corrosion protection properties of cerium layers formed on tinplate

Corrosion Science 46 (2004) 563–578

www.elsevier.com/locate/corsci

Corrosion protection properties of ceriumlayers formed on tinplate

N. Mora a, E. Cano a, J.L. Polo b, J.M. Puente c,J.M. Bastidas a,*

a CENIM-National Centre for Metallurgical Research, CSIC, Avda. Gregorio del Amo 8,

E-28040 Madrid, Spainb School of Industrial Engineering, University of Castilla-La Mancha, Avda. Carlos III s/n,

E-45071 Toledo, Spainc ACERALIA, P.O. Box 90, E-33480 Aviles, Asturias, Spain

Received 10 June 2002; accepted 3 July 2003

Abstract

Corrosion protection properties of cerium-passivated, unpassivated and chromium-passi-

vated tinplates in contact with a 0.1 M citric-citrate buffer solution were studied using po-

larisation (DC) and electrochemical impedance spectroscopy (EIS) methods. The morphology

and composition of the passive layer were studied using scanning electron microscopy (SEM),

energy dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS) in conjunction

with argon-ion sputtering. Cerium-passivated tinplate showed similar corrosion properties to

chromium-passivated tinplate in DC measurements, while the latter yielded the best corrosion

resistance results in EIS measurements. SEM and EDX illustrated spherical particles with

iron, cerium and tin contributions. XPS measurements indicated that the cerium-rich coating

was a hydrated oxide with a composition of cerium(III): Ce(OH)3 and cerium(IV): CeO2 and

Ce(OH)4.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Tinplate; Cerium- and chromium-passivation; Corrosion properties

* Corresponding author. Tel.: +34-91-553-8900; fax: +34-91-534-7425.

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

0010-938X/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0010-938X(03)00171-9

mail to: [email protected]

564 N. Mora et al. / Corrosion Science 46 (2004) 563–578

1. Introduction

Tinplate is a non-homogeneous material with a stratified structure, basically con-

sisting of a thin sheet of carbon steel coated with pure tin on both faces. Despite the

increasing use of new alternative materials in the canning industry, such as aluminium

and chromated steel sheet, tinplate continues to be used in more than 80% of cases.In order to stabilise the tinplate surface it is important to apply a passivation

treatment during its manufacturing and before lacquering. The general aim of the

passivation treatment is: (i) to prevent tin oxide growth on the surface, as excessive

oxide can cause discolouring during prolonged storage and in stoving operations

associated with lacquering and printing, especially in the curing of the lacquer; (ii) to

prevent the appearance of sulphide staining by sulphur contained in certain canned

foods; (iii) to improve lacquer adhesion to the metal substrate; and (iv) to improve

corrosion resistance after lacquering [1].Passivation treatments are commonly used in tinplate protection systems in the

canning industry. The application of these treatments is a relatively simple process.

Code 311 (3.5–9.0 mg/m2) is the most widely used and consists of a process of ca-

thodic treatment in a solution of sodium dichromate, while code 300 (1.0–3.0 mg/m2)

is a simple chemical dip treatment in a sodium dichromate solution. However, the

recent recognition that chromium in hexavalent state is both highly toxic and car-

cinogenic has led to a search for alternative non-toxic passivation treatments [2].

Rare earth elements have been proposed as a good alternative for hexavalentchromium, especially in the case of cerium and praseodymium which have two

higher oxidation states and are reasonably abundant [3]. In the case of aluminium

and aluminium alloys, the corrosion protection mechanism afforded by cerium

incorporated in the surface oxide is believed to involve a decrease in the rate of

cathodic oxygen reduction by the precipitation of trivalent cerium hydroxide

(Ce(OH)3) at regions of high pH [4], to form a complex hydrated cerium oxide layer

on the metallic surface [5].

Cerium ions can be used in one of two ways: (i) they can be added to an aqueousenvironment in low concentrations (100–1000 ppm); or (ii) they can be incorporated in

the passive oxide film on metal surfaces to form a protective conversion coating [6–8].

