chakraborty banerjee ea 3790-2011

7/26/2019 Chakraborty Banerjee EA 3790-2011

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Electrochimica Acta 56 (2011) 37903798

Contents lists available atScienceDirect

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

Electrochemical impedance spectroscopic investigation of the role of alkalinepre-treatment in corrosion resistance of a silane coating on magnesium alloy,ZE41

P. Chakraborty Banerjee a,b, R.K. Singh Raman a,c,

a Department of Chemical Engineering, Monash University, Clayton, VIC-3800, Australiab CAST Cooperative Research Centre, Hawthorn,VIC-3122, Australiac Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC-3800, Australia

a r t i c l e i n f o

Article history:

Received 6 November 2010

Received in revised form 7 February 2011

Accepted 10 February 2011

Available online 17 February 2011

Keywords:

Silane coating

Electrochemical impedance spectroscopy

Magnesium alloys

a b s t r a c t

Theprotective performance of the coatings of bis-1,2-(triethoxysilyl) ethane (BTSE) on ZE41 magnesium

alloywith different surface pre-treatments were evaluated using potentiodynamic polarizationand elec-

trochemicalimpedance spectroscopy(EIS)in 0.1 M sodiumchloride solution. Electricalequivalentcircuits

were developed based upon hypothetical corrosion mechanisms and simulated to correspond to the

experimental data. The morphology and cross section of the alloy subjected to different pre-treatments

and coatings were characterized using scanning electron microscope. A specific alkaline pre-treatment

of the substrate prior to the coating has been found to improve the corrosion resistance of the alloy.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

Magnesium alloys are the lightest metallic material with excel-

lent physical and mechanical properties [1]. High strength to

weight ratio makes them very attractive to the automobile and

aerospace industries. But the use of magnesium alloys is restricted

due to their poorcorrosion resistance. Thesurface filmformed upon

immersion of magnesium in aqueous solution consists of a non-

compact duplex layer comprising of an inner MgO layer next to

the metal and an external porous layer of Mg(OH)2 [2].The pro-

tection offered by this film is highly dependent on the conditions

of exposure[3].Moreover, the presence of several anions such as,

chloride, sulphate and nitrate in the aqueous solution hinder the

formation of the surface film and also disrupts the existing film,

which in turn accelerate the corrosion rate[3].The partially pro-

tective surface film, low standard reduction potential, presence

of certain impurities (viz., Fe, Cu, Ni) and microgalvanic couplingbetween the secondary phases and the -solid solution, as well as

galvanic coupling with other metals accelerate the corrosion prob-

lems of magnesium and its alloys. Thus magnesium alloys undergo

different types of corrosion, such as, general, galvanic, pitting and

granular corrosion[3]. It is believed that desired corrosion resis-

Corresponding author at: Department of Mechanical and Aerospace Engineer-

ing, Monash University, Clayton, VIC-3800, Australia. Tel.: +61 3 9905 3671;

fax: +61 3 9905 1825.

E-mail address:[email protected](R.K. Singh Raman).

tance of magnesium alloys will necessitate application of suitable

surface barrier, such as chemical treatments and/or coatings.

Silanes are emerging as an attractive environmentally friendly

alternative for improving the corrosion resistance of the metal-

lic substrates as well as for enhancing the compatibility of the

metal surface with the paint systems [48]. The coupling abil-

ity of silanes is attributed to their unique chemical structure,

which is given by, R(CH2)nSiX3, where R is any organofunc-

tional group, CH2 is the linker and X is the hydrolysable group.

When a silane is used for modification of a polymericinorganic

interface, the organofunctional group as well as the hydrolysable

groups of silane independently bond with the polymer and inor-

ganic surfaces. Silanes form oxane bonds with the hydroxyl groups

of inorganic substrates as well as can form covalent bonds with

suitable functional groups of different polymers.

