cerium-based conversion layers on aluminum alloys

Cerium-based conversion layers on aluminum alloys

Manuele DabalaÁa,*, Lidia Armelaob, Alberto Buchbergera, Irene Calliaria

aD.I.M.E.G., UniversitaÁ di Padova, Via Marzolo 9, 35131 Padova, ItalybCNR-CSSRCC, Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, UniversitaÁ di Padova,

Via Marzolo 1, 35131 Padova, Italy

Received 1 July 2000; accepted 28 October 2000

Abstract

Mixed layers of Al and Ce oxides have been obtained in conversion coatings on two aluminum alloys (AA6061 and

AA2618). The microstructure and the chemical composition of the protective ®lms have been examined by scanning electron

microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The corrosion

resistance in NaCl solution was investigated and a comparison with a traditional chromate conversion coating on the same

alloys is given. The coatings obtained by immersion in boiling solution of Ce(III) salts cover uniformly the alloys surface with

a protective layer, whereas the H2O2±cerium conversion coating process produces a thick, but cracked layer with `̀ dry-mud''

morphology, which preferentially covers the intermetallic particles on the samples surface. The boiled cerium conversion

coating treatments are as effective in corrosion resistance as the traditional chromating process, but the interest for industrial

purposes is limited because of long application times. The H2O2±cerium conversion coating process produces effective

corrosion resistance layers, thinner than the ones obtained by traditional processing, but very interesting for industrial

applications due to the limited environmental impact. # 2001 Elsevier Science B.V. All rights reserved.

PACS: 81.65 K; 81.15; 82.65 J

Keywords: Chemical conversion coating; Aluminum alloys; Cerium chloride; SEM; XPS; SIMS; Corrosion resistance; EIS; Hydrogen

peroxide

1. Introduction

Aluminum, a very reactive metal, forms in moist-

ured air a thin solid protecting ®lm of oxide which

prevents a further and generalized corrosion of the

material. However, in contact with solutions contain-

ing complexing agents (i.e. halides), aluminum under-

goes to localized corrosion. For this reason, to prevent

degradation processes of the metal and its alloys,

many different protection methods have been devel-

oped. Chemical conversion coatings are commonly

produced on various engineering alloys, to improve

paint adhesion and as adequate corrosion protection.

The most widely used conversion coating is produced

by chromating processes. Chromate protective layers

are usually obtained by immersion of the cleaned alloy

in an acid or alkaline solution containing chromate

ions. Due to its toxicity and carcinogenic nature,

chromium is very dangerous for health and far from

being environmentally friendly. Consequently, the

research for alternative non-toxic corrosion inhibitors

has led to the development of various novel conversion

Applied Surface Science 172 (2001) 312±322

* Corresponding author. Tel.: �39-049-827-5499;

fax: �39-049-827-5500.

E-mail address: [email protected] (M. DabalaÁ).

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 7 3 - 4

coating processes [1±10]. Recent patents cover a wide

variety of new coating formulations [11±14]. One of

the most promising systems is based on rare earth

elements. In a series of papers [15±17], several

researchers demonstrated that treatments with aqu-

eous solutions of rare earth salts (cerium, lanthanum,

neodymium and yttrium) effectively inhibit the corro-

sion of aluminum alloys. These studies revealed that

corrosion protection can be attributed to the formation

of an hydrate rare earth oxide ®lm on the metallic

surface. In the earlier studies the conversion coatings

were obtained by prolonged immersion in hot aqueous

solutions of rare earth ions [18±20]. Recently, a con-

version coating process based on the use of aqueous

solutions of CeCl3 and hydrogen peroxide was devel-

oped to produce passivating ®lms in much shorter time

[21±22]. In this paper, different Ce-based conversion

coatings have been obtained for two aluminum alloys,

AA6061 and AA2618. The chemical composition and

the microstructural features of the protective layers

were examined by SEM, XPS and SIMS. The corro-

sion behavior in NaCl solution was investigated and a

comparison with a chromate conversion coating on the

same alloys is given.

2. Experimental

Different conversion coatings were obtained on two

aluminum alloys, AA6061 and AA2618, whose che-

mical compositions are summarized in Table 1.

