Breakdown of Passivity of Nickel by Fluoride

Vol. 131, No. 4 -f 4....__'

T H E N I C K E L ELECTRODE I

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Fig. 7. Resistance R, as a function of voltage for a sintered nickel electrode (closed circles) and a sintered nickel electrode with 10% cobalt additive (closed squares) during a C/100 discharge of residual capacity. The open symbols indicate the diffusion resistances Rd, that were also measured at the higher voltages.

about 70 mV. This has significant effects on the rechargeability of the nickel electrode. The high peak in the resistance during the transition from the upper to the lower voltage plateau appears to be eliminated by the cobalt additive. The peak and how cobalt additives act to eliminate it are the subject of continuing study.

Acknowledgments The support of the Space Division of the United States Air Force is gratefully acknowledged (contract no. F04701-81-C-0082). We also wish to thank P. Riley for invaluable technical assistance in the chemical analysis of the electrodes. Manuscript submitted March 7, 1983; revised manuscript received Nov. 7, 1983. This was Paper 40 presented at the San Francisco, California, Meeting of the Society, May 8-13, 1983.

The Aerospace Corporation assisted in meeting the publication costs oS this article. REFERENCES 1. H. Bode, K. Dehmelt, and J. Witte, Electrochim. Acta, 11, 1079 (1966). 2. R. S. Schrebler Guzman, J. R. Vilche, and A. J. Arvia, J. Appl. Electrochem., 9, 183 (1979).

713

3. R. S. Schrebler Guzman, J. R. Vilche, and A. $. Arvia, This Journal, 125, 1578 (1978). 4. R. Barnard, C. F. Randell, and F. L. Tye, J. AppL Electrochem., 10, 109 (1980). 5. D. Tuomi, This Journal, 112, 371 (1965). 6. D. M. MacArthur, ibid., 117, 422 (1970). 7. D. M. MacArthur, ibid., 117, 729 (1970). 8. Z. Takehara, M. Kato, and S. Yoshizawa, Electrochim. Acta, 16, 833 (1971). 9. R. E. Carbonio, V. A. Macagno, M. C. Giordano, J. R. Vilche, and A. J. Arvia, This Journal, 129, 983 (1982). 10. R. Barnard, C. F. Randell, and F. L. Tye, J. EIectroanal. Chem., 119, 17 (1981). 11. J. F. Jackowitz, in "The Nickel Electrode," R. Gunther and S. Gross, Editors, p. 48, The Electrochemical Society Softbound Proceedings Series, Pennington, NJ (1982). 12. R. Barnard, G. T. Crickmore, J. A. Lee, and F. L. Tye, J. Appl. Electrochem., 1O, 61 (1980). 13. B. Kalpste, J. Mrha, K. Micka, J. Jindra, and V. Marecek, J. Power Sources, 4, 349 (1979). 14. J. Zedner, Z. Elektrochem., 11, 809 (1905); 12, 463 (1906) ; 13, 752 (1907). 15. S. U. Fatk, This Journal, 107, 662 (1960). 16. H. Bode, K. Dehmelt, and H. V. Dohren, Abstract 6, in "Proceedings of the Second International Symposium on Batteries," D. I-I. Collins, Editor, Oriel Press, Brighton (1966). 17. I. A. Dibrov, Elektrokhimiya, 14, 114 (1978). 18. F. Foerster, Z. Elektrochem., 13, 414 (1907); 14, 17 (1908) ; 14, 285 (1908). 19. O. Glemser and J. Einerhand, Z. Elektrochem., 54, 302 (1950). 20. O. Glemser and j. Einerhand, Z. Anorg, Allg. Chem., 261, 26 (1950). 21. J. A. Cherepkova, V. A. Kasyan, V. V. Sysoeva, N. N. Milyutin, and A. L. Rotinyan, Elektrokhimiya, 11, 443 (1975). 22. V. A. Kasyan, V. V. Sysoeva, N. N. Milyutin, and A. L. Rotinyan, Elektrokhimiya, 11, 1427 (1975). 23. R. Barnard, C. F. Randell, and F. L. Tye, J. Appl. Electrochem., 10, 127 (1980). 24. D. F. Pickett and J. T. Maloy, This Journal, 125, 1026 (1978). 25. A. H. Zimmerman, M. R. Martinelli, M. C. Janecki, and C. C. Badcock, ibid., 129, 289 (1932). 26. R. Sabapathy, P. V. Vasudeva Rao, and H. V. K.Udupa, J. Electrochem. Soc. India, 16, 135 (1967).

