An XPS study of passive film formation on iron in chromate solutions

Corrosion Science, Vol. 28, No. 6, pp. 559-576, 1988 Printed in Great Britain

0010-938X/88 $3.00 + 0.00 Pergamon Press plc

AN XPS STUDY OF PASSIVE FILM FORMATION IRON IN CHROMATE SOLUTIONS

ON

E . MCCAFFERTY, M . K . BERNETT a n d J. S. MURDAY Naval Research Laboratory, Washington DC 20375-5000, U.S.A. Abstract--X-ray photo-electron spectroscopy (XPS) has been used to monitor the surface composition of iron passivated in aqueous chromate solutions varying in concentration from 10 -4 to 10-t mol/l. For iron in 10 -4 M chromate, passive film formation is complete after 20 min immersion, reaching a plateau value of 15 at. % Cr in the film, which remains constant up to the longest immersion time of 2 h. For up to 20 min immersion, the uptake of Cr is rapid and follows a logarithmic rate law. Chromium is present in the passive film as Cr 3+, iron as Fe 3+, and oxygen both in metal-oxygen and metal-hydroxyl bonds. The outermost surface of the passive film is hydroxylated and the hydroxyl species is contained in the first 30--60/~ of the film, although at a lower concentration than at the surface. The outermost 15 ,~ of the passive film is enriched in Cr 3+, with Fe 3÷ being prevalent in the inner portion of the film. INTRODUCTION

THERE HAVE been numerous studies on the passivation of iron in chromate solutions. H9 The passive films have been analysed by various techniques, including chemical analysis of stripped oxide films,3'4 radiotracer methods, 5-x°ellipsometry11"12 Mossbauer spectroscopy, 13 Auger analysis, t4 X-ray photo-electron spectroscopy (XPS),15"16 and extended X-ray absorption fine structure (EXAFS). 17-19 These studies all show that chromium is incorporated in the passive films which are formed, although there is disagreement regarding the chemical composition of the films. For example, values for Cr/Fe in the films have been reported as 0.33,1° 1.0 4 and between 0 and 1.0, depending upon the pH and oxygen content of the solution.9 Modern surface analysis techniques have been used in numerous studies involving oxide films on stainless steels, 2°-27 but only in a few studies of passive film formation on iron in inhibitor-containing solutions. 14-19'28-31 Using XPS, Rozenfel'd and coworkers 15and Asami and co-workers x6found that Cr was present as C r 3+ but not C r 6+ in passive films formed from chromate solutions. Asami et al. also reported that enrichment of the surface film with C r 3+ for iron, immersed in chromate-containing hard and soft water, reached levels similar to those for films on stainless steels containing 12% or more chromium.16 Based on EXAFS measurements, Long and Kruger 17-19 suggested that the incorporation of H and Cr in in-situ films promotes greater bond flexibility with the formation of a 'more glass-like structure'. The purpose of this communication is to report on our XPS studies of passive film formation on iron in chromate solutions. Attention is given to the kinetics of passive film growth, composition of the film, effect of chromate concentration, depthconcentration profiles and the presence of water in the passive film. The present work provides more extensive information than has been given previously in the literature. Rosenfeld et al. 15 reported on only a single CrO:4Manuscript received 27 March 1987; in amended form 14 September 1987. 559

560

E. McCAFFERTY, M. K. BERNETr and J. S. MURDAY

concentration ( - 6 x 10 -4 mol/l), and Asami e t al. 16 considered two CrO~- concentrations (~2.5 x 10 -4 mol/l and ~5.0 × 10 -4 mol/l). Neither group utilized sputtering in order to report depth-concentration profiles. By contrast, the present work considers a wide range of CrO42- concentrations (10 -4 to 10 -1 mol/l), reports depth-concentration profiles and shows the effect of immersion time on passive film formation. EXPERIMENTAL

METHODS

Materials Ferrovac E high purity iron with the following impurities in wt% was used in this study: 0.007 C, 0.01 Mn, 0.002 P, 0.007 S, 0.006 Si, 0.025 Ni, 0.007 Cr. Samples were approximately 1 x 1 x 0.5 cm in size. Immediately before immersion into solution, surfaces were prepared in one of two ways. In most cases, the specimens were dry polished through 4/0 emery paper, rinsed with methanol and dried in a stream of argon. In a limited number of experiments, the specimens were electropolished in a 20:1 mixture of glacial acetic acid and perchloric acid 32 at 125 mAJcm 2 for 3 min. After electropolishing, the samples were also rinsed with methanol and dried with argon gas. Solutions Aqueous solutions were prepared from reagent grade Na2CrO4 and distilled water prepared in a Barnstead still. Because of an earlier interest in the crevice corrosion of iron, 33-36 solutions were de-aerated with argon to simulate conditions within a crevice. Following immersion for a given time, each sample was rinsed with a stream of distilled water, then with methanol, dried with argon, mounted on a sample holder and inserted into the spectrometer system. The elapsed time between mounting and insertion into the spectrometer vacuum system varied from 5-30 min. Two main sets of immersion conditions were used: (1) 10-4 M CrO 2-, with immersion times varying from 6 s to 2 h (all surfaces mechanically polished). (2) 10-4 to 10-l M CrO42-, fixed immersion time of 2 h (surfaces either mechanically polished or electropolished). X P S measurements XPS spectra were taken with a Physical Electronics combined XPS/AES spectrometer using either Mg Ka radiation (hv = 1253.6 eV) or AI Ka radiation (hv = 1486.6 eV). The background pressure in the sample chamber was 10-9 torr. Depth-concentration profiles were determined for a selected number of immersion conditions by sputtering with a 2 keV rastered beam of Ar ÷ ions positioned at an incident angle o f a b o u t 3 5 ° w i t h r e s p e c t t o t h e p l a n e o f t h e s a m p l e i n a n a r g o n a t m o s p h e r e o f 5x 10-Storr. Over125 sets of XPS spectra were recorded in this study, so that individual spectra shown later may be considered to be representative observations. The area under the various XPS peaks was determined by first drawing a linear base line, as has been done previously,37-39and then fitting the peaks to a simple Gaussian function. A digitizer interfaced to a personal computer was first used to digitize the individual spectra and to store the data in a text file. Between 40 and 98 points were digitized, depending on the height of the peak. A simple Gaussian function was fitted to a single peak or to overlapping constituent peaks using non-linear least squares techniques and the Newton-Raphson method. 4~43 In brief, the method minimizes the sum: Q2 = ~, [yi(x, q) - yexptl]2

