Electronic Structure and Pitting Behavior of 3003 Aluminum

8/11/2019 Electronic Structure and Pitting Behavior of 3003 Aluminum

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Electrochimica Acta 54 (2009) 41554163

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Electrochimica Acta

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a

Electronic structure and pitting behavior of 3003 aluminum

alloy passivated under various conditions

Y. Liu a, G.Z. Meng a,b, Y.F. Cheng a,

a Dept. of Mechanical & Manufacturing Engineering, University of Calgary, Calgary, AB T2N 1N4, Canadab College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

a r t i c l e i n f o

Article history:

Received 5 December 2008Received in revised form 19 February 2009

Accepted 20 February 2009

Available online 3 March 2009

Keywords:

Aluminum alloy

Passivity

Pitting corrosion

Chloride ions

Electrochemical measurements

a b s t r a c t

Passivity of aluminum (Al) alloy 3003 in air and in aqueous solutions without and with chloride

ions was characterized by electrochemical measurements, including cyclic polarization, electrochemi-

cal impedance spectroscopy (EIS), localized EIS and potential of zero charge, MottSchottky analysis and

secondaryion massspectroscopy(SIMS) technique. Stability, pitting susceptibility and repassivation abil-

ity of Al alloy 3003 under various film-forming conditions were determined. Results demonstrated that

passive filmsformed on3003 Alalloy inair and inNa2SO4 solution without andwith NaCl addition show

an n-type semiconductor in nature. The passive film formed in chloride-free solution is most stable, and

that formed in chloride-containing solution is most unstable, with the film formed in air in between.

Pitting of Al alloy 3003 passivated both in air and in aqueous solutions is inevitable in the presence of

chloride ions. There is the strongest capability for the air-passivated Al alloy 3003 to repassivate, and

the weakest repassivating capability for Al alloy 3003 passivated in chloride-containing solution. The

resistance of the passivated Al alloy 3003 to pitting corrosion is dependent on the competitive effects

of pitting (breakdown of passive film) and repassivation (repair of passive film). According to the dif-

ferences between corrosion potential and potential of zero charge, passive film formed in air has the

strongest capability to adsorb chloride ions, while the film formed in chloride-containing solution the

least. Chloride ions causing pitting of passivated Al alloy 3003 in air and in chloride-free solution come

from the test solution, while those resulting in pitting of passivated Al alloy 3003 in chloride-containingsolution mainly exist in the film during film-forming stage.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Aluminum (Al) alloys of 3xxx series, due to their favorable

strength-to-weight property, high thermal conductivity, excellent

formability, as well as good corrosion resistance, have been widely

used in automobile heat exchange systems, replacing more tra-

ditional materials like stainless steels and copper alloys [1,2].

However, Al alloys are prone to experience pitting corrosion dur-

ing service in cooling system [35]. It has been acknowledged [6,7]

that corrosionresistance of aluminum (Al) alloy depends on forma-tion of a layer of passive film on its surface. However, halide ions,

especially chloride ions (Cl), show a strong attack to passive film,

resulting in pitting corrosion of Al alloy. It was reported[8,9]that

3xxx series Al alloys containing 11.5%manganese (Mn) andAl/Mn

intermatellic compounds might undergo the attack of chloride ions

at vulnerable defect sites. The role of Cl in pitting processes and

its interaction with passive film have been studied extensively, and

Corresponding author. Tel.: +1 403 220 3693; fax: +1 403 282 8406.

E-mail address: [email protected](Y.F. Cheng).

models have been developed to illustrate pitting corrosion [1013].

In particular, point defectmodel(PDM)is a relatively mature model

to describe the growth mechanism and kinetics of passive film as

well as pit initiation and growth in the presence of Cl [1416].