Breakdown of Passivity of Nickel by Fluoride II. Surface

Analytical

Studies

B. P. L~chel

Institut fi~r Physikalische Chemie, Freie Universit6~t Berlin, D-1000 Berlin 33, Germany H.-H. Strehblow*

Institut fi~r Physikalische Chemie, Universit(~t Dfisseldorf, D-4000 Di~sseldorf 1, Germany ABSTRACT Passive layers on nickel and their change by the action of fluoride have been studied by surface analyses such as x-ray photoelectron spectroscopy (XPS) and low energy ion scattering (ISS). The thickness of the layers is deduced from the height of the XPS signals Ni2p3/2, O ls, and Fls and their attenuation by covering layers. Argon sputtering gives information on the in-depth structure of the passivating films in combination with XPS and the higher depth resolution of ISS. Their thickness and chemical composition change with the electrode potential and the time of exposure to HF. A multilayer structure is found with an inner oxide and outer hydroxide film and, in a higher potential range, a fluoride layer in between. The layer structure shows a close correspondence to the results of the electrochemical examination. The thickness and composition of surface layers are of decisive importance for processes at the solid* Electrochemical Society Active Member. K e y words: passivating layers, breakdown of passivity, nickel, fluoride, XPS, ISS, l a y e r thickness.

electrolyte interface. Especially in the field of corrosion, the analysis of the metal surface is necessary to gain a deeper insight into the phenomena and a better interpretation of the electrochemical results. Therefore, we started a combined study of x-ray photoelec-

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J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y

714

t r o n spectroscopy ( X P S ) , ion scattering spectroscopy ( I S S ) , a n d electrochemical examinations. I n a previous paper, the influence of fluoride on the passive b e h a v i o r of nickel was studied (1). A p r o m u n d e x p l a n a t i o n of the findings was only possible on the basis of the surface a n a l y t i c a l results. We e x a m i n e d the films of p a s s i v a t e d n i c k e l and those which have been a t t a c k e d b y fluoride. F o r this purpose, the specimens are p r e p a r e d u n d e r w e l l - d e f i n e d electrochemical conditions. Experimental

The s p e c i m e n p r e p a r a t i o n was described in detail p r e v i o u s l y (1). A f t e r the i n d i v i d u a l p r e t r e a t m e n t , the specimens were r i n s e d with t r i p l y distilled w a t e r a n d t r a n s f e r r e d into the wacuum c h a m b e r as q u i c k l y as possible, t y p i c a l l y w i t h i n 1 rain. F o r the analysis, we used an E S C A L A B 5 s y s t e m ( V a c u u m Generators, L i m i t e d ) w i t h a fast e n t r y airlock, a p r e p a r a t i o n chamber, a m a i n a n a l y t i c a l vessel with an o p t i m u m p r e s s u r e of 5 9 10 -11 m b a r and an o p e r a t i n g pressure of some few 10 -10 m b a r after sample induct. The X P S analysis was r u n w i t h a Mg Ka source with 300W power. F o r s p u t t e r i n g and ISS analysis, a fine focusing scanning ion gun was used. F o r d e p t h profiling, an argon b e a m w i t h 3 keV p r i m a r y e n e r g y and a cont r o l l e d specimen c u r r e n t of 0.7 ~A was scanned over an a r e a of 8 • 8 ram. I S S was p e r f o r m e d w i t h a h e l i u m b e a m of 1 keV p r i m a r y e n e r g y a n d 10 n A c u r r e n t to avoid a n y changes d u r i n g analysis. A gas inlet system facilitated the r a p i d change of gases for the ion gun. Phases of argon s p u t t e r i n g and analysis with h e l i u m followed a l t e r n a t i v e l y , thus p r o v i d i n g an I S S d e p t h profile. A f t e r each s p u t t e r period, an X P S analysis of the most r e l e v a n t p e a k s was taken. The sputter a r e a was l a r g e enough to get o n l y contributions of e q u a l l y etched p a r t s of the surface. F o r ISS, the signal was t a k e n from a small p a r t of the s p u t t e r e d a r e a in the center. W i t h the I S S spectrum, one m a y distinguish b e t w e e n the t h r e e m a i n elements, Ni, O, and F p r e s e n t at the surface. This m e t h o d has an i n - d e p t h resolution of a monolayer. However, d e p t h profiles are affected b y the different changes caused b y argon ion sputtering. Therefore, we used ISS o n l y to d e t e r m i n e q u a l i t a t i v e l y t h e sequence of m u l t i l a y e r structures. F o r X P S analysis, w e used the O l s signal at the b i n d i n g energies EB ---- 529.3 and 531.5 eV to distinguish b e t w e e n o x i d e and h y d r o x i d e , the F l s signal at EB ---685 eV, and the Ni2p3/2 peaks in the r a n g e EB -- 852868 eV. A t EB ---- 852.8 eV, the metallic Ni p e a k is found. The p e a k at 856.5 eV is an o v e r l a p of s e v e r a l signals, Ni(OH)2, NiO, and t h e "shake up" of the metallic nickel. NiO is found a t 853.4 and 855.4 eV a n d N i ( O H ) 2 at 855.8 eV. In the presence of fluoride, a p e a k at 857.2 eV is a t t r i b u t e d to NiF2 after c o m p a r i son with standards. The r e l a t e d "shake up" is found at 863.5 eV. As nickel is p r e s e n t s i m u l t a n e o u s l y at the surface in different valence states, the peaks and their satellites o v e r l a p to a complicated s t r u c t u r e w i t h a base line not w e l l defined. Unfolding this structure is affected w i t h a large error. Therefore, we did not use

