Indian Journal of Chemical TechnologyVol. 4, March 1997, pp. 94-lO0

Electrochemical behaviour of niobium and niobium passive films in nitricacid solutions

W A Badawy & F M AI-KharafiDepartment of Chemistry, Faculty of Science, University of Kuwait, POBox 5969, Safat 13060, Kuwait

Received 5 October 1995; accepted lO June 1996

Electrochemical behaviour of bare niobium and phosphoric acid anodized niobium electrodes isinvestigated in nitric acid solutions. Electrochemical impedance spectroscopy and polarisation tech-niques have been used to investigate the open-circuit growth of the passive film. The stability of theanodic oxide film has been studied as a function of the formation voltage, formation current densityand concentration of the ambient electrolyte. The results show that the Nb-Nb205-1M HN03 doesnot behave as a perfect dielectric. The flat band potential and donor concentration of the semicon-ducting anodic oxide film have been calculated from the Mott-Schottky plots.

The electrochemical behaviour of niobium andgenerally valve metals is of high technical import-ance because of the use of these materials in thepreparation of passive anodic films and in thegrowth of protective and decorative films. Niob-ium is subjected to extensive studies, concerningthe kinetics of oxide film growth, stability of theoxide film and its photoelectrochemical propert-ies'-6. The passive film on niobium was known tobehave as a weakly dissociated extrinsic semicon-ductor? It consists mainly ofamorphous or glassyNb20s with a density of 4.74 g cm-3 and staticdielectric constant of 45. It has a high concentra-tion of oxygen vacancies (10' ~ ern - 3), acting asdonor states". Crystalline Nb205 was found tohave a band gap of 3.4 -v. The low voltage pas-sive films and the behaviour of the metal in com-monly used media seems to be little studied.Several techniques such as galvanostatic, poten-tiostatic, cyclic polarisation and electrochemicalimpedance spectroscopy (EIS) have been used tostudy the electrochemical behaviour of valve me-tals. EIS is used to specify the analogy occurringbetween an interface undergoing an electrochemi-cal reaction and a pure electronic model consist-ing of specific combination of resistors and capa-citors 10. The advantage of this analogy can beused for the characterisation of the electrochemi-cal system in terms of its equivalent circuit. TheEIS provides a new and more detailed informa-tion concerning corrosion protection by anodicfilms such as anodized AI, Ta, Ti and Zr'n-13. Inthis paper the electrochemical behaviour of niob-ium and its passive film in nitric acid solutionshave been reported. The effect of formation pot-

ential, formation current density and the concen-tration of the test electrolyte (HNOJ solution) onthe behaviour of niobium and its passive film hasbeen investigating. Moreover, the flat band poten-tial and donor concentration of the semiconduct-ing anodic oxide film are also calculated.

Experimental ProcedureBulk cylinder of spectroscopically pure niobium

(Aldrich-Chemie) was used as working electrode.A thick copper wire was employed as electricalcontact. The electrodes were fitted into glass tub-ing of appropriate internal diameter by an epoxyresin, leaving a front surface area of 0.34 em? tocontact the electrolyte. The electrochemical cellwas an all glass three electrode cell with a largearea platinum counter electrode and Agi AgCI/Cl- reference electrode. The electrolytic solutionswere prepared from analytical grade reagents andtriple distilled water. Galvanostatic and potentios-tatic measurements were carried out using a PAR273 galvanostat/potentiostat coupled with PS3IBM. The impedance data were measured andcalculated using the IMSd-AMOS- system (Zah-ner Elektrik GmbH, Kronach, Germany). A 10mV peak to peak amplitude signal and a frequen-cy range of 0.1 Hz-IOO kHz was used.

