An electrochemical impedance spectroscopy study of passive zinc and low alloyed zinc electrodes in alkaline and neutral aqueous solutions

Corrosion Science, Vol. 32, No. 5/6, pp. 621-633, 1991 Printed in Great Britain.

0010-938)(/91 $3.00 + 0.00 © 1991 PergamonPress plc

AN ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDY OF PASSIVE ZINC A N D LOW ALLOYED ZINC ELECTRODES IN ALKALINE AND NEUTRAL A Q U E O U S SOLUTIONS A . E. BOHE,* J. R. VILCHE,* K. Jf~TTNER,~" W . J. LORENZ,t W . KAUTEK~ a n d W . PAATSCH:~ *Instituto de Investigaciones Fisicoquimicas Teoricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, Sucursal 4-C. C. 16, 1900 La Plata, Argentina t Institut fiir Physikalische Chemie und Elektrochemie, Universit~it Karlsruhe, F.R.G. SBundesanstalt for Materialforschung und -priifung (BAM), Berlin, F.R.G. Abstract--The electrochemical behaviour of passive films on Zn and low alloyed Zn-Co coatings (Co < 1.0%) has been studied in alkaline and neutral aqueous solutions (8.4 -< pH <- 12.9) by means of electrochemical impedance spectroscopy (EIS). For comparison, measurements on ZnO single crystals were also performed. From Mon-Schottky plots the flat band potential Eva and the donor concentration N D were determined as a function of pH and Co content. A Nernstian dependence of EFB on pH and an increase of N D proportionallyto the Co content were found. The passive current of Zn coatings decreases with decreasing pH value and increasing Co content. Photocurrent and photovoltage measurements are in satisfactory agreement with the electrochemical results. The decrease of the passive current density observed with increasing Co content can be explained in terms of a chemical stabilization of the passive film. INTRODUCTION THE FORMATION, c o m p o s i t i o n a n d p r o p e r t i e s of a n o d i c passive layers on Z n in a q u e o u s m e d i a h a v e b e e n t h e m a t t e r o f m a n y p u b l i c a t i o n s in t h e last d e c a d e s , p a r t i c u l a r l y d u e to the fact t h a t Z n is o n e of the m o s t i m p o r t a n t a n d w i d e l y u s e d m e t a l l i c c o a t i n g s for c o r r o s i o n p r o t e c t i o n o f i r o n o r steel u n d e r a t m o s p h e r i c o u t d o o r a n d i n d o o r e x p o s i t i o n . T h e l i t e r a t u r e on the s u b j e c t is r e v i e w e d in R e f s 1-4. D e s p i t e t h e r e l e v a n t w o r k d o n e , t h e r e a r e still b a s i c q u e s t i o n s r e l a t e d to the p r o t e c t i n g p r o p e r t i e s of surface l a y e r s w h i c h d e s e r v e f u r t h e r i n v e s t i g a t i o n . T h e k n o w l e d g e o f the c o n d u c t i o n b e h a v i o u r of p a s s i v e layers on e l e c t r o d e p o s i t e d Z n c o a t i n g s is i m p o r t a n t for a t t e m p t i n g a c o r r e l a t i o n with Z n O c h e m i c a l l y p r e p a r e d . It is well k n o w n t h a t p u r e Z n O is an n - t y p e s e m i c o n d u c t o r with a large b a n d g a p of a b o u t 3.2 e V at r o o m t e m p e r a t u r e . 5-8 T h e b a n d s t r u c t u r e o f single crystal a n d p o l y c r y s t a l line Z n O p h a s e s h a v e b e e n e x t e n s i v e l y s t u d i e d b y p h o t o - e l e c t r o c h e m i c a l t e c h n i q u e s . 9-11 T h e n - t y p e c o n d u c t i o n results f r o m the n o n - s t o i c h i o m e t r y of t h e e l e m e n t s d u e to t h e p r e s e n c e o f Z n 2+ at i n t e r s t i t i a l p o s i t i o n s in t h e lattice, c r e a t i n g shallow d o n o r levels just b e l o w t h e c o n d u c t i o n b a n d . 12 U n d e r a t m o s p h e r i c e x p o s u r e c o n d i t i o n s t h e c o r r o s i o n r a t e o f Z n c o a t i n g s is closely r e l a t e d to t h e o x y g e n r e d u c t i o n r e a c t i o n at t h e i n t e r f a c e o f t h e s y s t e m o x i d e surface l a y e r / a q u e o u s film. I n this r e s p e c t , it was s u g g e s t e d t h a t it is p o s s i b l e to r e d u c e t h e c o r r o s i o n r a t e if t h e o x i d e surface l a y e r on Z n is m o d i f i e d to r e t a r d t h e Manuscript received 1 May 1990; in revised form 8 June 1990. 621

