a surface analytical and electrochemical cerio en stainles steel (s vitanen).pdf

8/19/2019 A SURFACE ANALYTICAL AND ELECTROCHEMICAL CERIO EN STAINLES STEEL (S vitanen).pdf

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Corr osion cience,

Vol. 39, No. l&l I, pp. 1897-1913. 1997

0 1997 Elsevier Science Ltd

Printed in Great Britain. All rights reserved

001&938X/97 17.00+0.00

PII: soolo-938x 97)ooo84-x

A SURFACE ANALYTICAL AND ELECTROCHEMICAL STUDY

ON THE ROLE OF CERIUM IN THE CHEMICAL SURFACE

TREATMENT OF STAINLESS STEELS

S. VIRTANEN *’ M. B. IVES,* G. I. SPROULE,+ P. SCHMUKI+ and

M. J. GRAHAM?

*Walter W. Smeltzer Corrosion Laboratory, McMaster University, Hamilton, Ontario, Canada

+ nstitute for Microstructural Sciences, National Research Council, Ottawa, Ontario, Canada

Abstract-The mechanism of oxide layer formation and modification during chemical cerium nitrate treatment of

stainless steel has been investigated. The aim of the work was to study the role of cerium in modifying the oxide layer

properties, especially the kinetics of the cathodic reactions. For this, electrochemical and surface analytical studies

were carried out. During exposure to hot (90°C) cerium nitrate solution, oxide film formation by chromium

passivation and an accompanying dissolution of iron oxide takes place, leading to an enrichment of chromium in

the oxide layer. Further, insoluble cerium species are precipitated at the cathodic sites of the surface. The oxygen

reduction reaction is inhibited on these films. The effect of the cerium treatment cannot be solely attributed to the

formation of a chromium-rich oxide layer, since the cathodic reactions are more strongly inhibited on the cerium-

treated stainless steel than on passivated pure chromium. Moreover, the cerium treatment is efficient in retarding

the cathodic kinetics on pure chromium. Studies with a redox couple present in the electrolyte clearly show that the

inhibition of the oxygen reduction reaction is not due to a lower electron conductivity of the oxide layer. The

cathodic inhibition effect can be attributed to a high resistance against reductive dissolution. This is partially due to

the chromium enrichment and in addition to the cerium precipitation at the weak sites of the oxide layer which

otherwise under cathodic polarization would lead to reductive dissolution, thus providing current paths for

electrons participating in the oxygen reduction reaction. Treatment parameters such as time, alloy composition,

solution chemistry and potential during treatment were studied. Clearly, all factors leading to a maximum

chromium enrichment and/or cerium precipitation increase the cathodic inhibition efficiency. 0 1997 Elsevier

Science Ltd

Keyw ords: A. stainless steel, B. galvanostatic, B. SIMS, B. XPS, C. passive film.

INTRODUCTION

Chemical treatment

in solutions containing cerium compounds has been widely studied for

the prevention of localized corrosion of aluminum and its alloys.‘4 In these studies, the

positive effect of cerium has been attributed to inhibition of cathodic reactions. Similarly,

cerium has been found to help in preventing crevice corrosion of stainless steels, either by

cerium implantations or a simple immersion in boiling cerium nitrate solutions.6 Also in the

case of stainless steels, the effect of cerium has been attributed to retardation of cathodic

reactions.5-7 However, in another study, cerium treatment was found to be far less efficient

in improving the corrosion behavior of stainless steel.’ The precise inhibition mechanism of

the oxygen reduction reaction has yet to be clarified.

’ On leave from the Swiss Federal Institute of Technology, Institute of Materials Chemistry and Corrosion,

ETH-H%ggerberg, 8093 Ziirich, Switzerland.

Manuscript received 20 November 1996; in amended form 21 January 1997.

1897


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S Vlrtanen cl al.

It has been shown by previous surface analytical studies that cerium treatment leads to a

chromium enrichment in the oxide films.’ Therefore the question arises as to what is the

specific role of cerium species in the surface modification process. The aim of this work was

to study the processes such as layer growth and changes in oxide composition taking place

during the cerium treatment as well as the role of cerium in modifying the oxide layer

properties; especially the kinetics of cathodic reactions. For this, the electrochemical

behavior of cerium-treated stainless steel was studied during cathodic polarization in

aerated and deaerated borate buffer solution. To obtain information on the electron transfer

reactions through the oxide films, studies with a redox couple [Fe(CN /Fe(CN ] present

in the solution were carried out. Surface analytical data regarding the composition of the

oxide layers were obtained to try to understand the special role of cerium in the oxide layer

formation and modification. Further, the influence of different treatment parameters on the

treatment efficiency was studied.