(1)

i

where y~Xptlare the experimentally observed intensities; and yi(x, q), the function to be fitted, depends on the energy x and on k independent parameters q. Here yi(x, q) is either a single Gaussian peak or the overlapping Gaussian peaks (e.g., OH and OM peaks). To minimize Q2: aQ 2 - 0 = 2 E [yi(x, q) - yeixptl](C~yi(x' q ) l . (2) Oqk i \ Oqk / AS described by Sherwood et al. 42 an initial estimate q° is made of each q, q k = q ~ + 6~

(3)

A n XPS study of passive film formation on iron in chromate solutions

561

U s e of E q u a t i o n (3) allows y~(x, q) to be e x p a n d e d in a Taylor's series, which is truncated after the linear terms to give a system of k simultaneous equations which can be solved for the corrections Ok. T h e new values of 6k then serve as new estimates for a n o t h e r iteration. To illustrate, for the case of two overlapping Gaussian peaks, yi(x, q) = A l e

l/2S~(xi v')2 + A2 e -lt2s~(x'-U:)'-

(4)

where Ul is the location of the first peak, A 1the peak height and S l the standard deviation; and (-12,A2 and S 2 are the corresponding p a r a m e t e r s for the second peak. If the peak locations Ut and/-/2 are considered to be fixed (e.g. for O M and O H peaks), then the set of qk consists of four parameters, A l, A2, S 1 and $2. Initial estimates are given by A t = A~) + a, A 2 = A~ + fl, etc. W h e n the resulting system of four simultaneous equations is solved for a, fl, y and 6, then the corresponding new values of A~, A2, St and $2 serve as new estimates for a n o t h e r iteration. In general 4 or 5 iterations were required, giving typical changes of AQZ/Q 2 = 0.001 or less, IAAt] or IAA21~ 0.05 and IAHI I = ]AS1/0.42461 ~ 0.02 and Ian21 = 1As2/0.42461 ~ 0.02. T h r e e different least squares programs were written: (1) Overlapping peaks of fixed kinetic energies, Ui and U2: T h e program fits the best peak heights, A 1 and A2, respectively, and the best full widths at half m a x i m a ( F W H M ) , H t and H 2, respectively. (2) Overlapping peaks: T h e program fits Uj, U2, A l, A2, HI a n d / / 2 . (3) Single peak: T h e program fits Ui, A t and H I . EXPERIMENTAL

RESULTS

AND

DISCUSSION

XPS Peaks Figure 1 shows typical XPS spectra for the passive film formed on iron by immersion in chromate solution. The binding energies E b were calculated from the observed kinetic energies E k in the usual way: E b = hv -

Ek -

tPsp,

(5)

where the work function ~Psp of the spectrometer was referenced to the Au 4f7/2 binding energy of 83.8 eV. This gave the spectrometer work function to be tpsp = 5.6. The peaks in Fig. 1 have been identified by comparison with the results of previous XPS studies on bulk iron and chromium oxides. 4.-49 Tables 1 and 2 are compilations of published binding energies for Fe 2p3/2, Cr 2p3/2 and O ls electron levels for various iron and chromium oxides.

BINDING ENERGY (eV)

120

72o, 71o, 7oo/;57o 58o, ~?o//53,s

01s Z - - OH

100 Fe 2P3/2

>-

~

so

Fe +3

Cr 2p3/2 Cr 2Pl/2 ]

F-

~

<

525 v

/i oM

6O

t-.- 4 0

,,z ~ z

20 O

750 7-~0 750~/e~0 960 9~0#9~s 9~5 KINETIC ENERGY (eV)

Fro. 1.

XPS spectra for iron i m m e r s e d in 10 -4 M CrO42 for 2 h.

562

E . MCCAFFERTY, M. K. BERNETT and J. S. MURDAY

TABLE 1.

BINDING ENERGIES FOR Fe 2p3~ oR Cr 2p3/2 LEVELS Binding Energy (eV)

A s a m i and H a s h i m o t o 44

Substance a-Fe203 ~,,-Fe203

Allen etal. 45"46

711.0 -711.0 711.4 711.6 706.8

711.4 __ 711.4 711.0 -707.3

Cr(OH)3 • 0 . 4 H 2 0 C r O 2-

576.5 577.0 --

CrO3

579.1

575.6 -577.7 to 578.6* 577.1

Fe304

a-FeOOH ~,-FeOOH Fe metal

Cr203

Mclntyre and Z e t a r u k 47

Brundle etal. 4a

Mills and Sullivan 49

711.0 711.0 -711.9

711.2 711.2 711.2 711.2

711.6 -710.8 --

--

711.2

--

706.9

707.0

706.5

* R a n g e of values for 7CrO 2- compounds.