622

A . E . BOHEet al.

o x y g e n r e d u c t i o n . 13 It has b e e n p o s t u l a t e d t h a t C o ions h a v e t h e ability to p o i s o n t h e Z n o x i d e surface for t h e c a t a l y t i c o x y g e n r e d u c t i o n r e a c t i o n which was c o n s i d e r e d a c a t a l y s e d o n e . T h u s t h e c h e m i c a l c h a r a c t e r o f t h e i n t e r f a c i a l r e g i o n d e t e r m i n e s the e l e c t r o c h e m i c a l r e a c t i v i t y . I n a r e c e n t p a p e r t h e solid s t a t e c h e m i s t r y o f a n o d i c a l l y f o r m e d Z n O p a s s i v e films o n p u r e Z n a n d low a l l o y e d Z n - C o a n d Z n - N i coatings has been studied by means of electrochemical impedance spectroscopy (EIS) and p h o t o e l e c t r o c h e m i c a l m e a s u r e m e n t s . 14 T h e e x i s t e n c e o f d e e p levels d u e to i n c o r p o r a t i o n o f t h e s e e l e m e n t s i n t o t h e p a s s i v e l a y e r was p r o p o s e d f r o m t h e m i d g a p i m p e d a n c e results a n d m e a s u r e m e n t s o f t h e o p t i c a l q u a n t u m efficiency. 14 T h e p r e s e n t p a p e r is d e v o t e d to a s t u d y o f t h e s e m i c o n d u c t i n g p r o p e r t i e s o f a n o d i c a l l y f o r m e d o x i d e l a y e r s o n e l e c t r o d e p o s i t e d Z n e l e c t r o d e s in r e l a t i o n to t h o s e o f Z n O single crystal e l e c t r o d e s b y m e a n s o f E I S , a n d l a t e r to an analysis of the effects d u e to t h e a d d i t i o n o f w e l l - d e f i n e d small a m o u n t s o f c o b a l t in zinc coatings o n t h e s e m i c o n d u c t i n g p r o p e r t i e s o f p a s s i v e zinc o x i d e layers. EXPERIMENTAL METHOD The working electrodes consisted of Zn and low alloyed Zn-Co (Co content -< 1.0%) films of 30 ~m thickness electrodeposited on a steel substrate according to the procedure already described.TM For the sake of comparison, some measurements were performed by using ZnO single crystals grown in the gas phase, as a working electrode. The crystallographicorientation of the ZnO single crystal was (0001). Prior to the electrochemical experiments the Zn surfaces were cleaned with acetone, whereas the ZnO surfaces were polished with fine grade diamond paste and etched for 3 min in 5 M HC1. Finally, the electrodes were thoroughly rinsed with four-fold distilled water and cathodically polarized for 5 min in the hydrogen evolution region. The counter electrode was a large area Pt sheet. Potentials, E(SCE), were measured against a saturated calomel electrode. For the impedance measurements a Pt probe was coupled to the reference electrode through a 6 ~tF capacitor. The following electrolyte solutions were used: 0.1 M NaOH,

pH 12.9

(solution A)

0.1 M Na3PO4 + 0.1 M Na2HPO4,

pH 11.7

(solution B)

0.15 M Na2B40 7 + 0.3 M H3BO3,

pH 8.4

(solution C).