2.1. Chemical conversion coatings

Before treatment, the samples surfaces were

degreased and polished according to the standard

metallographic techniques [23]. Subsequently, a che-

mical pre-treatment of the alloys was performed as

follow: `̀ degreasing treatment'' by immersion in an

aqueous solution of NaOH at pH 12 for 1 min at room

temperature; rinse in deionized water for 5 min; `̀ mor-

danting treatment'' in a 20 g/l commercial solution of

BONDER (BONDER S.p.A., Milan, Italy) AL10

(H2SO4 between 35 and 40%, H3PO4 between 5

and 10%, HF between 3 and 5%) for 1 min at room

temperature and rinse in deionized water for 5 min.

Immediately after the chemical pre-treatment a cer-

ium-based conversion coating was obtained by

immersion (2 h at 1008C) in an aqueous solution of

Ce(NO3)3 (5 mM), rinse with deionized water, immer-

sion (2 h at 1008C) in an aqueous solution of CeCl3(5 mM) and ®nal rinse in deionized water (Ce1). A

different cerium-based conversion coating was

obtained as described above except both for the

immersion time (40 min) and the concentration of

cerium salts in the solutions (0.1 M) (Ce2). A third

cerium-based conversion coating was obtained by

immersion (10 min, room temperature) in 40 mM

CeCl3 aqueous solution to which 100 ml of 33vol.%

H2O2 was added per liter and followed by rinse in

deionized water. The solution pH was adjusted to 2

with HCl and was kept constant during the coating

processes (Ce3). To compare the corrosion resistance

results, a chromate conversion coating was obtained

on the pre-treated samples by immersion in a 30 g/l

commercial solution of BONDER AL 710 (CrO3

12ÿ15%, HF < 1%, HNO3 < 1%, H2SiF6 8±10%),

followed by rinse in deionized water.

2.2. Surface analysis

Scanning electron microscopy (SEM) was per-

formed using a Cambridge Stereoscan 440 electron

microscope. An electron accelerating voltage of 25

and 15 kV was used in backscattered electron and

secondary electron investigation, respectively. All the

specimens were coated with �20 nm of carbon by

evaporation before analysis. A Philips PV 9800 sys-

tem was used for energy dispersive X-ray (EDS)

analysis.

XPS analyzes were performed on a Perkin-Elmer F5600-ci spectrometer using non-mono-chromatized

Table 1

Chemical composition (wt.%) of the examined alloys

Alloy Mg Cu Si Cr Fe Ni Ti Mn

AA6061-T6 1.00 0.30 0.60 0.20 <0.70 ± <0.15 <0.15

AA2618 T6 1.60 2.30 0.18 ± 1.10 1.00 0.07 ±

M. DabalaÁ et al. / Applied Surface Science 172 (2001) 312±322 313

Mg Ka radiation (1253.6 eV). The working pressure

was <5�10ÿ8 Pa. The uncertainty in the binding

energy (BE) was 0.15 eV. Survey scans were obtained

in the 0±1100 eV range. Detailed scans were recorded

for the Al2p, Ce3d, C1s and O1s regions. The atomic

compositions were evaluated using sensitivity factors

supplied by Perkin-Elmer [24], taking into account the

geometric con®guration of the apparatus. The samples

were introduced directly into the XPS analytical

chamber by a fast entry lock system. The analyzed

area of XPS measurements was of 800 mm2.

Secondary ions mass spectrometry measurements

were done using O2� as primary ions in a HYDEN

EQS 1000 SIMS equipped with an HYDEN HAL IV

processing system operated in a vacuum of 10ÿ8 Torr.

Analyzes were performed by a primary ion energy of

4 keV with an ionic current of 1 mA. Surface char-

acterization was carried out recording mass spectra of

positive and negative ions. Subsequently, ion

species distributions were determined from the sur-

face to the ®lm/substrate interface by following the

signal of interest as a function of sputtering time.

Rastering of 750 mm2 and 20% electron gate were

used in order to avoid the problems due to crater wall

effects.

2.3. Electrochemical measurements

Polarization curves were performed by immersion

of the specimens in a 3.0% NaCl aqueous solution at

pH 7 and recorded with an AMEL 551 potentiostat

equipped with AMEL 567 function generator. The

corrosion current density values were obtained by the

Tafel extrapolation method [25]. The Electrochemical

impedance spectroscopy (EIS) measurements were

made in 3.0% NaCl aqueous solution at pH 7 and

recorded with Solartron Schlumberger 1255 FRA

spectrometer coupled with EG&G 273 A potentiostat.

