The formation of the passive layer on Cr in 0.5 M H2SO4 A combined electrochemical and surface analytical study

365

J. Electrvanal. Chem., 228 (1987) 365-392 Else&r

Sequoia S.A., Lausanne

THE FORMATION

- Printed in The Netherlands

OF THE PASSIVE LAYER ON Cr IN 0.5 M H,SO,

A COMBINJZD ELECTROCHEMICAL STUJIY *

AND SURFACE

ANALYTICAL

S. HAUPT and H.-H. STREHBLOW

Institut fiir Physikdische Chemie, Universitiil Diisseldorf, D-4ooO Diisseldvrf I (F.R G.) (Received 2nd March 1987)

ABSTRACT The passivation of Cr has been studied by a close combination of electrochemical and surface analytical methods. The transfer of the specimen in a closed system permits the examination of short oxidation times ( 3 1 ms) and negative electrode potentials just at the beginning of the passive range with a well-defined specimen pretreatment. The anodic oxide contains large amounts of hydrogen-containing species, including sulfate from the electrolyte. It grows according to a high field mechanism. The constants for the rate laws have been determined from the coincidence of quantitative XPS and electrcchemical results. Detailed evaluations require the formation of ca. 0.5 mC/cm’ of a species of lower valency which is oxidized in the course of an anodic passivation transient. The passive potential range may be subdivided in two parts with a change of the constants of the growth kinetics at 0.7 V, where the capacity data indicate a variation of the composition of the anodic oxide with presumably a content of higher-valent Cr ions in a water-containing C203 matrix. Studies with the rotating ring-disc electrode yield only a negligible contribution of the formation of soluble corrosion products ( < 10%) for very non-stationary conditions. For later stages of a transient, the corrosion current density i, is determined indirectly by a comparison of the anodic current and the growth of the oxide deduced from XPS measurements. i, for Cr is about 2 orders of magnitude smaller compared to the values of Fe, with a maximum of ca. 10 PA/cm2 with a still increasing layer formation. These data explain the improvement of the passive behaviour of Fe by its alloying with Cr as a major constituent of stainless steel.

INTRODUCTION

The passivity of Cr has been studied in detail for many years [l]. However, as in the case of other metals there are still many questions open to discussion [l]. To get reliable information, the application of surface analysis in addition to electrochem-

In honour of Professor H. Gerischer on the occasion of his retirement as Director of the Fritz-Haber Institute.

l

0022-0728/87/$03.50

0 1987 Elsevier Sequoia S.A.

366

ical methods is a necessary requirement. Resides several in-situ examinations those methods working in the ultra-high vacuum (UHV), like X-ray photoelectron spectroscopy (XPS), are very valuable because they give detailed information. However, they require the emersion of the electrodes from their environment with the loss of potential control and of contact with the electrolyte, with all sorts of possible changes as a consequence. A close combination of the methods of the two fields, however, and a detailed variation of the experimental parameters provides a mutual confirmation of the results. To exclude surface contamination and additional oxidation by the laboratory atmosphere, specimen transfer within a closed system has been applied, thus extending the experiments to short oxidation times and negative electrode potentials corresponding to very thin oxide layers [2-41. Thus XPS studies yield results for the composition of passive films with a time and potential resolution which enable the interpretation of the electrochemical results [5]. The correlation between electrochemical and XPS results for passive Cr can be used for the interpretation of results obtained in passivity studies on Fe/Cr alloys and stainless steel. EXPERIMENTAL

99.9% Cr was used for these studies. For surface analysis discs of 10 mm diameter were machined and fixed to a specimen stub. The surface was polished with diamond spray to a 1 pm final grading. Usually the specimens were sputtercleaned with Ar (5 kV, 50 PA, 10 mm) to remove any impurities including preexisting oxide. The electrodes were transferred from the preparation chamber to the electrochemical chamber filled with a protecting purified Ar gas atmosphere (Oxisorb, Messer Griesheim). After the specimen was turned upside down, its front plane was contacted to the electrolyte surface within a small glass vessel of 2 cm3 content provided with a Pt counter electrode and a connection to the Hg/Hg,SO,/O.S M H$O, reference outside the electrochemical chamber. The electrodes were polarized for oxidation times in the range of t = 1 ms up to > 1000 s. The oxidation was finished by disconnecting the counter electrode via a mercurywetted Reed relay before the specimen was emersed at open-circuit conditions. The sample was usually &aned by filling the vessel with water repeatedly and dipping the specimen before its introduction into the analyser chamber. The time necessary for the transfer from the electrolyte via the preparation chamber to the analyser chamber was > 1.5 mm, typically 3 min. The pressure was in lo-’ mbar range and decreased within 1 h to lo-* mbar. The closed system prevented the formation of further oxide after the electrochemical preparation. During these studies the performance of the system was controlled routinely with standard specimens like sputtered Hf, Fe, and Cu to detect any traces of oxygen or other unwanted contaminants (31. Ca. 0.3 to 1 nm of oxide were formed under these conditions, which is the smallest film all studies have to start with. The details of the equipment and its performance have been described previously 12-51. The electrochemical equipment included fast potentiostats, a function generator