They were prepared from analytical grade (p. a. Merck) reagents and four-fold distilled water. All experiments were carried out at T = 298 K in aerated solution. Details on the electronic equipment employed for the electrochemical measurements have been given elsewhere. 15 Impedance spectra were measured on samples pre-anodized for 2 h at a high potential, which was fixed depending on the composition of both the electrodeposited metal film and the electrolyte solution, and proceeded by potential steps of 0.10 or 0.15 V towards lower potentials within the passive region. Photo-electrochemical experiments were carried out at a constant photon energy of h v = 3.5 eV by using chopped light techniques. 16 Photocurrent and photopotential measurements were performed using a Xenon lamp (150 W) and a monochromator with 4 mm slit width (band width = 16 rim). The light was interrupted by means of an optical chopper at a frequency of 33 Hz and the current was recorded under both illuminated and dark conditions using a lock-in amplifier, the difference being the photocurrent/'ph. The photopotential Eph was measured between the intermittently illuminated working electrode and an unpolarized, dark platinum reference electrode. EXPERIMENTAL RESULTS T h e v o l t a m m o g r a m s o f e l e c t r o d e p o s i t e d Z n a n d low a l l o y e d Z n - C o e l e c t r o d e s r e l a t e d to t h e a c t i v e - t o - p a s s i v e t r a n s i t i o n in still s o l u t i o n s , m e a s u r e d at a scan r a t e o f 1 m V s -1 o n freshly p o l i s h e d a n d c l e a n e d surfaces, s h o w a t y p i c a l N e r n s t i a n p H - d e p e n d e n c e o f t h e p a s s i v a t i o n p o t e n t i a l w h i c h p r e c e d e s a w i d e passive c u r r e n t r e g i o n (Fig. 1). T h e p r e s e n c e o f C o in t h e e l e c t r o d e p o s i t e d Z n c o a t i n g s affects t h e

Electrochemical impedance of zinc

623

O.SO 025

Zn / Solution C

0 -025 ,

E -o5o Zn I Sotution

E -~.

B

0 -0.25 Zn-0.6% Co/Solution A

-OSO 02S

/

Zn/Solution

A

.,~-'~'

0 -Q2S

-2 Esc E / V FIG. 1.

Cyclic voltammograms of zinc ( ( . . . . ) in solution A;

) in different electrolytes and of Zn~,).6% Co 1 mV s u.

]dE(SCE)/dt[ =

voltammetric profile only at potentials E(SCE) --- 0.7 V where an additional anodic peak was found (dashed curve in Fig. 1), and the onset of the anodic current is comparable to that recorded on pure Co electrodes in the same electrolyte.IV The steady-state current density-potential curves, Jss vs E(SCE), obtained in the passive potential region of the different systems after a prepolarization of 2 h at each potential, show that the passivating current for electrodeposited Zn electrodes increases according to the solution pH (Fig. 2). Likewise, Zn coatings with a low Co content exhibit lower Jss values than those observed in the absence of Co under comparable conditions. However, it is interesting to note that the slope of the polarization curve, (ajJOE(SCE)), which is inversely related to the low frequency 10

, '

8

S

Zn/Soiution A

::L

Ii~ ll°

.....-.-

° ~

°1.o

-os

I

o

-o's

I

1.5

EscE/V FIG. 2.

Steady state polarization curves for the different systems in passive potential range.

624

A.E. BOHEet al. 61

r

.o 9,, °'~ : a ~

~'

i

(o) Zn/So[n.A, ESCE= 0 V (A)Zn/SolrtB, ESCE=Q~V (o)Zn/Soln. C, ESCE=0.3 V

-

1

t

i

o

t

0

.90.1

0,

- ---..r--.~1 2, tog (f/s -1)

3,

Fl~. 3. Bode diagramsof passive filmsformedon zinc in the differentelectrolytesand operation potentialsE(SCE).