The Fe 2p3/2 peak at a binding energy of 711 eV in Fig. I is due to an Fe 3÷ oxide, but it is not possible to differentiate between F-Fe203, a-Fe203 (or their hydrates) or F e 3 0 4 o n the basis of X-ray photo-electron spectroscopy. The peaks at binding energies of ~576 eV and ~586 eV are the Cr 2p3/2 and Cr 2pl/2 peaks, respectively, for C r 2 0 3 [ o r C r ( O H ) 3 ]. In the computer resolution of the Cr peaks, only the Cr 2p3/2 peak was considered. The oxygen ls signal in Fig. 1 consists of a peak at a binding energy at 531.5 eV due to M - O H bonds and a shoulder at 530.0 eV due to M - O bonds. As will be seen later, with argon ion sputtering the hydroxyl peak at 531.5 eV decreases and the shoulder due to metal--oxygen bonding grows into a peak, indicating that the outer layers of the oxide film are hydroxylated. Figure 2 shows the least-squares Gaussian curves computer-fitted to the spectra in Fig. 1. For the particular Fe 2p3/2 peak in Fig. 1, there is no evident contribution from the underlying iron substrate. The Cr 2p3/2 peak was fitted to a single curve for Cr 3÷ and there was n o C r 6÷ species in the passive film, as has been observed by previous investigators.15'~6 However, it is likely that the adsorption of CrO42 is a first step in the formation of the Cr3÷-containing passive film. 5-7 Figure 2 clearly shows that the oxygen peak consists of M - O H and M - O constituent peaks. As may be seen in Fig. 2, the experimentally observed XPS curves are fitted satisfactorily with simple Gaussian curves. It was unnecessary to include a Lorentzian

TABLE 2.

BINDING ENERGIES FOR THE O l s LEVEL IN IRON OXIDES OR C r 2 0 3

Binding Energy (eV) A s a m i and H a s h i m o t o 44

etal. 4s'46

McIntyre and Z e t a r u k 47

Brundle

Species

Allen

etal. 48

Mills and Sullivan 49

OM OH

530.1 + 0.2 531.4

530.1 531.8

530.0 + 0.2 531.4

530.1 ~531.1

530.1 + 0.2 531.4

0

~-I0

15

I

I

715

I

I

I

I

711

I

I

I

I

70:3

I

I

I

×PS SCALE: IOK

I

707

FIG. 2.

764 766 768 770 772 774 776 778 KIHETIC EHERGY (eV)

I

I

- , - - BINDING ENERGY (eV)

900

898

902

|

I

904

I

I

577 I

57:3

906

I

908

I

XPS SCALE,

I

KINETIC ENERGY(eV)

I

I

I

581

CAue

B~

_=

o

er~

Ok

950

945

955

I

KINETIC ENERGY (eV)

I

I

==

3

0

0 .... C

1

0

I

X "O

lOI-

I

BINDING ENERGY (eV) 535 530 525

,~,~

•~

20k

~z 30F

o) 401-

50k

GOk

701-

801.-

Computer-resolved XPS peaks corresponding to the spectra in Fig. 1.

0

I0

20

30

40

I

* - - BINDING ENERGY (eV)

>

564

E. McCAFFERTY,M. K. BERNETrand J. S. MURDAY

BINDING

ENERGY

(eV)

290 I

285 I

280 I

"1 I

50 I--

Z 1.1.1 I-Z i.-t

:'::PSF3:.: SCfiLE :

40 ._3",0

k i

20

10 o I

I

I

1190 1195 1200 KINETIL'. EtIERGY (eV) FIG. 3. Typicalcomputer-resolvedXPS peak for carbon.

component, as is often done 42'43because the experimental XPS peaks are sufficiently broad in the present work so as to allow description by Gaussian functions. The Chi square test was used to test the goodness of fit for each of the peaks shown in Figs 2, 3, 8 and 11; in each case the fit was significant at the 95% confidence level. Figure 3 shows typical computer resolved carbon peaks due to carbon contamination. The major peak at a binding energy of 284.6 eV indicates an aliphatic C - C linkage caused by an atmospheric hydrocarbon layer, while the smaller peak at a binding energy of about 287 eV is due to a C ~ O bond.5° With sputtering, the smaller peak at 287 eV generally disappeared and the larger peak at 284.6 eV decreased.

Effect of immersion time Figure 4 shows XPS spectra for different immersion times in 10 -4 M Na~CrO4. With increasing immersion time, the Cr 2p3/z peak at - 5 7 6 eV (CrzO3) increases and the Fe 2p3/z peak at 711 eV (Fe203) decreases. Figure 5 shows the uptake of chromium as a function of immersion time. The atomic fraction xj of a given element in the passive film was calculated from: 5t

i, xj --

cti

li'

(6)

za-7 where I/is the integrated peak intensity and a / t h e corresponding photo-ionization cross section of the given energy level for the given X-ray energy. Equation (6) is based on the assumption that the surface is homogeneous over the depth sampled by XPS, so that the equation can be used to estimate changes in surface composition with time. For a Gaussian peak, the area under the curve I i is given by: I i = 0.7526 x Ai x Hi. The atomic fractions in Fig. 5 have been calculated using photo-ionization

565

An XPS study of passive film formation on iron in chromate solutions Ols

Cr 2 p 3 / 2

Fe 2P3/2

I--

I---

1

.),o.,

Z

~-~-'---O.

