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Applied Surface Science 253 (2007) 6922–6931

Characterization of rare-earth conversion films formed on the AZ31

magnesium alloy and its relation with corrosion protection

M.F. Montemor a,*, A.M. Simoes a, M.J. Carmezim a,b

a ICEMS, Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugalb ESTSetubal, Instituto Politecnico de Setubal, 2910 Setubal, Portugal

Received 2 January 2007; received in revised form 2 February 2007; accepted 2 February 2007

Available online 13 February 2007

Abstract

Two pre-treatments were studied for AZ31 Mg alloy substrates, consisting of immersion in cerium nitrate and lanthanum nitrate solutions for

various immersion times. The surface composition was investigated by X-ray photoelectron spectroscopy and Auger electron spectroscopy that

revealed the presence of a surface film containing the rare-earth cation, with a composition which was time dependent in the case of the cerium pre-

treatment.

The corrosion behaviour of the pre-treated substrates in 0.005 M NaCl solutions was assessed by potentiodynamic polarization, open circuit

potential monitoring and the scanning vibrating electrode technique (SVET). The electrochemical results show that the pre-treatments reduced the

corrosion activity of the AZ31 Mg alloy substrates in the presence of chloride ions. The corrosion protection efficiency is dependent on the

treatment time.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Magnesium; Cerium; Lanthanum; XPS; AES; Corrosion

1. Introduction

During the past few years a considerable increase on the use

of magnesium alloys has been noticed, namely for applications

in the automotive, aeronautical and electronic industries. The

growing interest on the use of Mg alloys is due to their

interesting mechanical properties, good castability, easy

machining and good recycling possibilities.

The wrought MgAlZn alloys, such as AZ31, have found

applications in the automotive industry, mainly in the

production of structural components. These alloys, however,

present very high corrosion susceptibility, which reduces the

lifetime of structural components. The corrosion resistance of

these alloys can be enhanced by the modification of their

surface chemistry. Such procedure requires detailed studies and

assessment of fundamental knowledge in order to understand

the corrosion behaviour of the modified and of the bare alloy.

The most effective method to protect the surface of the Mg

* Corresponding author.

E-mail address: mfmontemor@ist.utl.pt (M.F. Montemor).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.02.019

alloys is a conversion pre-treatment based on formulations

containing chromate ions. However, due to the high toxicity of

these ions, this procedure has been abandoned and new

technical solutions, showing environmentally friendliness need

to be investigated. Surface treatments based on organofunc-

tional solutions are becoming attractive procedures to reduce

the corrosion rate of the substrate. Functional silanes [1] and

self-assembled monolayers [2] have been successfully tested on

magnesium substrates. These pre-treatments provide corrosion

protection, introduce surface functionality, enhance the

adhesion properties and consequently increase the lifetime of

painted substrates.

Other corrosion protection systems that have been proposed

are based on the formation of chemical conversion layers using

modified permanganate solutions or rare-earth salts. These lead

to the formation of nearly protective uniform coatings [3–8].

Rare-earth salts, especially cerium and lanthanum salts, are

known to inhibit the corrosion processes on several substrates

such as steel [5], galvanised steel [6,7] and aluminium and its

alloys [8,9]. Rare-earth salts can be used to deposit surface

conversion films, or incorporated in the alloy or even be used

as dopants in sol–gel or multifunctional silane pre-treatments

M.F. Montemor et al. / Applied Surface Science 253 (2007) 6922–6931 6923

[10–14]. It is generally accepted that rare-earth salts are

effective corrosion inhibitors, constituting a promising alter-

native to the use of pre-treatments based on Cr(VI)-containing

formulations.

Rare-earth conversion films are obtained by dipping the

metallic substrate in a solution of rare-earth salt. This procedure

has shown to produce a protective surface film, which provides

corrosion protection of metallic substrates such as galvanised

steel and aluminium [6,7,15]. However, very little is known on

the behaviour of magnesium alloys pre-treated with these rare-

earth salts. Rudd et al. [16] report that the corrosion rate of pure

magnesium and of magnesium alloy WE43 in borate buffer

solutions suffers an important drop when the substrates are

treated with cerium, lanthanum and praseodymium salts.