The electrode surface was pretreated and passi-vated, following the same procedure, which is me-chanical polishing using successive grades of em-ery papers and diamond spray down to 1 urnandwashing with triple distilled water. With thisprocedure, the electrode acquired a reproduciblysilvery bright surface. Passivation of niobium wascarried out in 0.33 M H3PO-l at a specified cur-

BADAWY & Al-KHARAFI:.ELECfROCHEMICAL BEHAVIOUR OF NIOBIUM 95

30K

7S 10K

3K60

°CD lK~ <:~45 '"c :; 300"=:

c

"0 ~ 10030 f.E- 30

15 10

'3

1 K

~ 300

~8. 100.§

30

10

3L- ~~~~~ __~~~~~~100m 2 5 10 30 100 300 lK 3K 10K 30K lOOK

Frequency, Hz

Fig. I-Bode plots of Nb in 1M HN03 after Smin (---),15 min (---), 30min(-.-.-.) and 60 min. or more (.....) of elec-trode immersion at open circuit potentials of - 248, - 223,

- 208 and - 192 mY, respectively.

rent density. After reaching the desired anodicformation potential, the current was interruptedand the measurements were carried out immedi-ately at a constant temperature of 25 ± 0.1 "C, Ailexperiments were carried out in unstirred natural-ly aerated solutions.

Results and DiscussionBehaviour ofthe bare metal

Mechanically polished niobium electrodes wereinvestigated in 1 M HN03 solution. The imped-ance spectra were taken at different intervals ofelectrode immersion and presented as Bode dia-grams 14 in Fig. 1. In this format all the experimen-tal data are equally represented and the phaseangle is a very sensitive indication of the presenceof additional time constants in the impedancespectra 15. The electrolyte resistance, R" is deter-mined at the highest frequency (100 kHz) and thepolarisation resistance, Rp' is obtained from thevalues of the real part of admittance at the lowestfrequency, where asymmetric values of Z are con-sidered ". The electrode capacitance, Cd' is calcu-lated from the Bode plot at a frequency of 0.158Hz where the impedance data give a straight lineof slope ~ - 1. It is also obtained from the valueof the imaginary part of admittance, ~m' at thesame frequency using the relation:~m = 1/2 Jt f Cd ... (1 )where fis the frequency.

The impedance data of niobium electrodes innitric acid solutions drift continuously towards in-creasing values which denotes progressive oxidefilm thickening on the electrode surface withtime". This means that the native oxide film istoo thin to impart complete passivity. Healing and

75

60°CD

~--~~~~~--~~~~~~~~O100m 1 2 5 10 30 100 300 1K 3K 10K 30K lOOK

Frequ<!'ncy, Hz

Fig. 2-Bode plots of Nb after 180 min of electrode immer-sion in nitric acid solutions of different concentrations. (----) 1

M, ( ) 0.1 M, (----) 0.01 M HN01.

oxide film thickening will continue until a steadystate is achieved which requires 120-180 min ofelectrode immersion in this solution. The effect ofconcentration of the test electrolyte was investi-gated. Fig. 2 presents the impedance spectra ofniobium electrodes immersed in HN03 solutionsof different concentrations. The electrode imped-ance increases as the concentration of HN03 dec-reases, which means that a thicker oxide film isgrown in dilute solutions.

Potentiodynamic data and open circuit mea-surements are in good agreement with the imped-ance measurements. Fig. 3a presents anodic andcathodic Tafel plots of the Nb electrode in differ-ent concentrations of nitric acid and the variationof the corrosion behaviour with time is shown inFig. 3b. It is clear (Fig. 3b) that the steady statepotential is getting more positive with time, thecalculated polarisation resistance increases andthe corresponding corrosion current decreaseswhich indicates that the barrier layer thickness in-creases from the moment of electrode immersionuntil a steady state is reached. The electrode pas-sivity (denoted by an increase in the oxide filmthickness) increases as the concentration of thetest solution decreases (Fig. 3a).