\0

1

0

u

7~,~ 7Is -~)s 660 s~o ~o s~o s~,o 5~o ~,o BINDING ENERGY (eV)

Effect of immersion time on XPS spectra for iron in 10 -4 M CrO 2 . (The numbers to the right of the peaks indicate immersion time in minutes.)

FIG. 4.

t~---Js

o ..J LL

0.15

z Z

o

I,-

¢J 0 . 1 0 ,,< n" LL

eo o 0.05 v o D ,~ M E C H A N I C A L L Y POLISHE[2 • ELECTROPOLISHED

O

t O

I

r 10

I

210

i

3tO

i

L 40

i

510

t

I 60

lJ

L 120

I M M E R S I O N T I M E (MINUTES)

FIG. 5. Uptake of Cr in the passive film on iron as a function of immersion time in 10 -4 M CrO ] . The different symbols refer to replicate experiments. (The atomic fraction of Cr is calculated on the basis that the passive film consists of Cr, Fe and O.)

566

E. MCCAFFERTY,M. K. BERNETr and J. S. MURDAY

cross sections given by Scofield,52 and are computed on the basis that the passive film consists of the measured elements of Fe, Cr and O. The fact that the passive film contains oxygen in metal-hydroxyl bonds (OH) as well as in metal-oxygen bonds (OM) implies that the passive film also contains H. However, the presence of hydrogen cannot be measured directly by XPS. As a first estimate one can assume that XH = XOHand can then scale the atomic fractions of the other constituents (OM, Fe 3+ and Cr 3+) accordingly. In addition to H in OH groups, however, the passive film can also contain additional amounts of H in chemisorbed or physically adsorbed molecular water. In view of these uncertainties and because of the lack of direct measurement of hydrogen, the atomic composition of the passive film is computed in terms of the measured components Cr 3+, Fe 3+, OH and OM (as is the usual procedure). As seen in Fig. 5, the uptake of Cr (as Cr 3+) in the passive film is complete after 20 min immersion. For longer immersion times, the atomic fraction of Cr is constant at approximately 15 at. %. The cationic fraction of Cr in the film, as measured by the quantity Xcr/(Xo: + XFe), lies in the range 0.6--0.8 for the plateau in Fig. 5. Asami, De Sa, and Ashworth I6 have recently reported similar ratios (-0.4-0.7) for iron in 2.5-5 × 10-4 M CrO42- solutions in distilled water and in hard and soft waters. The values in the present work and those in reference 16 are in good agreement with the earlier results of Asami and co-workers 23for binary Fe-Cr alloys. Asami and co-workers z3 observed that there is an enrichment of Cr in the passive film at the critical Cr content of 13 at. % necessary to confer passivity on binary Fe-Cr alloys. 53 Asami and co-workers23reported values of approximately 0.6-0.8 for Xcr/(Xcr + XFe) for the passive film on 13 at.% Cr. Thus, the present results and those in reference 16 show that the extent of Cr 3+ enrichment is similar for oxide films passivated in chromate and for oxide films on passive Fe-Cr binary alloys. It should be noted that the ratio Xcr/(Xcr + XFe) is the same whether atomic fractions are computed including or excluding the presence of hydrogen. Figure 6 shows the uptake of Cr (as Cr 3+) in the passive film in the early stages of film formation. Prior to the completion of film growth, it may be assumed that the film thickness is less than the photo-electron escape depth, so that the Cr composition is approximately linear with film thickness and thus may be used to monitor film growth. This assumption is reasonable in that it is shown later in this paper that the

u.

0.20

Z o Z

o

I-.tJ

,¢ 0 . 1 0

_(2 o

l--

i

0.1

i

i

~

i

llll

i

L

i

J

iiiil

1.0 10 IMMERSION TIME (MINUTES)

i

~

i

i

i~l

10o

FIG. 6. Logarithmic rate law for the early stages of passive film growth. (The atomic fraction of Cr is calculated on the basis that the passive film consists of Cr, Fe and O.)

A n XPS study of passive film formation on iron in chromate solutions

567

thickness of the Cr-containing layer (computed in the limit as an overlayer of Cr203 on the oxide-covered iron) is approximately 15 ,~, whereas the electron escape depth for the Cr 2p3/2 signal is about 10 A,.51 Thus it can be seen that in the early stages the film grows according to a logarithmic rate law, in agreement with the previous earlier results of Brasher 9 who performed radiotracer measurements and with the results of Kruger 54 who utilized ellipsometry. Using ellipsometry for iron in chromate solutions, Smialowska and Staehle 12 also found that the kinetics of iron oxide and iron and chromium hydroxide formation was logarithmic, but that the kinetics of iron hydroxide at constant potential were cubic. A logarithmic rate law can be explained by mechanisms in which the rate determining step is either electron transport across the film or transport of cations across a p-type semiconductor. 55

Effect of chromate concentration Figure 7 shows the atomic fraction of C r 3+ in the passive film as a function of chromate concentration for a fixed immersion time of 2 h. The amount of C r 3+ in the passive film decreases with increasing CRO24- concentration, although all films formed in 10-4-10 - I M CrO]- were passive and displayed low current densities ( - 1 / z A / c m 2 or less) at anodic overpotentials. A similar result has been reported by Asami, De Sa and Ashworth, 16 who reported that passive films formed in 50 ppm CrO]- ( - 5 x 10-4 M) are thinner than those formed in 25 ppm CrO~- (-2.5 x 10 -4 M ) .