361

for potential scans, pulse generators (Tektronix 26 G3) and recorders. For studies with the rotating ring-disc electrode (RRDE) a non-commercial tripotentiostat was used equipped with differential amplifiers at the input to drive independently three working electrodes (2 concentric rings and 1 disc) with a common grounded Pt counter electrode and a Hg/Hg,SO,/O.S M H,SO, reference electrode (E = 0.68 V). The current was measured with an non-commercial autoranging differential amplifier. The measuring resistances were switched automatically within 1 ms by Hg-wetted Reed relays. A change of the resistors in the range of 2 0 to 500 kQ provided a sufficient record of the currents between 100 mA/cm2 and 1 pA/cm2. Thus oxidation transients could be measured on a large time scale of 10 ps to > 1000 s without the current limitation of the potentiostat due to high resistances in the measuring counter electrode circuit. The Cr electrodes were embedded in epoxy resin. The RRDE with two concentric glassy carbon rings for the detection of soluble Cr(III) has been described in detail elsewhere [6]. In addition a simple rotating disc electrode @DE) was used, with a 6 mm diameter Cr disc. The electrodes were mounted on a modified commercial disc rotator (Pine Instruments ASR2). Its rotation frequency could be modulated with the potential programme of a function generator (Wavetek 180). The details of the equipment and the preparation of the RRD electrodes have been described previously [6]. The electrochemical data were transferred on line and processed with a personal computer (Apple IIe) via a 1Zbit AD converter. Capacity data were taken with a lock-in amplifier (Ithaca Dynatrac, model 393) with a 5 mV, 1 kHz sinoidal modulation (Wavetek 180) of the electrode potential. All solutions were prepared with pure substances (pro analysis) and ultrapure deionized water (Millipore, Milli Q water purification system). All potentials given refer to the standard hydrogen electrode (SHE). RESULTS AND DISCUSSION

Specimen preparation with a well-defined pretreatment Electrochemical passivation studies require the same pure situation at the surface as specimen preparation for XPS analysis. Especially the removal of contaminants and oxide residues formed during polishing of the sample is necessary for reliable and reproducible oxidation transients with well-known starting conditions. Preexisting oxide layers may be removed within the potential range of active dissolution at < -0.4 V. For better understanding the current density of Cr in 0.5 M H,SO, and the partial current density of Cr2+ dissolution for a potentiostatic study with the RRDE are presented in Fig. 1. The total current density as a function of the potential corresponds to the data in the literature with small deviations for the roughness factor (< 2) under the experimental conditions specified [1,7-91. The same holds for the Cr2+ dissolution in the active range obtained by the oxidation to Cr 3+ at a glassy carbon ring. For our measurements, in the potential range - 0.4 V to -0.2 V, the Cr2+ corrosion current density i,(Cr2’) exceeds the total current density i,. This may be explained by the additional formation of hydrogen. Other

368

Cr,O.SMH,SO,

0 It A lcr*. q

I

I

I

I

-0.6

-0.L

-02

0

E(SHE)

Fig. 1. Stationary

I

IH,,pass eLectrode

I

0.2

V

current density-potential curve in 0.5 M H2S04 in the range of active dissolution, iCrz+ = dissolution of Cr*+, I H2= hydrogen evolution at passive Cr electrode.

i, = total current density,

authors [8] found a positive value for the difference i, - i,(Cr”) with the RRDE technique and postulated the dissolution of undetectably small amounts of Cr3+. Because of the uncertainty of the collection efficiency and the influence of the ring material on the oxidation process of Cr2+ [6,9], a detailed interpretation of the RRDE results in this potential region seems impossible on the basis of the difference of two currents of similar size. The formation of Cr3+ as proposed [8] cannot be found experimentally by a detailed analysis of the bulk electrolyte after a sufficiently long corrosion in this potential range [9]. The electrochemical passivation transient cannot be started with a completely oxide-free oxide. The dissolution of Cr 2+ during a pretreatment within the active potential range and its later reoxidation in front of the electrode complicates the interpretation of the transient current density. In the range of more cathodic currents the formation of hydrogen and its later oxidation may cause problems. Therefore the polished specimen was reduced at -0.6 V for some 10 s to reduce the oxide, pulsed for some seconds to -0.4 V to get rid of hydrogen and remaining

369

contaminants with an anodic current as indicated in Fig. 1 and then was finally prepassivated at -0.12 V for ca. 30 s to remove any Cr’+ by its diffusion into the bulk electrolyte. In these cases where an anodic current at - 0.4 V was not achieved the cathodic reduction at -0.6 V was repeated. The dissolution at -0.4 V must be short in order to avoid surface roughening by too much Cr’+ dissolution. A reliable pretreatment yields passivation transients which coincide with a maximum deviation of ca. 20% for identical conditions. Furthermore this procedure yields the same change of the oxide thickness (XPS) with time for times > 1 ms as specimens pretreated by Ar ion sputtering and introduced, within the closed system, into the electrolyte immediately at the potential of oxide formation. The extrapolation of the oxide thickness determined by XPS yields a value of 0.3 to 0.5 nm for specimens pretreated at -0.12 V.

Concept for the interpretation of the results Passivation studies with various metals demonstrate that the growth of oxide films with barrier character is controlled by a high electrical field strength according to eqn. (1) [lo-131.

i, = A exp( BAE/d)

(I)

where i, is the total passive current density, A is a constant, B the field strength coefficient, AE = Ep - E,, Ep is the passivation potential, E, the oxide formation potential and d the oxide thickness. The passivation of Cr follows the same relation over a wide time and potential range. The kinetic parameters of eqn. (1) are determined by measurements with the different methods and are listed in Tables 2-4 below. A detailed interpretation of the results by an appropriate model is not intended because this requires knowledge of the potential distribution over the Cr/Cr,O, and C,O,/electrolyte interfaces and across the passive layer itself [lo-121. However, this conclusion may not be drawn on the basis of these experiments without speculation. A computer simulation with the parameters A and B of eqn. (1) yields passivation transients with a high coincidence with the experimental results. Starting with an initial oxide thickness d, or a related initial charge Q,, eqn. (1) yields an ionic current, which can be assumed to be constant for short time intervals At (e.g. 10 ps). Integrating the current of layer formation, one obtains the increase of the charge of oxide formation Q with time, which has to be added to the initial charge Q, (eqn. 2). Repeated application of these steps yields the time dependence of the charge, the related oxide thickness and the current density for various conditions (eqn. 3). For example, potentiodynamic conditions can be simulated by varying AE according to eqn. (4). With the assumption of a negligible contribution of corrosion, i, may be identified with the layer formation current density i, which leads to eqns. (2) and (3).

