Corrosion Protection Mechanism of 2-Mercaptibenzothiazole and its

Potential Synergistic Effect with Cerium Ions for treatment of AA

2024-T3

A. C. Balaskas*, M. Curioni and G. E. Thompson

Corrosion and Protection Centre, School of Materials, The University of Manchester,

Manchester, M13 9PL, United Kingdom

Abstract

The corrosion protection mechanism of 2-mercaptobenzothiazole (2-MBT)

and its potential synergistic effect with cerium chloride (CeCl3) for sustainability of

the aerospace alloy AA 2024-T3 was investigated. The corrosion inhibitors 2-MBT

and CeCl3 and their potential synergistic properties were assessed with image-assisted

electrochemical noise, the split-cell technique, potentiodynamic polarization,

electrochemical impedance spectroscopy (EIS) and scanning electron microscopy

(SEM) observations. It is suggested that 2-MBT inhibits the dealloying of S-phase

particles and prevents the formation of Cu-rich particles on the surface of AA 2024-

T3. 2-MBT protects the second phase particles from corrosion and offers

sustainability in 3.5% NaCl solution by formation of a protective layer on the surface

of AA 2024-T3. Additionally, 2-MBT is adsorbed on the surface of AA 2024-T3

during anodic dissolution of aluminium. A synergistic effect between 2-MBT and the

Ce3+ based corrosion inhibitor was not found. It is suggested that 2-MBT protects the

intermetallic particles from corrosion from the early immersion time and blocks the

precipitation of cerium hydroxides over the second phase particles. 2-MBT revealed

better corrosion protection properties compared with CeCl3 and 2-MBT + CeCl3 in the

tested time scale.

*Corresponding Author: andrbalaskas@gmail.com

Keywords: AA 2024-T3, electrochemical noise, EIS, corrosion, corrosion inhibitors

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1. Introduction

Chromate containing treatments are widely used for corrosion protection of

aluminium alloys (AA). In the latest years, significant effort has been focused on new

environmentally-friendly treatments for the replacement of hexavalent chromium,

since it is toxic and causes environmental issues [1].

The AA 2024-T3 aerospace alloy contains Cu-rich intermetallic particles such

as the S-phase (Al2CuMg) and the θ-phase (Al2Cu) particles and is susceptible to

pitting corrosion induced by the galvanic connection between Cu-rich particles and

the aluminium matrix which is less noble than the intermetallic particles. Pitting

corrosion sites can initiate from the dealloyed S-phase [2–6] and Al-Cu-Fe-Mn

particles [2,7] and the aluminium matrix close to the Cu-rich remnants from the

dealloyed S-phase particles [2–6]. The S-phase particles before the selective

dealloying of the Mg and Al elements are anodic with respect to the aluminium

matrix. The Cu-sponge remnants after selective dealloying of the Mg and Al elements

are cathodic with respect to the aluminium matrix [2–6]. The trenches in proximity of

the dealloyed S-phase particles are formed from the alkaline pH values induced from

the oxygen reduction reaction (equation 1) dissolving the aluminium oxide layer

(equation 2) [5]. The electrons from the aluminium oxidation are consumed by the

oxygen reduction reaction and hydroxyl anions are produced. The electrons can also

be consumed by the reduction of water producing hydrogen gas and hydroxyl ions

(equation 3). The products of the cathodic reactions, the hydroxyl ions, lead to further

increase of the pH to more alkaline values [5,6].

O2 + 2H2O + 4e– → 4OH– eq. 1

Al → Al3+ + 3e– eq. 2

2H2O + 2e– → 2OH– + H2 eq. 3

Al3+ + 3H2O → Al(OH)3 + 3H+ eq. 4

2H+ + 2e– → H2 eq. 5

Further on the corrosion process, the oxidation of aluminium reduces locally the pH

(equation 4). Consequently, the stability of the aluminium oxide layer is decreased

[2,5,6] and a pit starts to form. As the pit grows the corrosive environment expose

2

new surface of aluminium. Inside the pit, the pH is more acidic at the bottom of the

pit and dissolves further the aluminium matrix and prevents the repassivation of

aluminium [5,6]. During the progress of the pit, new Cu-rich particles are revealed

which are cathodic with respect to the aluminium matrix. As the aluminium further

dissolves more H+ is produced decreasing the pH. The reduction of the H+ inside the

pit might produce hydrogen evolution (equation 5) [5,6].

Corrosion inhibition of Ce3+ for aluminium alloys was first introduced by

Hinton [8]. The concentration of 100 ppm CeCl3 in the corrosive environment of 0.1

M NaCl solution decreased significantly the corrosion rate of the aluminium alloy. A

layer of hydroxide precipitates is formed over the intermetallic particles decreasing

the rate of cathodic reactions [9,10]. The formation of the layer of precipitates takes

place in alkaline environment over the dealloyed S-phase particles [10,11]. The

corrosion inhibition of Ce(NO3)3 for AA 2024-T3 was studied with image-assisted

electrochemical noise and potentiodynamic polarization. The results indicated that

Ce(NO3)3 is an effective corrosion inhibitor in concentration of mM in 3.5% NaCl

solution. Potentiodynamic polarization indicated decreased cathodic current density

for AA 2024-T3 electrodes immersed in Ce(NO3)3 in the concentration of mM

compared with specimens immersed without inhibitor [11,12]. Ce based conversion

coatings on aluminium alloys and their effect on protection of intermetallic particles

have been reported in the literature [13].

The inhibitor 2-MBT has been studied for corrosion protection of aluminium

alloys [14–16]. Potentiodynamic polarization tests indicated that 2-MBT in 0.05 M

NaCl [16] and 3.5% NaCl [14] solution decreases both anodic and cathodic reaction

rates compared with the AA 2024-T3 electrodes immersed without inhibitor.

Electrochemical impedance spectroscopy (EIS) measurements revealed the increase

of the low frequency impedance values with the addition of 2-MBT in the corrosive

environment. The results of fittings on the spectra indicated a decrease in double layer

capacitance suggesting the presence of the formation of a protective film on the top of

AA 2024-T3 [14]. The effect of 2-MBT on copper electrodes has been reported in the

literature [17–19]. Raman spectroscopy [19] and cycle voltammetry measurements

revealed the formation of a film between the ionized form of 2-MBT and copper ions.

This film prevents the corrosion of the electrodes [18,19]. The formation of the

protective film of the ionized form of 2-MBT and copper oxides during dissolution of

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copper takes place in an environment with various pH values [17]. The film is water

insoluble and it is a barrier to corrosive species [17].