EXPERIMENTAL METHOD

The sample material was a high-purity stainless steel AISI 304 (S < 0.001%) in sheet

form. Prior to treatment the surfaces were mechanically ground to a 600 grit finish for

electrochemical measurements or to a 0.25 pm diamond finish for surface analytical studies.

After polishing, the samples were rinsed in acetone and ethanol and dried in nitrogen gas.

The treatment solution was prepared from Ce(N0&.6Hz0 or NaN03. When desired, the

pH of the solution was adjusted by additions of diluted HN03 or NaOH. The natural pH of

the nitrate solutions was slightly acidic (pH 4.8 for 0.05 M cerium nitrate). The cerium

chemical treatments were carried out at 90°C in a water bath. After the treatment the

samples were rinsed in distilled water and dried with nitrogen gas.

The electrochemical experiments were carried out with an EG&G Part model 273

potentiostat. The scan rate for the potentiodynamic polarization curves was 0.2 mV s-‘.

Galvanostatic reduction was carried out with a current density of - 5 PA cmp2. The

electrolyte solution was borate buffer, pH 8.4 (8.17 g 1-l Na2B407.10H20+ 7.07 g l- ’

H,B03) to which 0.05 M K3Fe(CN)6+ 0.05 M K4Fe(CN)6 was added for the electron

transfer measurements. The solutions were prepared from reagent grade chemicals and

distilled water.

X-ray photoelectron spectroscopy (XPS) measurements were carried out in a Perkin-

Elmer PHI 5500 system with a monochromated Al K source. Cr 2p, Fe 2p, Ni 2p, Ce 2p and

0 1s core levels were collected using a pass energy of 29.4 eV and, if not otherwise stated,

with a take-off angle of 75”. The background was subtracted in the integrated mode. The

spectra were deconvoluted in order to separate the contribution of metallic and oxidized

species in a similar manner as described earlier by other authors.“,” The relative

concentration c(A) for an element A in the oxidized layer composed of i constituents is

defined by c(A) = [~(OX)~/S,]/[CZ(OX)i/~~], where 1(0x) and S represent the photoelectron

intensities and the relative sensitivities for oxidized species of an element (for Z(Ox) the sum

of the intensities of the different oxidized species was taken). The relative sensitivities were

taken from literature data.12

Thickness values were estimated by using a treatment

described elsewhere. I3

For secondary-ion mass spectrometry (SIMS) a Perkin-Elmer PHI 590 scanning Auger

microprobe with a SIMS II attachment was used. The species measured were mass 68

(52Cr0i), 72 (56Fe0+), 74 (NiO+), and 156 (CeO’). Mass 168 (Fe;) was recorded to


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Role of cerium in treatment of stainless steels

1899

indicate the position

of the oxide/substrate interface.

Further, masses 69

(52CrOH+ + 53Cr0+) and 73 (56FeOH+ + 57Fe0+) were measured in order to check for

the presence of hydroxide in the oxide films. In all oxide films some hydroxide was found in

the outermost part of the film. Relative sensitivity factors for the different signals were

determined by calibration with the XPS data. Sputtering yas performed with 1 keV Xe

corresponding to a sputter rate of approximately 4 A min-‘.‘4 The sputter time

corresponding to the interface location was determined at 50% of the mass 168 (Fez)

signal. The oxide layer thicknesses determined from SIMS depth profiles were in very good

agreement with the values of the layer thickness calculated from XPS data. A more detailed

description of the experimental procedure is provided in 15.

EXPERIMENTAL RESULTS

Inhibition of the cathodic reactions

The first set of experiments was aimed to clarify what different reactions take place on

cerium-treated stainless steel samples during cathodic polarization; specifically to

distinguish between oxide reduction and oxygen reduction reactions. This is important in

understanding the cathodic inhibition mechanism, since oxygen reduction takes place in the

potential range where reduction of the passive film is also possible. For this, cathodic

polarization experiments were carried out on as-treated samples in deaerated and aerated

solutions. Figure l(a) shows a comparison for samples after treatment for 1 h in 0.05 M

cerium nitrate solution at 90°C. It is evident that in the potential region between the open-

circuit potential and

z - 700 mV (SCE), a cathodic peak is present in both solutions. This

potential region corresponds to reduction of passive films on iron and therefore the cathodic

peak can be ascribed to reduction of ferric species in the film. An indication of reduction of

the oxide film on stainless steel during cathodic polarization was also found in an earlier

rotating disc study by Lu and Ives6

showing the existence of a mass-transport-independent

contribution to the total cathodic current.