(a) Potentiostatic conditions:

(2)

370

where Q, is the charge for the initial oxide layer and K is the stoichiometric constant relating oxide thickness and charge of layer formation. n-l

c i,(jAt)At

+ Q,

J=o

(b) Potentiodynamic

conditions: eqn. (3) combined with eqn. (4)

v = AE/At

AE = AE, + i vjAt

(4)

J=o

where AE, is the initial potential difference and v the scan rate.

AEIV

calculated

5*/

0.2

I

-3

-2

-1

I

r

1

I

0

1

2

3

log [t/s) Fig. 2. Time dependence of potentiostatic anodic charge of layer formation (without corrosion) for different potentials as indicated, starting with oxide-free surfaces; computer simulation according to eqn. (3), applying experimental values for the constants A and B for passive Fe as indicated (Table 4 below). (a) Anodic charge vs. log(r); (- - - - - -) starting with prepassivated electrodes. (b) Increase of anodic charge for various time intervals with the change of the electrode potential AE = Ep - E,. (c) Variation of the inverse anodic charge with log(t) for various potential changes.

371

(b)

loN E u

‘; 0 Q

d2

ai

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calculated ic)

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312

As a basis for further discussion, Figs. 2a-2c show the results of a computer simulation for the oxide growth with time and potential neglecting any contribution from the formation of soluble corrosion products and applying the parameters for Fe [5] of Table 4 below. The charge of oxide formation Q increases independently of the amount of preexisting oxide (charge Q,) after some induction time. After this period the current for a passivation transient should be independent of Q,. The increase of Q for a prepassivated specimen merges into the curve for a specimen starting with an oxide-free surface. The intercept shifts to shorter times with decreasing Q, and increasing oxidation potential. The charge and the proportional oxide thickness increase for definite times linearly with the deviation AE (Fig. 2b). According to eqn. (2) Q should follow an inverse logarithmic time dependence as shown in Fig. 2c. The inverse slope of these lines will lead again to the constant B [lo]. Equations (l)-(3) and the related figures are discussed frequently in the literature [lo-131 and are shown here mainly for a better understanding of the experimental results for Cr and their comparison with those for Fe. XPS studies All specimen preparations

started with a sputter-cleaned surface. Contrary to Fe, Cr forms stable passive layers in 0.5 A4 H,SO, which are neither dissolved during emersion nor suffer from self activation of the emersed electrode which cannot be embedded in resin for UHV studies as is usually done for electrochemical experiments. We had to restrict our examinations to the passive range (-0.12 to 1.10 V SHE). In the potential range of active dissolution ( < - 0.2 V) and transpassive behaviour (> 1.10 V) soluble Cr(I1) and Cr(V1) products are formed which are removed by rinsing with water. The thinning of the passive layer in the transpassive range as described in the literature [14,15] cannot be examined with the XPS technique we used, because adhering oxidizing Cr,O;- may cause the formation of additional oxide during the loss of potential control. The XPS signal contains a metal and a Cr oxide part with an average chemical shift of 3.0 eV (Fig. 3). The chemical shift in dependence of the oxidation or reduction potentials, was less than 0.5 eV so that no direct correlation between the chemical shift and a definite Cr valence was possible, although higher- or lower-valent products than Cr(II1) must be postulated according to the electrochemical results. The examination of bulk specimens with known stoichiometric composition yields a linear dependence of the chemical shift on the oxidation state [16]. Figure 3 demonstrates qualitatively the increase of the 01s and the Cr2p,,, oxide signal and the related decrease of the Cr2p,,, metal signal with increasing oxidation potential E and oxide thickness. The quantitative evaluation of the XPS signals involves background correction according to Shirley and Bishop [17-191 and a linear least-squares fit according to refs. 3 and 19 using appropriate standards. The signals are described by Gauss-Lorenz curves with a tail function of the asymmetric shape [3]. The standard for the metal signal was a sputter-cleaned Cr electrode, the oxide signal was fitted with slightly varying parameters for different oxidation potentials to get the closest

313

Cr 2p42

570

red: 0.68V+*O.Z2V 575 EB IeV

560

Fig. 3. Cr2~~,~ and related 01s potentials I&.

0

red.

528 530 532 5% 536 Ee I eV peaks of Cr passivated for tp = 300 s in 0.5 A4 H2S04

at indicated

fit. With the model of a simple homogeneous layer of Cr,O,, the oxide thickness was calculated from the intensity ratio according to eqn. (5). The parameters for the thickness evaluation are listed in Table 1. To ensure comparability with the data of other authors, Seah and Den&s values for the escape depth were applied [20], although they might be somewhat too large. L/k

= (D,,X,~/D,X,)(exp(d~~)

- 1)

(5)

Results for this evaluation for Cr specimens passivated for 300 s are given in Fig. 4. The oxide thickness increases linearly with the electrode potential Ep. The minimum thickness amounts to 1.0 run because of film formation by water decomposition. The extrapolation meets the abscissa at -0.2 V. About the same value is obtained from the extrapolated anodic charge Q for the passivation transients of the preactivated specimen. At - 0.2 V the formation of soluble Cr(II) is finished (Fig. 1) and the electrode becomes passive. The slope of the oxide thickness and its intersection with the potential axis at d = 0 depends on the density of the oxide and the escape depth X of the photoelectrons. It will be shown in this paper that despite

374 TABLE 1 Parameters used in the calculation of the thickness of the oxide layer Cr2p,,z XPS signal intensities of the metal and oxide molar densities of Cr and Cr,O, [21], D, = 0.14 mol/cm3, D,, = 0.069 mol/cm3 escape depth of the photoelectrons of the metallic and oxidic signal, calculated according to ref. [20], h, = 1.6 MI, h,, = 2.9 nm Binding energies:

Cr metal, Cr2p,,, passive oxide, CrZp,,, 0 oxide, 01s 0 hydroxide, 01s

574.3 eV 577.5 f 0.25 eV 530.5 eV 532.0 eV

Photoionisation cross sections [22,23]: 8.97 3.61

(Cr2p3J (01s)

35Cr. 0 SMHzSOl tp=3oos 30-

6

2.5-

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04

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2

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Fig. 4. Oxide thickness d according to XPS evaluations, total anodic charge Q and inverse capacity l/C vs. the passivation potential Ep and for passivation time t, = 300 s in 0.5 M HzS04. d,,(H,O) is the oxide thickness formed on oxide-free sputtered Cr by contact with water.