The synergistic effect of inhibitors has been evaluated for corrosion protection

of AA 2024-T3 in the literature [11,16,20–23]. The effect of Ce(NO3)3 with 2-MBT

for protection of AA 2024-T3 in 50 mM NaCl was evaluated with electrochemical

methods [16]. It was reported that better corrosion protection was offered with

combination of inhibitors compared with the individual inhibitors. 2-MBT has been

encapsulated into nanocontainers and used in coated specimens for corrosion

protection of aluminium and other alloys [24–27]. Double shell structured CeO2

nanocontainers [25], crystalline CeO2 nanocontainers and CeMo [24] nanocontainers

loaded with 2-MBT were added in coated AA 2024-T3 specimens for corrosion

protection. Additionally, EIS rtests revealed that treated with 2-MBT anodized AA

2024-T3 revealed improved corrosion protection properties compared with anodized

AA 2024-T3 electrodes [28]. X-ray photoelectron spectroscopy studies indicated the

formation of insoluble film of 2-MBT with copper oxides on the surface of copper

electrodes. This layer inhibited the corrosion of copper [17].

In this study, the corrosion protection mechanism of the inhibitor 2-MBT and

potential synergistic properties with CeCl3 was studied for sustainability of AA 2024-

T3. It was found that 2-MBT offers very good corrosion protection properties to AA

2024-T3 by inhibition of dealloying of S-phase particles and protection of the Cu-rich

intermetallic particles from corrosion. No synergistic effect between 2-MBT and

CeCl3 was found. It is suggested that the inhibition of dealloying of S-phase particles

and corrosion protection of Cu-rich second phase particles by 2-MBT prevents the

precipitation of cerium hydroxides. The precipitation of the cerium hydroxide layer

over dealloyed intermetallics and the necessary local alkaline environment was

revealed in the literature [10]. In order to reveal the corrosion mechanism of 2-MBT

and to investigate possible synergistic effect of 2-MBT with CeCl3 advanced and

traditional electrochemical techniques were used. Furthermore, SEM micrographs and

energy dispersive X-ray spectroscopy (EDX) measurements indicated the morphology

of AA 2024-T3 immersed in 3.5% NaCl solution with the presence the inhibitors and

the composition of second phase particles. With electrochemical noise no external

perturbation is applied to the electrodes and coupled with the images from the surface

provide real-life estimation of the inhibitors corrosion protection [29].

Potentiodynamic polarization revealed the effect of inhibitors on the anodic and the

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cathodic reaction rates. The split-cell technique indicated the effect of the inhibitors

on the reactions rate in the scale of seconds after the addition of the inhibitor in the

corrosive environment. EIS tests revealed the corrosion protection mechanism of

inhibitors and the phenomena on the surface of AA 2024-T3 in the presence of

inhibitors.

2. Experimental details

2.1. Materials and Reagents

The AA 2024-T3 specimens have nominal composition in wt.% of Si <0.5, Fe

<0.5, Cu 3.8-4.9, Mn 0.3-0.9, Mg 1.2-1.8, Cr <0.1, Zn <0.25, Ti <0.15 and Al matrix.

2-MBT, CeCl3, sodium hydroxide (NaOH) pellets and nitric acid were of analytical

reagent grade and purchased from Sigma-Aldrich, St. Louis, USA.

2.2. Specimen preparation and corrosion evaluation

The AA 2024-T3 specimens were pretreated by etching and desmutting. The

procedure includes immersion in 10% w/v NaOH for 30 seconds at 60oC with stirring,

then rinsing with distilled water and immersion in 30 vol.% HNO3 for 30 seconds at

ambient temperature. Finally, the specimens were rinsed with deionized water.

Beeswax was used for masking the edges of the electrodes with exposed area of 2.25

cm2.

For the image assisted electrochemical noise experiments, the method

described in detail elsewhere was used [29]. Briefly, in this method two AA 2024-T3

electrodes were coupled with a 10 kohm resistor. A NI-USB6009 (National

Instrument) analogue to digital converter was used to measure the potential of each

electrode with respect to a saturated calomel electrode (SCE). The Labview

programming language was used to develop a software to record the noise potential

and to calculate the noise current. The noise resistance values were calculated

according to the following procedure described in [14,29]. In the first step, a segment

of 1024 points was extracted on the initial data sets of potential and current. Then, the

square root of the variance of the potential divided by the variance of the current was

calculated on the first point of the segment (equation 6 [29]) until the end of the

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segment. A new segment was extracted with a step of 250 points and the calculations

were continued until the end of the dataset. The final Rn(t) diagrams were smoothed.

Rn(t )=√ σ2[V (t ) ]σ2[ I ( t )]

eq. 6 [29]

Maplin USB microscopes were used to record the images from the surface of

the AA 2024-T3 electrodes immersed in 3.5% NaCl solution in the presence of

inhibitors.

A SI 1287 Solartron Electrochemical interface was used for potentiodynamic

polarization and the split-cell tests. The AA 2024-T3 electrode was the working

electrode, a platinum electrode was the counter electrode and a saturated calomel

electrode (SCE) was the reference electrode in a three-electrode cell. The corrosive

environment was 1 litre of naturally aerated 3.5 wt.% NaCl with the addition of the

corrosion inhibitors. The potential was scanned from the open circuit potential (OCP)

to -1 V vs SCE in the case of the cathodic polarization and form -0.005 V vs SCE to -

0.5 V vs SCE in the case for the anodic polarization with the scan rate of 0.10 mV/s.

Two AA 2024-T3 electrodes connected with a zero resistance ammeter (ZRA)

and a SCE were used for the split-cell technique. The electrodes were immersed in

naturally aerated 3.5% NaCl solution in two different compartments connected with a

porous glass frit. After initiation of nitrogen gas in one compartment and oxygen gas

in the other compartment, the AA 2024-T3 electrode in the first compartment became

a net anode and the electrode in the compartment filled with air became a net cathode.

The time of addition of the inhibitors in the compartments is indicated with arrows

(Figure 8).