The aim of this work is to characterise a chromium-free passivation treatment of

tinplate based on cerium nitrate. The corrosion properties and composition of the

passive layer were studied using DC polarisation and electrochemical impedance

spectroscopy (EIS). The surface morphology, microstructure and composition of the

coatings were investigated using scanning electron microscopy (SEM), energy dis-

persive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS). The results forcerium-passivated tinplate were compared with unpassivated and conventional

chromium-passivated tinplates.

2. Experimental

Tinplate specimens with a tin coating weight of 2.8 g/m2 on each face were studied

using three types of passivation treatments: (i) the original tinplate specimens were

N. Mora et al. / Corrosion Science 46 (2004) 563–578 565

galvanostatically polarised in an aerated solution of 10,000 ppm cerium(III) nitrate

hexahydrate (Ce(NO3)3 Æ 6H2O) using a cathodic current density of 1 · 10�3 A/cm2

for 30 s, (ii) the original tinplate specimens were tested as-received, without passi-

vation treatment; and (iii) a conventional passivation treatment with a total chro-

mium coating weight of �5.5 mg/m2 (code 311) was performed by the manufacturer

on the original tinplate specimens. Passivation treatment (iii) was tested as referenceand for comparative purposes. The specimens were of the same batch, of comparable

surface roughness, and were all in the reflowed condition.

Three electrochemical tests were conducted using a 0.1 M citric-citrate buffer

test model solution at pH 4.3, commonly used in tinplate food packaging studies:

(i) potentiodynamic polarisation curves were plotted from )1.0 VSCE to +0.5 VSCE

at a polarisation scan rate of 0.4 mV/s; (ii) linear polarisation resistance (Rp) was

applied using an interval of ±20 mV from the rest potential (Ecorr); and (iii) EIS

measurements were performed in the frequency range from 1 · 104 to 1 · 10�3 Hzwith a logarithmic sweeping frequency of 5 steps/decade. EIS involved the im-

position of a 10 mV amplitude sine-wave and the measurements were carried out

at the Ecorr.

For electrochemical tests a Solartron 1250 Frequency Response Analyser and an

EG&G PARC 273A potentiostat/galvanostat controlled by a computer were used.

All the measurements were conducted with a conventional glass cell in citric-citrate

buffer solutions, purged with purified nitrogen to simulate anaerobic conditions

found inside cans for 1 h prior to the experiments and throughout their duration.A platinum gauze of large surface area (4 · 9 cm2) was the counter electrode and

a saturated calomel electrode (SCE) was the reference.

XPS experiments were conducted to characterise the passive films formed

using cerium nitrate. The spectra were obtained with a VG Microtech electron

spectrometer, model MT 500, using an Mg Ka1:2 anode X-ray source (hm ¼1253:6 eV) with a primary beam energy of 15 kV and an electron current of

20 mA. The pressure in the analysis cabinet was maintained at 1 · 10�9 Torr

throughout the measurements. The regions of interest were C 1s, O 1s, Sn 3d5=2

and Ce 3d. The binding energy (BE) scale of the spectrophotometer was peri-

odically calibrated using Ag 3d5=2 (368.3 eV, BE) and Au 4f7=2 (84.0 eV, BE)

substrates. The full width at half-maximum (FWHM) obtained for the Ag 3d5=2

line is 0.9 eV. A Shirley background subtraction was made to obtain the XPS

signal intensity. The concentration of each element was estimated by measuring

the respective peak area in the high-resolution XPS spectrum and using ele-

mental sensitivity factors.

The specimens were analysed by means of argon-ion sputtering on the originalsurface and after 30, 90 and 390 s argon-ion sputtering was carried out with a pri-

mary beam energy of 5 kV and an ion density of 1 lA/cm2, recording the corre-

sponding XPS spectrum for each sputtering cycle.

SEM (secondary electrons) and EDX techniques were performed using a JEOL

JXA-840 equipment with a Link AN 10000 system for the EDX analysis. This latter

was carried out on the whole surface area shown in the SEM images and at the same

voltage (15 kV).