The polarization curves thus suggest that the cathodic current for as-treated samples

stems from two reactions: oxygen reduction and reduction of Fe3+ species present in the

oxide film. Hence, cathodic polarization experiments for as-treated samples do not give

direct information on the kinetics of the oxygen reduction reaction, since it is difficult to

separate the contributions of the oxide and oxygen reduction. This is a particular

disadvantage as the amount of ferric species in the oxide layer can vary depending on the

process of oxide formation. On the other hand, if the cathodic polarization measurements

are carried out on cerium-treated samples after total reduction of the ferric species present in

the film, then the cathodic current stems only from reduction of redox species in the

electrolyte, e.g. from the oxygen reduction reaction. Figure l(b) shows a comparison of the

cathodic polarization curve of a cerium-treated sample

[

1 h, 0.05 M Ce(N03)s, 90°C] prior

to and after galvanostatic reduction treatment (2 h, - 5 uA cm-*) compared with the

corresponding polarization curves of an untreated sample. It is clear that after reduction,

the current peak corresponding to reduction of ferric ions is absent. Nevertheless, before

reduction of the samples it is clearly evident that the rate of oxygen reduction is retarded on

the cerium-treated sample in comparison with the untreated sample. The same effect can be

seen if the potential decay curves during galvanostatic reduction experiments in aerated

borate buffer of the untreated and treated samples are compared (Fig. 2). Clearly, the lower-

end potential of the cerium-treated sample indicates an inhibition of cathodic reactions.


4/17

1900

S. Virtanen YIal

10-s

-LL-

-1400-1200-1000 -800 -600

-400 -200 0 200

Potential mV SCE)

-1400-1200-1000 -800 -600 -400 -200 0

200

Potential mV SCE)

Fig. I. Potentiodynamic cathodic polarization curves in borate buffer, pH 8.4. (a) AISI 304

samples treated for

1

h in 0.05 M Ce(NO& in deaerated and aerated solutions. (b) Cerium-treated

[I h in 0.05 M Ce(NO&] and untreated AISI 304 samples measured directly from open-circuit

potential or after galvanostatic reduction treatment (2 h. -5 PA cme2).

The influence qf treatment parameters

In order to understand the role of cerium in the layer formation process, the influence of

various parameters such as time, solution chemistry, alloy composition, and potential

during treatment on the properties of the oxide layer was investigated.

Treatment time Figure 3 shows the potential decay curves during galvanostatic

reduction in aerated borate buffer for samples treated for different times. A remarkably

lower end potential is found for samples treated for longer times. In agreement with this, the

cathodic current density in potentiodynamic experiments in aerated borate buffer solution

decreased with increasing treatment time. In Fig. 4, SIMS profiles for samples treated for

various times in 0.05 M cerium nitrate/90”C are compared with the untreated sample and a

sample treated in pure water at 90°C. The amount of oxidized nickel was found to be


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1901

-800 1

-2000 0

2000

4000

6000

8000

Time (s)

Fig. 2. Potential decay curves during galvanostatic reduction (- 5 uA cm-*) for untreated and

cerium-treated [l h in 0.05 M Ce(NO,)s] AISI 304 samples in borate buffer.

- -500

> -600

5

xj -700

s

j -800

\

,\

_.

‘\

,.*’

_.X’

_._.-.

.\.__-.

i

-900 1

-2000 0 2000

4000 6000

8000

Time s)

Fig. 3. Potential decay curves during galvanostatic reduction in aerated borate buffer

(-5 uA cm-*) for AISI 304 samples treated in 0.05 M Ce(NO&) for various times.

negligible in the oxide layers and is therefore not shown in the graphs. Treatment in hot

water [Fig. 4(b)] leads to only minor changes in the oxide composition compared with the

air-formed film on the untreated sample [Fig. 4(a)]. During exposure to cerium nitrate,

however, very significant modification of the oxide film takes place. It is clearly evident in

Figs 4(c), 4(d), 4(e) and 4(f) that cerium treatment leads to a gradual chromium enrichment

and iron depletion in the oxide layer. Both SIMS and XPS data clearly show that a longer

exposure does not lead to a thickening of the film, the thickness remaining being x40 A.

After all treatments in cerium nitrate, cerium was found to be present in the oxide film, but

the amount of cerium does not change as a function of treatment time. The SIMS profiles

indicate that cerium is not present only as a surface contamination but is incorporated in the

(Fe,Cr) oxide film.


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I902

S. Virtanen et al

Sputter time (min)

1

1

‘-1

Sputler time (min)

Sputter time (min)

Sputter time (min)

Sputter time (min)

Sputter time (min)

Fig. 4.

SIMS profiles of AISI 304 stainless steel after various treatments. (a) Polishing (untreated);

(b) 30

min in

Hz0,‘90”C: (c f, 0.05 M Ce(N0&/90”C: (c) I5 min; (d) 30 min; (e) 1 h; (f) 3 h.