limit of the overall impedance Z(s) (as will be discussed below), slightly increases with increasing Co content for the low-alloyed Zn electrodes independent of the particular potential. Typical Bode plots of the electrodeposited Zn electrodes, which were prepolarized for 2 h at high potentials 1.2 V -< E(SCE) -< 1.5 V depending on the pH to avoid interference with the oxygen evolution reaction, in the different solutions are depicted in Fig. 3. For the sake of comparison, potentials indicated in Fig. 3 were set taking into account the Nernstian dependence of the passive layer formation according to the shift of the potential of the anodic passivation current peak and also due to the typical pH-dependence of the flat band potential usually found for various semiconducting oxides, is In the case of low alloyed Zn-Co electrodes EIS measurements were performed by using samples which were pre-anodized at 0.6 V for 2 h. This upper potential limit is necessary to avoid interference with the Faradaic reactions detected under steady-state conditions at potentials E(SCE) > 0.6 V (cf. Fig. 2). For the sake of comparison, pure Zn electrodes were also prepolarized under the same conditions prior to the impedance measurements to eliminate effects due to possible changes of the passive layer thickness. Figure 4 shows Bode diagrams of Zn and Zn-1.0% Co electrodes at E(SCE) = 0 V in solution A. The impedance spectra obtained with low-alloyed Zn-Co electrodes are similar to those recorded for the pure Zn electrodes, except that the contribution of the polarization resistance, Rp, at lower frequencies is more clearly observed. In particular the phase angle tends to zero at low frequencies. The Rp value'of the Zn-1%Co coating is obviously lower than that of the pure Zn sample in accordance with the higher slope (Oj~flOE(SCE)) of the d.c. polarization curve of the Zn-Co alloyed sample in the same potential range (cf, Fig. 2).

Electrochemical impedance of zinc

625

(o) Zn (=) Zn - 1.0% Co

A 5 "0 o n

ESCE =0 V

U

Q

3

Q

o_

a

QO O

2

0 00

O00ol

0

-90 FIG. 4.

0

1 2 log(f I s "1)

3

Bode diagrams of passive films on Zn ([2) and on Zn-1.0%Co (x) in solution A at E(SCE) = 0 V.

G e n e r a l l y , the electrochemical i m p e d a n c e of the semiconductor/electrolyte interface can be described by the following transfer function: Z(s) = Rf~ + R H (1

+

SRHCH) -1 q- Rsc (1 + sRscCsc) -1

(l)

w h e r e s = jto is the c o m p l e x variable for a sinusoidal system p e r t u r b a t i o n with ~o = 2~rf. T h e transfer function (1) c o r r e s p o n d s to the electrochemical system depicted in Fig. 5, w h e r e a Rs~Csc parallel c o m b i n a t i o n of the resistance and capacitance of the space charge layer is in series with a Rrr-CH parallel c o m b i n a t i o n

Semiconductor I Electrolyte

Space charge- ~ / / ~ / / ~ region - ~ ~ ~ d o u b l e

Csc

Helmholtz

(a)

~-

(b)

layer

CH

R~

FIG. 5.

Rsc R. Semiconductor/electrolyte interphase model: (a) scheme of the interphase region; (b) equivalent circuit corresponding to the transfer function (1).

A.E. BOHEet al.

626

of the resistance and capacitance of the Helmholtz layer, and an ohmic resistance, Rn. The latter can be measured as the high frequency limit (s ~ oo), whereas the low frequency limit (s---, 0) is related to the sum of the ohmic resistance and the polarization resistance, Rn + Rp, being in this case Rp = RH + Rsc. The experimental impedance spectra can be well described by the transfer function (1) by an appropriate choice of the parameters as seen from the simulation curves 1 and 2 in Fig. 6, which resemble the experimental Bode plots of both electrodeposited Zn and Z n - l . 0 % C o electrodes in Fig. 4, respectively. The parameters used for the simulation of curves 1 and 2 differ only by one order of magnitude in Rsc. The lower polarization resistance Rp found in the experiments for the Co-doped Zn deposits may therefore be explained in terms of a decrease of Rsc in the space charge region corresponding to an increase of the electronic conductivity of the semiconductor due to the incorporation of Co into the Z n O layer. It should be mentioned that the two RC-time constants in the transfer function (1) do not differ significantly and therefore the simulation does not show a clear separation of both time constants. Therefore it is not possible to determine all the parameters in equation (1) with sufficient accuracy from the experiments by using a non-linear least squares fit procedure. Alternatively, the analysis was restricted to the determination of Cs~ from high frequency data where the transfer function (1) can be approximated by slim Z(s) = Rn + (SCH) -1 + (sCsc) -1 = Rn + (sC) -1.