Potentiodynamic polarization results showed that a consider-

able ennoblement in the corrosion potential was observed for

all the treated samples. Literature [17,18] also reports that

cerium-based conversion coatings improve the corrosion

resistance of pure magnesium and magnesium alloys in

chloride media. The thickness of the cerium conversion layer

grows up in the first seconds, remaining nearly constant

afterwards [18]. Results published in literature agree that rare-

earth conversion films can be used as pre-treatments to protect

magnesium and its alloys in corrosive environments. However,

little is known about the mechanisms involved in such

processes, crucial for the development of new pre-treatment

formulations containing cerium or lanthanum ions as corrosion

inhibitors.

In this work electrochemical and analytical techniques were

combined to study the rare-earth conversion films formed on

the AZ31 Mg alloys and their ability to provide corrosion

protection.

2. Experimental procedure

2.1. Materials and solutions

AZ31 Mg alloy with the nominal mass composition of 96%

Mg, 3% Al and 1% Zn was obtained from Goodfellow Metals

(UK). The coupons were polished with SiC paper up to grit

2400, ultrasonically cleaned using acetone, washed with

deionized water (Millipore) and dried in air.

Solutions of cerium nitrate, Ce(NO3)3, and lanthanum

nitrate, La(NO3)3, with concentrations of 1.0 � 10�3 M and pH

6 were prepared. The metallic coupons were immersed in the

pre-treatment solution at room temperature (�22 8C) for

varying times: 1 min, 10 min and 60 min. Following immer-

sion, the treated coupons were dried in the oven for 30 min at

80 8C.

2.2. Analytical tests

The X-ray photoelectron spectroscopy and the Auger

electron spectroscopy measurements were performed using a

Microlab 310 F (from Thermo Electron, former Vg Scientific).

The Auger spectra were taken using an electron beam

accelerated at 10 keV. The target current was 5 nA.

The XPS spectra were taken in CAE mode (30 eV), using an

Al (non-monochromate) anode. The accelerating voltage was

15 kV. The quantitative XPS analysis was performed using the

Avantage software. The relative atomic concentration (Ax) was

calculated using the following relation:

Ax ¼normalised peak area � 100P

inormalised peak areas

where the subscript (x) refers to the quantified specie and the

subscript (i) refers to the other species detected in the XPS

spectra. The normalized peak area was obtained by dividing the

intensity of the XPS peak of the species (after background

subtraction) by the sensitivity factor of the corresponding

specie.

The background subtraction was performed using the

Shirley algorithm, which gives a curve S shaped and assumes

that the intensity of the background is proportional to the peak

area on the higher kinetic energy side of the spectrum.

The quantification was performed after peak fit. The peak fit

function used was a Gaussian–Lorentzian product function and

the algorithm was based on the Simplex optimization as used in

the Avantage software.

2.3. Electrochemical measurements

The open circuit potential measurements and the potentio-

dynamic polarization curves were performed using an

AUTOLAB PGstat 20 apparatus. Open circuit potential

measurements were carried on the samples immersed in the

conversion bath, i.e. during conversion film formation and also

on the samples exposed to the aggressive 0.005 M NaCl

solution, i.e. during corrosion experiments.

The potentiodynamic polarization curves were performed

using a scanning rate of 1 mV/s. The area of the working

electrode was 3.0 cm2, in a three-electrode electrochemical

cell, consisting of the working electrode, the saturated calomel

electrode as reference and the platinum as counter.

The scanning vibrating electrode technique (SVET)

measurements were performed using Applicable Electronics

equipment, controlled by the ASET program (Sciencewares).

The vibrating electrode was made of platinum–iridium covered

with polymer, leaving only an uncovered tip with a diameter of

40–50 mm. The surface was scanned at a distance of 200 mm.

The scanned area was 2 mm � 2 mm. Experiments were

performed in 0.005 M NaCl solutions.

3. Results

3.1. Characterization of the conversion film

Fig. 1 shows the evolution of the open circuit potential

(OCP) of the AZ31 Mg alloy substrates immersed in the

conversion baths for 1 h. The evolution was characterized by a

sharp increase towards more positive values mainly during the

first minute of immersion. For the substrate immersed in the

cerium nitrate solution the open circuit potential rose from

Fig. 1. Open circuit potential evolution for the bare alloy immersed in the

0.001 M Ce(NO3)3 and La(NO3)3 solution and magnification for the first

instants. Points A–D correspond to the times selected for film characterization.

