experimental difficulties inprobing the thin passive film of few nm thick and also due to thepossibility of compositional variation when the film is brought out forex-situ surface analysis.15 Thus ex-situ analytical studies for passivefilm characterization are not sufficient to reveal the real structure ofpassive film. Recently, Mott-Schottky analysis based on impedancemeasurements is proved to be a powerful technique for in-situ anal-ysis of passive films on metals and alloys.15–17 These techniques arebased on the fact that passive film acts like a semiconductor, so used tomeasure the semiconducting properties of the passive films, which areintimately related with the physicochemical structure of the passivefilm, i.e. to the anticorrosion properties.Though there have been few studies on the role of Si on corrosion

resistance properties of stainless steels, but how silicon affects thepassive film defect density and composition was rarely investigated

∗ Electrochemical Society Active Member.z E-mail: [email protected], [email protected]

before. So the objective of this study was to investigate the effectof Si on the defect density of Fe–20Cr–xSi (x = 0, 1, 2) alloys inpH 8.5 borate buffer solution using Mott-Schottky analysis, which isclosely related with corrosion resistance properties. X-ray photoelec-tron spectroscopy (XPS) investigations were also carried out on thepassive films.

Experimental Procedures

Alloys used in this study (Fe–20Cr–xSi (x = 0, 1, 2) were preparedin vacuum-arc-melting furnace. The cast was homogenized for 120min at 1200◦C, and then hot rolled into 3 mm thick plate. Specimenswere prepared after cold rolling the hot rolled plates into 1.6 mm thicksheets and solution annealing for 1150◦C for 60 min, followed by wa-ter quenching. For electrochemical tests, the specimens were polishedto 2000 grit emery paper and then ultrasonically cleaned with distilledwater. Prior to electrochemical tests, the specimens were cathodicallycleaned for 10 min at −0.8VSCE to remove the air formed oxide film.A three electrode cell composed of a specimen as a working electrode,a Pt counter electrode and a saturated calomel reference electrode wasused for the tests. Polarization tests were carried out at a scan rateof 1 mV/s in pH 8.5 borate buffer solution 18 at room temperature(25◦C).The in depth chemical composition profile of the passive films

formed on Fe–20Cr–1Si alloy formed potentiostatically at 0.4 VSCEfor 24 hwas examined usingX-ray photoelectron spectroscopy (XPS).XPS measurements were performed using Al-Kα and Mg Kα X-raysource (15 kV, 20 mA, 300 W), and a pass energy of 20 eV. Thereference energies were the C1s signal at 284.5 eV and the O1s signalat 531.0 eV. Characterization of the passive films was performed bymeans of high resolution scan and sputtering depth profiles. Sputterdepth profiles were obtained with argon ions (PAr = 5 × 10−7 torr,base pressure = 5 × 10−10 torr, energy: 5 kV, current: 3.0 μA/cm2).For Mott-Schottky analysis, the specimen were passivated at se-

lected film formation potential, Uf (0.4 ∼ 0.8 VSCE) for 9 h, be-fore capacitance was measured by sweeping the applied potential ata rate of 1 mV/s from the film formation potential to –1.0 VSCE.The excitation voltage was 10 mV (peak-to-peak) and the frequencywas 1 kHz.19

Results and Discussion

Polarization tests.— Fig. 1 shows the polarization behaviour of theFe–20Cr and Fe–20Cr–1Si alloys in deaerated pH 8.5 borate buffersolution at 25◦C with scan rate of 1 mV/ s. Corrosion potential (Ecorr)of the two alloys were almost equal; −0.77 VSCE for Fe–20Cr and−0.78 VSCE for Fe–20Cr–1Si alloy, respectively. The first anodic cur-rent peak at −0.57 VSCE was ascribed to the anodic dissolution or

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C392 Journal of The Electrochemical Society, 158 (11) C391-C395 (2011)

Figure 1. Potentiodynamic polarization response of the Fe–20Cr and Fe–20Cr-xCu alloys in deaerated pH8.5 borate buffer solution at 25◦C and a scanrate of 1mV/s.

the oxidation of Fe to Fe2+, the second anodic current peak at −0.35VSCE was related to the oxidation of Fe2+ to Fe3+, and the thirdanodic current peak at 0.72 VSCE was related to the transpassive oxi-dation of Cr3+ to Cr6+.12 It appears that there is close similarities inthe polarization behaviors between Fe–20Cr alloy and Fe–20Cr–1Sialloy, however passive current density was decreased in case ofFe–20Cr–1Si containing alloy (Fe–20Cr= 1.01E−6 and Fe–20Cr–1Si= 7.15 E−7). Also it is clear from the Fig. 1 that addition of silicondecreased the anodic dissolution of Fe to Fe2+ and of Fe2+ to Fe3+.From the polarization data of Fig. 1, film formation potential (Uf) wasselected in between (0.2 ∼1.0 VSCE) for subsequent Mott-Schottkyanalysis and XPS measurements.