The EIS measurements carried out with a SI 1287 Solartron Electrochemical

interface connected with a SI 1250 impedance/frequency response analyser (Figure 9a

and 9b) and an Autolab PGSTAT302N – Metrohm potentiostat equipped with FRA

module impedance/frequency response analyser (Figure 11a and 11b). The EIS

spectra acquired with a three-electrode cell configuration at the open circuit potential

(Figure 9a and 9b) and at -0.610 V vs SCE (Figure 11a and 11b). The electrochemical

cell consisted of the working electrode, a saturated calomel electrode (SCE) and a

platinum foil with area of 2 cm2 was the counter electrode. The frequency range was

6

between 10 kHz and 5 mHz (Figure 9a and 9b) and between 100 kHz and 10 mHz

(Figure 11a and 11b), with rms voltage at 10 mV. All EIS spectra were fitted with the

Z-view software.

2.3. SEM

In the first step, the surface of the AA 2024-T3 specimens was smoothed by

grinding and polishing. The mirror polished surface was achieved with polishing with

1 micron diamond paste. Afterwards, the specimens were immersed in the tested

environment. After the immersion, the specimens were rinsed and dried. The

specimens for the cross-section observations were prepared with the method of

ultramicrotomy. A glass knife was used for the first cuts of the specimen. The final

polishing was achieved with dry cuts with daemon knife. The surface and the cross

section of the specimens were investigated with ZEISS Ultra 55 field-emission gun

scanning electron microscope. The microscope was operated in the acceleration

voltage of 1 kV and the working distance was below 3 mm at 0 degrees angle of tilt.

3. Results

3.1 Image assisted electrochemical noise

The corrosion protection performance of 2-MBT and CeCl3 and their potential

synergistic effect for corrosion protection of AA 2024-T3 has been evaluated with

image assisted electrochemical noise. Figure 1 shows the time evolution of the noise

resistance of AA 2024-T3 electrodes in 3.5% NaCl in the presence of a) 2-MBT in

saturated conditions, b) 0.075 mM CeCl3 and c) 2-MBT + 0.075 mM CeCl3 until 40

hours of testing. Electrodes immersed with 2-MBT, CeCl3 and 2-MBT + CeCl3 reveal

similar corrosion protection properties. AA 2024-T3 electrodes immersed with 2-

MBT had noise resistance values of 110 kohm cm2 after 1 hour and close to 105 kohm

cm2 after 10 hours of immersion. AA 2024-T4 specimens without inhibitor had noise

resistance values of 8 kohm cm2 after 1 hour and 13 kohm cm2 after 10 hours of

immersion. The values of the noise resistance with CeCl3 were 23 kohm cm2 after 1

hour and increased to 100 kohm cm2 after 10 hours of immersion. Specimens

immersed with 2-MBT + CeCl3 revealed slightly decreased noise resistance values

7

compared with specimens with 2-MBT, 80 kohm cm2 after 1 hour and 100 kohm cm2

after 10 hours.

The optical images recorded during the electrochemical noise experiments are

shown in Figure 2. The images from the first hour until the 40 th hour of immersion

show the changes in the morphology of the surface during immersion in the

corresponding environment. AA 2024-T3 specimens reveal initiation of darkening in

the first hour and more intense darkening and pitting with increasing immersion time.

The electrodes immersed with 2-MBT reveal hydrogen bubbles in the first hours of

immersion indicating increased cathodic activity. The surface of AA 2024-T3 reveals

no darkening and pitting. With addition of 0.075 mM CeCl3 in the NaCl solution

initiation of pitting is observed on the surface of the electrodes after 40 hours of

immersion. The addition of 2-MBT + CeCl3 in 3.5% NaCl solution reveals increasing

number of hydrogen bubbles with time of immersion (Figure 2). However, no

darkening and pitting is observed until 40 hours of immersion.

Figure 3 shows the time evolution of noise current for AA 2024-T3 electrodes

immersed in 3.5% NaCl solution without inhibitor and immersed with 0.075 mM

CeCl3, with 2-MBT and 2-MBT + CeCl3. The diagrams indicate the large fluctuations

of current for AA 2024-T3 electrodes immersed without inhibitor compared with the

significantly decreased current fluctuations for electrodes immersed in the presence of

inhibitors. More specifically, large fluctuations of the noise current are observed for

AA 2024-T3 without inhibitor in the rage of 0.2 μA in the first hour and increased to

0.75 μA after 24 hours. The noise current for the electrodes immersed with addition of

all inhibitors reveal significantly decrease values compared with the specimens

immersed without inhibitor. In the case of addition of CeCl3 in the corrosive

environment, in the first hour of immersion the noise current of AA 2024-T3 reveal

increased values compared with the addition of 2-MBT and 2-MBT + CeCl3. AA

2024-T3 with CeCl3 has noise current values of 0.1 μA after 1 hour and values of 0.02

μA after 24 hours. The electrodes immersed with 2-MBT reveal noise current values

in the range of 0.01 μA in the first hour of immersion and the range of 0.005 μA after

24 hours. The specimens immersed with 2-MBT + CeCl3 have similar behaviour as in

the case of addition of 2-MBT. The noise current values are in the range of 0.02 μA in

the first hour of immersion and the range of 0.005 μA after 24 hours.

3.2 Effect of the inhibitors on the second phase particles

8

SEM observations indicate the morphology of the second phase particles after

immersion of AA 2024-T3 in 3.5% NaCl solution for the specimens immersed

without inhibitor and for the specimens immersed in the presence of 2-MBT. Figure 4

shows backscattered images from the cross section of AA 2024-T3 immersed in 3.5%

NaCl solution without inhibitor. Figure 4a shows trenching in proximity of second

phase particles and Figure 4b shows a cavity possibly after removal of second phase

particles. The backscattered image reveals the presence of copper inclusions and the

dissolved aluminium matrix near by the intermetallics. Figure 5 shows backscattered

images from the surface (Figure 5a and 5b) and the cross section (Figure 5c) of AA

2024-T3 immersed in 3.5% NaCl solution with 2-MBT. An Al-Cu-Fe-Mn and a S-

phase particle are indicated in Figure 5a. A cluster of intermetallic particles is

presented in Figure 5b. The S-phase, the Al-Cu-Fe-Mn and the θ-phase particles are

revealed uncorroded and the S-phase particles are not dealloyed from Al and Mg after

24 hours of immersion. Furthermore, no cavities and trenches around the second

phase particles are revealed on the surface of AA 2024-T3. The EDX spectra taken at

the indicated areas on Figure 5a and 5b reveal the elemental weight percentages. The

Al-Cu-Fe-Mn particle is indicated with the symbols Xi (C: 23.6, Al: 33.7, Mn: 3.0,

Fe: 11.2, Cu: 28.5) and Xvi (C: 6.2, Al: 50.0, Si: 1.35, Mn: 7.5, Fe: 12.3, Cu: 23.1).