The results of the SIMS analysis were confirmed by XPS studies showing a continuous

increase of chromium content as a function of time (Fig. 5). The cerium content, on the

other hand, is more or less constant with treatment time. In all cases the cerium content is

relatively small (max. 5-6 at%). In agreement with the SIMS data, the amount of oxidized

nickel in the oxide layer determined by XPS is very small and was thus not included.

Al loy composit ion. Because of the finding that chromium enrichment takes place during

the cerium treatment, it must be considered whether the main effect of the treatment is to


7/17

80

z

60

5

40

20

0


1903

at-% Fe

t-% Ce

0.05 M Ce(No,), 190°C

0 min 15 min 30 min

Ih 3h

Fig. 5. Composition of the oxide layers determined by XPS.

increase the chromium content of the oxide film. Since chromium oxides are very stable and

will not be reductively dissolved under the conditions used in this work, the presence of a

chromium oxide layer could provide a more efficient barrier to the oxygen reduction

reaction than a passive film on stainless steel. Further, it has been shown earlier that pre-

passivation of stainless steel in acidic solutions can lead to a strong improvement in the

resistance to localized breakdown and this was interpreted to be due to a high chromium

content of the passive film formed in acidic solutions.‘6

A comparison of the cathodic reduction characteristics of a cerium-treated stainless steel

with those of an untreated pure chromium sample is shown in Fig. 6(a). The slow potential

decay in the case of the stainless steel is due to reduction of ferric species in the passive film.

After this reduction wave, however, a higher overpotential for the cathodic current is needed

for the cerium-treated stainless steel than for passivated chromium. This indicates that

cathodic reactions (oxygen reduction and/or hydrogen evolution) are less inhibited on the

passive film on chromium than on the oxide layer of cerium-treated stainless steel [Fig. 6(a)].

This finding was confirmed in potentiodynamic experiments. In addition, cerium treatment

of pure chromium leads to a retardation of cathodic reaction kinetics on the chromium

passive film [Fig. 6(b)]. Clearly, cerium plays a specific role in the inhibition mechanism of

chromium-rich oxide films.

Another question concerns the effectiveness of the treatment on Fe-Cr alloys of varying

chromium content. For this, Fe-Cr alloys with a chromium content from 5 to 30 at% were

treated for 1 h in 0.05 M cerium nitrate and in 0.15 M NaN03, and the samples were

subsequently galvanostatically reduced in aerated borate buffer. Figure 7 shows a summary

of the results. Presented is the end potential of galvanostatic reduction (after 5000 s) as a

function of the chromium content for untreated, NaNOs-treated and Ce(NO&-treated

samples. This end potential qualitatively represents the inhibition efficiency of cathodic

reactions. Clearly, for low chromium contents in the alloy, neither nitrate treatment is

particularly efficient in inhibiting cathodic reactions. On the other hand, for higher

chromium contents, the cerium nitrate treatment leads to a more significant retardation of

the cathodic reactions than sodium nitrate. SIMS profiles of Fe-l 5Cr and Fe-30Cr samples

treated for 1 h at 90°C in 0.05 M Ce(NOs)s and in 0.15 M NaNOs are shown in Fig. 8. As

expected, in both solutions, the oxide layer on Fe-30Cr [Figs 8(b) and 8(d)] contains

significantly more chromium oxide than the layer on Fe-15Cr [Figs 8(a) and 8(c)]. The


8/17

1904

S. Virtdnen et ul.

\“““‘~“-‘-j

- AISI 304 Ce-treated

----...-- Cr (non-treated)

2000

4000

Time (s)

-700

-750

t

-800

-900 t

-850

-2000 0 2000

4000

6000 8000

Time s)

Fig. 6.

Galvanostatic reduction (- 5 PA cm

‘)

in aerated borate buffer for: (a) cerium-treated [3 h

in 0.05 M Ce(NO&] AISI 304 and unWedted passive film on pure chromium; (b) untreated and

cerium-treated [30 min in 0.1 M Ce(NO&] chromium.

chromium content in the oxide layers is very similar for samples treated either in sodium or

cerium nitrate. The cerium content does not seem to depend on the alloy composition. These

findings indicate that a high chromium content in the oxide film is necessary for the

achievement of the cerium effect on the kinetics of the cathodic reactions.

Solution chemistry

Since the above findings show that chromium enrichment is an

essential factor to achieve a good surface treatment, it is possible to try to maximize the

chromium enrichment, for instance by decreasing the pH of the treatment solution.