Passive films formed on Al alloy under various conditions are

associated with different structures. For example, a thin layer of Al

oxide film formed immediately in air is observed to be amorphous,

while the passive film formed in aqueous solution is usually dense,

coherent and compact[6].It is expected that there are significant

effects of the structure of passive film on its electrochemical andsemiconducting properties, and thus the pitting corrosion resis-

tance. To date, there has been limited work to investigate and

compare mechanistically the electrochemical and semiconducting

properties and pitting susceptibilitiesof passive filmsformed under

the various conditions[1720].For example, Bockris and Kang[17]

measured MottSchottky plots of the passive-film-covered pure Al

and its alloys to categorize the passive film on Al and Al alloys are

n-type semiconductors. Fernandes et al.[18]investigated the elec-

tronic properties of oxide film formed on 99.5% Al and 2024-T3 Al

alloy in a sulphuric-boric bath. The results indicated that the film

shows an n-type semiconductive behavior, with bandgap energies

0013-4686/$ see front matter 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.electacta.2009.02.058
http://www.sciencedirect.com/science/journal/00134686http://www.elsevier.com/locate/electactamailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.electacta.2009.02.058http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.electacta.2009.02.058mailto:[email protected]://www.elsevier.com/locate/electactahttp://www.sciencedirect.com/science/journal/00134686


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4156 Y. Liu et al. / Electrochimica Acta 54 (2009) 41554163

and semiconducting characteristics depending on the environmen-

tal conditions. Levinea et al. [19] employed MottSchottky analysis

to study the properties of oxide film of Al alloy 2024-T3, and deter-

mined that it is a p-type semiconductor, with non-stoichiometric

defects or substitutions existing in the ultra-thin layer. Koboti-

atis et al. [20] studied the electronic properties of passive layer

grown anodically on Al 70775 in chromate and oxalate solutions

using electrochemical impedance spectroscopy. It was found that

the oxide developed in the presence of chromate (good inhibitor)

exhibits a less-nobleflat-band potential anda lower averagedensity

of state.

It has been acknowledged [1416] that the electronic struc-

ture and properties of passive films were responsible for the film

breakdown and the initiation of pitting. A high pit density occur-

ring on the metal surface was generally associated with an n-type

oxide. However, the actual correlation between the semiconduc-

tive behavior of passive films and the pitting susceptibility was

nonexistent. Furthermore, passive films formed under different

conditions are expected to show different semiconducting prop-

erties and have distinct electronic structures, which would result

in different pitting susceptibilities. In this work, passive films

formed on Al alloy 3003 either in air or in aqueous solutions

without and with Cl were characterized by various electro-

chemical techniques, including cyclic polarization, electrochemicalimpedance spectroscopy (EIS), localized EIS (LEIS) and potential

of zero charge (PZC), MottSchottky analysis, and secondary ion

mass spectroscopy (SIMS). Electrochemical corrosion behavior of

the passivated 3003 Al alloy electrode was determined, and the

composition and electronic structure of the film was studied. The

adsorption, penetration and distribution of Cl in passive film and

the role of Cl in pitting of Al alloy 3003 were discussed. It is

anticipated that this research provides an essential insight into the

mechanistic understanding of passive film formation and break-

down as well as pitting corrosion of Al alloy 3003 under various

conditions.

2. Experimental

2.1. Electrodes and solutions

Specimens for electrochemical tests were cut from a round bar

of 3003 Al alloy supplied by DANA Canada Corporation, with the

chemical composition (wt%): Cu 0.20, Fe 0.70, Si 0.60, Mn 1.50,

Mg 0.05, Cr 0.05, Zn 0.10, Ti 0.05 and Al balance. Specimens were

machined and embedded in epoxy resin manufactured by LECO,

leaving a circular working area of 0.4cm2. The working surface was

ground with emery papers up to 1200 grit, cleaned by deionized

water and degreased in acetone.

2.2. Formation of passive films on 3003 Al alloy

Three types of passive film were formed on Al alloy 3003 under

controlled conditions. The first type was formed in air naturally

when 3003 Al alloy electrode was exposed in air. The second and

third types were formed in 0.25 M Na2SO4 solution, without and

with 0.5 M NaCl, respectively. Preparation of oxide film in solution

was not simply to immerse the air-exposed 3003 Al alloy electrode

tothe aqueous solution.The electrodesurfacewas groundfirstwith

a 1200-grit emery paper that is installed inside the film-forming

solution in order to remove completely the air-formed film before

the new film was generated in the solution. The ground electrode

continued to stay in solution for 2 h, and there was external poten-

tial applied at this stage. After then the film-covered electrode was

transferred rapidly to the test solution for electrochemical charac-

terization.