315

some uncertainty of these values, the thickness of the passive layer on Cr may be correlated with the electrochemical results in a quantitative way. The inverse electrode capacity increases linearly with the potential up to 0.6 V and consequently with the oxide thickness d (Fig. 4). For Ep > 0.6 V, l/C decreases whereas the XPS thickness increases with an unchanged slope. At the same potential the slope for the total anodic charge Q gets steeper (Fig. 4). This observation suggests that higher-valent Cr species like e.g. Cr(IV) and Cr(V1) are formed and incorporated within the film, thus changing its electronic properties and consequently the dielectric constant. The 01s signal is rather complicated and contains contributions of oxide, hydroxide, sulfate and water (Fig. 3). The presence of ca. 10% sulfate (with the assumption of its homogeneous distribution) is deduced from a weak Sls signal. These SOi- contaminations are presumably incorporated within the fihn because they cannot be washed off. The passive layer on Cr contains much more water or OH- than an equivalent film on Fe. The loss of water in UHV is a very slow

Cr 05M H,SO,. t,=300s 100

5

s

50 -(air

oxidized)

0 20

tl1.E ," * 910 ----_(a~ oxidized) ------------_

-

Cr,O,

05

E,lSHE) IV

Fig. 5. (Upper part) Hydroxide content of the oxide layer; (lower part) ratio of the 01s to the Cr2p,,, signal in dependence on the oxidation potential, passivation time r,, = 300 s in 0.5 M H,SO,. (- - -) Ratio for Cr,O, calculated with the sensitivity factors of Table 2 below, in addition ratio for air-oxidized sample (20 o C, 5 min).

316

process: the 01s signal does not change significantly within 15 min. The pressure in the analyser chamber was lo-’ mbar after ca. 10 min and the release of water was followed by residual gas analysis. Most of the XPS data for a quantitative evaluation were taken after ca. 5 min of vacuum exposure. The 01s signal can be split into two peaks at 530.5 eV and 532 eV (Table 1). Air-oxidized Cr samples show only the peak at 530.5 eV, which should be correlated to an 02- species. The 01s signal of samples covered with a very thin oxide layer (E < 0.2 V) has a binding energy of 532 eV, which is attributed to H,O, OH- and sulfate and is regarded here for simplicity as an OH- part (Fig. 5). The composition of the passive layer as represented in Fig. 5 was calculated assuming a homogeneous distribution of the 0 species within the film. A location of the 02- ions in the inner parts and the accumulation of OH-, H,O and SOi- at the outer parts of the layer seems more likely, however. As we could not get reliable data e.g. by angle-dependent XPS, showing a more complicated structure we assumed a homogeneous film for simplicity. Any further details of the XPS examinations will be discussed in close combination with the electrochemical results. Electrochemical

examinations

and comparison with the XPS data

Figures 6a and 6b compare current transients of prepassivated Cr and Fe electrodes in 0.5 M H,SO,. When a potential step is applied to a prepassivated electrode, a current plateau is expected for short times. If the anodic oxide grows according to eqn. (1) with a high field mechanism, the oxide thickness remains nearly constant during the initial stage. The time intervals for the current plateau values are equal to those of constant charge of layer formation in Fig. 2a. The length of the plateaus decreases with increasing AE and decreasing thickness of the preformed oxide film. The transients for Fe in Fig. 6b demonstrate the validity of this expected behaviour. For relatively small changes of the electrode potential (AE < 0.3 V) a current minimum precedes the plateau. Apparently, mobile ions have to be formed first e.g. by vacancies or ions at interstitials including polarisation phenomena [lo-121 to maintain an increased current so that the plateau value is reached only after some time. Similarly well-pronounced plateaus are not found for Cr, an observation which cannot be explained by eqn. (1) and which will be discussed later in detail. The minimum for small AE in the plateau range is found also for Cr. For sufficiently long times the current decreases linearly with time in a double logarithmic plot as a consequence of the growing oxide barrier and is independent of the potential of prepassivation and consequently of the initial oxide thickness (Fig. 6a,b). Thus the anodic transient of a prepassivated specimen consists of four stages. During the first 10 to 100 ps the passive electrode has to be charged due to the potential increase. Then the number of mobile ions has to be increased under the influence of the large field strength, e.g. ions are formed at interstitials, which leads to the plateau current. This process is apparently too fast for the high field strength during large potential changes so that the current minimum is not resolved for these conditions. Finally, the linear decrease is reached due to the growth of the anodic oxide. The plateau current densities for a set of experiments

377

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E,/V

Cr.0

5MH2S04

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Es-0.68V

2

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(a)

1

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1

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1

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log (t/s)

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1.38 120 1.18 1.08 0.98

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0

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log (t/s)

Fig. 6. (a) Double logarithmic plot of the total current density i of Cr in 0.5 M H,SO, vs. the passivation time during potentiostatic transients starting from various prepassivation potentials Es(tp = 300 s) to EP = 0.68 V. (b) Double logarithmic plot of the total current density i of Fe in 0.5 M H,SO, vs. the passivation time during potentiostatic transients from a prepassivation potential E, = 0.88 V (rP = 300 s) to the passivation potentials EP indicated.

with constant prepassivation potential Es and time of preoxidation, i.e. a constant oxide thickness at the beginning of each transient, and different oxidation potentials Ep, permit the determination of the parameters of the current-potential relation of eqn. (1) as listed in Table 4 below [5]. The transients for Cr with variable Ep and

Cr.05MHzS0~

(0)

E,=-012v

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alps-lms l Ims-1Oms olOms-IOOms

1

Cr.OSMH,SO,

30-

lb)

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loo-lOOOr

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l.O- \ o5 ~slOs

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0

0.2

0.4

0.6

08

IO

E,(SHE) I V

Fig. 7. Increase of charge AQ for various time intervals during potentiostatic oxidation at E,. (a) Short times, starting potential E, = -0.12 V; (b) long times, various starting potentials E, in a range as indicated, values for ca. 100 transients within the error bars. (- - -) indicates larger scatter of data.