3. Results and discussion

Fig. 1 shows three typical polarisation curves for the cerium-passivated unpass-

ivated, and chromium-passivated tinplates tested in a 0.1 M citric-citrate buffer so-

lution. The Ecorr were )0.670 VSCE for cerium-passivated tinplate; )0.684 VSCE for

unpassivated tinplate; and )0.682 VSCE for chromium-passivated tinplate. Similarbehaviour can be observed for all the tested tinplates, with an intense dissolution

process and the formation of an oxide film indicated by the presence of a current

minimum at about )0.460 VSCE (Fig. 1), which may be attributed to the formation of

tin(IV) hydroxide or the hydrated oxide (SnO2 ÆH2O): the most stable species under

anodic polarisation [9,10].

The Rp and corrosion current density (icorr) results, obtained from the polarisation

resistance measurements, for the tinplates tested in a 0.1 M citric-citrate buffer so-

lution were as follows. For the cerium-passivated tinplate: Rp ¼ 1:3 kX cm2 andicorr ¼ 17 lA/cm2. For the unpassivated tinplate: Rp ¼ 1:1 kX cm2 and icorr ¼ 21 lA/

cm2. And, finally, for the chromium-passivated tinplate: Rp ¼ 1:2 kX cm2 and

icorr ¼ 19 lA/cm2. The repeatability of these experiments was high. The Rp of the

unpassivated tinplate was lower and the icorr higher than for the cerium-passivated

specimen, which may explain its poor corrosion resistance behaviour. The cerium-

passivated and chromium-passivated tinplates showed similar behaviour. This result

is of practical importance, since it is probable that the cerium film formed favours

industrial relevant properties such as lacquer adhesion, oxidation resistance duringstoving, and sulphide staining resistance. It should be noted that the results were

obtained using bare tinplate and do not reflect the performance of coated tinplate.

Fig. 2 presents three representative Nyquist plots for passivated and unpassivated

tinplates tested in a 0.1 M citric-citrate buffer solution. The shape of the Nyquist

plots is similar for the cerium- and chromium-passivated tinplates, showing a

Fig. 1. Polarisation curves for the cerium-passivated, unpassivated and chromium-passivated tinplates.

A 0.1 M citric-citrate buffer was the test solution.

Fig. 2. Nyquist plots for (a) cerium-passivated tinplate; (b) unpassivated tinplate; and (c) chromium-

passivated tinplate. A 0.1 M citric-citrate buffer was the test solution.


capacitive behaviour: a depressed semicircle at high frequencies, with the centre lying

below the real axis, which is associated with the frequency dispersion of impedance

data; and a capacitive loop drawing a straight line at low frequencies (see Fig. 2(a)

and (c)). For the unpassivated tinplate (Fig. 2(b)) the Nyquist plot shows a capacitive

behaviour with some differences to the passivated tinplates: (i) a depressed semicircle

at high frequencies; (ii) a capacitive loop at intermediate frequencies, which resem-bles an inductive-type loop over the real axis; and (iii) a third capacitive loop

drawing a straight line or a portion of a second semicircle at low frequencies.

The simulation fitting procedure was performed using the equivalent circuit of

Fig. 3(a) for cerium- and chromium-passivated tinplates and that of Fig. 3(b) for

unpassivated tinplate. The parameters used are indicated in Tables 1 and 2. As can

be seen from Fig. 2, excellent agreement between experimental and simulated data is

observed. The negative values of C1 and R1 (see Table 2) may be related with the

passivation film growth [11]. Negative pseudocapacitance arises when the surfacecoverage by species decreases. The negative resistance results from interplay between

CPE

ZwRct

Rs

Rs

ZwRct

C1

R1

C2

R2

CPE

(a)

(b)

Fig. 3. Equivalent electrical circuits used to model: (a) cerium- and chromium-passivated tinplate/citric-

citrate buffer solution systems; and (b) unpassivated tinplate/citric-citrate buffer solution system.