Therefore, a comparison of the galvanostatic reduction behavior was carried out on AISI

304 and chromium samples treated for 1 h in 0.1 M Ce(NO& or in 0.3 M NaNOs at various

pH values. The results are summarized in Fig. 9 (end potential of the galvanostatic

reduction as a function of solution pH). In the case of stainless steel [Fig. 9(a)], in near-

neutral solutions treatment in cerium nitrate leads to a retardation of the kinetics of


9/17


1905

G -400 : 0

:: -500 1 A I @ f

2 -600 1 W

w

3 -700 :

z

2 -800 :

g

-900 1

-1000 -~‘~“‘~~“~.~~‘~‘~~‘(“~‘((~~~

5 IO 15 20 25 30 35

%

Cr

Fig. 7. The end potential (after 5000 s) of the galvanostatic reduction (-5 uA cm-*) in aerated

borate buffer for Fe-Cr alloys with a varying chromium content for untreated samples as well as for

samples treated for 1h in 0.05 M Ce(NOs)s/90”C or in 0.15 M NaNOs/90”C.

0.8

5

‘J 0.6

0

5

.Y 0.4

E

3

m

0.2

0.8

0 5 10 15

20

Sputter

time (min)

0

5 IO 15

20

Sputter time (min)

0.8

0 5

10

15

20

Sputter time (min)

0 5

10

15

20

Sputter lime (min)

Fig. 8. SIMS profiles of: (a) Fe-1SCr treated in O.iS M NaNOs; (b) Fe-30Cr treated in 0.15 M

NaNO,; (c) Fe-1SCr treated in 0.05 M Ce(NO&; (d) Fe-30Cr treated in 0.05 M Ce(NO&.


10/17

1906

S. Virtanen et al.

-400

I non-treated

-5oo-’ - -

------------_

,.D Na

c _600_1 (a)

::

j.

,,.’

> -700-r

,I.

.E

,

:’

,,, -800-r

.I.

.1’

_9oo_i

.:::/-

. . ._.._. ..‘_‘.

Ce

: ii::.”

-1000.1

,

/

I

I

1 2 3 4 5 6 7

PH

s -840

:: -860

>

J_

-880 i 1

‘.

w -900.~

‘b Ce

2 3 4 5 6 7

PH

Fig. 9. Effect of treatment solution pH on the end potential of galvanostatlc reduction

( -

5 PA cm -*) in aerated borate buffer for samples treated for

1

h at 90°C in 0.1 M Ce(NO& or in

0.3 M

NaNO?: (a) AISl 304 stainless steel; (b) chromium.

cathodic reactions, whereas the sample treated in NaN03 shows a behavior identical to that

of the untreated sample. In both nitrate solutions, the inhibition effect is stronger after

treatments in lower pH solutions. At pH 2, treatment in both nitrate solutions leads to an

identical galvanostatic reduction behavior. For pure chromium [Fig. 9(b)], a different pH

dependence is found. In this case the cathodic inhibition effect increases with increasing pH,

but again the largest difference between the cerium and sodium nitrate can be found at pH 6.

AISI 304 samples treated at pH 2 and pH 6 were further analysed by SIMS and the

profiles are shown in Fig. 10. Clearly, chromium enrichment is significantly stronger after

treatment in the solutions of low pH [Figs 10(a) and 10(b)]. On the other hand, very little

cerium is found on the surface of the sample treated at pH 2 in cerium nitrate [Fig. 10(b)],

and the composition of the oxide layer on this sample is thus almost identical to that of the

sample treated in NaN03 at the same pH [Fig. IO(a)]. Therefore it is not surprising that the

samples showed an almost identical electrochemical behavior during galvanostatic

reduction. Samples treated in solutions of pH 6 [Figs 10(c) and 10(d)] show a lower

chromium content, as can be expected. In this case, treatment in cerium nitrate leads to

significant amounts of incorporated cerium [Fig. 10(d)].

Treatment under polarization To study cerium incorporation on the stainless steel

surface, the treatment was carried out under cathodic or anodic polarization and the current


11/17

Role of cerium n treatment f stainless teels

1907

0.8

0.8

0.4

0.2

0

5

10 15 20

0 5 10

15 20

Sputter time (min)

Sputter time (min)

0

5

10 15 20

0 5 10

15 20

Sputter time (min) Sputter time (min)

Fig. IO. SIMS profiles of AI’S1304 stainless steel after various treatments for 1h at 90°C: (a) 0.3 M

NaN03, pH 2; (b) 0.1 M Ce(NO&, pH 2; (c) 0.3 M NaNOJ, pH 6; (d) 0. I M Ce(NO&, pH 6.

was monitored. Figure 11 shows the current density as a function of time for both

cathodically (a) and anodically (b) polarized samples. If cerium nitrate is added to the

solution during exposure to NaNOs under cathodic polarization, the current slowly

decreases indicating a gradual blocking of the cathodically active surface. Smaller currents

are also observed, if the sample is initially exposed to cerium nitrate. If the corresponding

experiment is carried out under anodic polarization, addition of cerium even increases the

passive current density. Since no decrease of the current densities can be observed even in

higher concentrated cerium nitrate solutions, it can be concluded that under anodic

polarization in cerium-containing solutions no blocking of the anodically active surface

takes place.