Darker: Ce(NO3)3; Ligther: La(NO3)3.

M.F. Montemor et al. / Applied Surface Science 253 (2007) 6922–69316924

�1.7 V (SCE) and stabilized at values above �1.35 V (SCE).

An identical trend was observed for the La-treated, although

with slightly less noble potentials. The formation of the

conversion film led to an important ennoblement of the open

circuit potential. The sharp rise observed during the first minute

of immersion is probably due to surface activation and growth

of the first layers of the conversion film. After approximately

10 min the open circuit potentials approached constant values,

revealing that a steady-state condition was achieved and that a

quite complete conversion process occurred.

Based upon the OCP evolution, samples treated for 10 s,

1 min, 10 min and for 60 min (points A–D in Fig. 1) were

studied by XPS.

For the film formed during 10 s the content of cerium was

low (Table 1), contrasting with the high content of Mg 2p and O

1s in the form of hydroxides, which reveals that the conversion

film provided low coverage. At this stage Al 2p was also

detected. This result shows that the cerium film was not

homogeneous, probably being very thin. Fig. 2 depicts the Ce

3d and O 1s spectra obtained on the substrates treated for the

Table 1

XPS Quantification table for the Ce(NO3)3 treated samples

Species At.%

10 s 1 min 10 min 60 min

OH� 68 55 39.4 47.9

O2� – 14 32.8 27.6

Mg2+ 21 9.5 6 1.4

Al3+ 3.5 1 – –

Ce4+ 4 13 16.5 17.6

Ce3+ 3.5 7.5 5.3 5.5

Ce4+/Ce3+ 1.1 1.7 3.1 3.2

longer times (points B–D, Fig. 1). The spectra show the Ce 3d5/2

and Ce 3d3/2 ionization and the corresponding satellites. The

spectra of cerium compounds exhibited complex features

related to hybridization with ligand orbital and partial

occupancy of the valence 4f orbital. The Ce 3d spectra

(Fig. 2A) revealed an important contribution from Ce4+ species

recognized by the satellite peak at approximately 917.0 eV.

This satellite arises from a transition from the 4f0 initial state to

the 4f0 final state and is exclusive of the presence of Ce4+ as

reported elsewhere [19].

Fig. 2. XPS spectra obtained on the AZ31 Mg alloy treated with cerium nitrate

for different times: (A) Ce 3d spectra; (B) O 1s spectra.

Table 2

Quantification table for the La(NO3)3 treated samples

Species At.%

10 s 1 min 10 min 60 min

OH� 64 64.2 69.7 67.8

Mg2+ 20.8 15 9.8 7

Al3+ 3.2 2.5 – –

La3+ 12 18.3 20.5 25.2

M.F. Montemor et al. / Applied Surface Science 253 (2007) 6922–6931 6925

The quantification of the cerium species was determined

after fitting procedures and was based on the fitting results

obtained for the Ce 3d5/2 ionization (between 875 eV and

894 eV). Peak positions and satellite assignments were made

according literature [20].

The shape of the O 1s spectra (Fig. 2B) showed that

hydroxides and oxides species are present in the surface film.

These are likely to be associated with cerium oxides/hydroxides

and magnesium hydroxides (Table 1).

The Ce(IV)/Ce(III) ratio increased with the treatment time

during the first 10 min of immersion. This trend was

Fig. 3. XPS spectra obtained on the AZ31 Mg alloy treated with lanthanum

nitrate for different times: (A) La 3d spectra; (B) O 1s spectra.

accompanied by an increase of the amount of oxides

determined from the O 1s spectra (low binding energy

shoulder). The amount of magnesium decreased with the

treatment time and the signal of aluminium was undetectable

for treatment times longer than 1 min.

The La 3d spectra are depicted in Fig. 3A. Lanthanum has

only one stable oxidation state, La3+, observed at 835.4 eV. The

O 1s peak (Fig. 3B) showed only one contribution at energies

around 531.5 eV that can be attributed to the presence of

hydroxides, thus revealing that the conversion film was mainly

composed of La(OH)3 (Table 2). With increasing treatment

time the signals of Mg and Al decreased and the La intensity

became stronger.