XPS analysis of the passive film.— Fig. 2a and 2b shows the XPSatomic concentration of the passive films formed at 0.4 VSCE onFe–20Cr and Fe–20Cr–2Si alloys, for 24 h in deaerated pH 8.5 boratebuffer solution at room temperature and Fig. 3 shows the relative con-centration of Cr, that is Cr/(Cr + Fe+ Si) [%], Si and Fe respectivelyin Fe–20Cr–2Si alloys, calculated from Fig. 2a and 2b. Fe concentra-tion in both passive films showed about 20∼30 at.% at the subsurface,and increased gradually with depth. The concentration profile of O inboth films exhibited peak value of 40∼45 at.% at the subsurface at1 nm depth, and then decreased with depth. The reduction of O con-centration was faster in the passive film on Fe–20Cr–2Si alloy thanthat on Fe–20Cr alloy. Appreciable amount of Si was detected in thepassive film formed on Fe–20Cr–2Si alloy. In addition, it is evidentfrom the lower O concentration profile for the passive film on Fe–20Cr–2Si than that Si is incorporated into the passive film, as reportedpreviously for a similar case of N containing stainless steel alloys.20

Figs. 2 and 3 show that at the surface of Si containing alloys, appre-ciable amount of Si is present in the form of SiO2 along with Cr2O3and these results are in agreement with those reported previously.21,22

Hong et al.21 based on AES analysis, reported that in case of Yitriumcontaining high silicon stainless steels, outer layer ismainly composedof SiO2 and inner layer is mainly Cr2O3. The formation of enrichedSiO2 protecting film promotes the passivation and improves corrosionresults. Xiuying et al.22 also reported that Si builds up in the surfacein the form of SiO2 in concentrated nitric acid environment and playsa main part as a passivating agent. Osozawa and Desestret et al.7, 8

prepared high silicon containing stainless steels (up to 4 wt.%) foruse in highly oxidizing environments and found that Si has increasedthe pitting resistance substantially. It has been reported that positiveeffect produced by Si is due to the increased stability of the passivestate, resulting from increase in the Si content of the protective film.6

Figure 2. XPS depth profiles for passive films formed at 0.4VSCE for 24 hoursin deaerated pH 8.5 borate buffer solution on (a) Fe–20Cr-0Si alloys and (b)Fe–20Cr–1Si alloys at room temperature.

Figure 3. Relative concentration of Cr, Cr/(Cr + Fe+ Si) [%], Si and Ferespectively in Fe–20Cr–xSi stainless steel alloys.



Journal of The Electrochemical Society, 158 (11) C391-C395 (2011) C393

Figure 4. Mott-Schottky plots for the passive films formed on Fe–20Cr andFe–20Cr–xSi alloys at (a) 0.4 VSCE for 9 h in deaerated pH 8.5 buffer solution,and (b) at 0.8 VSCE for 9 h in deaerated pH 8.5 buffer solution.

This Si enrichment is responsible for increased passive film stabilityand repassivation kinetics.10 Middleton et al.23 have reported recentlywhile investigating the role of Si in Ti-Si alloys, that Si was present inthe film as SiO2 and enriched/incorporated in the TiO2 oxide, as it waseasier for Si to replace Ti because of small size. Similar is the casein Fe–20Cr–xSi alloys, in which due to smaller size, Si can replaceiron and can form (Si+Cr) enriched γ-Fe2O3. So therefore, based onXPS atomic concentration depth profiles, it can be stated that Si wasfound enriched in the passive film of Fe–20Cr–xSi alloys and it willbe further discussed in section 3.3 based on Mott-Schottky analysisresults.