Elect roni c propert i es of the oxi de l uyers

Since cerium nitrate treatment clearly leads to a retardation of the oxygen reduction

reaction, it is interesting to investigate the electronic properties of the oxide layers. To study

electron transfer reactions, polarization curves were measured with a Fe(CN)i-/Fe(CN)i-

redox system present in the borate buffer solution. This redox system is well known and

widely used to study electron transfer reactions.““*

Fig. 12(a) shows a comparison of the

polarization curves of an untreated sample with a sample treated in hot water and in hot

cerium nitrate. Clearly, electron transfer reactions are accelerated after both surface


12/17

1908

S. Virtanen et

al

(a):

E = -500 mV SCE 90

1J

i” ,. .A=,“-w/

+if-

ddhon of Ce(NO,),

? I

- .-0.1 M NaNO,

0.05 M Ce(NO,),

-500 0 500 1000 1500 2000 2500 3000

Time (s)

25 ,,,,,,,,,,,.,,/,,,.,~,,/

E = +400 mV SCE

90°C :

-- .1 NaNO,

0.05 M Ce(NO,),

addition of Ce(NO,),

ot,

I*‘l”““‘L”,..*

I”‘.1

0

500

1000 1500 2000

2500

Time (s)

Fig. I 1. Current density as a function of time during treatment under polarization for AISI 04

stainless steel: (a) E= ~ 500 mV SCE);b) E= + 400 V WE).

treatments and even more strongly by the cerium nitrate treatment. The finding of an

increased electron conductivity after film formation in high-temperature water is in good

agreement with earlier work by other authors.‘9720

Further, if the redox system studies are

carried out on samples which have been treated for various times in cerium nitrate, an

identical behavior is found for all treatment times [Fig. 12(b)]. This indicates that the

retardationf he xygen reduction reaction after longer treatment times cannot be due to a

hindered electron transfer through the oxide film. A comparison of the electron transfer

kinetics on the untreated and treated passive film of chromium shows only a very slight

change in the kinetics after the cerium treatment. Further, electron transfer is faster on

passive chromium than on untreated stainless steel surfaces. This can be attributed to the

thinner oxide film in the case of chromium, which increases the probability of electron

transfer via a tunneling mechanism through the oxide.18

DISCUSSION

Oxide layerjbrmation

The role of cerium on the oxide layer formation can be best understood if the surface

analytical data are considered in more detail. Clearly, during exposure to hot cerium nitrate


13/17

Role of cerium in treatment of stainlesssteels

1909

~ non-treated

- - H,O - treated , , a .

I ...- Ce-treated

-400 -200 0 200 400 600

Potential

(mV SCE)

10-2 4

-400 -200 0 200

400

600

Potential (mV SCE)

Fig. 12. Polarization curves in deaerated borate buffer containing the redox couple

Fe(CN)i-/Fe(CN)i- for AISI 304 stainless steel: (a) untreated sample, samples treated for 1 h at

90°C in Hz0 or in 0.05 M Ce(NO&; (b) samples treated at 90°C in 0.05 M Ce(NO& for various

times.

solution, a gradual enrichment of chromium in the oxide

film takes place. The SIMS profiles

show a similar depth distribution of iron and chromium in cerium-treated samples (Fig. 4),

whereas in the air-formed oxide as well as in oxide films formed in hot water, or in NaN03,

chromium is present in the inner part of the layer and a much higher iron content is found in

the outer part. If the amount of hydroxide species is considered in the SIMS profiles, then a

higher Cr(oxide + hydroxide)/Fe(oxide + hydroxide) ratio is found in the outer part of the

cerium-treated samples. This is confirmed by angle-resolved XPS measurements, which

indicate for the cerium-treated samples a higher amount of oxidized Cr(hydroxide + oxide)

in spectra measured at a low angle (157 corresponding to a higher surface sensitivity. These

findings suggest that cerium treatment leads to a gradual dissolution of the iron oxide out of

the air-formed film. Simultaneously, oxide growth by chromium passivation takes place.

The overall layer thickness is only slightly changed. The dissolution of iron is most probably

due to the slightly acidic pH of the cerium nitrate solution (pH 4.8). By decreasing the pH of

the solution, chromium enrichment is stronger due to acceleration of iron dissolution.