Fig.1. Cyclic polarization curves of thepassivated 3003 Al alloy electrodein 0.25M

Na2SO4+ 0.5 M NaCl solution (potential scanning rate: 0.333mV/s).

All solutions were made up from analytical grade reagents and

ultra-pure deionized water (18 Mcm in resistivity).

2.3. Electrochemical measurements

Electrochemical measurements were performed through a

Gamry Reference 600 electrochemical system by using a three-

electrode cell, with 3003 Al alloy as working electrode (WE), a

saturated calomel electrode (SCE) as reference electrode (RE) and

a Pt wire as counter electrode (CE). All electrochemical tests were

conducted in 0.25M Na2SO4 + 0.5 M NaCl solution.

Prior to cyclic polarization measurement, 3003 Al alloy WE was

immersed intestsolutionat least1 h untilcorrosion potential(Ecorr)

reached a steady-state value. Anodic polarization scan was per-

formed at a potential sweep rate of 0.333 mV/s, with a reverse in

scan direction when anodic current density reached 0.1mA/cm2.

Pitting potential (Epit) was determined when anodic current den-sity deviated abruptly from the stable passive current density, as

indicated inFig. 1.

The conventional EIS measurements were conducted on the

macroscopic Al alloy 3003 WE at Ecorr or Epit, with the measur-

ing frequency ranging from 20 kHz to 0.001Hz and an applied AC

disturbance signal of 10 mV.

LEISmeasurements wereperformed on WE through a PAR Model

370 scanning electrochemical workstation, which was comprised

of a scanning Pt microprobe with a 10m tip, a 370 scanning con-

trol unit, an M236A potentiostat, an M5210 lock-in amplifier and a

video camera system. For LEIS mapping, the Pt microprobe, which

was set above the electrode surface at 50m, was stepped over a

designated area of the electrode. The probe scanning took the form

of a raster inxy plane. The step size during LEIS scanning was con-trolled to obtain a plot of 32 lines24 lineswith a scanning area of

1000m750m. An ACdisturbance signal of 10 mV was applied

to WE that was at Ecorr. The measurement frequency was fixed at

10 Hz.

Inmeasurements ofPZC of3003Al alloyelectrode,a frequency of

18 Hz and an ACdisturbance signal of 10 mV were applied. Double-

charge layer capacitance was obtained from the measured EIS.

All the tests were performed at ambient temperature (22 C)

and open to air.

2.4. SIMS characterization

Negative and positive SIMS characterizations were performed

through a ToF-SIMS IV instrument manufactured by IonTOF GmbH.


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Y. Liu et al. / Electrochimica Acta 54 (2009) 41554163 4157

A chopped 15 keV Ga+ ion beam of about 200 nm in diameter was

used to generate secondary ions, which were then separated by

the time-of-flight mass analyzer. In the imaging mode, maps of the

lateral distribution of elements across the target surface were col-

lected from an area of 40m80m. For the depth profiling, a

dual ion beam technique was used, where the Ga analytical ion

gun was scanned over an area of 30m30m near the centre

of a crater of 200m200m, created by another, sputter ion

gun. The sputter guns used either Cs+ or O ion beams of 80 nA at

1 keV initial energy for negative and positive secondary ion profiles

correspondingly.

In order to characterize the permeation and diffusion of chlo-

ride ions in passive films formed under various conditions, i.e., in

air and in aqueous solutions without and with chloride ions, the

films were polarized at an anodic potential of0.65 V (SCE) in

0.25M Na2SO4 + 0.5 M NaCl solution for 3 h, and then character-

ized by SIMS. For comparison, a blank specimen that was filmed

in 0.25M Na2SO4 solution without further anodic polarization in

chloride-containing solution was also under SIMS characteriza-

tion.