Behaviour of the anodically passivated electrodesAnodic oxide films were formed on niobium at

a constant current density of lmAcm-2 in natu-rally aerated 0.33 M H3P04 solution up to the de-sired formation voltage. The stability of the-formed oxide iilm was investigated as a functionof the formation voltage, formation current densi-ty and concentration of the test electrolyte.

Effect of [ormation voltage-oxide films corre-sponding to 2,4.7 and lOV were prepared as de-

96 INDIAN 1. CHEM. TECHNOL., MARCH 1997

-0,650

-0'050

-0·150 I

"--/-)-_................ .....

--------.:=--:.:~- ....~----:~:.::".- .

:::::::::::"":....-"', ......•................."<,"t-. ~

...... ", ..•...-

w -0.350

-0'450

-0,550

......~

Fig. 3(a)-Anodic and cathodic polarization of Nb electrodesin nitric acid solutions of different concentrations after 5 minof electrode immersion. (----) 1 M, (-----) 0.1 M, (.....)

om M HN03•

=,

0·000

-0·100

-0200 ---------~---....:-.:.:...--.=:~.. ,.

'----------~~-0400

-0·500

Fig. 3(b)-Anodic and cathodic polarization of Nb electrodesin I M nitric acid. (1) After 5 min. of electrode immersion

(----) (2) After reaching steady state (1 h) (-----).

scribed previously and then investigated in 1MHNO~ solution. The impedance spectra weretraced over a period of' 3 h from the steady statewhich was normally achieved for the anodicallypassivated electrodes within 15-30 min from elec-trode immersion in the test solution. The imped-ance spectra for the anodically passivated elec-trodes did not show remarkable variation withimmersion time, indicating that the formed oxidefilm is stable'? The collective impedance spectracorresponding to the four different formation pot-entials are presented in Fig. 4. For comparison,the impedance data corresponding to the bare ni-obium electrode in the same electrolyte and afterthe same time of immersion is also included.

Unlike tantalum, the phase angle maximum0max, deviates markedly from 90° and no ideal im-pedance behaviour could be obtained. This meansthat the passive films formed on niobium does notbehave as a perfect dielectric like some other

lOOK ..", ... ~_--_~

75

3K0'1:

~ .... 0.u IK cc 00 1,5 ••'tl•• 300 onQ. 0

E s:100 a,

3030

10

-2-0

~--~~~~~~~~--~~ __ ~~~o100m 1 2 5 10 30 roo 300 IK 3K 10K 30K lOOK

Frt'qu.ncy, Hz

Fig. 4-Bode plots of passivated Nb electrodes after 180min of electrode immersion in 1 M HN03 at different for-mation voltages: (1) 2V, (2) 4V, (3) 7V, (4) 10 V and for com-

parison (5) bare Nb surface.

(0) ( b)

"5'0e0

e

10·0Ec_

oO

o se 6'

0-02'0 4'0 6·0 8'0 10·0

VF ,VOlts

30·0

'0·0

50

---------------------------------------0·0 L--""'2""S----S:.&.0::----:},l::-s ----:'.1;:-0-::-0-~I~2S~---:-;' S~O:--""""'7,~,5

Time) min

Fig. S-~) Variation of the oxide film thickness with the im-mersion time of the passivated Nb-electrode in 1 M HNO) atdifferent formation voltages: (-----) Nb-bare surface, (0) 2V,(e) 4V, (~) 7V, and·(T) 10 V. (b) Final (0), &, and initial (e),bO

, film thickness of passive niobium electrodes as a functionof the formation voltage.

valve metals'<':'. As the formation voltage in-creases, the electrode impedance increases due tothe increase in the oxide film thickness. It is wellknown that the thickness of the formed anodicoxide film on valve metals is a function of the for-mation voltagev". The overall capacitance of theelectrode is given by the double layer capacitanceCdl, the capacitance of the surface states, C", andthat of the space charge layer of the semiconduct-ing oxide, Csc !H,19. Neglecting the capacitance of

BADAWY & Al-KHARAFI: ELECTROCHEMICAL BEHAVIOUR OF NIOBIUM 97

o • 0 0...