The fraction of rare-earth cation in the surface of the

conversion film was calculated from Tables 1 and 2 and the

results are depicted in Fig. 4 as a function of the square root of

the time. The fraction of rare-earth cations showed a very

important increase during the first minute of immersion and

then stabilized. This evolution suggests that the conversion film

was formed during the first minute of immersion in good

agreement with the evolution of the open circuit potential

(Fig. 1). For longer treatment times (10 min and 60 min) the

fraction of rare-earth cation remained approximately constant,

reflecting mainly film thickening.

Fig. 4. Fraction of rare-earth cations in the conversion film as determined by

XPS.

Fig. 5. Auger depth profiles obtained on the AZ31 substrates pre-treated with

cerium nitrate for 10 s (a), 10 min (b) and 1 h (c).

Fig. 6. Evolution of the cerium fraction in the conversion film with the etching

time as determined by Auger.

M.F. Montemor et al. / Applied Surface Science 253 (2007) 6922–69316926

The thickness of the cerium conversion film was evaluated

by Auger depth profiling. Fig. 5 depicts the depth profiles

obtained for the samples treated with cerium nitrate for 10 s,

10 min and 1 h. The results showed the presence of a Ce-rich

surface film formed on the top of the magnesium substrate. The

etching time necessary to remove this cerium rich layer

increased by about six to seven times when the treatment time

increased from 10 s to 1 h, revealing an important thickening of

the conversion film. The same trend was observed for the

lanthanum conversion films (not shown).

The films formed for 10 s already showed the presence of a

surface film. However, the amount of magnesium was high

(above 30%), in good agreement with the XPS results.

The fraction of the cerium in the conversion films as a

function of the etching time is depicted in Fig. 6. The surface

films seemed to present two layers: an outer cerium-rich layer,

where the fraction of cerium was maximum and an inner layer,

in which a gradual decrease of the cerium content could be

observed (below dotted line in Fig. 6). The amount of cerium in

the outer layer strongly increased with increasing treatment

time and the outer layer also showed an important thickening.

The fraction of cerium in the inner layers as well as the

thickness associated with this layer did not show significant

change with increasing conversion time.

3.2. Corrosion behaviour

Fig. 7 depicts optical images of the substrate treated for

1 min and for 60 min in the Ce(NO3)3 or La(NO3)3 conversion

baths and for the blank substrate after 6 h of immersion in the

0.005 M NaCl solution. The images were registered during

immersion. Strong cathodic and anodic activity developed on

the surface of the blank substrate and hydrogen evolution was

observed in this case. For the same time of immersion, in the

NaCl solution, the samples treated with either Ce(NO3)3 or

La(NO3)3 for 1 min also showed some corrosion activity and

hydrogen evolution could be observed in some cases, showing

that corrosion activity was not completely inhibited. Never-

theless, the surfaces are clearly less attacked than the blank

substrate. The longer treatment (60 min) was capable of

producing a more protective layer, which resisted Cl� attack for

the duration of the experiment. The observations were

confirmed by SVET mapping of the surface. For the

La(NO3)3-treated samples, the SVET maps (Fig. 8a) show

some corrosion activity, with the surface shared between anodic

Fig. 7. Video-microscope images obtained on the blank AZ31 Mg alloy and on the same alloy treated with lanthanum nitrate and cerium nitrate for 1 min and for

60 min. Images obtained after 6 h of immersion in 0.005 M NaCl. Area 2 mm � 2 mm.

M.F. Montemor et al. / Applied Surface Science 253 (2007) 6922–6931 6927

and cathodic activity. The samples treated for 60 min revealed a

distinct behaviour, showing practically null activity (Fig. 8b).

The anodic and cathodic current densities were around zero,

revealing that the corrosion processes were inhibited. An

Fig. 8. SVET maps obtained on the AZ31 Mg alloy pre-treated for (a) 1 min and (b

Scan size = 2 mm � 2 mm.

identical trend was observed for the Ce(NO3)3 conversion film

(not shown).