319

constant ES are similar to those shown in Fig. 6a, as may be seen also in Fig. 17 below with some increase of the current density with Ep for the linearly falling part. The increase of the anodic charge with potential is rather complicated and reflects the various processes in the different time ranges (Fig. 7a). For very short times the charging of the prepassivated electrode is dominating for low electrode potentials, although the contribution of oxide growth will increase with potential. The charge for 1 to 10 ms already shows two linear parts with an increase of the slope at 0.7 V, where the change of the electrode capacity indicates a change in fihn properties. The charges for larger time intervals show a plateau which is not expected for the growth of a simple homogeneous oxide according to a high field mechanism. We compare this observation to the oxidation of fihn components of lower valency formed in an initial stage of the transient (Cr(I1) * Cr(II1)) as found for Fe in alkaline solution [5]. A detailed XPS analysis of these processes was not possible for Cr, because in this case the chemical shifts and the amounts of these

A

Cr 0 SMHzSQ 3.0 I-

o -120mV,tp:300s~680mV(SHE) ,, 2~)m~,t~:300~~680mV(SHE)

25

2.0

I-

1.5

I-

E ‘p

1.0

05

Fig. 8. Increase of the total anodic charge Q, the inverse capacity l/C and the XPS thickness d with log(t) during potentiostatic oxidation at E,, = 0.68 V, starting with two different initial oxide layers formed at -0.12 V and 0.28 V (tP = 300 s).

380

0 0

0

0

0

~,I’<

,*’

,,’ rn’

0’

381

components are too small to be detected. Lower potentials require longer times for this effect with an expected shift of this feature in Fig. 7a. Figure 7b depicts the increase of anodic charge for three time intervals within the range of the linearly falling current density of double logarithmic plots like those in Fig. 6a. About 100 transients with different Es and Ep combinations have been evaluated with the computer and the results fit within the indicated error bars. Data measured at low potentials Ep < 0.1 V and times > 10 s show a larger statistical scatter, as indicated, which might be a consequence of the poor protecting properties of the passive layer growing in this potential range. Equation (1) (Fig. 2b) suggest a linear relation of AQ and Ep for definite time intervals. For Cr two linear parts are obtained with a change at 0.7 V, where a variation of the composition of the passive layer is expected according to the inclination of the l/C-E, relation of Fig. 4. This feature should be seen as a variation in the stoichiometry of the passive layer and a related change in the growth kinetics. According to the XPS results, the oxide thickness is not affected and continues with the same slope for Ep > 0.7 V also (Fig. 4). The time dependence of the oxide thickness obtained from XPS measurements is also independent of the preformed film, i.e. the conditions of prepassivation (Fig. 8). Plots of the oxide thickness and l/C versus log(t) yield straight lines. The corresponding total charge increases non-linearly with log(t). This deviation reflects the influence of the contribution of corrosion. Figures 9a and 9b present results for the XPS thickness as a function of log(t) for different passivation potentials Ep. Identical results are obtained for Ar-sputtered, oxide-free specimens introduced into the electrolyte at the same potentials E,.Both plots yield a linear relation. The inverse logarithmic representation reveals an opposite change of the slopes with Ep as expected according to eqn. (1) (Fig. 2~). l/C increases linearly with log(t) for all oxidation potentials in the passive range up to Ep = 0.88V (Fig. 10) [24]. A plot of the inverse capacity versus the XPS thickness (Fig. 11) for different times and potentials of oxide formation yields one straight line according to the simple condenser model. From the slope a relative permittivity of 54 and from the extrapolation to vanishing thickness a Helmholtz capacity of C, = 56 pF/cm* is obtained. Apparently the capacity fits to a simple condenser equation for oxidation times of 1 to 1000 s and potentials in the range of 0 to 0.6 V and may be seen as an in-situ measure for the oxide thickness. The potential dependence of l/C for definite oxidation times f > 1 s yields lines which meet, when extrapolated, in the potential region at which passivation starts (Fig. 1) and a vanishing oxide thickness is expected (Fig. 12). The corresponding plot of the XPS thickness yields for t > 100 s an intersection with the potential axis at E = -0.2V, corresponding to d = 0 MI. For shorter times, the extrapolation of d yields ca. 0.3-0.5 nm at a passivation potential of E = -0.2V. The presence of an oxide layer equivalent to 0.5-0.8 mC/cm2 close to the passivation potential (e.g. E = - 0.12 V) must also be postulated from the analysis of potentiostatic transients, starting with this potential. An extrapolation of the A0 values for an interval t = 1 to 10 ms meets E = -0.2 V at e= -0.5 mC/cm’, a value equivalent to the pre-existing film (Fig. 7a). This layer does not influence the capacity (Figs. 11, 12)

382

Cr, O.SMH2S04

-O.l2V-E,, 6-

EplV

-I I

1

0

1

I

,

3

2 log(t/s)

Fig. 10. Inverse capacity vs. log(r) for various paasivation

6-

potentials.

Cr,05M &SOL

5-

v o.18v~Ep~o.58v,

tp=3oos

- - E,s0.38’/, tPzlOs extrapolated

‘\/f , 05

(

,

,

IO

15

20

dxps

215

1

1nm

Fig. 11. Correlation between the inverse capacity and the XPS thickness d for different potentials timra IICinAintc4~ Aachhrrllinm frnm ertrnnolntinn nf A and 1 /C tn low nntentinlc in Fiu 17

and

383

Cr,05MH2S04 -olZV-EP 50

65

40

100s

2.0 / E 1.5 D -:fY* 1.0

0

,I Ii,/

I, -II -02

0

l

,2.5

.20 ‘ICtl

d2

dL EpiSHE) I

Fig. 12. Dependence passivation times.