On the basis of the number of the hydrolysable groups, silanes

are of two types, viz., monosilane, having three hydrolysablegroups and bis-silane, having six hydrolysable groups. Though

monosilanes have been widely used on different metal substrates,

bis-silanes are reported to provide superior corrosion protection

[9].As bis-silanes can form more silanol groups, in the reticula-

tion stage, these silanol groups can react with each other to form

siloxane bonds, and thus can build robust hydrophobic polysilox-

ane layers (SiOSi linkages), which, in turn, enhances durability

of silane coatings in aqueous media[10].

The interaction of silanes with metal substrates also depends

on the nature of the inorganic surfaces[9].Plueddamann[11]sug-

gested thatoxide surfaces witha higher number densityof hydroxyl

0013-4686/$ see front matter 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.electacta.2011.02.050
http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.electacta.2011.02.050http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.electacta.2011.02.050http://www.sciencedirect.com/science/journal/00134686http://www.elsevier.com/locate/electactamailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.electacta.2011.02.050http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.electacta.2011.02.050mailto:[email protected]://www.elsevier.com/locate/electactahttp://www.sciencedirect.com/science/journal/00134686http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.electacta.2011.02.050


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P. Chakraborty Banerjee, R.K. Singh Raman / Electrochimica Acta 56 (2011) 37903798 3791

Table 1

Nominal composition of ZE41.

Elements present in ZE41

Mg Zn Ce La Nd Pr Zr Fe Cu Ni

w t% 94 .4 3.82 0.64 0.27 0.1 4 0.06 0.69 0.002 0.002


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3792 P. Chakraborty Banerjee, R.K. Singh Raman / Electrochimica Acta 56 (2011) 37903798

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

-1.2

-1.1

-2-3-4-5-6-7-8

log (j/A cm-2

)

EvsSCE/V

Tested after keeping in desiccator for 24 h

Tested after polarizing in hydroxide for 600 s

Tested after dipping in hydroxide for 48 h

Untreated ZE41

Fig.1. Potentiodynamic polarization plotsof thealloywith differentpre-treatments

and untreated alloy.

3. Results and discussion

3.1. Alloy with different pre-treatments: electrochemical

characterization

The potentiodynamic polarization plots of the alloy with dif-

ferent pre-treatments and the untreated alloy are shown in Fig. 1.

Bothcathodic andanodiccurrentdensities of all thepre-treatedand

untreated specimens were respectively similar. The similar shapes

of the anodic plots as well as the similar magnitudes infer that

the dissolution rate and the dissolution mechanism are analogous.

However, EIS were conducted to further assess the protection, if

any offered by different pre-treatments. The Nyquist plots (Fig. 2)

of the alloy with different pre-treatments and the untreated alloy

are characterizedby two capacitive semicircles. The highfrequency

semi circles correspond to the charge transfer processes whereas

the medium/low frequency semicircles correspond to the masstransport relaxation (Mg+) in the solid phase, i.e., in an aggregating

layer [17]. The corrosion resistance is determined by the com-

bined diameter of these two semicircles[18].A marginal increase

in the corrosion resistance was observed in case of the pre-dipped

and pre-polarizedspecimens, whereas the specimen withnaturally

developed hydroxide film (upon storing in a desiccator for 24 h)

showeda similar corrosion resistance as that of the untreated alloy

(scatter in the data was 500 cm2).

0

500

1000

1500

2000

2500

3000

300025002000150010005000

Z'/ cm2

Z"/

cm

2

Tested after keeping in desiccator for 24 h

Tested after polarizing in hydroxide for 600 s

Tested after dipping in hydroxide for 48 huntreated ZE41

Fig. 2. Nyquist plots of the alloy with different pre-treatments and untreated alloy.