Mott-Schottky analysis.— The defect density of the alloys wasanalyzed by Mott-Schottky analysis, which provides precise informa-tion on the electronic properties of the passive films. The analysis wascarried out on the passive films formed at 0.4 VSCE and 0.8 VSCE,respectively, for 9 h in pH 8.5 buffer solution at room temperature, asshown in Fig. 4a and 4bFor theMott–Schottky analysis, the specific interfacial capacitance

(C), is obtained from C = 1/ωZ ′′, where ω is the angular frequencyand Z ′′ is the imaginary part of the specific impedance. Assumingthat the double-layer capacitance is sufficiently high that it can beneglected in a series combination with the space-charge capacitance,

Table I. Donor and acceptor density of the Fe–20Cr and Fe–20Cr–xSi alloys at Uf (0.4 & 0.8)VSCE, in deaerated pH 8.5 boratebuffer solution at room temperatures, based on data of Fig. 4.

Uf = 0.4VSCE Uf = 0.8VSCEND NA ND1 ND2 NA

Fe20Cr 4.82E20 2.10E21 2.78E20 1.27E21 2.10E21Fe20Cr1Si 3.90E20 1.26E21 2.80E20 1.24E21 1.25E21Fe20Cr2Si 3.05E20 1.20E21 1.77E20 9.50E20 1.20E21

the measured capacitance,C, is equal to the space-charge capacitance,CSC. According to the Mott-Schottky theory, the CSC of n-type andp-type semiconductor are given by Eq. 1 and Eq. 2, respectively;

1

C2= 2

εε0eND

(E − EF B − kT

e

)[1]

1

C2= − 2

εε0eNA

(E − EF B + kT

e

)[2]

where ε is the dielectric constant of the oxide (15.6 for Cr-substitutedγ-Fe2O3),24 ε0 is the vacuum permittivity (8.854*10−14 F cm−1), e isthe charge of an electron,ND andNA are the donor and acceptor densityin the passive film, respectively, E is the applied potential. Eapp andEFB is applied potential and flat band potential, respectively, and k isthe Boltzmann constant Thus, for n-type and p-type semiconductors,C−2 versus E should be linear with positive and negative slopes thatare inversely proportional to the ND and NA, respectively.The Mott–Schottky plots for the passive films formed on Fe–20Cr

and Fe–20Cr–xSi alloys were taken at 0.4 VSCE (passive region) and0.8 VSCE (transpassive region), as shown in Fig. 4a and 4b. Mott–Schottky plots for the passive films of Fe–20Cr and Fe–20Cr–xSi werevery similar in shape, confirming that the passive films on these alloysare very similar and Si addition did not change the base structure of thepassive film in Si containing alloys. Mott–Schottky plots at 0.4 VSCE(Fig. 4a) exhibited two linear regions. Positive slope in the potentialregion higher than −0.4 VSCE demonstrated n-type semiconductivity,related to (Fe, Cr)-oxide for both of the alloys. On the other hand, thenegative slope in the potential region lower than −0.4 VSCE showedp-type semiconductivity attributed to an inner Cr enriched oxide forboth alloys.25,26 The dominant and detectable donor species in n-typepassive film were oxygen vacancy, VO2+ and the dominant acceptorspecies in p-type passive film were metal vacancy, VCr−3.19,25 Whenthe passive film was formed at 0.8 VSCE in Cr-transpassive region, twolinear regions with two positive slopes above −0.4VSCE and 0 VSCEwere observed respectively, as shown in Fig. 4b. The second linearregion at potentials higher than 0 VSCE was attributed to the existenceof Cr6+ ions acting as the deep donor in the passive film.25 Table Ishowed the values of acceptor density (NA, VCr−3) and donor density(shallow level (ND1, VO2+) and deep level (ND2, VCr+6)) calculated onthe basis of Eqs. 1 and 2 at 0.4 VSCE and 0.8 VSCE respectively.The flat band potential (EFB), represents a region where predomi-

nant electronic defects establish the transition between p-type and n-type semiconductivity. It determines the position of energy bands withrespect to redox potentials of electro active ions in the electrolyte. Thecharge transfer in turn governs these positions across the semiconduc-tor/electrolyte interface, the contact potential between semiconductorand electrolyte, interface, and the stability of the semiconductor. Flatband potential of the passive films formed at 0.4 VSCE and 0.8 VSCEwas measured based on equation 1, for n- type passive film, where asequation 2 can be used to measure EFB for p-type passive films. A tan-gent was drawn on X-axis from the linear region with positive slope(−0.4VSCE to 0VSCE) and the corresponding valuewas taken as EFB forn-type passive films and it was found to be nearly similar (−0.32 VSCE∼-0.36 VSCE) irrespective of the film formation potential. A positiveslope higher than flat band potential indicated that the passive film



C394 Journal of The Electrochemical Society, 158 (11) C391-C395 (2011)