100

30

10 ..100m 1 2 5 10 30 100300 lK 3K 10K 30K lOOK

FrfQufllcy, Hz

Fig. 6(a)-(0 0) Experimental Bode plot of Nb-Nb20S-( v,= 10V)-l M HN03 after 180 min of electrode immersion. ( - - )The-oretical values calculated according to R, = 5.87 Q, R; = 139 kQ

c= 9.81lF

the surface states C", the total capacitance, Cd' isgiven by:lICd = lIC,c + lICd ••• (2)Compared to an upper frequency of f= 100 kHz,no pronounced changes in the Mott-Schottkycharacteristics could be observed. The thicknessof the formed passive film, 6, was calculated fromthe capacitance data according to:6 = E EO/Cd ... (3)in which E is the dielectric constant of the oxidefilm taken as 46, EO the permittivity of free space(8.85 x 10 - 14 F em - 1) and Cd is the electrodecapacitance". The variation of the oxide filmthickness with time either for the bare metal sur-face or the anodized electrodes is presented inFig. 5. The rate of the oxide film thickening orthinning can be presented by:6 = 6° + ~ t ... (4)

where ~ is the rate coefficient of oxide filmchange, 6° is the initial oxide thickness and t isthe time of the electrode immersion in the test so-lution. If the oxide film dissolves in the electro-lyte, ~ acquires negative value. Consideration of theresults presented in Fig. 5 show that oxide films for-med at Vf:5:2 V are very stable and undergo thicken-~g with a very small growth rate in the test solution(~= + 0.018 nm min - 1). Oxide films correspond-ing to Vf~ 4V undergo a dissolution process inthe first 30 min of electrode immersion with al-most the same rate coefficient (~= - 0.22 nmmin - 1). After this period the oxide film tends tobe stable and the rate coefficient of its dissolution~ reaches - 2.5 pm min - 1. This means that, ox-ide films formed at higher formation voltages arecosnisting of two layers, an adherent very stable

7530K

75

60 '1».••0.c

45 a::as:II..

o

60 0<1>..S/

'"ca45 ••••as:II..

10KG

• 3K..~ lK~ .~ 300E

- .o )J

100

30 155

10

L---~~~~~~~--~~~~~~O100m 1 2 5 10 30 100 300 11\ 3K 10K 30K lOOK

Frequency ,Hz

Fig. 6(b)-(OO) Experimental Bode plot of Nb-NbPs-1 MHNOJ and (-) computer field data of R, = 5.36 Q, R; = 370 kQ

andC = 3.151lF.

inner layer corresponding to the second segmentof the 6 vs t relation, which is responsible for thehigh passivity of the metal and another more sus-ceptible to dissolution of very small thicknesscorresponding to the first segment of the 6 vs trelation (Fig. 5a).

The initial oxide thickness, 6°, is obtained byextrapolating the 6 vs t relation to t= 0 whichcorresponds to the initially formed anodic oxidefilm at the corresponding formation voltage. Aplot of 6 and 6° vs 'V! gives straight lines with aslope representing the rate of oxide film growthas a function of formation voltage (Fig. 5b). A va-lue of the growth rate of 1.4 nm V- 1 was ob-tained. The reported value of 22 AV-l is onlyobtained if the initially formed oxide film is con-sidered without regard to the relatively fastchange in the first 30 min from electrode immer-sion-".

The deviation from ideal capacitive behaviourof the formed oxide film can be understood hycorrelating the impedance data of the electro-chemical system (Nb/ oxide/electrolyte) to theoret-ical capacitor/parallel resistor model as was donebefore for other electrochemical systems": Datacorrelation and fitting were carried out to imped-ance data after 180 min of electrode immersion in1 M HN03 for a 10 V phosphoric acid anodizedniobium electrode. Fig. 6a represents experimen-tal and theoretical data of the electrochemical sys-tem correlated to numerical values of R, = 5.87Q, representing the solution resistance, ~ = 139kQ, representing the polarisation resistance andCd = 9.8 f-lF, representing the electrode capacit-ance. The impedance data were then fitted to the-oretical values according to the proposed capaci-tor/parallel resistor model using a fitting program.