The potentiodynamic curves obtained with the blank coupon

showed high current densities due to the dissolution of the

) 60 min in La(NO3)3. Maps obtained after 6 h of immersion in 0.005 M NaCl.

Fig. 9. Potentiodynamic polarization curves obtained for the AZ31 Mg alloy

pre-treated for different times in Ce(NO3)3. All curves performed in 0.005 M

NaCl. Scan rate 1 mV/s.

M.F. Montemor et al. / Applied Surface Science 253 (2007) 6922–69316928

substrate and strong anodic activity with current densities

reaching 1 mA/cm2 for potentials above �1.5 V (Fig. 9). The

Ce(NO3)3 conversion treatment reduced the currents by more

than one order of magnitude. The lowest anodic currents and

the largest potential shift were recorded for the substrates

treated during 60 min. There were no significant differences

between the substrates treated during 1 min and the substrates

treated during 10 min.

Fig. 10. Potentiodynamic polarization curves obtained for the AZ31 Mg alloy

pre-treated for different times in La(NO3)3. All curves performed in 0.005 M

NaCl. Scan rate 1 mV/s.

Fig. 11. (a) Open circuit potential of the AZ31 Mg alloy pre-treated for

different times in Ce(NO3)3; (b) zoom of the curves for the first minute of

immersion. Curves performed during immersion in 0.005 M NaCl.

The substrates pre-treated with La(NO3)3 also showed an

important drop of the anodic currents, more marked for the

substrates pre-treated during 60 min (Fig. 10). All the pre-

treated substrates revealed a shift of the corrosion potential

towards more negative values. This trend can be related with

cathodic inhibition effects (Fig. 10).

The evolution of the open circuit potential for the blank

sample in NaCl solution was characterized by an asymptotical

curve towards nobler values during the first instants of

immersion (Figs. 11 and 12). The initial values were around

�1.85 V and increased by more than 0.25 V, reaching stable

values after 2–3 min of immersion, possibly due to dissolution

of the substrate followed by precipitation of a magnesium

hydroxide layer also containing small amounts of aluminium

oxide.

For the treated samples the evolution of the open circuit

potential was different and depended on the treatment time in

the conversion bath. The open circuit of the 1 min Ce(NO3)3-

Fig. 12. (a) Open circuit potential of the AZ31 Mg alloy pre-treated for

different times in La(NO3)3; (b) zoom of the curves for the first minute of

immersion. Curves performed during immersion in 0.005 M NaCl.

M.F. Montemor et al. / Applied Surface Science 253 (2007) 6922–6931 6929

treated samples was characterized by a small but fast increase

during the first seconds of immersion followed by a sharp drop

(Fig. 11). The values reached a minimum and than started to

increase again towards nobler values, reaching a stable plateau

at around �1.7 V after about 5 min of immersion in the NaCl

solution. For the substrates pre-treated for longer times (10 min

and 60 min), the potential evolution was also characterized by a

slower increase, reaching a maximum after 25 s. The potential

minimum occurred after about 50–60 s and was again followed

by a rise towards nobler values and stabilization at �1.7 V. For

the substrates treated with La(NO3)3 the evolution was identical

(Fig. 12).

The small increase observed during the first instants of

immersion can be due to activation of the metallic substrates in

the presence of the NaCl solution, this process being faster for

the samples treated during shorter times (1 min). Such

behaviour can be explained by the different thicknesses of

the conversion film. The film formed during 1 min is thinner

and therefore surface activation occurred earlier. The drop of

the potential readings and their minimum can be associated

with the cathodic inhibition mechanism of the rare-earth ions,

whereas the rise towards nobler values indicates that the

conversion film becomes saturated with electrolyte, and surface

oxidation starts. Nevertheless, the film provides corrosion

protection and the potential raise is delayed comparatively to

the blank sample.