3.2. Alloy with different pre-treatments: surface characterization

Since the structure of the hydroxide film determines the bond-

ing of silane with the metal as well as the performance of silane

coatings in corrosive media, it became essential to develop a broad

understanding of the hydroxide film formed on the metal surface

upondifferentpre-treatments. SEM of the surface and crosssection

of the alloy after different pre-treatments provided some useful

information.Fig. 3shows a thick (average thickness 20m) and

uniform hydroxide layer on the specimen that was pre-dipped in

hydroxidefor 48h. Thesurfacehydroxide films formedon theother

two specimens (polarized in hydroxide and held in desiccator for

24 h) were too thin to be observed by SEM. Surface topography

of the polarized specimen (Fig. 4(b)) confirmed that the surface

hydroxide layer was indeed very thin, which allowed imaging of

the intermetallics at the grain boundaries of the underlying alloy

microstructure. The topographic features of the specimen that was

held in desiccator for 24 h were similar to those shown inFig. 4(b).

However, the very thick hydroxide layer that developed over the

specimen dipped in hydroxide for 48h (Fig. 3) did not allow obser-

vation of the underlying microstructure (Fig. 4(c)).

3.3. Coated alloy with different pre-treatments: surface

characterization

As shown in Fig. 5, the silane coated specimens with differ-

ent pre-treatments have different surface morphologies. Thickness

of the coatings that are developed upon the silane treatments is

expected to be limited to a fewhundreds of nanometres [19], which

allows the features underneath the coating to be observed. There-

fore, it was possible to observe intermetallic precipitates present at

the surface of the coated specimenswithoutalkaline pre-treatment

(Fig. 5(a)) and, to some degree, even in the case of the alloy with

presumably very thin hydroxide layer as developed upon polariza-

tion (Fig. 5(b)). In this context, it is interesting to observe a few

precipitates on the alloy pre-dipped in hydroxide for 48h and then

BTSE coated (Fig. 5(c)), in spite of the considerably thicker hydrox-

ide layer developed upon this pre-treatment (Figs. 3 and 4(c)).However, a careful observation of the cross section of the spec-

imen pre-dipped in sodium hydroxide (Fig. 3(a)) would suggest

that though the alloy matrix had reacted with sodium hydroxide

to produce a thick hydroxide layer, the intermetallic particles had

not reacted and were embedded into the hydroxide layer. In fact,

some of these particlesextend right up to the surface of the hydrox-

ide layer, which explains the observed particles under the silane

coated specimens(Fig.5(c)).Examinationof thecross section of this

specimen (Fig. 6)confirms the silane coating to be thinner in com-

parison with the thickness of the hydroxide layer and it is barely

distinguishable.

3.4. Polarization and electrochemical impedance spectroscopy

tests: uncoated and different coated specimens

Fig. 7 depicts the potentiodynamic polarization plots for the

uncoated, only hydroxide pre-dipped (without any coating) and

BTSE coated (with and without different alkaline pre-treatments)

specimens. The anodic current densities of the BTSE coated alloy

with prior alkaline pre-treatments (both pre-dipped and pre-

polarized) were higherthan othersup to1.2VSCE, which confirms

that the alkaline pre-treatments and then BTSE coating were effec-

tive in impeding the anodic dissolution reaction for these coated

specimens. Beyond this potential, anodic current densities of all

the specimens were similar, implying that the protective coatings

formed on thealkalinepre-treated andthen BTSEcoated specimens

disintegrate at these high anodic over potentials. The effectiveness

of the combined role of alkaline pre-treatment and BTSE coating is


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Fig. 3. Back scattered electron (BSE) image of the cross section of the alloy pre-dipped in hydroxide.