3. Results

3.1. Cyclic polarization measurements

Fig. 1shows the cyclic polarization curves measured on Al alloy

3003 with passive films formed in air and aqueous solutions with-

out and with chloride ions in 0.25 M Na2SO4 + 0.5 M NaCl solution,

where the solid arrows indicated the potential scan direction. It is

seen that all passivated electrodes showed a stable passive region

where the low passive current density was independent of poten-

tial. Current density then increased abruptly at Epit, followedwith a

positive hysteresis loop during reverse potential scanning. The val-

ues ofEpit were 0.60V, 0.53V and 0.50V (SCE) for electrodes

with passive films formedin air, in Na2SO4 solution,and in chloride-

containing Na2SO4 solution, respectively. There was similar Erp of

about 0.70 V (SCE) for all electrodes. Furthermore, although thecurrent density was set at 0.1 mA/cm2 for scan reversion, it did not

decrease immediately after the potential was reversely scanned,

but continued to increase. The different current densities resulted

in different sizes of the hysteresis loop forthe three types of passive

film, which were correspondent with different repassivation abili-

ties of the film. There was the biggest loop for passive film formed

in chloride-containing solution, while the smallest loop forthe film

formed in air.

3.2. Conventional EIS measurements on the macroscopic electrode

Fig. 2shows the Nyquist diagrams measured on Al alloy 3003

electrodes with passive films formed in air and in solutions with-

out and Cl

and solution, respectively, in 0.25M Na2SO4 +0.5MNaCl solution (atEcorr). There was a common characteristic for all

curves, i.e., a capacitive semicircle in the high-frequency range and

a diffusive tail in the low-frequency range. There was the biggest

semicircle for passive film formed in solution without chloride

ions, and the smallest semicircle for the film formed in chloride-

containing solution.

Fig. 3 shows the Nyquist diagrams of the passivated 3003 Al

alloy electrodes at Epit. It is seen that, at Epit, there was a sig-

nificant decrease of the semicircle size. Moreover, an inductive

loop was observed in low-frequency range in all diagrams. Fur-

thermore, there was the biggest diameter of the semicircle for

passive film formed in chloride-free solution (1300), and thesmallest one for passive film formed in chloride-containing solu-

tion(12 only). Observationof electrodemorphologies afterLEIS

Fig. 2. Nyquist diagrams measured on 3003 Al alloy with passive films formed

in air (a), chloride-free solution (b) and chloride-containing solution (c) in 0.25 M

Na2SO4+ 0.5 M NaCl solution at individual corrosion potential.

measurements in Fig. 4 showed that the electrode passivated in

chloride-containing solution suffered severe pitting corrosion withdeep pits (Fig. 4c), while the passivated electrode in air and in

chloride-free solution had slightpitting with smalland shallow pits

(Fig. 4aand b).

The EIS measurements were performedin aerated solutions and

thus described the sum of cathodic and anodic processes proceed-

ing on heterogenous surface of oxide and pits. However, at Epit,

anodic reaction including pitting corrosion dominated the elec-

trode behavior, and the cathodic response was too small to be

ignored.

3.3. LEIS measurements

Fig. 5 shows the LEIS maps measured over 3003 Al alloy

electrodes with passive films formed in air and the solutions

without and with chloride ions, respectively, at Ecorr in 0.25M

Na2SO4 + 0.5M NaClsolution. Inthexyzthree-dimensionalspace,

|Z|represents the measured impedance amplitude, which usually

refers to the resistance of electrode to localized corrosion at indi-

vidual measuring point. Thus, the fluctuating plane in the 3D figure

represents the distribution of local impedance over the scanned

surface of the electrode. The 3D impedance distribution was also

projected on xy plane, where the impedance amplitude of indi-

vidual point was represented with different colors. It is seen that

there were frequent fluctuations of impedance value measured on

passive film formed in air (Fig. 5a). The impedance distribution was

the most uniform on electrode passivated in chloride-free solution

(Fig. 5b).