10s 1s

O/", / / ,' ,' ,/ 8" I/ / / I / A ', I' I/,// , / / / ’ /“,,‘,’ / / ,’ ;, /’

o 5_ / J’ ;; &” / “, / 1’ ’ ‘, I ‘, I /

.c

of reciprocal

d6

08

15

V capacity

and XPS thickness

on passivation

potential

for different

and may be seen as a 1 to 2 monolayers thick Cr(II)-oxide or -hydroxide or Cr-hydride film [25-291. With increasing potential the layer is oxidized to Cr(II1). The presence of a duplex hydroxide film with Cr(I1) species on the inside and Cr(II1) on the outside at potentials slightly more positive than the passivation potential has already been postulated before for thermodynamic reasons [27]. For a better interpretation of passivation transients, the subdivision of the anodic current density in a layer-formation part i, and a corrosion part i, is a necessary requirement. The application of a CRRDE for the analysis of dissolved Cr3+ ions with a reduction to Cr2+ at the inner ring and its reoxidation to Cr3+ at the outer concentric ring is sufficiently sensitive to detect a corrosion current density of 5 PA/cm2 [6]. The time resolution is limited to 1 s because of the hydrodynamic modulation which has to be applied to increase sensitivity. For fast potentiodynamic scans (0.1-l V/s) no soluble Cr3+ was found although the total current density was in the range of 1 mA/cm2. According to these measurements one has to conclude

384

that even for very non-stationary conditions (i, > 100 PA/cm*) the corrosion current density in 0.5 M H,SO, is smaller than 10 PA/cm* or less than 10% of the total anodic current density. This information permits us to determine the corrosion current density at long times indirectly by comparison of the electrochemical and XPS results of the transients. As already shown in Fig. 8, a plot of the XPS thickness and the inverse capacity versus log(t) yields straight lines in the time range of 1 to 1000 s, while the total anodic charge increases non-linearly with log(t), which should be seen as a consequence of the contribution of the formation of soluble corrosion products. The differentiation of the charge of oxide formation (Q xrs a dx,,) with time yields the current of oxide formation which therefore will fit a straight line with a slope of - 1 in a log(i,)/log(t) diagram. The experimental time dependence of the total anodic current density i, is represented in the same

r

Cr.OSMH,SO,

-LI

7.

E

=a

t-b

I, a=W‘,08~bi0.7

Y

1, ; g

-002V5Ep' 068V

ic -6a

-5

It

a=l+(ls) , - I,(ls),b=l , “.:::\

,

_-3

-2

-1

4

0

'I 1

2

3

-C!12V-Ep

3 ? E Y 2

2

a 1

?

-~-I -2

-1

0 log (t/s)

,

1

-1

2

3

Fig. 13. (Upper part) Double logarithmic plot of total i, and layer formation current density i, vs. time. (Lower part) Time dependence of total anodic charge for various passivation potentials; for f > 1 s the charge of layer formation is presented, deduced from XPS measurements from the results of Fig. 9a with the correlation 1 mC/cm2 = 0.7 nm (line without points).

385

plot again by lines for times t > 1 s and potentials 0.1 V
L

1

3-

2N 5 G 1 l; .F O-

-l-

I

1

4

-1

0

*

1

I

'cvFe1

2

3

Iogllrorr I pA cm ‘1 Fig. 14. Double logarithmic plot of layer formation current density vs. corrosion current density for Fe and Cr in 0.5 M H2S0.,, according to ref. 32; values for Fe are taken from refs. 31 and 32.

386

Cr,05MHzS04 -OlZV-E> 30

E,IV x 0.08 A028 0.0.38 0 0.48

2.5,

"E 7" 20, Y :

a 1.5

10

0.5

-3

,I

0

log (t/s) Fig. 15. Reciprocal

total anodic charge vs. log(t) for various passivation

potentials.

linear relation is obtained. For Cr a maximum of i = 10 PA/cm2 is found with a further increase of i, only. For Fe i, may be larger by 2 orders of magnitude, which can be taken as an indication of the better protecting properties of Cr and its stabilizing properties for passive layers on Fe/Cr alloys. The similar feature of the curves of Fig. 14 for both metals indicates that the discussions in the literature for passive corrosion and layer formation for Fe [12,32] are also valid for Cr. The inverse logarithmic plot of the total anodic charge versus log(r) yields straight lines for t > 1 s (Fig. 15). Their inverse slopes are presented in Fig. 16 versus the passivation potential. The slope of this plot should increase linearly with the potential drop within the passive layer [lo] and should be proportional to the constant B (eqn. 1) [ll]. Figure 16 depicts two linear parts with a change of the slope at 0.7 V where a variation of the composition is indicated by the capacity and the anodic charge (Fig. 4). It seems reasonable that at 0.7 V the growth kinetics changes according to the variation of the film composition. Extrapolation of the values for E z 0.7 V yields a value E,, = 0.4 V with a constant B as listed in Table 2. l/Q = Ku - K ln(lO) log(t)/( BAE) (6) where a is a constant.

38?

16-

Cr,0.5MHZSf&

l/a=a-blogt

-0.l2V+E,,trlOs I&-

12-

4-

2-

I

-02

I

0

0.2

04

0.6

I

0.8

1.0

E,lSHE) IV

Fig. 16. Reciprocal slopes of Fig. 15 for I > 1 s vs. passivation potential.