98 INDIAN J. CHEM. TECHNOL., MARCH 1997

i '90

<: 3KI lK

1300E- 100

75

60•c»~

-30

10IS

, , I I I I , I I I I I 10100m 1 2 5 10 30 100 300 lK 3IC 10K 30M lOOK

FI'Iq~ltz

Fig. 7-Bode plots of 10 V passivated niobium in 0.33 MH)P04 at different current densities after 180 min of elec-trod~ immersion in 1 M HN03 solution. (- - -) I, (-.-.- 5, ( )

10 and (-----) 20 mA cm",

The fitting procedure was applied to computergenerated data of R,; = 5.36 Q, ~ = 370.2 kQ,and Cd = 3.15 IlF such data fitting is presented inFig. 6b. The fitting produces shows clearly thatthe Nb/Nb20/ HN03 system deviates from theideal capacitor behaviour, The mean error in thephase angle is 2S and in the absolute impedanceis 8.9%. These values indicate that the Nb/oxide/HNO, system deviates apparently from the capa-citor/parallel resistor model. This can be attribut-ed to a non-ideal capacitor behaviour of the oxidefilm and a complicated nature of this passive film.In nitric acid solutions NO; and N02- are pres-ent in parts of the film. The N01/NOi redoxsystem from impurity band in the energy gap ofNh20S as was found with TiOF, and it changesthe structure of the oxide layer, especially at theoxide/electrolyte interface, to a porous film. Thestructure changes are the essential cause of the in-flections obtained in the 6 vs t relation (Fig. 5a).This inflection is not present for the 2V anodizedelectrode, since the oxide film thickness at thisanodization potential is small (5-7 nm) and tendsto increase rather than to dissolve.

Effect of formation current density- Anodic ox-ide films were formed on Nb up to 10 V in 0.33M H1P04 at different current densities in therange' ] -20 mA ern - 2. The electrodes were theninvestigated in 1 M HNO) solution. The collectiveimpedance spectra of the different electrodes arepresented in Fig. 7. The impedance data showthat the oxide film formed at lower current dens-ity (::::; 1 mA em - 2) is more stable than thatformed at higher current densities (~ 5 mAem -1). The general behaviour of the oxide film

90

lOOK ...---- "--"'\"- 160.c»~, ..\ ~, 0

\ 45 II

" ;g, L.. \,_. 30 e,

75

<: 10K

••~ 31\v~ III

~. 300- 100

30

10

, I , I I , , I , I I , '01 2 5 10 30 100100 III 31\ lOll 10K lOOK

Fr~qu ••ncy,Hz100m

Fig. 8-Bode plots of JOV passivated Nb electrodes in 0.33M H)PO. at 1 mA cm - 2 after 180 min of electrode immer-sion in nitric acid solutions of different concentrations. (----) 1M,

(----) 0.1 and ( ) 0.01 M HNO).

did not change with time, especially for thoseformed at 1 mA em - 2. The increased stability ofthe passive film formed at low current densities isdue to the increased stoichiometry and the dec-rease in the number of defects in the formed ox-ide film+". Calculation of the oxide thickness hasshown that the steady state oxide thickness calcu-lated after 30 min of electrode immersion variesslightly with the formation current density, arange of 12-15 nm for the lOV anodized elec-trode at 1 to 20 mA cm", respectively, was ob-tained.