Table 1

Parameters used in the simulation of impedance data

Tinplate Ecorr

VSCE

Rs

(X cm2)

Yp(lF cm�2 s�ð1�aÞ)

a Rct

(X cm2)

rw

(X cm2/s1=2)

Ce-passivated )0.670 54 214 0.75 283 26

Cr-passivated )0.734 48 21 0.79 9630 536

See Fig. 3 for symbols meaning.


adsorption and electrodissolution processes given by þC2, �R2 (see Table 2). The

best corrosion properties are shown by the chromium-passivated specimens (see scale

of Fig. 2(a) and (c), and Rct parameter of Table 1). The Warburg coefficient (rw)

(Table 1) is of the same order as Rct, possibly indicating ionic diffusion through thesolid products precipitated [12].

Figs. 4–6 show Kramers–Kronig (K–K) analysis of the transformation of the

impedance data in Fig. 2: the plots in Figs. 4–6(a) compare the results of the

imaginary-to-real transform with the real component of the experimental imped-

ance; and the plots in Figs. 4–6(b) compare the result of the real-to-imaginary

transform with the imaginary component of the experimental impedance. The al-

gorithm used has been described elsewhere [12]. The experimental data satisfy K–K

transforms for cerium- and chromium-passivated tinplates. On the other hand, theexperimental data do not satisfy K–K transforms for unpassivated tinplate, sig-

nifying instabilities and relaxation processes for the unpassivated tinplate/citric-

citrate buffer system.

Figs. 7–9 show typical SEM and EDX results for cerium-passivated, unpassivated

and chromium-passivated tinplates, respectively. SEM photos consisted of different

sized spherical particles, in which larger particles were dotted with smaller ones. This

Table 2

Parameters used in the simulation of impedance data

Tinplate Ecorr

VSCE

Rs

(X cm2)

Yp(lF cm�2 s�ð1�aÞ)

a Rct

(X cm2)

rw

(X cm2/s1=2)

C1

(lF cm�2)

R1

(X cm2)

C2

(lF cm�2)

R2

(X cm2)

Unpassivated )0.684 59 382 0.74 464 15 )32,737 )322 32,532 )207

See Fig. 3 for symbols meaning.

N.Mora

etal./Corro

sionScien

ce46(2004)563–578

569

Fig. 4. Comparison of experimental impedance (�) and impedance calculated using K–K relations (þ)

for cerium-passivated tinplate/citric-citrate buffer system at the rest potential.


for unpassivated tinplate/citric-citrate buffer system at the rest potential.


is particularly visible for the cerium treated specimen. The passive layer was prob-

ably initially formed by the deposition of smaller particles on the surface of the


for chromium-passivated tinplate/citric-citrate buffer system at the rest potential.

Fig. 7. SEM and EDX results for the cerium-passivated tinplate.


Fig. 8. SEM and EDX results for the unpassivated tinplate.


tinplate. The growth of smaller particles and the integration of larger particles (as a

result of the growth of smaller ones) led to the formation of the background. EDXspectra show iron, cerium and tin contributions.

Fig. 10 shows a high resolution C 1s XPS spectrum obtained for the cerium-

passivated tinplate. Deconvolution of the spectrum shows three components corre-

sponding to carbon in form of OC@O located at 289.61 eV BE, ACAO located at

285.6 eV BE, and CAC located at 284.9 eV BE. When the sputtering time increased,

the carbon contribution disappeared, indicating that it can be attributed to surface

contamination.

Fig. 11 depicts a high resolution O 1s XPS spectrum obtained for the cerium-passivated tinplate. Oxide and hydroxide groups can be observed as the major

components, suggesting that the cerium-rich coating was a hydrated oxide. The O 1s

BE in cerium oxide can vary from 529.6 to 530.3 eV BE, depending on the cerium(III)/

cerium(IV) ratio [13]. Species such as hydroxyl groups fall around 531.5 eV BE. Three

component curve fits were made to the O 1s spectrum for the hydrated cerium oxide

coatings, where the components represented oxygen anions (O2�) in cerium oxide

(Ce–O) at �529.7 eV BE, hydroxyl groups (Ce–OH) at �532.0 eV BE, and adsorbed

water at �533.0 eV BE (see Fig. 11). When argon-ions sputtering was performed, thewater and hydroxide contributions disappear but the oxide forms increase.

Fig. 10. Carbon 1s XPS spectra for the cerium-passivated tinplate.

Fig. 9. SEM and EDX results for the chromium-passivated tinplate.