(2)

The overall capacitance C of the passive Zn electrode at different potentials was estimated from the imaginary component lm {Z(s)} = (sC)-1 of the impedance data at frequencies in the range 1 kHz -< f-< 10 kHz using equation (2). The C value at a

" {2} 3 oc~

2 I

I

0

a, -30k

I\(1)

~\

I

\

I \',.

-90 _1200r -1

,~ 0 1

I

,

/

/

i

/

, , 2 3 /, i09 (f IS "z)

5

FIG. 6. Simulation of the experimental impedance spectra in Fig. 4 using the transfer function (1) corresponding to the equivalent circuit in Fig. 5b. Simulation parameters: CH = 20#F cm-2, C~c= 4.5pFcm-2, RH = 104£~cmz, Rn = 20 g'/cm2;(1) Rsc = 2 x 105 cmz and (2) Rsc = 104l~ c m 2.

Electrochemical impedance of zinc

0.25

i

i

627

v

( a ) Zn/Solution A ( = ) ZnlSo[ution B ( o )ZnlSolution C 0.20

e~

E U

0.15

6 (.3

oo

0.10

0

A

/ II i

0.05

I

°1.o

•

,

n

-o.s

6

o'.s

11o

I

is

20

Esc E / V

Fie,. 7.

Mon-Schottky plots of passive layers on zinc obtained from impedance measurements in different solutions.

given potential was obtained from extrapolation of C vs f - i and C vs f 2 plots at f__+ 0o, yielding practically the same result with both procedures. The Csc values of the space charge layer were calculated from the experimental C values properly corrected for CH assuming a conservative constant value of CH = 20/*F cm-Z. The Csc values were found to be about one order of magnitude lower than the CH value. The potential dependence of Csc should follow the well known Mott-Schottky relation: 19,20 C s c 2 ~--

2 (eNDeeO)-' (E(SCE) - EFB -- kT/e)

(3)

where N D is the concentration of donor states in the semiconducting passive film and EFB is the flat band potential. All other symbols have their usual meaning. The Mott-Schottky plots C~ 2 vs E(SCE) for the anodically formed passive films on Zn electrodes studied in the different solutions A-C exhibit well defined linear portions over a wide potential range (Fig. 7). The donor concentration ND obtained from the slope of the linear part of the C~c2 vs E(SCE) plots diminishes with decreasing pH value. The ND values are found to be relatively high, about 1 x 102°-3 × 102o cm -3, but typical for anodically formed semiconducting passive layers. The observed increase of ND with increasing pH value from neutral to alkaline solutions can be explained by the amphoteric character of zinc leading to a slight variation of the

628

A . E . BortE et al.

(e) ($) (e) (s)

#l

Zn Zn- 01,'/, CO Zn- 0.6 "/o Co Zn- tO '/, Co

i / 9

/ iI / !

¢ I /

J

I.L

I

I

I

/ !

3

r /

0

I

..7..-/ .,,'..- .. .'...... ~..,. .--.

;,'.y

0

-015

()

I

05

EscE/V FIG. 8.

Mott-Schottky plots of passive layers on Zn and different Zn-Co coatings in solution A.

stoichiometry and the properties of the Z n l + 6 0 . A typical Nernstian pHdependence of the fiat band potential EFB was observed despite of the differences in the donor concentration. For further interpretation of the results on low-alloyed Zn electrodes, typical Mott-Schottky plots are depicted in Fig. 8 for different Co contents. It can be seen that the low-alloyed Z n - C o electrodes follow also the Mott-Schottky relation sufficiently well, but as the Co content increases the data slightly deviate from the theoretical straight line at relatively high positive potentials. Thus, with increasing Co content the linear portion of the Csc vs E(SCE) plots are getting shorter. This effect can be explained by the existence of deep energy levels within the band gap which become partially ionized only at high potentials. 21 On the other hand, with increasing concentration of the doping element Co the donor concentration No increases linearly, as shown in Fig. 9, whereas the fiat band potential EFB remains practically unchanged. In attempting to compare the EFB values of the anodically formed passive layers on Zn with those obtained on ZnO single crystals, electrochemical impedance measurements were carried out on a Z n O single crystal electrode in different electrolyte solutions. Figure 10 shows a Bode diagram obtained in borate buffer solution, pH = 8.4, at E(SCE) = 1.4 V which exhibits a behaviour similar to that of

Electrochemical impedance of zinc

629

f

¢J

o

% z

o16 0'.8 11o % Co FIG. 9.