3.4. PZC measurements

Fig.6 showsthe double-chargelayer capacitance of passive films

formed in air, 0.25M Na2SO4, and 0.25M Na2SO4 + 0.5 M NaCl solu-

tions, respectively, as a function of applied potential. It is seen

that there is a common feature for the three curves, i.e., a mini-

mum of double-charge layer capacitance that is considered as PZC

of the electrode was observed. In addition, steady-state corrosion

potential (Ecorr) of the passivated 3003 Al alloy electrode was also

included in each diagram. Generally, PZC ware more negative than

Ecorrfor all passivated electrodes.The differences between Ecorrand

PZC (E= Ecorr PZC) for passive films formed in air, Na2SO4 solu-

tion and Na2SO4 + NaCl solution were 0.119 V, 0.093 V and 0.036 V,


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Fig. 3. Nyquist diagrams of 3003 Al alloy with passive films formed in air (a),

chloride-free solution (b), and chloride-containing solution (c) at individualEpit.

respectively. Thus, there was a smaller potential difference, E, forpassive film formed in aqueous solutions than that formed in air,

and further, passive film formed in chloride-containing solution

hada smaller potential difference than that formedin chloride-free

solution.

Fig. 4. Surface morphology of 3003 Al alloy with passive films formed in air (a),

chloride-free solution (b), and chloride-containing solution (c) at individualEpit.

3.5. Capacitance measurements and MottSchottky analysis

Potential dependence of the capacitance of space-charge layer

(Csc) is expressed by MottSchottky relationship[21]:

for n-type semiconductor

1

C2

SC

=2

er0ND

Efb

T

e

(1)


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Fig.5. LEISmapsmeasured on 3003Al alloy electrodes withpassive filmsformed in

air (a), chloride-free solution (b), and chloride-containing solution (c) at individual

Ecorrin 0.25M Na2SO4+ 0.5M NaCl solution.

for p-type semiconductor

1

C2SC=

2

er0NA

E fb

T

e

(2)

where e is electron charge (1.61019 C), r is dielectric con-stant of Al oxide, taken as 10[22], 0 is the vacuum permittivity

(8.851014

F cm1

), ND is the donor density, NA is the acceptor

Fig. 6. Relationship between double-charge layer capacitance vs. applied potential

for 3003 Al alloy with passive films formed in air (a), chloride-free solution (b) and

chloride-containing solution (c).

density, E is the applied potential, fb is flat-band potential, isBoltzmann constant (1.381023J K1) andTis absolute temper-

ature.ND andNA can be determined from the slope of the linear

relationship between C2SC

and E, while fb is obtained from the

extrapolation toC

2

SC = 0.


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Fig.7. MottSchottky curves forthree types of passivefilm formedon 3003Al alloy

measured at 1000 Hz in 0.25M Na2SO4+ 0.5M NaCl solution.

Fig. 7shows the MottSchottky curves for the three types ofpassive film measured at 1000 Hz in 0.25 M Na2SO4 + 0.5 M NaCl

solution. To demonstrate the consistence of capacitances measured

by EIS and MottSchottky at Ecorr, the film formed in chloride-

containing solution was usedas an example. Thecapacitances were

0.36Fm2 and0.5Fm2, respectively, indicatingthat the measured

capacitance corresponded to the capacitance of passive film. It is

seenFig. 7that all passive films behaved like an n-type semicon-

ductor, with a positive slope of the linearC2SC E. The curved form

of the lines indicated the highly disordered nature of passive film

where highly localized states existed between the valence and the

conduction bands[23,24].The fitted values of flat-band potential

and donor density for three passive films are shown inTable 1.It is

clear that passive film formed in air had a more negative fband a

higherND, and there were similar values offband NDfor passivefilms formed in aqueous solutions.

3.6. SIMS characterization

Fig. 8 shows the chloride concentration profiles of the three

passive films formed under various conditions and a blank 3003

Al alloy specimen measured by SIMS. It is seen that the concen-

tration of chloride ions decreased continuously with the sputter

depth in the electrode. As expected, there was the lowest or even

zero chloride concentration forpassive filmformed in Na2SO4solu-

tion without a further anodic polarization in chloride-containing

solution. The permeation depth of chloride ions into passive film

followed the order: film formed in chloride-free solution < film

formedin air< filmformed in chloride-containing solution.Despitethe slightirregularityof chloride concentrationdetermined bySIMS

for passive film formed in chloride-free solution, generally, the con-

centration of permeatedchloride ions wasranked as: filmformed in

air< filmformed in chloride-free solution< filmformed in chloride-


Table 1

Flat-band potential,fb, and donor density,ND, for passive films formed under vari-

ous conditions.