Figure 13 shows the average composition of the oxide layer as a function of the

chromium content of the alloy (a) or solution pH for AISI 304 (b) for sodium and cerium


14/17

1910

S. Virtanen er al

12

100

80

;

s 60

.L

m

40

20

0

120

100

RO

;:

s 60

2

10

20

0

(a)

Fe- ISCr Fe- 150

Fe-30Cr

Fe-3OCr

NaNO:

Ce(NO?)?

NaNO 1

Ce(NO-c)j

(b)

Fe

pH 2

PH 2

NaNO;

Ce(NO 311

PH 6

PH 6

NaNOl

WNO3)3

Fig. 13. Average composition of the oxide layer determined from the SIMS data: (a) as a function

of the chromium content of Fe-Cr alloys; (b) as a function of solution pH for AISI 304 stainless steel.

nitrate treatments.

It is

clear from the figure that for

a fixed pH or fixed alloy composition,

the Cr/(Fe + Ce) ratio is constant for both sodium and cerium nitrate. In the case of samples

treated in cerium nitrate, cerium seems to replace part of the iron oxide in the oxide layer.

The SIMS data further indicate that the oxide layers formed in cerium nitrate are in all cases

thinner than the corresponding oxide layers formed in sodium nitrate. This suggests that

cerium in the oxide film makes the film more protective, thus hindering further film growth.

In this way, the presence of cerium in the solution leads to a higher Cr/Fe ratio in the oxide

film, which is generally beneficial for the stability of passive films on Fe-Cr alloys.

The influence of cerium on the kinetics of cathodic reactions

It is evident from the cathodic polarization measurements that cerium treatment leads to

a retardation of the oxygen reduction kinetics. The electrochemical studies in the presence of

the redox system clearly indicate that this is not due to an increased electron resistivity of the

oxide layer. Since oxygen reduction takes place in the potential range where reduction of the

oxide film can take place as well, it is therefore suggested that the inhibition of the oxygen


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191

reduction reaction is associated with the behavior of the oxide layer under cathodic

polarization. It has been shown elsewhere that reduction of thin Fez03 films in borate buffer

leads to a complete dissolution of reduced Fe2+ ,2’

whereas in the case of mixed (Fe,Cr)zOs

part or all of the Fe 2+ is trapped in the oxide without dissolution22 depending on the Fe/Cr

ratio. Furthermore, chromium oxide generally cannot be reduced under moderate cathodic

polarization in borate buffer.22 Therefore it can be concluded that the oxide films which are

highly enriched with chromium are more resistant to reductive dissolution than the passive

film on untreated stainless steel.

Accordingly, chromium enrichment itself leads to a higher resistance to reductive

dissolution. Since it was shown that cerium treatment of pure chromium is efficient in

retarding the oxygen reduction kinetics, the effect of cerium is not solely based on chromium

enrichment of the oxide. Even though chromium oxides are very stable it is possible that

weak sites of the oxide are dissolved under cathodic polarization and these sites would then

become low-resistivity paths for the current, leading to an increase of the oxygen reduction

kinetics. It has been discussed earlier that the passive film on chromium contains local sites

of a higher electron conductivity due to variations in the film thickness and defectiveness.23

It is then possible that cerium is blocking such weak sites, which otherwise would become

reductively dissolved.

In the case of an untreated passive film the cathodic polarization leads to reductive

dissolution of the oxide film (overall dissolution of iron oxide and local dissolution of

defective sites of chromium oxide) and, as a consequence, the oxygen reduction is less

hindered

on a bare steel surface than on the treated steel surface covered by chromium-rich

oxide and cerium species. It should be pointed out that the electron transfer kinetics on a

bare metal surface are very fast compared with an oxide-covered surface and therefore the

increase of the electron conductivity of the oxide layer by cerium treatment is of minor

significance compared with the enhancement of electron transfer kinetics due to dissolution

of the untreated oxide layer.

The mechanism of ceri um incorporat i on i n he oxide l ayer

The mechanism of cerium incorporation in the oxide layer is clearly cathodic

precipitation, as indicated by the studies carried out under polarization. The cathodic

potential used in this study [E= - 500 mV (SCE)] . s ar above the equilibrium potential of

the Ce(III)+Ce(O) reaction [Eo= -2.48 V (NHE)], thus the species responsible for the

blocking of the cathodically active sites must be Ce(II1) species. XPS spectra and a

comparison with standards indicate that cerium is indeed present as Ce(III) on the surface

(see Table 1 for binding energy positions for a cerium-treated stainless steel sample and for

standards). According to Pourbaix,24

an increase of pH, which can be expected to take place

at cathodic sites of the surface, will then lead to a precipitation of Ce(OH)3. During cathodic

polarization, the surface pH of the solution will increase and thus insoluble Ce(II1) species

are precipitated. During open-circuit treatments, cerium precipitation will take place at

cathodic sites of the surface. Therefore cerium will be incorporated in the oxide films exactly

at those sites which otherwise would lead to current paths during cathodic polarization. The

finding that the amount of cerium is always relatively low in the oxide film is in good

agreement with the concept of an inhomogeneous cerium distribution in the film.