M/I 10 - 4

10 - 3

CrO~ 10 - 2 =

10-1 i

1

"r

8!-

7I 85 I L

L

It.

8

Z

o F- 0 . 1 0 n~ li. (.)

O I-<

0.05 o MECHANICALLY POLISHED • ELECTROPOLISHED 10 -4

10 .-3

M/I

10 -2

10 -1

Cr04 =

FIG. 7. Bottom: A t o m i c fraction of Cr in the passive film as a function of CrO4~concentration. Top: p H of the a q u e o u s solution as a function of CRO4-2 concentration. (The atomic fraction of Cr is calculated on the basis that the passive film consists of Cr, Fe, and O.)

568

E. MCCAFFERTY, M. K. BERNETr and J. S. MURDAY

Figure 7 also shows that the pH of the CrO 2- solutions increases with CRO24concentration. Brasher and Mercer 9 and Mayne and Ridgway4 have shown that the C r a+ uptake (at constant [CrO2-]) decreases with increasing pH over the pH range applicable in the present work. From a thermodynamic point of view, the incorporation of Cr 3÷ into the passive film is given by: 2CRO42- + 10H ÷ + 6e- ~ Cr203(film) + 5H20

(7)

CrO24- + 5H ÷ + 3e- ~ Cr(OH)3(fijm) + H20

(8)

or:

so that the atomic fraction Xcr of Cr 3+ in the passive film is a function of the CrO42concentration, pH and electrode potential E. That is:

XCr f([CrO24-], pH, E),

(9)

=

so that

/ XCr/ d[ CrO2-] = ,

,a[CrO42-I p,.E

/0XCr

dpH

\OPH]tcrO~-I.EX diCrO24_]

(Oxcr~ x dE + \ OE ]lCrO~-l,pH d[CrO42-]"

(10)

In order to explain in detail the decrease in Cr uptake with increasing CrO42concentration, it is necessary to evaluate the three partial derivatives in Equation (10) by varying in turn the CrO 2- concentration, the pH and the electrode potential, while keeping the other two variables constant in each case. Such work was outside the scope of the present study, but it is interesting to note that the amount of Cr 3+ in the film needed to maintain passivity decreases with increasing CrO42- concentration.

Depth-concentration profiles Depth-concentration profiles were obtained for passive films formed on iron immersed in 10-4 M chromate for (a) 20 min, (b) 1 h, (c) 2 h and (d) for iron immersed iin 10-l M chromate for2 h. Figure 8 shows the effect of argon ion sputtering on the Fea+/Fe° peak, the Cr 3+ peak, and the OH/OM peak for iron immersed in 10-4 M chromate for 2 h. With increased sputtering time, the Fe 3÷ peak decreases while the contribution from underlying Fe ° increases. With increased sputtering time, the OH peak also decreases while the OM peak increases. Figure 9 shows the depth-concentration profile corresponding to Fig. 8. As seen in Fig. 9, the passive film contains a mixture of Cr 3÷ and Fe 3+ species, with Cr 3+ predominant near the outer surface and Fe 3+ prevalent in the inner part of the oxide film. The oxygen species in the outermost part of the film is predominantly the OH group. As will be seen in the next section, the sputtering rate is approximately 6 A/min, so that the OH group is predominant in the outermost 3 A. This means that the passive film on iron contains a surface hydroxylated layer, similar to that existing on bulk oxide surfaces. 56'57 After 2 rain sputtering, the atomic fraction of OH decreases from approximately 0.7 to approximately 0.2 and remains near that value for up to 10 rain sputtering.

An XPS study of passive film formation on iron in chromate solutions BINDING EN£RGY {eV) 715 711 TO7

[lO t ,:,o

:g?, ~l: ~LE I,)~

10 minute - . sputter ~ ~,~

.

.

BINDING ENERGY ( I V ) 51111

703

0

577

569

BINDING ENERGy (eV) 535 530 525

573

~'~

. zo

zO I

o

89~:

~

sputter

!

~02

904

~06

~0~

~4S

950 ~SS

945

250

20

xPS SCALE

6O

5 minute

~00

50

i

*0

3O ~0 I

764 766 768 770 772 774 776 778

955

40 a ~ W L E

tsputter minutei'ifi j

4O

I

........ .o:[

,

764 766 76.~ 7713 772 7 7 4 : 7 6 775

15

zero sputter

[

~L 10

SSL .E

eO

70

~ zo

i*-'

1

:64 : ~

FIG. 8.

o I ~ I X PSS CALE

xPs S t a L l tO;"

;'68 ;'70 :'TZ 774 7 ~ 7;'8 KItIETIC ENEE'°'/ (eV)

898

(eV)

~00 902 904 3 0 g KINETIC ENEP°~

908

~45

KH I ET£ E ]NER(G eV ,) 950

955

Effect of sputtering on the FeS+/Fe ° peak, the Cr3÷ peak and the OH/OM peak for iron immersed in l0 -4 M CrO~- for 2 h.

0"80 L

r- ~ + O M I E z

0.60 "13

Z

o_ I-U <

0.40 OH

Fe +3

_o

0.20

0.00( 3

2

4 6 8 SPUTTER TIME (MINUTES)

10

FIG. 9. Depth-concentration profiles corresponding to the spectra in Fig. 8. (The atomic fractions are calculated on the basis that the passive film consists of Cr, Fe and O.)