ApdZ 1984

this p a r t of the Ni2p3/2 signal for q u a n t i t a t i v e analysis. The m a i n qualitative a n d q u a n t i t a t i v e i n f o r m a t i o n was t a k e n from the metallic l~lzp3/Z, the Ols, and the F l s peak. As references, we used a s p u t t e r - c l e a n e d Ni specimen, 1~iO, Ni(OH)2, a n d Ni~'2 p o w d e r pressed in a tin foil to reduce surzace charging (2) and a Ni sample oxidized in a i r at '100~C a n d so c o v e r e d with a visible NiO film thick enough to p r e v e n t a n y contribution of the metallic Ni peak. Table I shows the r e l e v a n t p e a k positions of different nickel compounds. In cases w h e r e surface charging was a p r o b l e m , the signals were c o r = r e c t e d b y re~erring t h e m to the C l s position with EB ---- 284.8 eV. Carbon was a l w a y s p r e s e n t in traces b e cause of the inevitable c o n t a m i n a n t s from the electro= lyte and the e x p o s u r e to the environment. The results of Table I show a good a g r e e m e n t with those from the l i t e r a t u r e (3-9) and helped in identification of the n a ture of the surface films. The Ni2p3/2 p e a k of the N i 2 0 8 specimen is close to the positions of NiO and Ni(OH)2. The distinction b e t w e e n Ni208 and NiO in the absence of N i ( O H ) 2 is possible b y using the different O l s p e a k positions (7). In presence of N i ( O H ) 2 this distinction is impossible on the basis of E S C A m e a s u r e merits. Therefore, w e could not find s m a l l a m o u n t s of Ni203 in a NiO m a t r i x as p o s t u l a t e d for the t r a n s passive p o t e n t i a l range for Ni. However, the presence of Ni208 cannot be r u l e d out. Results and Discussion