Film formation medium fb vs. SCE (V) ND (1027 m3 )

In-air 1.509 32.39

In 0.25M Na2SO4 solution 0.833 7.73

In 0.25M Na2SO4+ 0.5M NaCl solution 0.815 7.48

Fig. 8. Depth profiles of chloride ions on passive films measured by SIMS.

4. Discussion

4.1. Passive films formed on 3003 Al alloy in air and in aqueous

solutions

The present work shows clearly (Fig. 2) that there are quite

different stabilities of passive films formed on 3003 Al alloy

electrode under various conditions. The measured EIS plots at cor-

rosion potential, i.e., a high-frequency capacitive semicircle and

a low-frequency diffusive tail, are fitted with an electrochemi-

cal equivalent circuit shown in Fig. 9a [25], where Rs is solution

resistance, CPE is constant phase element, Rf is charge-transfer

resistance of passivated 3003 Al alloy electrode, andWis Warburg

diffusive impedance. The high-frequency capacitive semicircle rep-

resents the charge-transfer reaction of passivated 3003 Al alloy,while the low-frequency diffusive impedance is associated with

the oxygen diffusion. Under stable passivation, the film formation

achieves an equilibrium state. Thus, the filmformation rate is equal

to the dissolution rate of 3003 Al alloy. Electrochemical parameters

fitted from EIS data are listed in Table 2.Apparently, there is the

largest resistance (thus the most stable) for passive film formed in

Fig. 9. Electrochemical equivalent circuits used for fitting EIS data measured at

individualEcorr(a) and atEpit(b).


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Table 2

Electrochemical parameters fitted from EIS data measured at individualEcorr.

Film-forming condition Rs () CPE-Y0 (S sn) n Rf(105 ) W(S s0.5)

In air 7.4 7.14 0.93 1.91 226

In 0.25M Na2 SO4 solution 9.6 7.48 0.80 5.71 24.9

In 0.25M Na2 SO4+ 0.5 M NaCl solution 9 1.58 0.91 1.03 247

chloride-free solution, the smallest resistance (most unstable) for

passive filmformed in chloride-containing solution,and the passivefilm formed in air in between.

In general, passive film formed in aqueous solution is usually

associated with a compact, uniform structure because of hydration

process occurring on 3003 Al alloy electrode. It is acknowledged

[26,27]that the hydrated passive film always shows a higher sta-

bility than that without hydration. For passive film formed in air,

the film structure is usually non-uniform, with different thickness

and compositional distribution. The LEIS mapping on air-formed

passive film shows significant fluctuations of local impedance on

the film (Fig. 5a), demonstrating the structural non-uniformity.

As a comparison, the LEIS mapping on passive films formed in

aqueous solutions (Fig. 5band c) is quite uniform. LEIS has been

demonstrated as a unique alternative to characterize the localized

corrosion behavior of metal at a microscopic scale [2831].Whilethe conventional EIS reflects an averaged impedance response of

a macroscopic electrode, LEIS provides information specific to the

individual microscopic site. Therefore, a LEIS mapping is capable of

detect local active spots where a low impedance is usually iden-

tified. It is thus concluded fromFig. 5that passive films formed in

aqueous solutions are muchmore uniform, with fewer localdefects,

than that formed in air.

For passive film formed in chloride-containing solution, it is

expected that chloride ions get involved in the film formation pro-

cess, as demonstrated by SIMS characterization results (Fig. 8)that

there is the deepest chloride sputter depth and the highest chlo-

ride concentration at individual depth. It is generallyacknowledged

[6,7]that Cl plays an important role in initiation and propagation

of pitting corrosion. The high concentration of Cl

existing in thepassive film formed in chloride-containing solution results in the

difficulty of film to be repassivated, as seen in cyclic polarization

measurement in Fig. 1. Upon initiation of the corrosion pit, Cl also

contributes to the rapid propagation of pitting.

