A surface analytical and electrochemical study on the role of cerium in the chemical surface treatment of stainless steels

CorrosionScience, 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 3*’ M. B. IVES,*

G. I. SPROULE,+

P. SCHMUKI+

and

M. J. GRAHAM? W. Smeltzer Corrosion Laboratory, McMaster University, Hamilton, Ontario, Canada +Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario, Canada

*Walter

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 ceriumtreated 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 Keywords: A. stainless steel, B. galvanostatic, B. SIMS, B. XPS, C. passive film.

INTRODUCTION 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. Chemical

’ 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

1898

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 PerkinElmer 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

Role of cerium in treatment of stainless steels

1899

of the oxide/substrate interface. indicate the position 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 opencircuit 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 lowerend potential of the cerium-treated sample indicates an inhibition of cathodic reactions.

S. Virtanen

1900

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-LL-800

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-600

-400

-200

0

200

0

200

Potential (mV SCE)

-1400-1200-1000

-800

-600

-400

-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

Role of cerium in treatment

-800

1

-2000

0

2000

of stainless steels

4000

6000

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8000

Time (s) Fig. 2.

Potential decay curves during galvanostatic reduction (- 5 uA cm-*) for untreated cerium-treated [l h in 0.05 M Ce(NO,)s] AISI 304 samples in borate buffer.

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Potential decay curves during galvanostatic reduction in aerated borate (-5 uA cm-*) for AISI 304 samples treated in 0.05 M Ce(NO&) for various times.

buffer

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.

S. Virtanen

I902

et al

Sputter time (min)

Sputter time (min)

1

1

‘-1 Sputler time (min)

Sputter time (min)

Sputter time (min) Fig. 4.

SIMS

profiles

of AISI

(b) 30 min in Hz0,‘90”C:

Sputter time (min)

304 stainless steel after various

(c f, 0.05 M Ce(N0&/90”C:

treatments.

(a) Polishing

(untreated);

(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. Alloy composition.

the cerium

treatment,

Because of the finding that chromium enrichment takes place during it must be considered whether the main effect of the treatment is to

Role of cerium in treatment of stainless steels

n

1903

at-% Fe at-% Ce 0.05 M Ce(No,), 190°C

80 z $

60

5 40 20 0

Fig. 5.

0 min

15 min

30 min

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3h

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 prepassivation 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

S. Virtdnen et ul.

1904

-AISI 304 \“““‘~“-‘-j

Ce-treated

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

2000

4000

Time (s)

-700 -750 t

-800 -850 -900 t -2000

0

2000

4000

6000

8000

Time (s) Fig. 6. Galvanostatic in 0.05 M Ce(NO&]

reduction (- 5 PA cm ‘) in aerated borate buffer for: (a) cerium-treated AISI 304 and unWedted passive film on pure chromium; (b) untreated cerium-treated [30 min in 0.1 M Ce(NO&] chromium.

[3 h and

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

Role of cerium in treatment

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-1000

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-~‘~“‘~~“~.~~‘~‘~~‘(“~‘((~~~ 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 1 h in 0.05 M Ce(NOs)s/90”C or in 0.15 M NaNOs/90”C.

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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&.

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the end

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( - 5 PA cm -*) in aerated borate buffer for samples treated for 1 h at 90°C in 0.1 M Ce(NO& 0.3 M NaNO?: (a) AISl 304 stainless steel; (b) chromium.

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)]. cathodic

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

Role of

ceriumin treatmentof stainlesssteels

1907

0.8 0.8 0.4 0.2 0 5

10

15

20

0

5

0

5

10

10

15

20

15

20

Sputter time (min)

Sputter time (min)

15

Sputter time (min)

20

0

5

10 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. Electronic properties of the oxide luyers

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)iredox 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

S. Virtanen et al

1908

(a):

E =

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1

-500 mV SCE

90%

1J

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.A=,“-

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-

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0.05 M Ce(NO,),

-500

0

500

1000

1500

Time

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2000

2500

3000

90°C

:

(s)

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mV SCE

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I*‘l”““‘L”,..*

1000

500

Time Fig.

I 1.

Current

1500

2000

2500

(s)

density as a function of time during treatment under polarization stainless steel: (a) E=

~ 500

mV (SCE); (b)

E=

+

for

AISI304

400mV (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 retardation of theoxygen 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

Role of cerium in treatment of stainlesssteels ~ - -

I

-400

10-2 4 -400

non-treated H,O - treated

...-

,

1909

, a.

Ce-treated

-200 0 Potential

-200

200

0

Potential

400

600

400

600

(mV SCE)

200

(mV SCE)

Fig. 12. Polarization curves in deaerated borate buffer containing the redox couple Fe(CN)i-/Fe(CN)ifor 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

S. Virtanen

1910

120

er al.

(a)

100

80 ;.

s 60 .L m 40

20

0

Fe- ISCr NaNO:

Fe- 150 Ce(NO?)?

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Fe 120

(b)

100

RO ;: s

60

2 10

20

0

pH 2 NaNO;

PH 2 Ce(NO 311

PH 6 NaNOl

PH 6 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

Role of cerium in treatment of stainless steels

191I

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 cerium incorporation in the oxide layer

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)] 1s . far 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

S. Virtanen

1912

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

3d3.2 3dm

905.13 886.88

Ce(NOh 905.25 886.75

&Ce(SO& 906.4 887.75

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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(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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