Figure 5. Defect densities based onMott-Schottky of the passive films formedon Fe–20Cr and Fe–20Cr–xSi alloys (a) acceptor density at 0.4 VSCE and0.8VSCE for 9 h in deaerated pH 8.5 buffer solution, and (b) shallow and deepdonor densities at 0.8 VSCE for 9 h in deaerated pH 8.5 buffer solution.

behaved like n-type semiconductor (donor density/oxygen vacancyand metal interstitials), while at potentials lower than this, a straightline with a negative slope was observed suggesting that a p-typesemiconducting behavior (acceptor density). Consequently the pas-sive film acts as a Schottky barrier above and below the flat bandpotential.The donor density (ND1, VO2+) of the Si containing alloys was

lower than those without silicon, as shown in Table I, and this meansthat concentrations of VO2+ of the passive films is decreased by Siaddition. A decrease in the acceptor density (NA) of silicon containingalloys was also observed, which means that Cr3+ concentration of thepassive film was increased (Fig. 5a). Similar trend was observed whenthe analysis was carried out at 0.8 VSCE. There was no significantchange in shallow donor density (ND1, VO2+), however deep donordensity (Cr+6, ND2) was decreased to compensate the charge neutralityin the film as shown in Fig. 5b.Mott-Schottky analysis alongwithXPSdata, suggest that Silicon is

incorporated/dissolved in the Cr-substituted γ-Fe2O3 spinel structureas discussed in the previous section based on XPS results. The resultscan be further explained by charge neutrality principle, according towhich, the substitution of Fe3+ by Si4+ in the passive film increased thepositive charge of the passive film. This increase in positive chargewas

compensated with the decrease in concentration of VO2+ and/or Cr6+

to satisfy the charge neutrality in the film. A similar trend has beenreported earlier while discussing the Mott- Schottky behavior of Niand Mo containing Fe–20Cr stainless steel alloys.27 An incorporationofNi2+ into theCr-substituted γ-Fe2O3 spinel structure by substitutingFe3+ would reduce positive charge in the film, thereby promotingthe formation reaction of VO2+ and/or the transpassive oxidation ofCr3+ to Cr6+ to supplement the positive charge in the passive film.In contrast, Mo4+ and Mo6+ dissolved in the Cr-substituted γ-Fe2O3by substituting Fe3+ cause excessive positive charge, which shouldreduce the concentration of VO2+ and/or Cr6+ to satisfy the chargeneutrality in the film. The results lead to the decrease in the shallowdonor density and/or deep donor density respectively in the passivefilm.Therefore, it is concluded that some silicon is enriched/

incorporated/dissolved in γ-Fe2O3 in Fe–20Cr–xSi alloys by substi-tuting Fe3+ and passive film on these alloys becomes (Si+Cr) substi-tuted γ-Fe2O3. So, the Si addition decreased the shallow donor densityand/or deep donor density of the passive film. The increase in Cr3+

concentration of the passive film, was confirmed by the lower acceptordensity (NA,VCr−3) of p-type passive films formed on Fe–20Cr–xSialloys than those on Fe–20Cr-0Si alloys. Therefore, the passive filmformed on Si containing alloys is more stable and protective in natureas compared to Si free alloys. Having such a passive film will in-crease the localized corrosion resistance, repassivation rate and stresscorrosion cracking resistance of the stainless steel alloys as reportedpreviously.10

Discussion

Point defectmodel ofMacdonald et al.25 states that theminor alloy-ing elements can form a solid solution with major alloying elements,when a passive film is formed. So these minor alloying elements, inother words, can modify the passive film properties by dissolving inthe film (as dopants) and will affect the defect chemistry of the pas-sive film. The physico-chemical properties of the passive film couldbe dominated by the point defects such as oxygen vacancies, metalinterstitials and metal vacancies, with high concentrations (usually1019–1021 cm−3), which are responsible for mass transport for the ox-ide growth. In terms of defect density of the passive film, the solubilityof Si in the passive film in Fe–20Cr–xSi alloys, either substitutionalor interstitial, will reduce or enlarge the point defect concentrations.Subsequently, the variation will improve or weaken the corrosion re-sistance through the respective inhibition or enhancement of the masstransport. The suppression of defects or imperfection will favor theformation of a more compact layer with a lower porosity, but the pro-motion leads to formation of a more porous, open and consequentlythicker layer. As in this study, it was found that the Si enrichment in thepassive film, decreased the defect density of the film, so it can be statedthat Si has improved the passive film stability and protectiveness.It is well known that γ-Fe2O3 has a spinel structure containing