4.2. Pitting susceptibility of 3003 Al alloy electrodes passivated in

air and in aqueous solutions

Electrochemical cyclic polarization measurement is capable of

predicting the susceptibility of passivated metal to pitting corro-

sion [6,32]. Generally, if the reverse anodic curve is shifted to lower

currents, i.e., negative hysteresis, or if the reverse curve essentially

retraces the ascending curve, i.e., neutral hysteresis, no pitting cor-

rosion will occur on the target metal or alloy. In contrast, if thereverse anodic curve is shifted to higher currents than the for-

ward curve, i.e., positive hysteresis, pitting corrosion will occur. It

is apparent fromFig. 1that positive hysteresis loops are measured

on 3003 Al alloy passivated under various conditions, suggesting

that pitting of 3003 Al alloy passivated in air and in aqueous solu-

tions is inevitable in the test system. The values ofEpit show that

it is earliest for 3003 Al allow passivated in air to occur pitting,

while it is relatively most difficult for 3003 Al alloy passivated inchloride-containingsolution to initiate pitting, withthat passivated

in sulfate solution in between. Thus, in accordance with the mea-

suredEpit, The pitting susceptibility of passivated 3003 Al alloy is

ranked as: in air > in chloride-free solution> in chloride solution.

Furthermore, the area of the measured positive hysteresis loop

indicates the repassivity capability of the metal or alloy, and a

smaller area indicates a stronger ability for metal or alloy to

repassivate. Therefore, there is the strongest capability for the

air-passivated 3003 Al alloy to repassivate,and the weakest repassi-

vating capabilityfor 3003 Al alloy passivated in chloride-containing

solution.

The resistance of a passivated metal or alloy to pitting is depen-

dent on the competitive effects of pitting (breakdown of passive

film) and repassivation (repair of passive film). Although the air-formed passive film is easy to initiate pitting in chloride solution

(the lowest Epit), it hasthe strongestcapability to repassivate, i.e., to

self-repair after pitting initiation. Thus, the overall ability of passi-

vated 3003 Al alloy to pitting is in moderate state. The passive film

formed in chloride-containing solution has the relatively most pos-

itiveEpit, but the weakest repassive ability. Consequently, it shows

the most active state. The passive filmformed in chloride-free solu-

tion is the most stable, which is attributed to the moderate Epitand

repassivating ability. The relative stability of passive films formed

under various conditions is demonstrated by EIS measurements on

passivated 3003 Al alloy electrodes at their individual Epit(Fig. 3).

Upon pitting, the roughness of the electrode surface increases, and

the electrode state thus becomes more non-uniform. As a conse-

quence, an inductive loop is observed in the low-frequency range,which is one of the typical features indicating pitting corrosion or

electrode roughening [33,34]. TheEIS feature is fitted with theelec-

trochemical equivalent circuit inFig. 9b, whereLis inductance and

RLis inductive resistance. The fitted electrochemical parameters are

shown in Table 3. It is seen that there is the highest charge-transfer

resistancefor passive film formed in chloride-free solution (Fig.3a),

and the lowest charge-transfer resistance for the film formed in

chloride-containing solution (Fig. 3b).

4.3. Pitting mechanism of passivated 3003 Al alloy electrodes

The present work demonstrates that passive films formed in air

and in aqueous solutions behave like an n-type semiconductor, as

indicated by a positive slopeof MottSchottky relationship in Fig. 6.According to point defect model [35], the main electron donors

in an n-type semiconductor are oxygen vacancies. Chloride ions

would occupy the positions of oxygen vacancies to generate cation

vacancies at solution/film interface, which transport towards the

film/metal interface to produce cation vacancy condensate, result-

ing in local depart of passive film and thus pitting. A complete

Table 3

Electrochemical parameters fitted from EIS data measured atEpit.

Passivation condition Rs () CPE-Y0(105 S sn) n Rf() RL() L(H)

In air 39 126 0.80 970.1 687 106

In 0.25M Na2 SO4 solution 35 1.93 0.71 1573 821 123 K

In 0.25M Na2

SO4

+ 0.5 M NaCl solution 2.9 1.64 1 10.15 0.97 2.22


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Fig. 10. Schematic diagrams of electric field distributions at the electrode/solution

interface when electrode is at Ecorr(a) and PZC (b).

description about the interfacial electrode reactions and mass-

transportprocesses are proposed by Macdonaldbased on PDM [35].