The parameters A and B of eqn. (1) may also be obtained experimentally from anodic transients of prepassivated electrodes by the evaluation of the current plateaus at short times (see Fig. 6b). In contrast to Fe, the transients for Cr do not show well-pronounced horizontal plateaus (Figs. 6a and 17). According to eqn. (1) and Fig. 2a one may draw a horizontal line starting at the point where the transient for a prepassivated electrode merges with the common linear decay of the transient for the same passivation potential beginning with an oxide-free electrode (Fig. 17, upper part). The shaded part of the transient corresponds to an excess charge AQ, (Fig. 17, lower part) which cannot be explained by eqn. (1). It may be attributed to the oxidation of some lower-valent species as Cr(I1) or to an oxide formation which

TABLE 2 Constants obtained from the evaluation of the linear relations in Figs. 15 and 16 K=0.7

um mC-‘cm2

AE=E,-E, (a) E-c 0.7 V: Et = -0.12 V, E = 17.0 mC cmm2 V-’ = 11.9 rim/v

(b) E>0.7V: E,e = 0.4 V, B = 47.4 mC cmm2 V-l = 33.2 rim/v

388

log (t/s)

z-

4

E,=038V,tp.300s EplV 048 068

E,,t@Os~E,

0.4-

E,/ V 0 058

? E 03E a" a 02-

Ol-

I

0

I

I

I

02

04

06

1

0.8

1

10

blSHE1 I V

Fig. 17. (Upper part) Typical evaluation of an anodic transient, starting with a prepassivated (Lower part) Excess charge AQ, vs. the passivation potential I&.

electrode.

does not occur according to a high field mechanism but presumably by water decomposition due to the presence of water within the film. AQ, does not exceed some few 100 PC/cm’ (Fig. 17, lower part). From the plateau values as indicated by the dashed line in Fig. 17 one obtains a plot of log(i,) versus the ratio of the potential changes for different Es and Ep combinations. The difference Es - E, is proportional to the oxide thickness for the prepassivation conditions according to Fig. 4. Thus ( Ep - Ef)/( Es - E,) refers to the increase in field strength and all log (it) values are expected to fit to one line which is shown in Fig. 18. From Fig. 18 the values A and B of eqn. (1) are obtained, according to eqn. (7), deduced from eqn. (1) with d = (Es - E,) M. The parameters are given in Table 3. log(i,)

= log(A) + [(E,

- &)B/{(E,

- E,)M

l@)}]

(7)

According to Fig. 14 the correlation between the current density of layer formation and corrosion is described by eqns. (8)-(10) for i, < 10 PA/cm’. These relations refer to a proportional change of the electrical field strength in the

389

Cr.0 5M HzSOL E,(t,=300s)-E,

W 0 0.58 l 0.48 A 0.38 A 0.28 0 0.18

I

1

2

1

3

0

4

(Ep-Et)/ (Es-Ed Fig. 18. Plot of the plateau values of the CutTentdensities obtained as described in Fig. 17 in dependence of the ratio (E, - I?,)/(& - I?,), which is proportional to the electrical field strength.

Helmholtz layer at the electrolyte side and within the oxide. W

=A, exp(WWQd~))

(8)

i,(t)

=A, w(&WQ&))

(9)

i,(t)

= it(t)

-i,(t)

(10)

where t is the time, A,, A,, Bt and B, are constants, i, is the total current density,

TABLE 3 Kinetic parameters of oxide growth for eqn. (1) deduced from Fig. 18 M = slope of XPS thickness vs. passivation potential for Et = - 0.12 V B = 10.2 MI/v A = 5.01 x 10m9 A/cm’

tp = 300 s (Fig. 4), it4 = 2.2 mn/V

390

Cr, 0 5M H,SO, 3

E,/V

iFI "x d 1

0

. & 1--: > 08

•~~8 measured r0.28 x008 I -calculated

Y2

AEI\

ALE,-Et

l O68

N 5

06

04

02

i--

-2

-1

0

1t

2

3I

4

AEl\ 08

/

0.6

3 N E " v _E2 a'

k: '1

x

1

-2

-1

x 0 log(tI

1

2

3

51

Fig. 19. Experimental charge (points) of layer formation (upper part) deduced from XPS data with K = 0.7 nm cd mC_’ and total anodic charge (lower part) in comparison to tbe values caiculated with eqns. (Q--(9) (lines) for transients starting at E, = -0.12 V.

i, the corrosion current density, QI the layer formation charge and AE = ,?I$, - E,; i, < 10 pA/cm’. Equation (11) follows from eqns. (8)-(10): i, = L[i,/i,(l

A cm-*)] ’

01)

where L and y are constants.

TABLE 4 Constants obtained from a least-squares fit of eqns. (6)-(q) to the experimental data (XPS and electrochemkal results) for t > 1 s and 0.2 V < E < 0.7 V A, = 5.01 X lo-’ A,&&, Bt = 16 mC cm-’ V-’ ( =11.2 MI/V) A,=l.l7XlO-*A/cm2, II,-13 mCcmv2 V-’ (=9.1 MI/V) L = 4.2 x 10’ A/cm’, y = 1.6 Constants for Fe [5,31]: IM NaOH: A,-A,=6.3X10-11A/cm2,

II,==B,=38nm/V

391

With these equations the time dependence of the total charge, the charge of oxide formation and the XPS layer thickness for t > 1 s can be calculated with sufficient accuracy similarly to the procedure described for eqn. (3). This computer simulation yields a good coincidence with the experimental data (Fig. 19). The values for the applied parameters are listed in Table 4. 1 mC/cm2 = 0.7 nm was used as a correlation of the XPS thickness and the charge of anodic layer formation. For all potentials a constant XPS thickness of 0.3 run was subtracted which could not be detected by charge measurements. This layer is presumably formed chemically during water contact. Comparison with ellipsometric data from the literature The XPS results for the increase of the oxide thickness with potential in strongly acidic electrolytes may be compared well with ellipsometric in-situ data from the literature [30,33]. Passivation for 1 h yields a linear increase of the oxide thickness with potential [33]. Its values are larger by a factor of about 1.6 compared to the results of this paper, whereas the potentials of oxide formation obtained by extrapolation to a vanishing oxide thickness coincide within ca. 100 mV [33]. Comparison of the data for shorter oxidation times of ref. 30 of 0.1 to 10 s permit an additional interesting comparison. A d(ellips.)/E plot for t = 1 s yields a subdivision of the passive range in three parts, in good agreement with the results of this work. For 0 V -z E < 0.6 V, d(ellips.) increases linearly with E and the extrapolation yields E,, in close agreement with the values found with XPS and electrochemical experiments. For -0.3 < E < 0 V, ca. 1 to 2 monolayers are found, which fits well to the Cr(II)-containing layer of the extrapolated XPS values. For 0.6 V < E < 0.9 V a deviation of the linear increase of d(ellips.) to lower values is observed, in contradiction to the anodic charge and the XPS thickness. These ellipsometric data may be explained by a change of the optical constants because of the formation of species of higher valency such as Cr(IV), which is expected according to the capacity data of Fig. 4. CONCLUSION