Furthermore, since cerium is found distributed throughout the film and its depth

distribution does not show any time dependence, it is more likely that cerium is

incorporated in the film by local destruction of the oxide film followed by a precipitation


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1912

S. Virtanen el al.

Table

1.

Binding energies (eV) of Ce 3d spectra

for AISI 304 treated in Ce(NO& and for Ce(II1)

and Ce(IV) standards

AISI 304

Ce(NOh &Ce(SO&

3d3.2

905.13 905.25 906.4

3dm

886.88 886.75 887.75

of cerium species from the solution than by a diffusion of cerium into an existing film, since

the latter would be expected to lead to a higher surface concentration of cerium.

The results on Fe-Cr alloys with a varying chromium content clearly indicate that

cerium is efficient only if the chromium content in the alloy and subsequently in the oxide

films is sufficiently high. Thus a low chromium-containing film contains too many sites,

which are prone to reductive dissolution, to be blocked with cerium.

Since cerium incorporation takes place by pH-induced precipitation, the influence of

solution pH on the effectiveness of the treatment can be well understood. In the case of

stainless steel, a low pH leads to a high chromium enrichment but, owing to the high

solubility of Ce(OH)s in acidic solutions,24

only a small amount of cerium is found on the

surface. Thus, both the Ce(NO& and NaNOs treatments lead to a similar behavior during

subsequent galvanostatic reduction. On the other hand, in near-neutral solutions (pH 6),

only cerium nitrate is efficient in inhibiting the cathodic kinetics, and in this case the effect is

solely due to the precipitation of cerium species. In the case of pure chromium, cathodic

inhibition efficiency increases with increasing pH, again due to easier cerium precipitation.

The finding of a different galvanostatic reduction behavior for chromium treated in NaNOX

solution of different pH values may be due to formation of oxide films with a pH-dependent

thickness or stoichiometry.

The oxygen reduction inhibition is thus most likely due to the formation of a highly

reduction-resistant oxide film-partially due to the chromium enrichment and in addition

to the precipitation of insoluble Ce(OH)s at cathodic weak sites of the oxide layer. In order

to achieve the optimum cathodic inhibition, both the chromium enrichment and cerium

precipitation should be maximized. The resistance against reductive dissolution can be of

major importance for localized corrosion resistance. During localized attack such as pitting

or crevice corrosion, the outer surface is under cathodic polarization. Thus an oxide film

which is highly resistant against reductive dissolution prevents high cathodic oxygen

reduction currents and hence suppresses anodic pit growth.

CONCLUSIONS

(1) During exposure of stainless steel to hot (90°C) cerium nitrate solution, a gradual

dissolution of iron oxide and an accompanying film growth by chromium passivation take

place, leading to an enrichment of chromium in the oxide layer. The chromium enrichment

increases with exposure time (15 min to 3 h) and with a lower treatment solution pH.

Further, insoluble cerium species are precipitated at the cathodic sites of the surface. The

amount of cerium incorporated does not depend strongly on the treatment time, but

decreases in acidic solutions. The modification of the oxide chemistry by the cerium

treatment leads to an inhibition of oxygen reduction kinetics on the stainless steel surface.


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1913

(2) The effect of the cerium treatment cannot be solely attributed to the formation of a

chromium-rich oxide layer, since cathodic reactions are more strongly inhibited on the

cerium-treated stainless steel surface than on passivated pure chromium. Moreover, cerium

treatment of pure chromium leads to a retardation of the oxygen reduction reaction on the

chromium passive film.

(3) The inhibition of the oxygen reduction reaction is not due to a lower electron

conductivity of the oxide layer. The effect can be attributed to the high resistance of the

cerium-treated oxide film against reductive dissolution. This is partially due to a higher

chromium content of the passive film and, in addition, to the precipitation of cerium at weak

sites in the oxide layer which otherwise under cathodic polarization would lead to reductive

dissolution thus providing current paths for the oxygen reduction reaction. Factors leading

to a maximum chromium enrichment and/or cerium precipitation and incorporation

increase the cathodic inhibition efficiency.

Acknowledgements-The authors would like to thank MRCO and Long Manufacturing Ltd for financial support

of this work, and Dr Mark Kozdras and Dr Brian Chiedle (Long Manufacturing Ltd) for helpful discussions and

comments.

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