evidenced from the observation that the anodic current densities

of the BTSE coated specimen (without alkaline pre-treatment) and

the only hydroxide pre-dipped specimen (without BTSE coating)

were similar to that of the uncoated specimen at all anodic over

potentials. Cathodic current densities for all the specimens were

similar. Thecorrosionpotentials(Ecorr) of the BTSE coatedspecimen(without alkaline pre-treatment) and the specimen polarized in

hydroxide andthen BTSE coated are more active than the uncoated

alloy, whereas, Ecorrof the specimen dipped in hydroxide andBTSE

coated is about 90 mV more positive than the untreated alloy. It

may be interesting to note that the Ecorr of the BTSE coated (with

and without alkaline pre-treatment) specimens varied over a wide

range (maximum variation is about 200mV). However, this kind

of difference in Ecorr between different coated magnesium alloys

has been observed in several other studies [10,2026].It is quite

plausible to assume that the nature (uniformity and thickness) ofthe silane films formed after different pre-treatments will be dis-

similar. In the present study, the observed variation in Ecorr(which

is a measure of the corrosion susceptibility[27]) of the different

BTSE coated specimens, may be attributed to the different corro-

Fig. 4. SEM images of the surface morphology of the alloy: (a) untreated, (b) polarized in hydroxide and (c) dipped in hydroxide for 48 h.


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Fig. 5. SEM images of the surface morphology of silane coated alloy: (a) without alkaline pre-treatment, (b) polarized in hydroxide for 600 s and (c) pre-dipped in hydroxide

for 48 h (arrows show the intermetallic particles underneath the coatings).

sion susceptibility, arising due to dissimilar nature of the silanecoatings formed on these specimens.

The Nyquist plots (Fig. 8)of all the specimens show that while

slight improvement in impedance was observed in the case of the

only hydroxide pre-treated and only BTSE coated specimens, the

highest improvement in impedance was observed in case of the

BTSE coated specimens with alkaline pre-treatments. A compar-

ison of the plots would suggest that the total impedances, which

arethe measure of the corrosionresistances, offered by the alkaline

pre-treated specimens to be 56 times superior to that of uncoated

alloy. The total impedance of thespecimenpre-dipped in hydroxide

and then BTSE coated is 6 k, and that of the specimen pre-

polarized in hydroxide and then BTSE coated is 5 k, whereas

the total impedance of the uncoated alloy is in the range of 1 k.

From the polarization and EIS results, it can be concluded thatthe improvement in corrosion resistance was not only due to the

alkaline pre-treatment or BTSE coating alone, but due to the com-

bined effect of both. Electrical equivalent circuits (EEC) can be

employed to obtain a mechanistic insight into the influence of the

different surface pre-treatments on the corrosion behaviour of the

BTSE coated specimens. In the present study, impedances of the

interfaces of substrate/surface hydroxide/NaCl solution and sub-

strate/surface hydroxide/silane film/NaCl solution were analysed

on the basis of identification of appropriate EEC. In corrosion pro-

cesses, each of these interfaces can be represented as a parallel

combination of a capacitance and a resistance. This concept forms

the basis of EECs that have been employed here. EECs were devel-

oped for different interfacial scenarios based upon hypothetical

corrosion mechanisms, and the simulation data were generated for

each EEC. The experimental data sets were compared with the cor-responding simulated data sets, in order to arrive at an appropriate

corrosion mechanism for each scenario. It was possible to calcu-

late interfacial resistances and capacitances that can be related to

the homogeneity of the films andcan describe the corrosion mech-

anisms. The relative magnitudes of these components provide an

estimation of the protection provided by a given coating.

In the present study, complex nonlinear least squares fitting

(CNLS) was used to analyse the impedance data. In this method, all

the data are simultaneously fitted to a givenEEC containing a set of

unknown parameters (for example, circuit elements), which may

enter nonlinearly in the formula for the measured function of fre-

quency and impedance. It also provides uncertainty estimates for

all the estimated parameters in a given EEC. Unlike the other meth-

ods (KronigKramers relations, ordinary nonlinear least squares,etc.[28])it allows to fit a very complicated model with large num-

ber of circuit elements. CNLS procedure implies minimization of

the sum of square functions[28],

S=

k

i=1

{wai[faeifat (;p)]

2+wbi[f

beifbt (;p)]