3. Results and discussion

Figs. 1 and 2 show the SEM backscattered electron

images of Ce1 conversion layers on the aluminum

alloys. The surface of both samples appears uniformly

covered by the protective layer, whose thickness,

estimated with a SEM backscattered electron cross-

section image, is about 100±150 nm. Thicker areas are

observed in correspondence of intermetallic particles

containing elements such as Ni, Fe, Cr, Mn, and

surrounded by an aluminum-based solid solution.

Fig. 1. SEM backscattered electron image of Ce1 treated AA2618 surface. The light areas reveal cathodic zones.

314 M. DabalaÁ et al. / Applied Surface Science 172 (2001) 312±322

Such particles, identi®ed as FeNiAl9 and (Fe,Cr,Mn)3-

SiAl12 for the AA2618 and the AA6061 alloy, respec-

tively, probably behave as cathodic zones and favour

an increase in the growth rate of the cerium-based

protective layer. This hyphotesis is con®rmed by EDS

measurements. Fig. 3 shows the EDS spectrum of the

AA2618 sample. Over the cathodic zones (light areas)

the signal deriving from Ce is higher than over the

surface areas of pure aluminum. With respect to Ce1

and Ce2 procedures, the Ce3 conversion process

produces very different coatings on the alloys. Con-

cerning the AA6061 sample, the conversion coating

process does not produce a signi®cant surface color

modi®cation, whereas, after treatment, the AA2618

alloy reveals a pale yellow colored surface related to

the presence of Ce(IV) species and darker small areas.

The morphology of Ce3-derived layers on AA2618

and AA6061 samples is shown in Figs. 4 and 5. In both

Fig. 2. SEM backscattered electron image of Ce1 treated AA6061 surface.

Fig. 3. EDS spectra of AA2618 samples both on matrix surface and on precipitate particles.


cases the intermetallic clusters on the surface are

covered with thick and cracked coatings with `̀ dry-

mud'' morphology. Such agglomerates appear thicker

and more diffused over the AA2618 surface since this

sample shows more intermetallic particles with

respect to the AA6061 alloy. In particular, large

agglomerates with an average diameter of ca.

50 mm, and a diffuse presence of little structures

(diameter few micrometers) can be observed on the

AA2618 surface.

Fig. 4. AA2618 surface morphology of Ce3 chemical conversion coating.

Fig. 5. Cerium oxide agglomerate on AA6061 surface.


Concerning the surface chemical composition,

Figs. 6 and 7 show the XPS Ce3d and O1s regions

for the AA2618 and AA6061 alloys, treated in boiling

cerium solutions. In both cases, the Ce3d region

showed a position (BE Ce3d5=2 � 885:7 eV [26])

and a band-shape typical for Ce(III) species [27].

The presence of Ce(IV) ions was excluded since no

peaks around 916.0 eV, typical of tetravalent cerium

species, were observed [27]. As regards the O1s

region, it was centered at 531.5 eV and it showed high

FWHM (full width half maximum) values (�3 eV),

thus, suggesting the coexistence of different chemical

environments attributed to aluminum and cerium oxi-

des (Al2O3: BE O1s � 531:5 eV [24,28]; Ce2O3 BE

O1s � 530:3 eV [28]). The presence of aluminum

oxide was furtherly con®rmed by the Al2p peak which

was centered at a position typical for alumina (BE

Al2p � 74:2 eV [24]). Finally, the Cl2p region was

observed in both cases and attributed to chloride

species adsorbed on the samples surface. A slight

different surface chemical composition was observed

for both alloys after immersion in the CeCl3/H2O2

solution. Besides Ce(III) species, the presence of

cerium in tetravalent state was con®rmed by the

presence of the satellite structure around 916.0 eV

[29] (Fig. 8), attributed to a `̀ shake-down'' charge-

transfer process [27]. In addition, the Ce3�/Ce4� ratio

appeared higher in the AA2618 alloy with respect to

the AA6061 one. Concerning oxygen (Fig. 9), the O1s

region was centered at 531.8 eV, ascribable to alumi-

num oxide, and it also showed a shoulder at 529.3 eV,

attributed to cerium oxide CeO2 [24]. A higher Al/Ce

ratio was observed for the AA6061 alloy with respect

to AA2618, thus con®rming the heavy deposition of

cerium oxides on the AA2618 surface, as evidenced

by SEM investigation.