Dependence of the donor concentration Nr~ on the Co-content in the zinc coating derived from the slope of the Mott-Schottky plots in Fig. 8.

the anodized Zn samples. However, the system behaves purely capacitively in the medium and low frequency range studied and no polarization resistance could be observed. These results are in accordance with the transfer function (1) for the case of a high Rsc value. A similar behaviour was found for the impedance spectra of the ZnO single crystal electrode over a wide potential range in the different solutions used. The capacitance values calculated from the imaginary component of equation (2) were found to be nearly constant in the 1-10 kHz frequency range. The measured Csc values are at least 400 times lower than the Helmholtz layer capacitance CH. Mott-Schottky plots for the ZnO single crystal electrode are shown in Fig. 11. The slope of the linear C~ 2 vs E(SCE) relation is found to be nearly independent of the

~-" u~

6

ZnO-single crystat/S01ution C ESCE= 1.4V °%°°

0%

o

N o

0oo %

3

i

i

i

i

•o

ooo i

o~

o°o2

-

FIG. 10.

0

1

2

log(f Is -~)

3

Bode diagram of Z n O single crystal in solution C at E(SCE) = 1.4 V.

630

A.E. BOnEet al.

(a]

8

Solution

A

(A) Solution B

(o) Solution C

',--

6

E U

It. p i

¢9

3

,,/,i

I

//

/'~' /~ i i /.

ZnO- Single Crystal

ii///11 /

01 /'" -o.5 ,(

6

65

1:o

1.s

2.0

ESCE/V

F[~. 11. Mott-Schottkyplots of ZnO singlecrystalsin differentsolutions. solution pH value and therefore the donor concentration No ~ 1.8 _+ 0.1 × 1016 cm -3 appears to be also unaffected by the pH value for the ZnO single crystal electrodes. The ND values are about four orders of magnitude lower than those obtained for the anodically formed ZnO layers. However, the flat band potentials EVB obtained on both types of electrodes are in close agreement in the corresponding solutions and exhibit the expected Nernstian pH dependence (Fig. 12). In addition, photo-electrochemical measurements with constant photon energy of hv = 3.5 eV were carried out on anodically formed ZnO layers to compare the onset-potential of the photocurrents with the EvB values obtained by extrapolation of the Mott-Schottky plots in Figs 7 and 11. Both, photocurrent and galvanostatic photopotential measurements in 0.1 M NaOH (pH 12.9, solution A) show that the onset of the photocurrent and the photovoltage appears at potentials around E(SCE) ~ - 0 . 7 V (Fig. 13). This is in accordance with the results obtained from the impedance measurements (Fig. 7) as well as with literature data. 22-25 Furthermore, the negative sign of the photovoltage confirms the n-type semiconducting property of the passive layer on Zn. DISCUSSION The experimental results presented for anodically formed passive layers on Zn coatings and for ZnO single crystals in aqueous electrolytes covering the pH range 8.4-
Electrochemical impedance of zinc

(v) Zn

-O.l. -0.5 >

631

"I

(v) ZnO-single crystal

"% ' % . %.

-0.6

%.

{D LL

"'I"

u.I -0.7

-0.8 -0.9

6

~

1'1

1'0

1'2

1'3

pH FIG, 12. Dependence of the flat band potential EFB on the solution pH for Zn ( V ) and ZnO single crystal ( V ) obtained from the extrapolation of the corresponding MottSchottky plots.

- ~2/' . ~Zn
0i

>E -15: uJ

_l:

-0.5

0

05

10

Esc E / V Fro. 13. Photocurrent, ]ph VS E(SCE), and pbotovoltage, Eph VS E(SCE), curves for passive layers on zinc in solution A. Photon energy Ehv = 3.5 eV and chopper frequency f = 33s -1.