The S-phase particle (Al2CuMg) is indicated with the symbol Xii (C: 21.2, Mg: 12.6,

Al: 32.5, Cu: 33.7). The θ-phase particles (Al2Cu) are indicated with the symbols Xiii,

Xiv and Xv. Xiii (C: 4.6, Mg: 1.3, Al: 87.5, Mn: 0.4, Cu: 4.7), Xiv (C: 8.0, Mg: 1.2,

Al: 84.6, Mn: 0.5, Cu: 4.3), Xv (C: 7.2, Mg: 1.2, Al: 85.1, Mn: 0.4, Cu: 4.7).

3.3 Effect of inhibitors on anodic and cathodic reaction rates

Figure 6 shows the polarization curves for the AA 2024-T3 immersed in 3.5%

NaCl without inhibitor after 1 hour (a) and 24 hours (b), AA 2024-T3 with addition of

2-MBT after 1 hour (c), 24 (d) and 72 hours (e) of immersion. The corrosion process

of AA 2024-T3 electrodes immersed without inhibitor is under cathodic control. The

cathodic part of the curves after 1 and 24 hours the current has little dependence on

potential. On the anodic part of the scan, after 1 hour of immersion the pitting

potential is very close to the OCP. After 24 hours, the OCP drops at –0.69 V vs. SCE

while the pitting potential remains at –0.59 V vs. SCE, very close to the pitting

potential after 1 hour of immersion. As the corrosion process proceeds the rate of the

anodic reactions increases and subsequently the OCP drops to more negative values.

9

For AA 2024-T3 electrodes immersed after 1 hour in the presence of 2-MBT the

pitting potential increases for about 30 mV to the value of –0.57 V vs. SCE compared

with the specimen without inhibitor. Additionally, the rates of anodic and cathodic

reactions are decreased. After 24 and 72 hours the pitting potential drops to values

close to the pitting potential of the control experiment. The anodic reactions rates and

the OCP reveal a small decrease while the rates of cathodic reactions are stable with

increased immersion.

In Figure 7 the cathodic polarization curves of AA 2024-T3 in 3.5% NaCl

solution a) without inhibitor with b) 1.5 mM CeCl3, c) 2-MBT and d) 2-MBT + 1.5

mM CeCl3 after 24 hours are shown. AA 2024-T3 electrodes immersed with 1.5 mM

CeCl3 show decreased cathodic current density values and decrease OCP compared

with specimens immersed without inhibitor revealing very good cathodic inhibition

properties. Additionally, AA 2024-T3 electrodes immersed in the presence of 1.5 mM

CeCl3 reveal decreased cathodic current density. Additionally, AA 2024-T3 electrodes

immersed with addition of 2-MBT and with addition of 2-MBT + CeCl3 reveal less

decreased cathodic current density compared with AA 2024-T3 immersed with 1.5

mM CeCl3. Furthermore, the OCP of AA 2024-T3 with immersion with 1.5 mM

CeCl3 is decreased compared with immersion with 2-MBT and with 2-MBT + CeCl3.

This behaviour indicates that CeCl3 decreased the cathodic reaction rates of AA 2024-

T3 in 3.5% NaCl more successfully than 2-MBT and 2-MBT + CeCl3.

The split-cell experiments are presented in Figure 8. The increase of the

current (after 16 minutes) is the result of initiation of nitrogen gas in one compartment

and oxygen gas in the other compartment. The AA 2024-T3 electrode in the

compartment filled with nitrogen gas becomes a net anode and the electrode in the

compartment with oxygen gas becomes a net cathode. Consequently, the rate of

anodic reactions increases and the current density increases as well [12,14]. After 35

minutes the inhibitors were added in the individual compartments. The time of

addition of the inhibitor is indicated with arrows. With addition of CeCl3 inhibitor a

gradual decrease of the cathodic current is observed. The addition of 1.5 mM CeCl3

results in the stronger decrease of the current compared with addition of 2-MBT. The

decrease of the current was 0.5 μA/cm2 after 40 minutes (time after the addition of

inhibitor). While, the addition of 2-MBT revealed a minor decrease of 0.09 μA/cm2.

3.4 Electrochemical impedance spectroscopy

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The corrosion protection properties of 2-MBT and 2-MBT with CeCl3 on AA

2024-T3 electrodes was evaluated with EIS measurements. Figure 9a shows the Bode

plots and Figure 9b shows the Nyquist plots for AA 2024-T3 electrodes in 3.5% NaCl

solution in the presence of 2-MBT (saturated) and in 2-MBT (saturated) + 0.075 mM

CeCl3 compared with AA 2024-T3 electrodes immersed without inhibitors.

Similar EIS spectra were revealed for AA 2024-T3 electrodes immersed in 2-

MBT and in 2-MBT + CeCl3. The low frequency impedance for electrodes immersed

in 2-MBT showed values close to 9 x 10-4 Ω cm2, while specimens immersed in 2-

MBT + CeCl3 revealed a small decrease in low frequency impedance with values

close to 7-8 x 10-4 Ω cm2. AA 2024-T3 electrodes immersed in both inhibiting

solutions have low frequency impedance values close to one order of magnitude

higher than AA 2024-T3 immersed without inhibitor. The linear part of the

impedance spectra from the high to medium frequency range is the capacitive

behaviour of the electrode due to the protective properties of the aluminium oxide

layer. The specimens immersed in both inhibiting solutions revealed increased

capacitive behaviour until lower frequency values and more broad and increased

phase angle values compared with the specimens immersed without inhibitor.

The electrical circuit presented in Figure 10 was used to extract quantitative

parameters from the EIS spectra. The Rsol accounts for the solution resistance,

connected in series is the capacitor associated with the capacitive behaviour of the

aluminium oxide layer (Coxide). The corrosion product resistance (Rcorr products) is

connected in parallel with the Coxide. Connected in series with Rcorr products is the constant

phase element (CPEdl) associated with capacitive behaviour of the double layer

capacitance. Connected in parallel with CPEdl is the polarization resistance (Rpol)

connected in series with a short Warburg element related with the diffusion prosses on

the surface of AA 2024-T3 electrodes [14]. The Warburg element parameters are the

Warburg resistance (WS-R), the length of the diffusion layer (WS-T) and the Warburg

exponent (WS-P) [30,31].