Therefore, adsorption and permeation of chloride ions into passive

film is usually the first step to cause pitting.

To understand fundamentally the sources of chloride ions to

result in pitting in passive film under different forming conditions,the potential of zero charge is measured and shown in Fig. 6.The

potential of zero charge at which the excess charge at the elec-

trode/electrolyte interface could be eliminated usually acts as a

reference in determining the type and amount of ions adsorbed

on the electrode surface[36]. If an electrode under its open-circuit

potential is positively charged and thus adsorbed with anions, the

PZC is a more negative potential applied to counteract the excess

charges at the interface,as schematically represented in Fig. 10. The

potential differences between Ecorrand PZC,E, for Alpassive films

formed under various conditions show positive values, suggesting

that the electrode surfaces are positively charged at Ecorr for all

types of passive films. Consequently, chloride ions are expected to

adsorb on electrodesurface. Furthermore, fromthe value ofE, itis

deduced that passive filmformedin air has the largest capability toadsorb chloride ions, while the film formed in chloride-containing

solution the least. Thus, passive film formed in aqueous solution,

especially the chloride-containing solution, has a weak capability

in anions adsorption, which is attributed to the mutual repulsion

among anions. It is expected that a high concentration of chloride

ions exists in passive film formed in chloride-containing solution,

and a further adsorption of chloride ions from the solution will be

repulsed. Therefore, chloride ions causing pitting of passive film

formed in air and in chloride-free solution come from the test solu-

tion, while chloride ions resulting in pitting of passive film formed

in chloride-containing solution are mainly thoseexisting in the film

during film-forming stage.

It has been demonstrated [20,37,38] that passive film with a

higher donor density is always associated with a lower resistance

to pitting corrosion. The present work shows that there is the high-

est donor density in passive film formed in air, as seen inTable 1,

providing potential sites for chloride ions to occupy. Moreover, it

is determined that there is the strongest capability for chloride ion

adsorption on passive film formed in air, it is expected that the

passive film formed in air has the lowest resistance to pitting, as

demonstrated by a lowestEpit.

Itis realized that thesize,shape anddistribution ofsecondphase

intermetallic particles influence the pitting corrosion behavior. For

example, it was found [39]that the adsorption of Cl in passive

film prefers at or around inclusions and second phase particles due

to weaker oxide film on these sites. This relevant subject will be

explored in more detail in the further work.

5. Conclusions

Passive film formed on 3003 Al alloy in air and in Na2SO4 solu-

tion without andwith NaCl addition show n-typesemiconductor in

nature. Passive filmformed in chloride-free solution is most stable,

and that formed in chloride-containing solution is most unstable,

with the film formed in air in between. Passive film formed in air

is associated with a non-uniform structure/composition and the

highest donor density in electronic structure, resultingin a reducedstability than those formed in aqueous solution. However, incorpo-

ration of chloride ions in passive film would decrease significantly

the resistance of the film to pitting when it is formed in a chloride-


Pitting of 3003 Alalloypassivated in airand in aqueous solutions

is inevitable in the presence of chloride ions in the test solution.

There is the strongestcapability for the air-passivated3003 Al alloy

to repassivate, and the weakest repassivating capability for Al alloy

passivated in chloride-containing solution. The resistance of the

passivated 3003 Al alloy to pitting is dependent on the competi-

tive effects of pitting (breakdown of passive film) andrepassivation

(repair of passive film).

The positive potential differences between Ecorrand potential of

zero charge for Al passive films formed under various conditions

suggest that the electrode surfaces are positively charged at Ecorr.

Consequently, chloride ions are expected to adsorb on electrodes.

Passive film formed in air has the strongest capability to adsorb

chloride ions, while the film formed in chloride-containing solu-

tion the least. Chloride ions causing pitting of passive film formed

in air and in chloride-free solution come from the test solution,

while those resulting in pitting of passive film formed in chloride-

containing solution exist in the film during film-forming stage.

Acknowledgements

This work was supported by Canada Research Chairs Pro-

gram, Natural Science and Engineering Research Council of Canada

(NSERC) and Dana Canada Corporation.

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Electronic Structure and Pitting Behavior of 3003 Aluminum