SIMS analysis con®rmed XPS results regarding the

chemical nature of the protective coatings on the

alloys surface. SIMS depth pro®les were obtained

by analyzing the following positive ions: 43 (AlO�),

54 (Al2�), 140 (Ce�) and 156 (CeO�). All the pro®les,

reported in Fig. 10, show an analogous behavior thus

con®rming that the composition of the Ce-conversion

coatings is a mixture of Al- and Ce-oxides. Concern-

ing AlO�/CeO� and AlO�/Al2� ratios, they remain

Fig. 6. XPS Ce3d region for AA2618 Ce1 coated sample.

Fig. 7. XPS O1s region for AA6061 Ce1 coated sample.

Fig. 8. XPS Ce3d region for AA2618 Ce3 coated sample.


almost constant along the coating thickness from the

surface to the ®lm/alloy interface. This behavior

suggests that the composition of the protective layer

is rather homogeneous and that aluminum is present in

the ®lm as oxide. As regards the estimation of the ®lm

thickness, problems arise from the insulating nature of

the protective layer with respect to the aluminum

alloy. As a matter of fact, sputtering rates are different

for conductive or non-conductive samples so that

depth determination is not simple. Anyway, the

®lm/alloy interface for the AA6061 Ce3 sample is

reached after 130 min of sputtering.

3.1. Electrochemical measurements

The electrochemical behavior of the treated Al

alloys was studied by examining the polarization

curves obtained in 3.0% NaCl aqueous solution. Such

curves are reported in Fig. 11 for the AA2618 samples

before and after the protective treatment. The free

corrosion potential (Ecorr) was recorded for all the

treated samples, at ca. ÿ700 mV. The anodic curves

are almost unaffected by the presence of the conver-

sion coatings, whereas the cathodic branches appear

strongly reduced. In particular, the O2 reduction limit

current is lowered and the water oxidation reaction

occurs at low potentials. In Table 2, the corrosion

current densities obtained for the untreated and Ce-

treated alloys are summarized. The data for a chro-

mate conversion layer are also reported for compar-

ison. The Ce3 coating process produces a higher free

corrosion potential with respect to others cerate pro-

cesses, due to the presence on surface of Ce(IV)

species. As regards the AA2618 alloy, the lowest

corrosion current density is obtained for the Ce1

coated sample, with a corrosion resistance even better

than for the chromate coating. Concerning the Ce3

coated sample, it shows a corrosion current density

higher than for chromated sample. This behavior is in

agreement with the morphological features of the

surfaces and can be related to the uniformity of the

protective layer. In fact chromating and boiled cerium

Fig. 9. XPS O1s region for AA2618 Ce3 coated sample.

Fig. 10. SIMS pro®le for AA6061 Ce3 coated sample.


processes produce uniform passive layers on the alloy

surface, thus showing an higher corrosion resistance,

i.e. a lower corrosion current density. In a different

way, the sample surface coated with the Ce3 conver-

sion process exhibits an inhomogeneous layer of

mixed cerium oxides only covering the intermetallic

particles on the aluminum surface: this one exhibits a

lower corrosion resistance. Similar results are

obtained for the AA6061 alloy. In this case, the best

corrosion resistance is obtained for the chromated

alloys. The limited presence of intermetallic particles

on the AA6061 surface reduces the precipitation of

cerium oxide agglomerates. On intermetallic particles,

which are more noble than the surrounding alloy, both

hydrogen evolution and oxygen reduction to OHÿ are

favored. Consequently, a local pH increase probably

occurs, thus, inducing the precipitation of hydrated

cerium oxide. These intermetallics sites are the main

responsible to the corrosion behavior of aluminum

alloys [30].

The impedance curves, obtained by EIS for the

AA2618 Ce-treated alloy, show two different time

constants (Fig. 12). One was attributed to the conver-

sion layer impedance, whereas the other is due to the

layer porosity polarization resistance. Fig. 13 empha-

sizes the equivalent circuit model developed for the

study of the conversion layer [6]. In this model Rs is

the resistance of the electrolyte, Rp is the polarization

resistance or charge transfer resistance which repre-

sents the corrosion resistance of the whole material,

Rcoat and Ccoat represent the intrinsic resistance and

capacity of conversion layer, respectively. Rcoat values

are correlated with the thickness of the protection

coating. Ws is a Warburg diffusional element, also

known as semi-in®nite transmission line, correlated to

the diffusion processes in the solution, and ®nally Qp

is a dispersed capacity, called constant phase element,

associated with the porosity of the conversion layer.