vacancies 28 and ferric ions in its tetrahedral or octahedral sites. Orgelet al.29 reported that Fe+3 ion in spinel structure resides at the sameenergy level irrespective of whether it is as octahedral sites or at tetra-hedral sites, but Fe+2 ions are more stable at octahedral sites than attetrahedral ones. Belo et al.30 reported the effect of Cr and Cu on thesemiconducting properties and discussed in detail the basic aspects ofcrystallographic spinels which are important in describing the elec-tronic structure of passive films formed on iron based alloys. Theyhave stated that that the way different ionic species distribute in tetra-hedral and octahedral sites contained in an oxide lattice, determinesthe defect density (donor, acceptor) energy levels in the band gap ofthe that particular oxide. It was also reported that chromium cationcan be incorporated in the octahedral positions of the spinel and thisphenomenon will lead to the formation of a larger range of spinelswith chemical composition situated between that of the magnetite in-verse spinel and that of the direct spinel (chromite). As mentionedearlier that, this study has found based on Mott-schottky analysisand XPS investigations, that Si was incorporated in the passive film.



Journal of The Electrochemical Society, 158 (11) C391-C395 (2011) C395

This silicon has occupied some of the tetrahedral and octahedral sitessubstitutionally or interstitially, previously being occupied by Fe+3

and Fe+2 sites respectively, keeping in mind that size of Si is less thanthat of Fe.Atrens et al.31,32 have discussed in detail the role Mo and oxygen

with various chemical binding states, on the passivity and its break-down in stainless steels. It was found that oxidized Cr and Fe formthe main components within the passive film on stainless steels andthere is generally more Cr oxide in the outermost layer of the passivefilm in comparison with oxides of iron and other alloying elements,which imparts these alloys excellent corrosion resistance propertiesdue to formation of protective passive film. A similar phenomenon,to that of Mo enrichment in the passive film,27,31 was observed inthe case of Si containing Fe–20Cr–2Si alloys in this study. The lowerconcentration of donor density in Si containing stainless steels indi-cates the inhibition of silicon on the generation of oxygen vacanciesand metal (iron) interstitials. Silicon substitution and interstitials canbe thought as follows; Si = Si+1

Fe+s + e−, Si = Si+2Fe+2 + 2e− and

Si = Si+4i + 4e−. As the silicon species (Si+1Fe+s , Si

+2Fe+2 and Si

+4i ) +

4e− have a positive charge, so the formation of silicon substitutionand interstitials will decrease the donor density of passive film dueto charge neutrality principle. Since the defect density or electronicproperties are ultimately related with the corrosion resistance of stain-less steels,25 so a decrease in defect density, can significantly improvethe corrosion resistance ability of a particular alloy, i.e. Fe–20Cr–xSi.So it can be concluded based on these investigations that Si incorpo-ration/enrichment in the passive film decreased the defect density ofthe passive film, which ultimately improved the passive film stabil-ity and corrosion resistance properties of Si containing stainless steelalloys.

Conclusions

Potentiodynamic polarization tests,Mott-Schottky analysis as wellas XPS investigations of the passive films on Si containing stainlesssteels revealed that:

(1) Si addition decreased the anodic dissolution or the oxidationof iron to iron oxide as well as the passive current density, indeaerated pH 8.5 borate buffer solutions.

(2) XPS analysis found silicon enrichment in the passive filmof Fe–20Cr–xSi alloys. Silicon was mainly present in theouter few nanometers and decreased as the depth of film wasincreased.

(3) Mott-Schottky analysis showed that, passive film on Si contain-ing alloys was (Si+Cr) substituted γ-Fe2O3. Also the additionof silicon decreased the acceptor density (increased the Cr+3

concentration) of the passive film at 0.4 VSCE & 0.8 VSCE re-spectively, which resulted in more protective and stable passivefilms on silicon containing alloys. Results also showed that boththe shallow donor and deep donor densities were decreased dueto Si addition in Fe–20Cr–xSi alloys. These results lead to theconclusion that Si presence in Fe–20Cr–xSi alloys will increas

their localized corrosion resistance properties, due to formationof a stable and protective passive film on their surface.

Acknowledgments

The author gratefully acknowledges the support provided by KingFahd University of Petroleum & Minerals (KFUPM) and KoreaAdvanced Institute of Science and Technology (KAIST) in the suc-cessful completion of this research work.

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mott-schottky analysis of passive films on si containing stainless steel alloys

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