Fig. 12 shows the Sn 3d5=2 XPS spectrum obtained from the cerium-passivated,

unpassivated and chromium-passivated tinplates. Tin was also present in the passive

Fig. 11. Oxygen 1s XPS spectra for the cerium-passivated tinplate.


layer as a small quantity of metallic (Sn0), seen by the insurgence of the peak at

485.0 ± 0.1 eV BE, and oxide, SnO (486.5± 0.3 eV BE) and SnO2 (486.8 ± 0.4 eV BE)

[14,15]. The Sn0 form increases with the sputtering time and the SnOx (SnO and/or

SnO2) form decreases. To determine the stoichiometry of the oxide the intensity ratio

of tin and oxygen, I(Sn 3d5=2)/I(O 1s), was used and the ratios for SnO and SnO2 for

chromium-passivated tinplate yields a value of 1/2, which suggests that tin oxide is in

the form of SnO2; using the unpassivated tinplates a relation of 1/1 was obtained,

indicating that the tin oxide is as SnO. Unfortunately, using the cerium-passivatedtinplate an I(Ce 3d)/I(O 1s) relation was not obtained, since the passive film is a

mixture of cerium(III) and cerium(IV) as oxide and hydroxide forms. As can be

observed, the intensity of the tin peak for the cerium-passivated tinplate is lower than

for the unpassivated and chromium-passivated tinplates, which may be attributed to

a uniform and thick layer of low porosity on the cerium-passivated specimen.

Fig. 13 shows a high resolution Ce 3d XPS spectrum obtained for the cerium-

passivated tinplate. Cerium in the form of cerium(III) and cerium(IV) was observed.

The high resolution XPS spectrum of Ce 3d is hard to analyse because of the presenceof a satellite structure due to hybridisation with the O 2p orbital and partial occu-

pancy of the 4f core levels in CeO2. The Ce 3d spectrum contains three characteristic

regions: from 880.0 to 890.0 eV BE being associated with Ce 3d5=2; from 890.0 to 910.0

eV BE being a mixture of Ce 3d5=2; and Ce 3d3=2 peaks and the peak at 917.0 eV BE

which is a satellite line of the Ce 3d3=2 line and arises exclusively from cerium(IV) [2].

The presence of cerium(IV) in the CeO2 oxide is to be expected as the yellow color-

ation of the passive film indicates a hydrated cerium(IV) oxide and cerium(III) oxide.

In summary, the cerium oxide layer composition is believed to contain a mixture ofcerium(IV) [CeO2 and Ce(OH)4] and cerium(III) Ce(OH)3 oxidation states [16]. In

aerated conditions the benefit of the cerium oxide layer may be attributed to a de-

crease in the efficiency of the cathodic reduction of dissolved oxygen. Nevertheless, as

has been indicated above the present study was performed in de-aerated conditions.

Fig. 12. Tin 3d5=2 XPS spectra for the (a) cerium-passivated, (b) unpassivated, and (c) chromium-passi-

vated tinplates.


Therefore, it is suggested that in these conditions a protective mechanism related with

the reduction of the dissolution process may take place.

Fig. 13. Cerium 3d XPS spectra for the cerium-passivated tinplate.

Fig. 14. Atomic concentration of carbon, oxygen, tin and cerium elements for the cerium-passivated

tinplate.


Fig. 14 depicts the atomic concentration of each element against the argon-ionsputtering time. The atomic concentration (ci) of the elements, as a percentage, was

determined using the expression:

ci ¼Ai=SiPðAi=SiÞ

� 100

where Ai is the peak area and Si is the sensitivity factor [17]. It can be observed in

Fig. 14 that cerium and tin increase with the sputtering time, and carbon decreases.In the experimental conditions of this study, the different techniques supplied

complementary information on the corrosion properties of the layers formed on

treated tinplates. In general, good concordance was obtained between the results of


the different techniques used. The uniform layers formed on cerium- and chromium-

passivated tinplates indicated by XPS and SEM analyses are responsible for the low

corrosion rates obtained with electrochemical methods.