4. Discussion

When the AZ31 Mg substrate is immersed in the NaCl

solution, initiation of the corrosion activity occurs, the most

important cathodic reactions at the surface being water and

oxygen reduction:

2H2O þ 2e� ! 2OH� þH2 " (1)

O2þ 2H2O þ 4e� ! 4OH� (2)

Simultaneously, and for the range of potentials measured,

the most important anodic reactions are magnesium and

aluminium oxidation:

Mg ! Mg2þ þ 2e� (3)

Al ! Al3þ þ 3e� (4)

Following corrosion initiation, the metallic cations react

with the hydroxyl ions producing soluble species or insoluble

precipitates (reactions (6) and (7), respectively), depending on

the pH:

Mg2þ þOH� ! MgðOHÞþ (5)

Mg2þ þ 2OH� ! MgðOHÞ2 # (6)

Al3þ þ 3OH� ! AlðOHÞ3 # (7)

Hydrogen release could be observed immediately after

immersion of the samples in the NaCl solution, as consequence

of the reduction of water. This process was however hindered in

the presence of the conversion films, indicating that the

conversion layers could provide corrosion protection. The

thicker conversion films seemed to be more effective, delaying

corrosion activity. The kinetics of the corrosion processes were

unaffected by the thickness or by the chemical composition of

the conversion film. All the conversion films shifted the

corrosion potential towards more negative values and decreased

the corrosion currents and the anodic current densities by more

than one order of magnitude. The shift towards more cathodic

values indicates that cathodic inhibition was likely to occur as

reported in literature for these type of conversion films [19,21].

The presence of a conversion layer containing Ce(III) and

Ce(IV) species has been widely discussed in literature.

Different theories/mechanisms have been proposed to explain

the presence of these species. It has been claimed that local

increase of the pH due to the oxygen reduction reaction leads to

the precipitation of Ce(OH)3 which can be oxidized to hydrated

CeO2 [20,22,23]. Aldykiewicz et al. [23] reported that the film

formation involves the oxidation of Ce(III) to Ce(IV) in

solution, which can, in turn, precipitate as insoluble CeO2 at the

M.F. Montemor et al. / Applied Surface Science 253 (2007) 6922–69316930

cathodic sites. The same explanation is reported by Yu and Li

[19]. Another study [24] reports that the cerium conversion

treatment on AA2024 does not involve reactions of the Ce(III)/

Ce(IV) couple at the surface, but involves the reduction of H2O2

and/or O2, which leads to a strong pH increase at the metallic

surface and subsequent cerium oxide precipitation. The

proposed model [24] suggests that the formation of the

conversion film takes place through the deposition and

coalescence of small round-shaped cerium oxide particles,

which form a very thin layer that grows during immersion in the

conversion bath, thickening and developing a cracked-mud

structure. The nucleation of the film is strongly accelerated by

the presence of the active copper particles that exist in the

AA2024 alloy.

Lin and Fang [25] proposed that after immersion in

Ce(NO3)3, the air-formed magnesium oxide film immediately

dissolves because of the pH values below 8.5, which make it

unstable. The dissolution of the magnesium oxide is also

accompanied by aluminium dissolution, leading to the

formation of Al3+ cations that are rapidly deposited as Al(OH)3

at the pH characteristic of the conversion bath (�5.5). If the pH

raises enough (pH > 8.5) Mg(OH)2 also starts to precipitate,

creating a porous layer on the surface. The same mechanism

also claims that after precipitation of magnesium and

aluminium, the major cation in solution is Ce3+ that precipitates

on the top of Mg/Al hydroxide layer, forming a compact layer.

Afterwards there is deposition of a fibrous outer layer. In the

same work a significant contribution from cerium oxides was

detected, being attributed to the oxidation of Ce(III) to Ce (IV)

species.

In spite of the discussions published in literature it is not

clear how the cerium conversion films growth and develop a

mixed oxide structure and the relation of the film chemical

composition with the corrosion protective behaviour.

In the present study important changes were noticed in the

composition of the Ce(NO3)3 conversion films with increasing

treatment time. The film thickens and its chemical composition

changes when the treatment time in the conversion bath

increases. The films formed during the first minute of treatment

were mainly composed of cerium hydroxides becoming richer

in cerium oxides (CeO2) for longer treatment times in the

conversion bath (10 min and 60 min).

When the metallic substrates are immersed in the conversion

bath (pH � 5.5) there is dissolution of the outer oxide layers as

postulated in literature [25]. This immediately leads to the

formation of cathodic activity and production of hydroxyl ions.