2} (1)

whereft( ;p) is a function of both the angular frequency () and

a set of EEC parameters p. i = 1,. . ., k represents the data points

associated withi.ft( ;p) can be divided into two parts,fat(;p)

andfbt(;p), (both depend on the same set of parameters).fat(;p)

and fbt(;p) can either represent Zreal and Zimaginary or |Z| and

(phase angle of impedance),respectively.wa

i andwb

i arethe weights


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Fig. 6. SEM images of the cross section of the pre-dipped in hydroxide and then BTSE coated specimen: (a) secondary electron image, (b) back scattered electron image and

(c) secondary electron image (higher magnification).

related to the ith data point. faei

and fbei

are the experimentally

obtained data values. Whenfbt(;p) = 0, CNLS reduces to ordinary

non linear least squares [28]. In the present study, since faei

and

fbei

values do not vary over several orders of magnitude, the fit-

ting procedurewas chosen to be unity weighted (i.e.,wai = wb

i = 1).

The circuit description code (CDC) used by Boukamp [29], has been

followed in the present study.

For the uncoated alloy, a simple EEC was used to simulate

the data corresponding to a hypothetical corrosion mechanism,

where electrical double layerand naturallyformed surface hydrox-

ide film behave as a set of capacitance and resistance in parallel

(as shown inFig. 9).Theoretical simulation was conducted usinga Rs(Cf[Rf(CdlRc)]) electrical equivalent circuit (Fig. 9), where the

solution resistance is represented byRs. The surface film, which is

represented by a parallel combination of a capacitance (Cf) and the

surface film resistance (Rf), is in parallel with the electrical dou-

ble layer (represented by the parallel combination of the electrical

double layer capacitance (Cdl) and charge transfer resistance (Rc)).

A comparison with the simulated results based upon other EECs

suggested that Rs(Cf[Rf(CdlRc)]) was the closest fit to the experi-

mental impedance data for the uncoated alloy with a relatively low

chi square value (2103) as well as a low error (4%)in impedance

measurement. The calculated parameters for the proposed EEC are

shown inTable 2.

The simulated results and the observed Bode impedance plots

forthe uncoated ZE41 alloy areshown in Fig. 10. Errorplots(Fig. 11)

for the uncoated alloy show that the maximum error in simulated

data is less than 7% for both |Z| and angle. Though the coated spec-

imens were found to conform to a different EEC, the trend of error

plots for all of them was similar to that shown inFig. 11.

-1.7

-1.6

-1.5

-1.4

-1.3

-1.2

-1.1

-2-3-4-5-6-7-8

log (j/A cm-2

)

E

vsSCE

/V

Uncoated

BTSE coated (without alkaline pre-treatment)

Dipped in hydroxide and BTSE coated

Polarized in hydroxide and BTSE coated

Only hydroxide pre-treated (dipped)

Fig.7. Potentiodynamic polarizationplots ofBTSE coated (withandwithoutalkaline

pre-treatment), hydroxide pre-treated (without any coating) and uncoated ZE41

alloy.


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0

1000

2000

3000

4000

5000

6000

6000500040003000200010000

Z'/ cm2

Z"/

cm

2

BTSE coated (without alkaline pre-treatment)

Polarised in hydroxide and BTSE coated

Dipped in hydroxide and BTSE coated

Uncoated

Only hydroxide pre-treated (dipped)

Fig. 8. Nyquist plots of the BTSE coated (with and without alkaline pre-treatment),

hydroxide pre-treated (without any coating) and uncoated ZE41 alloy.

RcCdl

Rf

Rs

Cf

Electrical

double layer

Surface film

Metal

Fig. 9. The electrical equivalent circuit fitted to the experimentally obtained

impedance data of the uncoated alloy.

Table 2

Calculated parameters for the components in Rs(Cf[Rf(CdlRc)]).