4. Conclusions

DC measurements for cerium- and chromium-passivated tinplate are of the same

order of magnitude: Rp � 1:2 kX cm2 and icorr � 18 lA/cm2. Unpassivated tinplate

shows poor corrosion resistance behaviour: Rp ¼ 1:1 kX cm2 and icorr ¼ 21 lA/cm2.

The shape of the Nyquist plots is similar for the cerium- and chromium-passi-vated tinplates, showing capacitive behaviour including a depressed semicircle at

high frequencies and a straight line at low frequencies, indicating a diffusion process

during oxide dissolution. The Warburg diffusion coefficient (rw) was �26 X cm2/s1=2

for cerium-passivated tinplate and 536 X cm2/s1=2 for chromium-passivated tinplate.

With unpassivated tinplate a capacitive behaviour is observed containing: (i) a de-

pressed semicircle at high frequencies, (ii) an inductive loop over the real axis at

intermediate frequencies, and (iii) a capacitive loop drawing a straight line at low

frequencies. EIS data shows the best corrosion resistance for the chromium-passi-vated specimens (9630 X cm2) as compared with cerium-passivated tinplate (283

X cm2) and unpassivated tinplate (464 X cm2). The experimental results satisfy K–K

relations for cerium- and chromium-passivated tinplates. On the other hand, the

experimental results do not satisfy K–K transforms for the unpassivated tinplate/

citric-citrate system, signifying instability and relaxation processes.

XPS results performed on the cerium-passivated tinplate show oxide (Ce–O) and

hydroxide (Ce–OH) groups as the major components of the cerium-rich coating. Tin

as Sn0 and SnO was also present in the cerium-passivated tinplate. The cerium filmof the cerium-passivated specimens is a mixture of cerium(III) [Ce(OH)3] and

cerium(IV) [CeO2 and Ce(OH)4].

Acknowledgements

The authors wish to express their gratitude to the Commission of the European

Communities (Agreements 7210-PB/193 and 7210-PC/193) for financial support.

References

[1] R. Catal�aa, J.M. Caba~nnes, J.M. Bastidas, Corros. Sci. 40 (1998) 1455.

[2] B.R.W. Hinton, Met. Finish. 89 (1991) 55.

[3] A.E. Hughes, J.D. Gorman, P.J.K. Paterson, Corros. Sci. 38 (1996) 1957.

[4] S.M. Cohen, Corrosion 51 (1995) 71.

[5] B.R.W. Hinton, N.E. Ryan, D.R. Arnott, Mater. Australas. 19 (1987) 18.

[6] A.J. Davenport, H.S. Isaacs, M.W. Kending, Corros. Sci. 32 (1991) 653.

[7] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Mater. Forum 9 (1986) 162.


[8] A.E. Hughes, R.J. Taylor, B.R.W. Hinton, L. Wilson, Surf. Interface Anal. 23 (1995) 540.

[9] S.D. Kapusta, N. Hackerman, Electrochim. Acta 25 (1980) 1625.

[10] C.M.V.B. Almeida, T. Rab�ooczkay, B.F. Giannetti, J. Appl. Electrochem. 29 (1999) 123.

[11] R.F.A. Jargelius-Petterson, B.G. Pound, J. Electrochem. Soc. 145 (1998) 1462.

[12] J.M. Bastidas, J.L. Polo, C.L. Torres, E. Cano, Corros. Sci. 43 (2001) 269.

[13] D.R. Arnott, N.E. Ryan, B.R.W. Hinton, B.A. Sexton, A.E. Hughes, Appl. Surf. Sci. 22/23 (1985)

236.

[14] T.Y. Kim, Y.S. Jin, K.Y. Kim, Surf. Coat. Tech. 99 (1998) 319.

[15] E. Paparazzo, G. Fierro, G.M. Ingo, N. Zacchetti, Surf. Interface Anal. 12 (1988) 438.

[16] D. Briggs, M.P. Seah, Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy,

second ed., John Wiley, New York, 1990, p. 615, 637.

[17] Y. Xingwen, C. Chunan, Y. Zhiming, Z. Derui, Y. Zhongda, Corros. Sci. 43 (2001) 1283.

corrosion protection properties of cerium layers formed on tinplate

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