The X P S s p e c t r a of electropolished n i c k e l r i n s e d w i t h t r i p l y d i s t i h e d w a t e r snow q u a l i t a t i v e l y the p r e s ence of a 1~ir film w i t h a N i ( O H ) ~ o v e r l a y e r (Fig. 1). The i~i2p3/2 shows a metallic i,~z p e a k (EB = 852.8 eV) a t t e n u a t e d b y the s u r I a c e film and a p e a k at EB = 856.5 eV with its origin from NiO, Ni(OH)2, and a "shake up" contribution. The O l s signal d e m o n s t r a t e s the presence of h y d r o x i d e (EB = ~31.5 eV) a n d a s h o u l d e r at EB -- 529.9 eV of oxide. A r g o n s p u t t e r i n g enlarges the s h o u l d e r at l o w e r binding e n e r g y w i t h a s i m u l t a n e o u s l y vanishing h y d r o x y l i c signal. A change from n i c k e l h y d r o x i d e to oxide or even its r e d u c t i o n to n i c k e l m e t a l b y p r e f e r e n t i a l s p u t t e r i n g d u r i n g argon ion b o m b a r d m e n t m a y be e x c l u d e d b y the r e sults of other authors (6, 7) and own tests w i t h N i ( O H ) 2 specimens. Consequently, the surface film has a double s t r u c t u r e with an i n n e r oxide and an o u t e r h y d r o x i d e layer. S p u t t e r i n g enlarges the m e t a l l i c Ni2p3/2 signal as a consequence of the t h i n n i n g of the surface film and, c o n t r a r y to Ni(OH)~, b y a p a r t i a l reduction of NiO to Ni m e t a l b y p r e f e r e n t i a l s p u t t e r ing. ISS shows o n l y a Ni and O peak. Both signals a p p e a r a f t e r a s h o r t a r g o n s p u t t e r i n g of some few m i n utes corresponding to a r e m o v a l of less t h a n one m o n o layer. This is a t y p i c a l effect of all I S S spectra w h e n w a t e r or OH groups are p r e s e n t at the surface. Back= scattering of h e l i u m at the l i g h t e r h y d r o g e n atoms does not occur. The n i c k e l specimen passivated in 1M HC104 (e = 1.4V) showed a s i m i l a r X P S s p e c t r u m (Fig. 2). The N i m e t a l p e a k is less pronounced, a n d at EB ---- 855 e V

Table I. Binding energies of XPS peaks and satellites of different nickel standards referred to Cls with EB - - 284.6 eV 01s Specimen Ni metal NtO Ni (OH) s N1F2

Ni2p3/2

852.8 853.4 855.8 857.2

NhO3 (4)

855.7

Ni, mech. p o l i s h e d Ni, electropolished Nl, h e a t e d in air to 700~

852.8 852.5 853.6

O~-

OH-

529.3

531.2 531.0 531.4

Om~.

Fle

858.6b 855.41

856.0 8553 a

860.5b 861.7 b 863.5~

861.4b

531.7

860.9 ~

529.3 529.3 529.3

531.2 531.5 531.2

684.5 532.8

9 Multiplet splitting 141. b Satellite ( s h a k e u p ) .


Vo|. 131, No. 4

P A S S I V I T Y OF N I C K E L

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XPS

Ni2P I II

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Fig. 1. XPS and ISS depth profile of a nickel specimen electropolished in a 57% H2S04 + 43% H20 mixture. I[ I

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I

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850

860

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a larger Ni2p3/2 signal is found. This is a consequence of a thicker oxide film. T w o distinct 01s signals indicate the presence of an oxide and hydroxide layer.

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Sputtering with argon ions again enlarges the Ni metal peak and reduces the oxide shoulder at EB ---- 855 eV. The Ols hydroxide signal also vanishes first within

IS.C

ISS

x3~,

~I0

mlrl

Fig. 2. XPS and ISS depth profile of a passive film on nickel formed in 1M HCI04 at e = 1.4V for 3h.

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J. E l e c t r o e h e m . Soe.: E L E C T R O C H E M I C A L