Potentiostatic EIS measurements at -0.610 V vs SCE for AA 2024-T3

electrodes immersed with 2-MBT and with 2-MBT + 0.075 mM CeCl3 after 24 hours

of immersion in 3.5% NaCl are shown in Figure 11a and 11b. EIS tests performed at

potential above OCP and below the pitting potential for AA 2024-T3 electrodes

characterization immersed with inhibitors under anodic dissolution. Similar work with

EIS spectra obtained at potential above pitting potential and in the passive region (-1.0

11

V vs Hg/Hg2SO4) for pure aluminium electrodes immersed in 10-3/10-5 M hydrofluoric

acid were reported in [32]. EIS spectra at -0.610 V vs SCE reveal the behaviour of

AA 2024-T3 immersed with and without inhibitors during anodic dissolution with

rate of 6 x 10-6 A/cm2. The results reveal a capacitive part from high to medium

frequency range and an inductive loop at low frequency range for AA 2024-T3

immersed with 2-MBT and with 2-MBT + 0.075 mM CeCl3. While, AA 2024-T3

immersed without inhibitor showed an incline trend of the resistive part at the low

frequency range related with diffusion phenomena [32]. It is suggested that the

inductive behaviour of AA 2024-T3 electrodes immersed with inhibitors is related

with adsorption phenomena at the electrode surface [33]. Similar results have been

reported for corrosion of carbon steel in the presence of inhibitors [34]. It was

suggested that the inductive loops could be attributed to adsorption phenomena of

monoethylene glycol during dissolution of iron [34]. Inductive behaviour and the

determination of Rp have been reported for iron in the presence of acidic media [35–

37]. The inductive loop in the low frequency range has been observed for carbon steel

electrodes immersed in the presence of acids and has been caused by the relaxation

phenomena of the absorbed species [35].

The potentiostatic EIS spectra at -0.610 V vs SCE of AA 2024-T3 electrodes

immersed with and without inhibitors were fitted with the electrical circuit presented

in Figure 12 to extract quantitative parameters during the corrosion process. The

electrical circuit in Figure 12 is similar with the circuit in Figure 10. The elements

Lanod and Ranod were added in parallel with Rct (charge transfer resistance) and Ws. Lanod

is associated with the variation of the active anodic sites on AA 2024-T3 and Ranod

represents the resistance related with adsorption phenomena of 2-MBT at the potential

of -0.610 V vs SCE [30]. A constant phase element (CPEoxide) (Figure 12) was used

instead of a capacitor (Figure 10) to associate the behaviour of the aluminium oxide

layer in this condition. The EIS spectra for AA 2024-T3 electrodes immersed without

inhibitor at -0.610 V vs SCE revealed on inductive behaviour and was fitted without

the parameters Lanod and Ranod.

4. Discussion

4.1 The corrosion inhibition mechanism of 2-MBT for AA 2024-T3.

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The corrosion process of AA 2024-T3 initiates from the selective dealloy of S

phase particles which are initially anodic to Al matrix. The resulting Cu-rich particles

are cathodic with respect to the matrix and the cathodic activity increases locally.

Trenches in proximity of second phase particles are formed (Figure 4a) and some of

the intemetallics may be removed from the surface (Figure 4b). The oxidation of

aluminium (anodic activity) increases with increase of the cathodic activity. This

behaviour is evident from the potentiodynamic polarization of AA 2020-T3 after 1

and 24 hours. The small dependence of the cathodic current on the applied potential

and the increased anodic current density with decrease of the OCP after 24 hours are

observed (Figure 6).

With addition of 2-MBT in 3.5% NaCl solution the anodic reactions rate of

AA 2024-T3 decreases and the pitting potential increases from the first hour of

immersion (Figure 6). It is revealed immediate inhibition of anodic reactions

indicating inhibition of the initial dealloying of the S-phase particles. For specimens

immersed with 2-MBT, dealloying of S-phase particles and trenches did not appear

after 24 hours as revealed from SEM observations (Figure 5a). The S-phase particle is

indicated with the symbol Xii in Figure 5a. Additionally, θ-phase particles in Figure

5b (indicated with the symbols Xiii, Xiv and Xv) and the Al-Cu-Fe-Mn intermetallics

revealed uncorroded. Corroded θ-phase particles have been reported in the literature

for clusters of intermetallic particles which consist of θ-phase and S-phase particles

[38]. Al-Cu-Fe-Mn particles are noble with respect to the aluminium matrix and

trenches may be observed after long term tests as a result from cathodic reactions over

these particles [39]. After 72 hours of immersion the anodic reactions rate is

maintained decreased indicating very good long-term corrosion protection properties

for 2-MBT inhibitor. Additionally, the noise resistance values were close to 9 times

higher compared to the specimens immersed without inhibitor (Figure 1). The split-

cell tests suggest that 2-MBT is a poor cathodic inhibitor in the very early immersion

time before the completion of the first hour (Figure 8). The hydrogen bubbles

observed in the early immersion time also suggest cathodic activity on the surface of

the AA 2024-T3 electrodes (Figure 2) [40–42]. The decreased cathodic current

density after 1 until the 72 hours is due to the prevention of formation of Cu-rich

particles. In the literature the formation of a protective film with the ionized form of

2-MBT and Cu oxides preventing the dissolution of Cu electrodes has been reported

[17].

13

EIS measurements at OCP for AA 2024-T3 electrodes immersed with 2-MBT

in 3.5 % NaCl indicated increased corrosion protection properties for AA 2024-T3.

The low-frequency impedance is increased for close to one order of magnitude

compared with AA 2024-T3 immersed without inhibitors. Additionally, AA 2024-T3

immersed with 2-MBT reveal increased capacitive behaviour and more broad and

increased phase angle values compared with electrodes immersed without inhibitor. It

is suggested that with addition of 2-MBT in 3.5% NaCl solution a protective film is

formed on the surface of AA 2024-T3 [14]. The formation of a film between the

ionized form of 2-MBT and copper Cu which prevents the corrosion of Cu electrodes

has been reported in the literature [18,19]. Fittings revealed significantly decreased

aluminium oxide layer capacitance (1.17 μF cm-2) and double layer capacitance values

(19 μF cm-2) for AA 2024-T3 immersed with 2-MBT compared with electrodes

immersed without inhibitor (Coxide: 55.4 μF cm-2, Cdl: 150 μF cm-2) (Table 1). The

value of Coxide and Cdl remained stable after 48 and 72 hours of immersion indicating

that the thickness of the aluminium oxide layer did not change significantly with

increasing exposure time. The low capacitance values for both aluminium oxide layer

and double layer were maintained low until 72 hours indicating that the protective

inhibitor film formed on the surface of AA 2024-T3 remained stable until 72 hours.