The fraction of the stable species in solution is related with pH

as published elsewhere [26] and all the cations start to

precipitate as soon as the pH starts to rise. Thus, the metallic

cations (reactions (6)–(9)), including Ce3+ and Ce4+ precipitate

as hydroxides:

Ce3þ þ 3OH� ! CeðOHÞ3 # (8)

Ce4þ þ 4OH� ! CeðOHÞ4 # (9)

This leads to the formation of a first layer where cerium,

magnesium and aluminium hydroxides are present. In fact,

the surface analysis results show the presence of significant

amounts of magnesium and traces of aluminium during the

early stages of treatment. The OCP results also show that

this layer seems to develop within the first minute of

immersion.

The E–pH diagram for cerium [27,28] shows that the

precipitation of Ce(IV) species occurs at much lower pH than

Ce(III) species. Therefore, as soon as the pH starts to increase,

there is precipitation of Ce(OH)4. However, the presence of

Ce(OH)3 in the conversion film shows that pH could raise to

values above 10, at the very early stages of film formation.

With increasing treatment time the cerium conversion film

became richer in Ce(IV) species with an increasing

contribution of oxides (Fig. 2). This evolution shows that

or hydroxides are converted into oxides or that oxides

precipitate preferentially. Literature [27,28] suggests that for

pH values high enough (and depending on the O2 content),

Ce3+ species can be oxidized to Ce(OH)4, which may explain

the increasing concentration of Ce(IV), but does not explain

the increased content of oxides. The conversion of Ce(OH)3

into CeO2 has been mentioned in literature [23]. However,

such reaction must occur in a pH range between 2.8 and 4.9

[28], which is very unlikely to exist on the surface. Thus, to

explain the formation of CeO2 a solid phase transformation

can be postulated [28]:

CeðOHÞ4 ! CeO2 # þ 2H2O (10)

On the other hand, the growth of the conversion film

certainly reduces the corrosion activity at the native substrate.

Thus, as the conversion film grows and thickens the changes in

the pH are not as marked as the changes observed during the

very early stages of immersion in the conversion solution. For

longer conversion times, the pH changes slow down and the pH

may not attain sufficiently alkaline values. Therefore, the

formation of Ce(OH)4 is favoured, comparatively to Ce(OH)3,

as well as the solid state reaction (10), which may produce the

insoluble Ce(IV) oxide.

The following mechanism can thus be proposed to explain

the growth of the cerium conversion layer on the AZ31 Mg

alloy (Fig. 13):

1. D

issolution of the native oxide, accompanied by formation

of hydroxyl ions and pH rise.

2. G

rowth of a first layer composed of Ce(IV) hydroxides and

Ce(III) hydroxides mixed with Mg and Al hydroxides during

the first instants of immersion.

3. T

hickening of the surface film and weaker pH changes with

preferential deposition of Ce(OH)4 and its conversion into

CeO2, forming an outer layer, richer in Ce(IV) species.

Concerning the growth of the lanthanum conversion layers,

an identical mechanism can be proposed. However, during step

2 only La(III) hydroxides are formed and during step 3 only

film thickening occurs. The differences in the chemical

composition of the films seem to have no direct relation with

the corrosion protection performance.

Fig. 13. Scheme of film formation: (a) at short conversion times; (b) at longer conversion times.

M.F. Montemor et al. / Applied Surface Science 253 (2007) 6922–6931 6931

5. Conclusions

The pre-treatment of the AZ31 Mg alloy in Ce(NO3)3 or

La(NO3)3 solutions leads to the formation of a conversion film,

whose thickness increases notably with the treatment time.

The lanthanum conversion films are composed of La(OH)3.

The cerium conversion films have a time-dependent composi-

tion; a clear enrichment in Ce(IV) oxide and hydroxides was

detected by XPS analysis.

Cerium and lanthanum conversion layers provide corrosion

protection on the AZ31 Mg alloy. The corrosion protection

seems to be independent of the chemical composition of the

conversion layer however it depends on its thickness.

Acknowledgement

Financial support by POCTI/CTM/59234/2004.

References

[1] F. Zucchi, V. Grassi, A. Frignanit, C. Monticelli, G. Trabanelli, Influence

of a silane treatment on the corrosion resistance of a WE43 magnesium

alloy, Surf. Coat. Technol. 200 (2006) 4136–4143.