The values of the different elements of the equivalent

circuit for different samples are given in Table 3. The

values obtained for the untreated samples are

Fig. 11. Polarization curves of different coated AA2618 samples.

Table 2

Corrosion current density and free corrosion potential obtained by

polarization curves of examined samples

Icorr (mA/cm2) Ecorr (mV)

Alloy AA2618

Untreated 2.5 � 10ÿ6 ÿ700

Chromate 9.1 � 10ÿ8 ÿ717

Cerium 1 treated 8.5 � 10ÿ8 ÿ703



Alloy AA6061

Untreated 4.4 � 10ÿ7 ÿ734

Chromate 1.2 � 10ÿ8 ÿ735





considerably different than those for treated samples,

because the nature of the surfaces are deeply modi®ed

by conversion coating process. However, the values of

the elements of the equivalent circuit obtained for the

treated samples are similar, except for the Rp values,

which represent the corrosion resistance of the mate-

rials.

The AA6061 untreated alloy shows a polarization

resistance (Rp) higher than the AA2618 sample in

agreement with a lower content in intermetallic clus-

ters (i.e. cathodic sites) which are considered as the

main responsible for the corrosion of aluminum alloys

[30]. In addition, all the treated samples show, a

corrosion resistance higher than untreated. The rela-

tive increase in corrosion resistance is higher in

AA2618 samples than in AA6061. This behavior

can be ascribed to the poor content in intermetallic

particles on the AA6061 samples surfaces, as pre-

viously described. Finally, the AA6061 chromated

alloy exhibits the highest Rp and Rcoat since the growth

of the corrosion protective layer occurs with a differ-

ent mechanism.

4. Conclusions

`̀ Chromium-free'' aluminum conversion coatings

were obtained using different aqueous baths of cerium

salts and their corrosion resistance was compared with

the one performed by chromate protective coating.

Two aluminum alloys, AA2618 and AA6061, were

investigated in order to evaluate the in¯uence of the

alloying elements. Treatments in boiling Ce(NO3)3

aqueous solutions (Ce1 and Ce2) produced uniform

protective layers on the alloys surface, whereas in the

CeCl3/H2O2 coated samples (Ce3) cracked deposits

with `̀ dry-mud'' morphology were observed and the

presence of Ce(IV) species was detected. In both cases

the conversion coatings improved the corrosion resis-

tance of aluminum alloys in marine-like environment.

Thicker passivating layers were obtained for the

Fig. 12. EIS `̀ bode plots'' of different coated AA2618 samples.

Fig. 13. Equivalent circuit model developed for the study of

conversion layer.


AA2618 alloy with respect to the AA6061 samples,

due to the more diffused presence of intermetallic

particles with cathodic characteristic on the AA2618

surface. For this reason, the relative increase in corro-

sion resistance was higher in AA2618 than in AA6061

samples. The boiled cerium conversion coating treat-

ments were effective in corrosion resistance as the

traditional chromating process, but the interest for

industrial purposes is limited by the long time of

application. The H2O2±cerium process produced coat-

ings with a good corrosion resistance: even if these

coatings are worse than those produced by the chro-

mating process, it is worthwhile to continue the

investigation for industrial purposes due to their

low environmental impact.

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Table 3

Parameters of equivalent circuit for different treated samples

Samples Rs (O) Ws (mF0.5) Rp � 103 (O) Rcoat (O) Ccoat (mF) Qp (mF1ÿnp) np

6061 38.6 2.91 13.3 18.3 8.1 1.25 0.88

6061 chromated 47.2 3.64 2260 26.5 8.2 1.73 0.89

6061 cerium 1 treated 45.5 3.34 1860 26.3 8.7 1.76 0.91

6061 cerium 2 treated 40.4 3.52 103 24.1 8.3 1.95 0.93

6061 cerium 3 treated 42.6 3.91 616 20.3 8.2 2.00 0.90

2618 38.7 2.56 1.71 135 1.96 19.1 0.89

2618 chromated 46.5 3.44 980 213 1.96 19.4 0.88

2618 cerium 1 treated 44.4 3.63 1030 180 1.93 19.9 0.86

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2618 cerium 3 treated 47.6 3.85 384 155 1.97 19.7 0.87


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cerium-based conversion layers on aluminum alloys

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