570

E. MCCAFFERTY,M. K. BERNETTand J. S. MURDAY

Depth-concentration profiles similar to Fig. 9 were obtained for the other cases investigated (10 -4 M CrO 2- at 20 min and 1 h immersion and 10-t M CrO42- at 2 h immersion) except for one detail. For two of the four passive films, the OH concentration decreased to zero after 5 min sputtering. Thus, in general, passive films formed on iron immersed in chromate solutions contain a hydroxyl species in the outermost 30-60/k.

The calculation of sputtering rate The sputtering rate was determined by sputtering the air-formed oxide film on clean iron, resolving the Fe 2p3/2 peak into its constituent Fe 3÷ and Fe ° peaks, and then calculating the sputtering rate from the ratio of the two peaks, as follows. The intensity, I, of electrons of a given energy emitted from a homogeneous material of depth x is given by: 58'59

dI = FaDk e -x/~ dx,

(11)

where F is the X-ray flux, a the photo-ionization cross section in a given shell of a given atom for a given X-ray energy, D is the atomic density in number of atoms per unit volume, k is a spectrometer factor and 2 is the escape depth of photo-electrons of a given kinetic energy. For an oxide film of Fe203 of thickness d on an iron substrate (Fig. 10a) the intensity of electrons emitted from the iron atoms in the oxide is:

IFe203=flFCtFeDFezo3ke-X/adx

(12)

where Dve2o3 is the atomic density of iron atoms in F e 2 0 3 (3.94 x 1022 atoms/cm3). For the underlying substrate:

Ive=f]FaFeDFeke-X/~dx

(13)

where DEe is the atomic density of metallic iron (8.45 x 1022 atoms/cm3). Evaluating (a)

Fe203

Id

o

////11/ ,//////////111//////~

d

Corb°n- 7

0

Iron//'

(b)

,?e203 id//: :liU/,ir,~nlll/i/i ii , do dc

+ d

(c)

Cr203 Fe203

ld I 0 ~d2 dl //////, "////////////////i/// dl + d2 / ron FIG. 10. Models for: (a) and (b) calculating the sputtering rate and (c) for calculating the limiting thickness of the Cr-enriched region for the passive film formed on iron in CrO42solutions.

An XPS studyof passivefilmformation on iron in chromate solutions

571

the integrals and combining Equations (12) and (13) gives: IF~O3 _ DFe203 [e~V~_ 1]. IFe DFe

(14)

Figure 11 shows the effect of sputtering on the Fe 2p3/2 peaks of electropolished iron. The constituent peaks were computer resolved, as described earlier. With increased sputtering, the Fe 3+ peak decreases and the Fe ° peak increases. Figure 12 shows the oxide film thicknesses, d, calculated from Equation (14) using an escape depth of 10/~ for the observed kinetic energy of ca 700 eV. 51 The slope in Fig. 12 gives the sputtering rate as -6/~dmin. A similar set of experiments with mechanically polished iron gave a sputtering rate of 7/~min. It should be noted that the presence of an overlayer of carbon contamination of thickness dc (Figure 10b) does not affect the calculated sputtering rate. If a carbon overlayer is considered, the numerator and denominator in Equation (14) should both be multiplied by the factor e -dd2, so that the ratio IFe,_o3llFe is the same whether or not the carbon overlayer is considered. The limiting thickness o f the chromium-enriched layer As shown in Fig. 9, the passive film contains a mixture of Cr203 and Fe203 (or their hydrates), with the outermost portion of the film enriched in Cr 3+. The maximum thickness of this Cr 3+ enriched layer can be determined by assuming that in the limit, the passive film is a bilayer consisting of an overlayer of Cr203 on an underlying layer of Fe203 (Figure 10c). For an overlayer of Cr203 of thickness dl, Equation (11) gives the intensity of the Cr peak as: Ic~ = Fac~Dcrk2[ l - e-d'/'t].

(15)

For the underlying layer of Fe203 of thickness d2; Equation (11) gives: IFe = FCtFeDFek2 e-d'/'l[ 1 -- e-d2/'!]

(16)

Icr acrDcr[1 - e-d'/a] IFe -- CtFeOFee:d'/'l[ 1 -- e-d#;t] "

(17)

so that

Figure 13 shows Icr/IFe for iron in 10 -4 M NazCrO4 as a function of immersion time. As seen in Fig. 13, the data are scattered between an upper limit oflcr/IFe = 3.5 and a lower limit of IO/IFe = 1.5. Use of these limits in Equation (17) along with the Scofield photo-ionization cross sections, ;tm 10 ,~,sl and the atomic densities of 4.13 × 1022atoms/cm 3 for Cr in Cr203 and 3.94 x 1022atoms/cm 3 for Fe in Fe203 gives the thickness dl of the Cr203 layer as a function of the thickness d Eof the Fe203 layer, as shown in Fig. 14. From Fig. 14, it can be seen that the thickness of the Cr203 overlayer is essentially independent of the thickness of the underlying Fe203 layer, for values o f d 2 1> 20 &. Assuming that the thickness of the incipient air-formed oxide film on iron is 25/~60 gives the thickness of the Cr203 overlayer as dl = 14 + 3 ~ . This value is the limiting thickness of the outer portion of the passive film which is enriched in Cr20 3. This result is consistent with the fact that it takes 1-2 rain to sputter through the CrzO3-enriched portion of the film. This sputtering time corresponds t o 7-14 ~ , based on the sputtering rates calculated earlier.