[2] A.L.K. Tana, A.M. Soutar, I.F. Annergren, Y.N. Liu, Surf. Coat. Technol.

198 (2005) 478–482.

[3] M. Zhao, S. Wu, J. Luo, Y. Fukuda, H. Nakae, A chromium-free con-

version coating of magnesium alloy by a phosphate-permanganate solu-

tion, Surf. Coat. Technol. 200 (2006) 5407–5412.

[4] H. Umehara, M. Takaya, S. Terauchi, Surf. Coat. Technol. 169/170 (2003)

666–669.

[5] C. Wang, F. Jiang, F. Wang, Corros. Sci. 46 (2004) 75–89.

[6] M.F. Montemor, A.M.P. Simoes, M.G.S. Ferreira, Prog. Org. Coat. 44 (1)

(2002) 79.

[7] M.F. Montemor, A.M.P. Simoes, M.G.S. Ferreira, Prog. Org. Coat. 43

(2001) 59.

[8] A.J. Aldykewicz, H.S. Isaacs, A.J. Davenport, J. Electrochem. Soc. 142

(1995) 3342.

[9] A. Aballe, M. Bethencourt, F.J. Botana, M. Marcos, J. Alloys Compd. 323/

324 (2001) 855–858.

[10] W. Trabelsi, P. Cecilio, M.G.S. Ferreira, M.F. Montemor, Prog. Org. Coat.

54 (2005) 276.

[11] M.L. Zheludkevich, R. Serra, M.F. Montemor, K.A. Yasakau, I.M. Mir-

anda Salvado, M.G.S. Ferreira, Electrochim. Acta 51 (2005) 208.

[12] M.L. Zheludkevich, R. Serra, M.F. Montemor, M.G.S. Ferreira, Electro-

chem. Commun. 8 (2005) 836.

[13] M. Cabral, R.G. Duarte, M.F. Montemor, M.G.S. Ferreira, Prog. Org.

Coat. 54 (2005) 322.

[14] M.F. Montemor, A.M. Cabral, M.L. Zheludkevich, M.G.S. Ferreira, The

corrosion resistance of hot dip galvanized steel pre-treated with bis-

functional silanes modified with microsilica, Surf. Coat. Technol. 200

(2006) 2875.

[15] B.F. Rivera, B.Y. Johnson, M.J. O’Keefe, W.G. Fahrenholtz, Surf. Coat.

Technol. 176 (2004) 349.

[16] A.L. Rudd, C.B. Breslin, F. Mansfeld, Corros. Sci. 42 (2000) 275.

[17] K. Brunelli, M. Dabala, I. Calliari, M. Magrini, Corros. Sci. 47 (2005)

989.

[18] M. Dabala, K. Brunelli, E. Napolitani, M. Magrini, Surf. Coat. Technol.

172 (2003) 227.

[19] X. Yu, G. Li, J. Alloys Compd. 364 (2004) 193.

[20] Y. Kobayashi, Y. Fujiwara, J. Alloys Compd. 408–412 (2006) 1157.

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

[22] S. Bohm, R. Greef, H.N. McMurray, S.M. Powell, D.A. Worsley, J.

Electrochem. Soc. 147 (2000) 3286.

[23] A.J. Aldykiewicz Jr., A.J. Davenport, H.S. Isaacs, J. Electrochem. Soc.

143 (1996) 147.

[24] P. Campestrini, H. Terryn, A. Hovestad, J.H.W. de Wit, Surf. Coat.

Technol. 176 (2004) 365.

[25] C.S. Lin, S.K. Fang, J. Electrochem. Soc. 152 (2) (2005) B54.

[26] K.A. Yasakau, M.L. Zheludkevich, S.V. Lamaka, M.G.S. Ferreira, J. Phys.

Chem. B 110 (2006) 5515.

[27] P. Yu, S.A. Hayes, T.J. O’Keefe, M.J. O’Keefe, J.O. Stoffer, J. Electro-

chem. Soc. 149 (2002) C623.

[28] S.A. Hayes, P. Yu, T.J. O’Keefe, M.J. O’Keefe, J.O. Stoffer, J. Electro-

chem. Soc. 153 (2006) C74.