632

A.E. BOHEet al.

previously identified as ZnO by electron diffraction, but also weak lines due to v-Zn(OH)2 were noted. 1 Recent in situ Laser Raman Spectroscopy on passive films of Zn in alkaline media confirmed the presence of ZnO. 26 For ZnO single crystals it has been proposed that the effect of pH on the double layer potential can be discussed in terms of amphoteric dissociation of - ZnOH surface groups on the basis of the pH dependence of the EFB values. 22 The n-type conductivity of zinc oxide arises from a Zn excess in the nonstoichiometric compound Znl+~O with 6 > 0. The large band gap of 3.2 eV was recently determined from the spectral response.14 The band model of anodic ZnO layers was already analysed two decades ago. 27 The Zn excess is also important for the instability of ZnO in aqueous solutions. In the absence of illumination, the rate of anodic dissolution of ZnO crystals is extremely small and is mainly determined by the rate of the chemical dissolution and the Zn excess in the ZnO specimens. 2s Recently, from sensitive measurements of the solution conductivity combined with colorimetric determination of zinc in solution, it was concluded that the calculated solubility of active Zn in water at pH = 8.54 and T = 298 K leads to a value of log Ksp = 11.22, which is lower than the values in the literature. 29 The well defined linear Mott-Schottky plots in Figs 7 and 11 suggest the absence of fast surface states on both the anodic ZnO films and on the ZnO single crystal substrate. The anodic ZnO film can be considered as a highly-doped amorphous ntype semiconductor. The incorporation of up to 1% Co into the matrix of the zinc coating leads to a proportional increase of the donor concentration No in the passive film (Fig. 9). Positive potential changes relative to E v ~ provoke a redistribution of charge and potential within the space charge layer obviously without affecting the Helmholtz layer. Increasing Co content promotes the oxygen evolution reaction at lower positive potentials (Figs 1 and 2). The addition of Co to a wide band semiconductor as TiO2 has been shown to increase significantly the spectral response in the visible range, but a reliable attribution to particular electronic transitions was not achieved. 3° The deep donor levels of Co detected by EIS measurements in the anodically formed passive film on low alloyed Zn-Co coatings do not exhibit a clear spectral activity. 14 Obviously these dopands represent filled metallic states within the band gap of the semiconducting zinc oxide which can not contribute to the spectral response. Recently, calculations of Zn-O bond-centered clusters, in which zinc and oxygen second nearest neighbours are present, indicate that oxygen vacancies form deep donors and zinc vacancies are predicted to form shallow acceptors. 31 From the viewpoint of corrosion protection, the results show that the decrease of the polarization resistance Rp of the Zn-Co samples (cf. Fig. 4) cannot simply be interpreted as an indication of an enhanced corrosion rate, because Rp is only correlated to the inverse slope of the d.c. polarization curve at the operational point of the EIS measurement. Experimentally it was found that the overall current density in the passive range is slightly diminished by the incorporation of small amounts of Co into the matrix of the Zn coating (cf. Fig. 2). From the above discussed influence of Co on the semiconducting properties of the passive film resulting in a decrease of the resistance Rsc of the space charge layer with increasing Co content, it is difficult to attribute the slightly improved corrosion resistivity of Z n Co coatings to a single process at the ZnO/electrolyte interface. It may be referred to a retardation of an electrochemical charge transfer or electrocatalytic step of the