572

E. McCAFFERTY, M . K . BEItNETT a n d J. S. MUltDAY

BINDING 715 I

ENERGY

711

I

I

(eV)

707

I

I

703

I

I

I

70 60 50

5 minute sputter

40 30 20 10 0 i

I

i

I

I

I

i

i

764 766 768 770 772 774 776 778 KINETIC ENERGY

IIO I00 90 80 70 60 50 40 30 20 I0 0

2 minute sputter

I

I

I

i

i

i

i

I

764 766 768 770 772 774 776 778 KINETIC ENERGY

8O 7O 6O 5O

I minute sputter

4O

3O 20

e °e

I0

°i

I

I

I

I

I

i

i

I

764 766 768 770 772 774 776 77@ KINETIC ENERGY

60

5O ~- 4O

zero sputter

~ 3o w i-z 2o lO o i

i

i

i

I

i

i

i

764 766 768 770 772 774 776 778 KINETIC ENERGY(eV)

FIG. 11.

E f f e c t o f s p u t t e r i n g o n t h e F e 2p3/2 p e a k f o r t h e a i r - f o r m e d o x i d e film o n electropolished iron.

A n XPS study of passive film formation on iron in chromate solutions

40 Z v

u_ 30 (

__

LEAST SQUARES LINE

20

X 0

q i

L

SPUTTER TIME (MINUTES)

FIG. 12,

Effect of sputtering on the calculated thickness of the air-formed oxide film on electropolished iron.

5 4 e

o 3 2 1

0

I

0

210

I

I

I

I

60

40

~

I

80

I

I

1 O0

I

L

120

IMMERSION TIME (MINUTES)

FIG. 13.

Ratio of the intensities of the 2p3/2 peaks of Cr and Fe as a function of immersion time in 10 -4 M CrO]-.

30 "< 25 Z

20

u_ 15 0 G Z I F-

....

1~0

2'0

3b

4'o

THICKNESS OF Fe203 (d 2) IN .A

FIG. 14.

Thickness d~ of the Cr203 overlayer as a function of the thickness d 2 of the Fe203 layer.

573

574

E. McCAFFERTY,M. K. BERNEI"rand J. S. MURDAY

Bound water in the passive film As shown in Fig. 9, the passive film formed on iron in chromate solution contains O H - ions, which are prevalent at the outer surface but which are also contained in the bulk of the film. This information is consistent with the results of previous investigations which have used radiotracer methods, 61'62 Mossbauer, 63 X P S 23'39'64'65 or SIMS data 65-67 to show that water is present in the passive film on iron. There is disagreement, however, as to whether water is contained in the outermost surface alone 67or in the bulk of the film as well. 66According to one model of passivity,68the passive film is crystalline and anhydrous, while according to another model, 63'65the passive film consists of amorphous polymer chains of iron atoms linked together by di-oxy or di-hydroxy bonds, with the chains further linked by water. The results of the present study are not conclusive, but the presence of water as OH- groups 30--60/~ into the passive film tends to support models based on the existence of bound water in the passive film.

Comparison with Fe-Cr alloys In a recent review on passivity, Kruger 69 pointed out that for the passive film on Fe--Cr alloys, (a) the outer part of the passive film is enriched in Cr, (b) the film is made up of a mixture of iron and chromium oxides and (c) the oxygen present in the passive film exists in two different bonding states, as M - O H or M - O O H and M-O. The present results show that these same three properties are also exhibited by the passive film formed on pure iron immersed in chromate solutions. As noted earlier, the enrichment of Cr in the passive film formed in chromate solutions, as measured by the ratio XCr/(XCr "q- XFe), is the same as that found in the passive film on Fe-13 at.% Cr (23), where passivity is induced in the Fe-Cr series. Thus, the effectiveness of chromate solutions as inhibitors for iron appears to lie in the ability to form passive films which are similar to those formed on Fe-Cr bulk alloys. CONCLUSIONS X-ray photo-electron spectroscopy (XPS) studies on the passive films formed on iron in chromate solutions show that: (1) Passive film formation is complete after 20 min immersion in 10-4 M chromate. (2) The concentration of Cr in the completed passive film is approximately 15 at. %, computed on the basis that the film consists of the elements Cr, Fe and O. (3) The passive film grows according to a logarithmic rate law. (4) Chromium is present in the passive film as Cr3+; iron is present as Fe 3+. (5) The amount of Cr in the passive film decreases with increasing concentration of chromate over the range 10-4 M to 10-1 M CrO 2-. For 10-4 M CrO24-, the passive film contains 15 at. % Cr and the amount of Cr decreases to 9 at. % for 10-1 M CrO24-. (6) The outer portion of the passive film is enriched in Cr 3+, with Fe 3÷ prevalent in the inner portion. (7) The limiting thickness of the chromium-enriched outerlayer is 14 + 3 ,~, computed as an overlayer of Cr203. (8) The oxygen ls signal consists of both M - O H and M-O contributions. The outermost surface of the passive film is hydroxylated, and the hydroxy species is contained in the first 30--60/~ but at a lower concentration than at the surface.