Consequently, the polarization resistance for the electrodes immersed with 2-MBT

ranged between values from 50 to 60 kΩ cm2, while electrodes immersed without

inhibitor had polarization resistance value of 0.9 kΩ cm2. Additionally, the Warburg

resistance was increased for electrodes immersed with 2-MBT (30 - 50 kΩ cm2)

compared with specimens immersed without inhibitor (20 kΩ cm2).

EIS tests acquired at -0.610 V vs SCE indicated inductive behaviour at low

frequency for specimens immersed with 2-MBT. On the contrary, electrodes

immersed without inhibitor did not reveal inductive parts on the spectra (Table 2). For

carbon steel electrodes immersed in the presence of inhibitors, the inductive

behaviour was related with adsorption phenomena of the inhibitor on the metallic

surface [34]. Lanod can be related with the extension of the anodic dissolution area of

aluminium and Ranod with the resistance associated with the adsorption phenomena of

2-MBT on the surface of AA 2024-T3 during dissolution of aluminium. Specimens

immersed with 2-MBT reveal increased Rct (20 kΩcm2) and Ws-R values (27 kΩcm2)

compared with specimens immersed without inhibitor (Rct: 0.7 kΩcm2, Ws-R: 1

kΩcm2). Moreover, the Coxide (30 μF cm-2) and Cdl (0.9 μF cm-2) values for specimens

14

immersed with 2-MBT are significantly decreased compared with specimens

immersed without inhibitor (Coxide: 150 μF cm-2, Cdl: 250 μF cm-2). This behaviour

indicates very good corrosion protection properties for 2-MBT by formation of a

protective layer on AA 2024-T3 and the adsorption of 2-MBT during anodic

dissolution of aluminium.

4.2 Investigation of potential synergistic effect between 2-MBT and CeCl3.

AA 2024-T3 electrodes immersed with 2-MBT + CeCl3 reveal decreased

values of noise resistance and low frequency impedance values compared with

electrodes immersed with 2-MBT. Additionally, the immediate effect of 2-MBT on

the corrosion protection of Cu-rich intermetallic particles and on the inhibition of

dealloying of S-phase particles and the gradual effect of CeCl3 on the protection of the

S-phase particles (precipitation of Ce hydroxides on the S-phase particles after initial

dealloy from Mg) indicates no synergistic effect of 2-MBT and CeCl3 on protection of

AA 2024-T3 in the tested timescale.

It is suggested that the immediate effect of 2-MBT on inhibition of dealloying

of S-phase particles and corrosion protection of Cu-rich intermetallics prevents the

deposition of Ce hydroxides over these particles. Cathodic potentiodymanic

polarization revealed decreased current densities and OCP values for AA 2024-T3

electrodes immersed with CeCl3 compared with electrodes immersed with 2-MBT and

with 2-MBT + CeCl3 and specimens without inhibitor. Additionally, similar cathodic

current densities are observed for specimens immersed in 2-MBT and in 2-MBT +

CeCl3 revealing that CeCl3 did not further decrease the cathodic current density when

2-MBT is present in the corrosive environment. In Figure 2, specimens immersed

with 2-MBT + CeCl3 revealed increased generation of hydrogen bubbles with

increasing immersion time. It is suggested that the cathodic activity is not inhibited

for specimens immersed with 2-MBT + CeCl3 indicating that CeCl3 with 2-MBT did

not effectively inhibit cathodic reactions.

Complementary, image assisted electrochemical noise (Figures 1-3) indicate

that CeCl3 in concentration of mM in 3.5% NaCl is an effective corrosion inhibitor of

AA 2024-T3. However, Ce3+ ions well known corrosion inhibitors in the literature are

protective after initiation of corrosion [10]. The cerium hydroxides precipitate at

alkaline pH on the dealloyed S-phase particles and Cu-rich intermetallics resulting in

15

the increase of the corrosion resistance of AA 2024-T3 [10]. The anodic dissolution of

the aluminium and the consumption of electrons by the cathodic reactions over the

Cu-rich particles provide the alkaline environment revealing that the corrosion

process has initiated [10].

The gradual effect of CeCl3 on inhibition of cathodic reactions in the very

early immersion time is indicated by the split-cell technique (Figure 8) with the

gradual decrease of the current with addition of the inhibitor in the corrosive

environment. Additionally, in the early immersion time addition of CeCl3 revealed

increased noise current values (0.1 μA after 1 hour) and were decreased (0.02 μA)

after 24 hours. After 24 hours of immersion cathodic potentiodynamic polarization of

AA 2024-T3 with 1.5mM CeCl3 revealed the lowest cathodic current density with

decreased OCP compared with addition of 2-MBT and 2-MBT + CeCl3 (Figure 7).

The decreased cathodic current density and the high noise resistance values reveal

very good corrosion protection properties. It is known that the corrosion process of

AA 2024-T3 is under cathodic control [5,6,43]. However, pitting is evident after 40

hours of immersion on the surface of AA 2024-T3 (Figure 2).

EIS measurements for AA 2024-T3 electrodes immersed with 2-MBT + CeCl3

revealed small decrease in low frequency impedance compared with specimens

immersed with 2-MBT. EIS measurements for AA 2024-T3 electrodes immersed with

2-MBT + CeCl3 revealed decreased polarization resistance values (30 – 44 kΩ cm2)

compared with specimens immersed with 2-MBT (50 – 60 kΩ cm2). Additionally,

specimens immersed with 2-MBT showed slightly decreased Coxide (6 – 0.5 μF cm2)

and Cdl (6 – 15 μF cm2) values compared with specimens immersed with 2-MBT +

CeCl3 (Coxide: 6 – 10 μF cm2, Cdl: 10 – 20 μF cm2) (Table 1). This behaviour indicates

the better corrosion protection of 2-MBT compared with 2-MBT + CeCl3 and the

presence of CeCl3 with 2-MBT did not inhibit the formation of the protective layer on

AA 2024-T3 from 2-MBT.