Effect of concentration of the test electrolyte=The 10 V anodized Nb-electrodes were investi-gated in nitric acid solutions of different concentr-ations. The impedance spectra (Fig. 8) show thatthe electrode impedance is higher in the more di-lute solution, viz., Z = 139 kQ in the 1 M HN03solution and 252 kQ in the 0.1 M HN03 mea-sured after 180 min of electrode immersion. Therespective values measured immediately afterelectrode immersion are 287 and 441 kQ. Thesevalues support the observation that the oxide filmthickness decreases after longer period of immer-sion of the anodized electrodes in solutions ofHN03 of higher concentration (lM). The effect ofthe electrolyte and also the passivation medium isreflected clearly in the Mott-Schottky plots ob-tained from capacitance measurements. The in-crease of concentration of nitric acid is accom-panied by a decrease in the donor concentration.Such change in the donor concentration is reflect-ed in the dissolution behaviour of the oxide filmand its passive nature.

BADAWY & Al-KHARAFI: ELEcrROCHEMICAL BEHAVIOUR OF NIOBIDM 99

10·0 •

8·0

..~ 6·0

N'"-~•...o-rIU

.•..•.

4·0

7·0

Pot."tiol ,V

Fig. 9-Mott-Schottky plots of passive niobium films in 0.33M H3PO. at a current density of 1 mA cm-z. (0) 2V, (e) 4V,

(~) 7 V and (.) 10 V.

Flat-band potential and donor concentrationImpedance measurements have shown that the

electrode capacitance is frequency independent inthe frequency range 1-40 kHz. Accordingly, fur-ther capacitance measurements were made at afixed frequency of 1.5 kHz and presented asMott-Schottky plots (Fig. 9). The electrode capa-citance is a function of the space charge capacit-ance of the semiconducting oxide film (Eq. 2).The flat-band potential was obtained from the in-tercept of the Mott-Schottky plot with the poten-tial axis. The donor concentration was calculatedfrom the slope of the plot according to:Nd = 2 (s fOe .dC-2/dEt 1 ••• (5)Where Nd is the donor concentration in em - 3, eis the electronic charge (1.60 x 10 - 19 As) anddC-2/dEis the slope of the Mott-Schottky plot.The value of the flat-band potential was found tobe independent of the formation current densityor the formation voltage for all measurements in1 M HN03. A value of about -175 mV was ob-tained. This value changes only with the change inthe concentration of the ambient electrolyte. It isshifted in the anodic (positive) direction as theconcentration of the acid increases, which means

Table l=-Donor concentration and flat band potential of 0.33M H3P04 anodized Nb- electrodes in naturally aerated nitric

acid solution.(a) Effect offormation voltage (~•. ,lmA cm? in 1 M HN03)

VJlV ~,nrn No,cm-3 Efb, V2 7.to 3.07x 1019 -0.1754 9.90 0.96 x io» -0.1707 12.6 0.31 x tol9 -0.180W- 14.9 0.18 x tol9 -0.175

(b) Effect offonnation current density (Vf~~O V in 1 M HNOJ)

i,.'JRAcm-z No, cm ":' EIb,V

0.18 X 1019

0.11 X 1019

0.10 X 10'"

1.0010.020.0

-0.175-0.180-0.170

(c) Effect of concentration of ambient electrolytesl (Vf=:l0 V,v: 1 mAcm-Z)

Concentration of HN03

0.18 X 101•

0.24 X io--0.175-0.320

I.OM0.1 M

that the flat-band potential is pH dependent. Thedonor concentration, on the other hand, decreasesby one order of magnitude when the oxide filmthickness increases from "'" 7 to "'" 15 nm. Theformation current density did not show such largeeffect. For the oxide film formed under the sameconditions at 1 and 20 mA ern - 2 a decrease inthe donor concentration by only 30 % was calcu-lated. The different values of donor concentrationof the different electrodes are presented in Table1.

AcknowledgementThis work has been supported by Kuwait Uni-

versity, Research grant No. SC059.

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100 INDIAN J. CHEM. TECHNOL., MARCH 1997

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