716

about 5 rain; the O l s oxide signal vanishes w i t h i n 20 min. The I S S signal does not contain f u r t h e r i n f o r m a f/on. The o x y g e n signal also disappears w i t h i n 20 roan of sputtering. A comparison with the electropolished Ni specimen shows a h y d r o x i d e l a y e r of similar t h i c k ness b y both the size of the O l s h y d r o x i d e signal and its d i s a p p e a r a n c e w i t h i n the same s p u t t e r i n g time. However, the oxide thickness has d o u b l e d a f t e r the results for the O l s signal. The X P S d e p t h profile shows again an o u t e r h y d r o x i d e and an i n n e r oxide l a y e r for p a s s i v a t e d n i c k e l electrodes. S i m i l a r results a r e o b t a i n e d within the whole passive r a n g e o f , -- 0.5-1.8V. In the transpassive region for ~ > 1.SV, an increase of the o x i d e thickness is found. A h i g h e r v a l e n t oxide, i.e., Ni2Oz, which is p o s t u l a t e d f r o m electrochemical results could not be detected. A clear indication of its presence is not possible because of the small chemical shift of the Ni2p3/2 p e a k (Table I ) . To obtain q u a n t i t a t i v e results, t h e thickness of the surface l a y e r s was calculated from the X P S signals. F o r base line substraction the b a c k g r o u n d at both sides of a signal was connected at the p e a k position (10). W i t h sufficient accuracy the p e a k heights b e t w e e n the m a x i m u m and the base line were t a k e n for a q u a n t i t a t i v e evaluation instead of the p e a k areas which were controlled in s e v e r a l cases. The calculation was p e r formed on a r e l a t i v e basis with p e a k h e i g h t ratios, w h i c h minimized the error. A n y changes b y unfolding the o v e r l a p p i n g p e a k s causes a l a r g e r inaccuracy. The s t r u c t u r e of the Ni2p3/2 signal is complicated b y the o v e r l a p p i n g peaks of the different n i c k e l compounds and t h e i r satellites. A n unfolding for q u a n t i t a t i v e e v a l u a t i o n w o u l d give r a t h e r uncertain results. T h e r e fore, w e used only the Ni m e t a l p e a k at EB -- 852.8 eV. and in addition the m u c h s i m p l e r O l s signal (EB _-829.3 and 531.5 eV). F o r the unfolding we assume a s y m m e t r i c a l p e a k shape with identical leading and tailing edges. The a t t e n u a t i o n of the Ni2p3/2 m e t a l signal is used to get a value for the thickness of the total layer. If y is the thickness of the oxide, x of the h y d r o x i d e , and 8 of the total l a y e r (Fig. 3), one obtains for the a t t e n u ated Ni m e t a l signal I1Me = KFerMe[Ni]~MeMe e x p ( - - 6 / l o Me) 6= x + y

[1] [la]

w h e r e F is the intensity of the x - r a y source and K a characteristic constant of the s p e c t r o m e t e r containing the transmission cl~aracteristic of the e n e r g y a n a l y z e r a n d the d e t e c t o r and the a n g u l a r acceptance of t h e analyzer. ~Me is t h e photoionization cross section for the Ni2p3/2 signal a f t e r Scofield (11), corrected for the a n i s o t r o p y of the photoelectron emission including the angle b e t w e e n the x - r a y source and the e n e r g y a n a l y z e r (12, 13). rNi] _-- 0.1516 moI cm -8 is the concentration of the n i c k e l atoms in the m e t a l (14), and ~oxMe the escape d e p t h of the Ni2p3/2 photoelectrons from a NiO matrix. In a first a p p r o x i m a t i o n , the escape d e p t h depends only on the kinetic e n e r g y after the r e l a t i o n

a)

Substrate Ni

Oxide NiO

Hydroxide Ni(OH)2

x

b)

Substrate Ni

Vacuum (Electrolyte)

y

Oxide NiO

Fluoride NiF2

Hydroxide Ni(OH)2

x

y

z

Vdcuum (Electrolyte)

Fig. 3. Model of the passive layer on nickel, a: without fluoride. b: with an intermediate fluoride layer.

SCIENCE AND TECHNOLOGY =

A p r i l 1984

Bk/Ekm

[Z]

for kinetic energies Ekia > I00 e V (15) so that the values of N i O and l~i[OH)2 do not differ (koxMe~ koHMe), kMe Me is the escape depth for the metal matrix, which is a i f t e r e n t f r o m that of the oxides w h e n considering the matrix effect to a first approxlmation (15). F o r a clean s p u t t e r e d nickel surface, one obtains for the Ni m e t a l signal I Me = KFr Me[Ni] ~MoMe

[3]

Dividing Eq. [1] and [3] yields the following relations for identical e x p e r i m e n t a l conditmns for the s p e c t r o m eter (K, F ) IMe

= exp (--8/Xo Me)

[4]

W i t h t h e m e a s u r e d i n t e n s i t y of the Ni2p3/2 m e t a l signal of the passivated s p e c i m e n J[1Me and the clean and u n covered s t a n d a r d IMe, one obtains the total thickness 8 -- x + y of the double l a y e r a f t e r r e l a t i o n [4]. F o r the subdivision in the two p a r t s x and y, one m a y use the ratio of the p e a k heights of the O l s h y d r o x i d e and oxide p e a k [ O H - ] [1 -- e x p (--x/~oOH) ]