EIS tests performed at -0.610 V vs SCE for electrodes immersed with 2-MBT

+ CeCl3 revealed inductive behaviour at low frequency range similar with specimens

immersed with 2-MBT. Electrodes immersed with 2-MBT and with 2-MBT + CeCl3

showed similar Rct (20 and 30 kΩ cm2), Ws-R (25 kΩ cm2) and Coxide (25 μF cm2)

values. However, Cdl values for specimens in 2-MBT (0.9 μF cm2) were decreased

compared with specimens in 2-MBT + CeCl3 (10 μF cm2) indicating the formation of

a thinner protective inhibitor layer on AA 2024-T3 surface (Table 2). Additionally,

16

Lanod values for AA 2024-T3 electrodes immersed with 2-MBT (100 kΩ s cm2) are

significantly increased compared with specimens immersed with 2-MBT + CeCl3

(750 kΩ s cm2). Ranod values for electrodes immersed with 2-MBT (5 kΩ cm2)

revealed small decrease compared with specimens immersed with 2-MBT + CeCl3 (10

kΩ cm2). This behaviour indicates that the addition of CeCl3 with 2-MBT reveals

decrease of the adsorption of 2-MBT on the surface of AA 2024-T3 and increased of

the dissolution area of aluminium compared with the specimens immersed with 2-

MBT.

Consequently, 2-MBT inhibits the anodic reactions from the early immersion

time indicating inhibition of the initial dealloying of the S-phase particles. SEM

observations revealed that the S-phase particles are not dealloyed from Al and Mg

after 24 hours of immersion. EIS measurements indicated the formation of a

protective layer on AA 2024-T3. Additionally, EIS tests indicate the adsorption of 2-

MBT on AA 2024-T3 during anodic dissolution of aluminium. Possible synergistic

effect of 2-MBT with cerium ions for protection of AA 2024-T3 was investigated

with addition of 2-MBT + CeCl3 in the corrosive environment. The results from

electrochemical noise, the split-cell technique and potentiodynamic polarization

indicated that the immediate effect of 2-MBT on the protection of second phase

particles blocks the precipitation of cerium hydroxides over the Cu-rich particles. The

addition of 2-MBT + CeCl3 in 3.5% NaCl resulted in domination of 2-MBT on the

surface of AA 2024-T3 protecting the second phase particles from corrosion. The

corrosion protection of Cu-rich particles did not provide the necessary alkaline

environment over the intermetallics for the precipitation of cerium hydroxide

protection layer. Therefore, addition of CeCl3 with 2-MBT in the corrosive

environment resulted in similar noise resistance values as in the case of addition of the

individual inhibitor 2-MBT revealing no synergistic effect between the inhibitors 2-

MBT and CeCl3. The addition of the individual inhibitor CeCl3 in the corrosive

environment revealed a gradual effect on the corrosion protection of AA 2024-T3 as

indicated from electrochemical noise and the split-cell tests. The noise resistance

values revealed increased to 100 kohm cm2 after 10 hours of immersion (23 kohm cm2

after 1 hour) and the split-cell revealed gradual decrease of the cathodic reaction rates.

The precipitation of the cerium hydroxide layer over the second phase particles is

formed in alkaline environment over the dealloyed S-phase particles [10,11].

17

5. Conclusions

The corrosion protection mechanism of 2-MBT and potential synergistic effect

with CeCl3 for sustainability of AA 2024-T3 was evaluated with traditional and

advanced electrochemical methods. It was found that 2-MBT inhibits the anodic

reaction of dealloying of S-phase particles from Al and Mg from the early immersion

time. The formation of Al-Cu particles from the dealloyed S-phase particles is

prevented and consequently the cathodic reactions are decreased. The Cu-rich and the

Al-Cu-Fe-Mn intermetallic particles are also protected from corrosion. Additionally,

EIS tests revealed the corrosion protection of AA 2024-T3 by formation of a

protective layer and the adsorption of 2-MBT on electrode surface during anodic

dissolution of aluminium. No synergistic properties found between 2-MBT and the

Ce3+ based inhibitor CeCl3. It is suggested that the protection of second phase particles

from dealloying prevents the precipitation of Ce hydroxides. However, the formation

of the 2-MBT protective layer on AA 2024-T3 and the adsorption of 2-MBT during

anodic dissolution is not prevented by Ce3+ ions. SEM observations reveal the

protection of the second phase particles when the AA 2024-T3 electrodes are

immersed with 2-MBT compared with the control test.

Acknowledgements: The authors acknowledge EPSRC for the support of the

LATEST2 Programme Grant

18

Figure 1. Time evolution of noise resistance of AA 2024-T3 immersed in 3.5%

NaCl solution without inhibitor, in the presence of a) 2-MBT (saturated), b)

0.075 mM CeCl3 and c) 2-MBT (saturated) + 0.075 mM CeCl3.

19

Figure 2. Optical images obtained during the EN measurements for AA 2024-T3

immersed in 3.5% NaCl solution without inhibitor, in the presence of 2-MBT

(saturated), 0.075 mM CeCl3 and 2-MBT (saturated) + 0.075 mM CeCl3.

20

Figure 3. Time evolution of a) noise current of AA 2024-T3 immersed in 3.5%

NaCl solution without inhibitor and in the presence of in the presence of 0.075

mM CeCl3, 2-MBT (saturated), and 2-MBT (saturated) + 0.075 mM CeCl3 and b)

magnification of diagram (a).

21

Figure 4. Backscattered electron images from the cross-section of AA 2024-T3

specimen immersed in 3.5% NaCl solution for 6 hours showing a) trenching in

proximity of second phase particles and b) a cavity on the surface.

22

23

Figure 5. Backscattered electron images from a,b) the surface after 24

hours and c) the cross-section after 6 hours of immersion of AA 2024-T3 in 3.5%

NaCl solution with 2-MBT. The marks on Figure 5a and 5b indicate the locations

where the EDX spectra were taken.

Figure 6. Potentiodynamic polarization of AA 2024-T3 electrodes in 3.5% NaCl

solution without inhibitor after a) 1 hour, b) 24 hours, with 2-MBT (saturated)

after c) 1 hour, d) 24 hours and e) 72 hours.

24

Figure 7. Cathodic potentiodynamic polarization of AA 2024-T3 electrodes in

3.5% NaCl solution a) without inhibitor, with b) 1.5 mM CeCl3, c) 2-MBT

(saturated) and d) 2-MBT (saturated) + 1.5 mM CeCl3 after 24 hours of

immersion.