Electrochemical impedance of zinc

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anodic and/or cathodic corrosion reactions probably associated with an improved chemical stability of the zinc oxide due to an increase of its amorphousness. It is also possible that the necessary transport of ions or vacancies through the oxide film is retarded by the incorporation of Co atoms into the oxide film. Further investigations may also be helpful to understand the influence of doping elements on the kinetics of passive film formation. Acknowledgements--The authors would like to thank the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen (AIF) for financial support of this work. One of us (J. R. V.) is grateful to the Alexander von Humboldt Foundation for making this cooperation work possible. REFERENCES 1. R. D. ARMSTRONGand M. F. BELL, in Special Periodical Reports on Electrochemistry, Vol. 4 (ed. H. R. THIRSK), p. 1. The Chemical Society, London (1974). 2. R. J. BRODELand V. E. LEGER,in Encyclopedia of the Electrochemistry of the Elements, Vol. V (ed. A. J. BARb), p. 35. Marcel Dekker, New York (1976). 3. H. LE1DHEISER, in Corrosion Mechanisms (ed. F. MANSFELD), p. 165. Marcel Dekker. New York (1987). 4. Y. E. GRAEDEL,J. electrochem. Soc. 136, 193C (1989). 5. G. HEILAND, E. MOLLWOand F. STOCKMANN,Solid State Phys. 8, 191 (1959). 6. J. F. DEWALD,in Surface Chemistry of Metals and Semiconductors. John Wiley, New York (1960). 7. A. K. VIJH, in Electrochemistry of Metals and Semiconductors. Marcel Dekker, New York (1973). 8. H. E. BROWN, in Zinc Oxide--Properties and Applications. International Lead and Zinc Research Organization, New York (1976). 9. H. GERISCHERand H. TRIBUTSCH, Ber. Bunsen. phys. Chem. 72,437 (1968). 111. F. CARDONand W. P. GOMES,Surf. Sci. 27,286 (1971). 11. D. FICHOV,J. POULIQUEU,J. KOSSANYI,M. JAKANI,G. CAMPETand J. CLAVERIE,J. electroanal. Chem. 188, 167 (1985). 12. Z. M. JARZELSKI,in Oxide Semiconductors. Pergamon Press, London (1973). 13. H. LEIDHEISERand I. SUZUKI,J. electrochem. Soc. 128,242 (1981). 14. J. R. V1LCHE, K. J/JTrNER, W. J. LORENZ, W. KAUTEK,W. PAATSCH,M. H. DEANand U. STIMMING,J. electrochem. Soc. 136, 3773 (1989). 15. J. EIITZIG, K. JC1"rNER,W. J. LORENZand W. PAATSCH,J. electrochem. Soc. 133,888 (t986). 16. W. PAAISCH,J. Physique 38,151 (1977). 17. H. GOMEZMEIER, J. R. VILCHEand A. J. ARVIA,J. electroanal. Chem. 138, 367 (1982). 18. S. R. MORRISON, in Electrochemistry of Semiconductors and Oxidized Metal Electrodes. Plenum Press, New York (1980). 19. W. SCHOrrKY, Z. Phys. 113,367 (1939). 20. N. F. Morr, Proc. R. Soc. A 171, 27 (1939). 21. M. H. DEAN and U. STIMMIN6,J. electroanal. Chem. 228, 135 (1987); Corros. Sci. 29, 199 (1989). 22. J. F. DEWALD, Bell Tech. J. 39,615 (1960). 23. F. LOHMANN,Ber. Bunsen. phys. Chem. 70, 428 (1966). 24. B. PEa'rINGER,H. R. SCHOPPEL,T. YOKOVAMAand H. GEmSCHER,Ber. Bunsen. phys. Chem. 78, 1024 (1974). 25. A. E. BonE, J. R. VILCHE, K, J~TrNER, W. J. LORENZand W. PAATSCH,Electrochim. Acta 34, 1443 (1989). 26. S. T. MAYER and R. H. MULLER, Ext. Abstract No. 511, p. 769, 177th Electrochemical Society Meeting, Montreal (6-11 May 1990). 27. W. P. GOMEZ, T. FREUNDand S. R. MORRISON,J. electrochem. Soc. 115,818 (1968). 28. H. GEmSCHER,J. electrochem. Soc. 113, 1174 (1966). 29. G. BOHNSACK,Ber. Bunsen. phys. Chem. 92,803 (1988). 30. Y. MATSUMOTO,J. KURIMOTO,Y. AMAGASAKIand E. SATO,J. electrochem. Soc. 127, 2148 (1980). 31. M. H. SUKKAR,K. H. JOHNSONand H. L. TULLER, Mater. Sci. Engng B6, 49 (1990).