IOH~

"Iox~ - - [O '~-] exp (--x/~OoH) [1 -- exp (--y/~Oox) ] [5] [ O H - ] -- 0.0896 tool cm-Z a n d [ 0 2 - ] -- 0.0839 tool cm -3 are the m o l a r concentrations of the O H - a n d 0 2 - ions of NiO and Ni(OH)2, r e s p e c t i v e l y (14). The factor in b r a c k e t s takes account of the s e l f - a t t e n u a t i o n of the signal w i t h increasing thickness of the h y d r o x i d e and oxide layer, respectively. The a d d i t i o n a l e x p o n e n tial factor in the l o w e r p a r t reflects the a t t e n u a t i o n of the O l s oxide signal by the h y d r o x i d e o v e r l a y e r . E q u a tion [5J requires identical e x p e r i m e n t a l conditions for the. s p e c t r o m e t e r again. Neglecting the m a t r i x effects on the escape d e p t h with ~Oox : kOoH : ko, one obtains the relation for the thickness x of the h y d r o x i d e l a y e r z = --Zo In

I~

-k I~

e x p ( - - d / ~ o)

Iono[O 2 - ] + IoxO[OH - ]

[6]

Equations [4] a n d [6] p e r m i t one to calculate the thickness 5 = x + y of the total layer, and the p a r t s x for the h y d r o x i d e a n d y for the oxide. The results of this e v a l u a t i o n are d e p i c t e d in Fig. 4. F o r a b e t t e r u n d e r s t a n d i n g , the p o t e n t i o d y n a m i c polarization curve of n i c k e l in 1M HC104 is r e p r e s e n t e d t o g e t h e r w i t h the total thickness 8 and the ~xide t h i c k ness y. The h y d r o x i d e thickness x equals the distance b e t w e e n both curves. The e r r o r of these results is most affected b y the u n c e r t a i n t y of the escape d e p t h ~. of the electrons. This m a y be e s t i m a t e d to be _ 2 0 % , thus leading to a s i m i l a r e r r o r for the thickness values a f t e r Eq. [4] a n d [6] w i t h 5 ,,, ~ and x ,,, L W e a k l y acidic and alkaline sol~tions.--The passive l a y e r in p h t h a l a t e (pH 5.0) and b o r a t e buffer (pH 8.0) has the same d o u b l e l a y e r s t r u c t u r e and o n l y slight changes in thickness a r e found. Table II c o m p a r e s some results of the t h r e e electrolytes. Marcus et al. found the same s t r u c t u r e b u t half the thickness for n i c k e l paasivated at , = 0.54V in 0.1M H2SO4 (3). T h e y used, however, the c o m p l e x Ni2p3/2 signal for t h e i r e v a l u a t i o n and did not t a k e into account the a t t e n u a t i o n b y t h e covering layers. O h t s u k a and H e u s l e r d e t e r m i n e d s i m i l a r values in 1M H2SO4 (, ---- 0.6-1.6V, 8 = 14-20A) for the t o t a l l a y e r (16) b y in situ e l lipsometry, and Sato in b o r a t e buffer p H 8.4 (17). The thickness of t h e h y d r o x i d e l a y e r does n o t change w i t h potential and p H and does not s e e m to h a v e a p r o n o u n c e d influence on the passive behavior. Its constant thickness for all electrolytes suggests t h a t it is


VoL I31, No. 4

P A S S I V I T Y OF N I C K E L

Table II. Layer thickness of passivated nickel at different potentials e in 1M HCI04, phthalate buffer pH 4.9 and borate buffer pH 8.0 and of electropolished nickel

,M HC, l

Time of

E 102" U

717

\ i0 o.

\

e/V

IM HCIO~

0.5

1.0

rain

8/A

140 120

19 18

Thlckness/A of O x i d e Hydroxide

11 12

8 6

200

120 12,0

25 33

23

15 16 ~6

9 7

Phthalate buffer

1.1

160

19

14

w

Borate buffer

0.5

170

24

17

7

- -

29

7

~,~

Nickel electropolished

9 Total layer

passivatlon

1.4

1.6 1.8

10-2-

30

Electrolyte

- -

7

o Oxide tendency to form precipitates. The oxide l a y e r d e t e r mines the passivating properties, as shown by its change with potential especially in the transpassive region, where a reduced dissolution, besides the evolution of oxygen, is found (18). The thinner oxide layer on electropolished nickel also supports this conclusion. These specimens are much better protected against aggressive electrolytes when they are prepassivated additionally for a sufficiently long period (19, 20), and achieve smaller stationary current densities with time.

20. ,,