An XPS study of passive film formation on iron in chromate solutions

575

Acknowledgements--One of the authors (E. McCafferty) is grateful to Peter G. Moore for generously making available his digitizing program and for helpful discussions, and also to Noel H. Turner for a number of helpful discussions. REFERENCES 1. Z. SZKLARSKA-SMIALOWSKA,in Passivity o f Metals (R. P. FRANKENTHALand J. KRUGER, eds), p. 443. The Electrochemical Society, Princeton, New Jersey (1978). 2. M. COHEN in ref. (1), p. 521. 3. T. P. HOAR and U. R. EVANS,J. chem. Soc. 2476 (1932). 4. J. E. O. MAYNEand P. RIDGWAY,BE. Corrosion J. 9,177 (1974). 5. R. A. POWERSand N. HACKERMAN,J. Electrochem. Soc. 100,314 (1953). 6. D. M. BRASHERand C. P. DE, Nature 180, 28 (1957). 7. D. M. BRASHERand A. H. KINGSaURV, Trans. Faraday Soc. 54, 1214 (1958). 8. O. KUBACHEWSKYand D. M. BRASHER, Trans. Faraday Soc. 55, 1200 (1959). 9. D. M. BRASHERand A. D. MERCER, Trans. Faraday Soc. 61,803 (1965). 10. M. COHEN and A. F. BECK, Z. Elektrochem. 62,696 (1958). 11. J. KRUGER,J. Electrochem. Soc. 110,664 (1963). 12. Z. SZKLARSKA-SMIALOWSKAand R. W. STAEHLE,J. Electrochem. Soc. 121, 1146 (1974). 13. G. M. BANCROFT,J. E. O. MAYNEand P. RIDGWAY,BE. CorrosionJ. 6, 119 (1971). 14. J. B. LUMSDENand Z. SZKLARSKA-SMIALOWSKA,Corrosion 34, 169 (1978). 15. I. L. ROZENFELD, L. P. KAZANSKII,A. G. AKIMOVand L. V. FROLOVA,Protection o f Metals, USSR 15, 280 (1979). 16. K. ASAMI, M. S. DE SA and V. ASHWORTH in Proceedings o f the 6th European Symposium on Corrosion Inhibitors, p. 769. Ferrara, Italy (1985). 17. G. G. LONG, J. KRUGER, D. R. BLACKand M. KURIYAMA,J. Electronal. Chem. 150,603 (1983). 18. J. KRUGERand G. G. LONG, in Surfaces, Inhibition and Passivation (E. MCCAFFERTYand R. J. BRODD, eds), p. 210. The Electrochemical Society, Pennington, N.J. (1986). 19. G. G. LONG, J. KRUGER, M. KURIYAMA,A. I. GOLDMAN, D. R. BLACK, E. N. FARABAUGHand D. M. SANDERS,J. Appl. Phys. (in press). 20. J. B. LUMSDENand R. W. STAEHLE, Scripta Met. 6, 1205 (1972). 21. I. OLEFORD,Corrosion Sci. 15,687 (1975); also: I. OLEFORD and H. FISCHMEISTER,Corrosion Sci. 15, 697 (1975). 22. J. E. CASTLEand C. R. CLAYTON,Corrosion Sci. 17, 7 (1975). 23. K. ASAMI, K. HASHIMOTOand S. SHIMODAIRA,Corrosion Sci. 17,713 (1977); Corrosion Sci. 18, 151 (1978). 24. R. P. FRANKENTHALand D. E. THOMPSON,in Passivity o f Metals (R. P. FRANKENTHALand J. KRUGER, eds), p. 262. The Electrochemical Society, Princeton, New Jersey (1978). 25. H. OGAWA, H. OMATA,I. ITOH and A. Or~oA, Corrosion 34, 52 (1978). 26. J. R. CAHOONand R. BANDY, Corrosion 38,299 (1982). 27. C. R. CLAYTON,K. DOSS and J. B. WARREN, in Passivity o f Metals and Semiconductors (M. FROMENT, ed.), p. 585. Elsevier Science, The Netherlands (1983). 28. R. BAILEYand J. E. CASTLE,J. Materials Sci. 12, 2049 (1977). 29. M. KOULDELKA,J. SANCHEZand J. AUGUSXVNSKI,J. Electrochem. Soc. 129, 1186 (1982). 30. J. C. WOOD and N. G.-VANNERBERG, Corrosion Sci. 18, 315 (1978). 31. R. D. GRANATA,P. G. SAYrXESTEaANand H. LEIDHEISERJr., in Surfaces, Inhibition and Passivation (E. MCCAFFERXVand R. J. BRODD, eds), p. 69. The Electrochemical Society, Pennington, New Jersey (1986). 32. P. B. SEWELL, C. D. STOCKBRIDGEand M. COHEN, Can. J. Chem. 37, 1813 (1959). 33. E. McCAFFERXV,J. Electrochem. Soc. 121, 1007 (1974). 34. E. McCAFFERXV,J. Electrochem. Soc. 126,385 (1979).~ 35. E. McCAFFERXY,in Equilibrium Diagrams: Localized Corrosion (R. P. FRANKENTHALand J. KRUGER, eds), p. 548. The Electrochemical Society, Pennington, New Jersey (1984). 36. E. MCCAFFERXV, in Proceedings o f the 6th European Symposium on Corrosion Inhibitors, p. 533. Ferrara, Italy (1985). 37. J. P. COAD and J. G. CUNNINGHAM,J. Electron Spectrosc. Related Phenom. 3,435 (1974).

576

E. McCAFFERTY,M. K. BERNETI"and J. S. MURDAY

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