25

Figure 8. Time evolution of the current density from the split-cell tests for AA

2024-T3 electrodes with addition of the individual inhibitor. a) without inhibitor,

with b) 1.5 mM CeCl3 and c) 2-MBT (saturated) in the cathodic side and 2-MBT

(saturated) in the anodic side (d). The arrows indicate the time of addition of the

inhibitor in the compartment.

26

Figure 9. a) Bode and b) Nyquist plots for AA 2024-T3 electrodes without

inhibitor, with 2-MBT (saturated) and 2-MBT (saturated) + 0.075mM CeCl3.

The inset graph in Figure b is the magnification of AA 2024-T3 without

inhibitor. The symbols present the EIS spectra and the lines present the

calculated spectra from the values reported in Table 1.

27

Figure 10. Schematic representation of AA 2024-T3 electrode and the electrical

circuit model used for fittings. All the fitted values are reported in Table 1.

28

Figure 11. a) Bode and b) Nyquist plots for AA 2024-T3 electrodes at -0.610 V vs

SCE without inhibitor, with 2-MBT (saturated) and 2-MBT (saturated) +

0.075mM CeCl3. The inset graph in Figure b is the magnification of AA 2024-T3

without inhibitor. The symbols present the EIS spectra and the lines present the

calculated spectra from the values reported in Table 2.

29

Figure 12. Schematic representation of AA 2024-T3 electrode at -0.610 V vs SCE

and the electrical circuit model used for fittings. All the fitted values are reported

in Table 2.

Table 1. The fitted parameters obtained from the circuit presented in Figure 10

for AA 2024-T3 electrodes without inhibitor, with 2-MBT (saturated) and 2-

MBT (saturated) + 0.075mM CeCl3.

AA 2024-T3 with 2-MBT (saturated)

Time 1 h 24 h 48 h 72 h

error error error error

Rsol Ω cm2 4.96 ±0.055 4.89 ±0.881 4.23 ±0.333 3.21 ±1.51

Coxide μF cm-2 6.18 ±0.064 1.17 ±2.16 3.72 ±1.92 1.02 ±1.48

Rcorr pr Ω cm2 111 ±14.8 0.59 ±1.39 2.44 ±1.6 2.3 ±0.95

Cdl μF cm-2 6.33 ±0.157 14.3 ±1.97 12.47 ±1.69 14.98 ±1.38

ndl 0.831 ±0.005 0.921 ±0.018 0.901 ±0.024 0.927 ±0.01

Rpol Ω cm2 61471 ±1515 60002 ±1646 60737 ±1560 50041 ±586.58

30

Ws-R Ω cm2 31385 ±3363 48314 ±5268 30731 ±3949 29818 ±1585

Ws-T sec-1 46.8 ±8.34 69.5 ±11.7 52.7 ±7.7 56.8 ±4.37

Ws-P 0.46 ±0.036 0.524 ±0.031 0.587 ±0.041 0.526 ±0.017

X2 1.42×10-3 2.37×10-3 3.82×10-3 4.19×10-4

AA 2024-T3 with 2-MBT (saturated) + 0.075mM CeCl3

Time 1 h 24 h 48 h 72 h

error error error error

Rsol Ω cm2 4.37 ±0.049 4.56 ±0.065 5.09 ±0.093 4.37 ±0.096

Coxide μF cm-2 6.76 ±0.066 8.29 ±0.079 9.44 ±0.12 8.8 ±0.624

Rcorr pr Ω cm2 91.6 ±9.66 172 ±9.15 308 ±33.4 23.9 ±37.7

Cdl μF cm-2 13.5 ±0.273 19.1 ±0.426 26 ±0.813 23 ±0.617

ndl 0.78 ±0.005 0.843 ±0.006 0.752 ±0.01 0.697 ±0.021

Rpol Ω cm2 31145 ±662 37968 ±1063 49915 ±2688 44278 ±3875

Ws-R Ω cm2 36772 ±1471 54755 ±3045 53163 ±5774 82294 ±9067

Ws-T sec-1 38.5 ±2.27 52.5 ±3.65 52.2 ±5.93 83.5 ±16.4

Ws-P 0.525 ±0.014 0.593 ±0.017 0.617 ±0.033 0.479 ±0.029

X2 1.07×10-3 2.47×10-3 4.9×10-3 1.74×10-3

AA 2024-T3 no inhibitor

Time 24 h

error

Rsol Ω cm2 3.826 ±0.078

Coxide μF cm-2 55.4 ±8.53

Rcorr pr Ω cm2 2.53 ±0.720

Cdl μF cm-2 150.5 ±10.8

ndl 0.892 ±0.01

Rpol Ω cm2 913 ±27.98

Ws-R Ω cm2 19352 ±2075

Ws-T sec-1 255.7 ±48.2

Ws-P 0.682 ±0.013

X2 5.23×10-3

31

Table 2. The fitted parameters obtained from the circuit presented in Figure 12

to fit the EIS spectra at potential -0.610 V vs SCE for AA 2024-T3 electrodes

without inhibitor, with 2-MBT (saturated) and 2-MBT (saturated) + 0.075mM

CeCl3.

AA 2024-T3

-0.610 V vs SCEAA 2024-T3 with 2-

MBT (sat)

-0.610 V vs SCE

AA 2024-T3 with 2-MBT

(sat) + 0.075mM CeCl3

-0.610 V vs SCE

Time 24 h 24 h 24 h

error error error

Rsol Ω cm2 4.21 ±0.034 3.53 ±0.124 5.1 ±0.069

Coxide μF cm-2 146 ±196 27.2 ±6.65 25.6 ±4.05

noxide 0.926 ±0.129 0.898 ±0.017 0.91 ±0.017

Rcorr pr Ω cm2 2.5 ±1.6 59.7 ±268.07 221 ±73.1

Cdl μF cm-2 252 ±207 0.86 ±5.46 9.55 ±3.94

ndl 0.881 ±0.058 0.991 ±0.55 0.941 ±0.047

Rct Ω cm2 650 ±18.9 19848 ±1176 31856 ±616

Ws-R Ω cm2 1084 ±45.7 26919 ±14751 24598 ±12103

Ws-T sec-1 22.4 ±1.52 22.3 ±3.2 77.6 ±3.11

Ws-P 0.525 ±0.014 0.811 ±0.034 0.967 ±0.019

Ranod Ω cm2 - - 5283 ±591 12761 ±4326

Lanod Ω s cm2 - - 105240 ±13172 658410 ±88596

X2 1.47×10-3 26.8×10-3 5.13×10-3

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