The application of specimen transfer in a closed system permits the examination of the formation of passive films starting with very short times and negative potentials of oxide formation, i.e. very thin films. Changes of the surface layers after a well-defined electrochemical preparation are optimally excluded. The pretreatment of the specimen’s surface may be controlled so that one may start with a well-known and reproducible situation. For this condition, XPS and electrochemical experiments may be correlated quantitatively to each other and lead to a mutual confirmation. RRD studies demonstrate a negligible contribution of the formation of soluble Cr corrosion products for non-stationary conditions. The analysis of XPS and electrochemical data show that corrosion consumes an appreciable part of the total current density when stationary conditions are approached. The results are interpreted with the high field mechanism for ionic conduction within the passive

392

layer. The data show that the passive behaviour is similar to that of Fe, however with a much smaller influence of the corrosion in the passive state. This situation may explain the enrichment of Cr in the passive layer of Fe/Cr alloys and stainless steel and the improved protection of the metal surface. ACKNOWLEDGEMENT

The support of this work by the Deutsche Forschungsgemeinschaft 200/2 is gratefully acknowledged.

to project Str

REFERENCES 1 K. Niki et al. in A.J. Bard (Ed.), Part B, Encyclopedia of Electrochemistry of the Elements, Vol. 9, Marcel Dekker, New York, 1986, p. 255. 2 S. Haupt, U. Collisi, H.D. Speckma~ and H.-H. Strehblow, J. Electroanal. Chem., 194 (1985) 179. 3 S. Haupt, C. Cahnski, U. Collisi, H.W. Hoppe, H.D. Speckmann and H.-H. Strehblow, Surf. Interface Anal., 9 (1986) 357. 4 S. Haupt, C. Calinski, U. Collisi, H.W. Hoppe, H.D. Speckmann and H.-H. Strehblow, DECHEMAMonogr., 102 (1986) 33. 5 S. Haupt and H.-H. Strehblow, Langmuir, in press. 6 S. Haupt and H.-H. Strehblow, J. Electroanal. Chem., 216 (1987) 229. 7 M.S. Basiouny and S. Haruyama, Corms. Sci., 17 (1977) 405. 8 R.D. Armstrong, M. Henderson and H.R Thirsk, J. Electroanal. Chem., 35 (1972) 119. 9 M. Okuyama, M. Kawakami and K. Ito, Electrochim. Acta, 30 (1985) 757. 10 K.J. Vetter, Electrochemische Kinetik, Springer, Berlin, 1961. 11 M.J. Dignam in J.G’M. Bockris, B.E. Conway, C. Yeager and R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 4, Plenum Press, New York, 1981, p. 247. 12 K.E. Heusler in A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, Vol. 9, Marcel Dekker, New York, 1986, p. 229. 13 C. Bartels, J.W. Schultze, U. Stimming and M.A. Habib, ELectrochim. Acta, 27 (1982) 129. 14 R.D. Armstrong and M. Henderson, J. Electroanal. Chem., 32 (1971) 1. 15 Th. Heumann and H.S. Panesar, J. Electrochem. Sot., 110 (1963) 628. 16 M.N. Shlepakov, A.M. Sukhotin, Y.P. Kostikov and E.G. Kuz’mina, Elektrokhimiya, 22 (1986) 125. 17 D.A. Shirley, Phys. Rev., 135 (1972) 4709. 18 H.E. Bishop, Surf. Interface Anal, 3 (1981) 272. 19 D. Briags and M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, Wiley, Chichester, 1983, p. 445. 20 M.P. Seah and W.A. Dench, Surf. Interface Anal, 1 (1979) 2. 21 Handbook of Chemistry and Physics, 60th ed., The Chemical Rubber Co., 1980. 22 H. Scofield, J. Electron Spectrosc., 8 (1976) 129. 23 R.F. Reilman, A. Msezane and ST. Manson, J. Electron Spectrosc., 8 (1976) 389. 24 B. Lovrecek and J. SefaJa, Ehxtrochim. Acta, 17 (1972) 1151. 25 A.M. Sukhotin, N.K. Khorera, Y.P. Kostikov, I.G. Vasil’kova and M.N. Shlepakov, Elektrokhimiya, 16 (1980) 1403. 26 M.N. Shlepakov, A.M. Sukhotin and Y.P. Kostikov, Elektrokhimiya, 18 (1982) 1433. 27 H. Weidinger and E. Lange, Z. Elektrochem., 64 (1960) 468.

28 29 30 31 32 33

M.N. Shlepakov, A.M. Sukhotin, Y.P. Kostikov and E.G. Kuz’mina, Elektrokhimiya, 21 (1985) 1149. N.K. Khorera and A.M. Sukhotin, Elektrokhimiya, 18 (1982) 20. M.A. Genshaw and R.S. Sirohi, J. Electrochem. Sot., 118 (1971) 1558. S. Haupt and H.-H. Strehblow, in preparation. K.J. Vetter and F. Gom, Electrochim. Acta, 18 (1973) 321. M. Sea, R. Saito and N. Sato, J. Electrochem. Sot., 127 (1980) 1909.