Parameters Estimated values Rel. std. error/%

Rs/ cm2 157 1

Cf/Fcm2 1.1105 5

Rf/ cm2 364 17

Cdl/Fcm2 1.3105 18

Rc/cm2 523 11

Fig. 10. Bode plots for measured and calculated values of uncoated ZE41 alloy.

Fig. 11. Error plots for measured and calculated values of uncoated ZE41 alloy.

Several EECs were employed to fit the experimental impedance

data for the BTSE coated specimens with different pre-treatments.

However the lowest chi square values and the minimum over-

all errors in impedance measurements were obtained with

Rs(Qsi[Rsi(CfdlRc)]) (Fig. 12).In this EEC, the solution resistance is

represented by Rs. The silane film is characterized by a constant

phase element (CPE) (Qsi) and pore resistance (Rsi). Other stud-ies [4,7,20,30,31]suggest that it is typical to represent the silane

film using a CPE, instead of a pure capacitor. The other time con-

stant (represented by the parallel combination of a capacitance

Cfdl and a resistance Rc) that is present in the EEC is assumed to

describe the combined responses of the electrical double layer and

the hydroxide film. This aspect will be explained in more details in

thefollowing sections.The calculated andthe measured Bode plots

for the specimens with different coatings are shown inFig. 13.

Table 3presents the calculated values of the different parame-

ters associatedwith the proposed EEC for differentspecimens. Both

Rsi and Rc for the specimen dipped in hydroxide and then BTSE

coated is higher than others. Also, the total impedance, which is a

measure of the corrosion resistance[18],has the highest value for

thepre-dipped specimen.Although Qsi forall specimens aresimilar,Cfdl of different specimens varies over several orders of magnitude.

Also,Cfdl determined for different coated specimens are different

from the typical electrical double layer capacitance for metal elec-

trodes, whichin general ranges inF/cm2 [32]. Thus it is suggested

that the time constant, consisting ofCfdland Rc did not arise due to

the sole response of the electrical double layer, but it did due to a

combined response of the surface hydroxide film and the electrical

double layer. TheCfdl of the BTSE coated specimen pre-dipped in

Rc

Rs

RsiQsi

CfdlElectricaldouble layer

Silane film

Metal

Fig. 12. The electrical equivalent circuit fitted to the experimentally obtained

impedance data of the different BTSE coated specimens.


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Fig. 13. Bode plots for measured and calculated values of the different coated specimens: (a) only BTSE coated, (b) pre-dipped in hydroxide and then BTSE coated and (c)

pre-polarized in hydroxide and then BTSE coated.

hydroxide was 34 orders of magnitude lower than the other two

specimens (Table 3).Since capacitance is directly proportional to

the area of the capacitor, the relatively lower capacitance indicates

a comparativelylowerexposureof themetal/hydroxide interface tothe electrolyte. This can be attributed to the fact that after immer-

sion of the specimen in 3 M sodium hydroxide (pH 12) for 48 h, a

considerably thicker and less defective surface hydroxide film was

developedon this specimen (Fig.3), which enhancedthe cross link-

age of silane with surface hydroxide during coating process, and

thus presumably produced a uniform and less defective coating.

Improvement in the corrosion resistance of the BTSE coated

specimensas a resultof the alkali pre-treatments is understandable

if one considers the Pourbaix diagram for magnesium [33],shown

in Fig. 14. In aqueous environment,magnesium formsa surface film

of magnesiumhydroxide. The protection offered by Mg(OH)2 filmis

highlydependent on the pH andpresence of differentanions in the

solution [34]. From the Pourbaixdiagram,it is evident thata passive

Mg(OH)2 film will form at pH >11. The nature and protectivenessof this hydroxide layer is reported to be profoundly influenced by

the alloying constituents of different magnesium alloys [3436].

For example, in the case of MgAl alloys, the major alloying ele-

ment, Al plays an important role in the surface film formation [35].

Therefore, it maybe importantto consider the role of Zn, the major

alloying element in ZE41 alloy. The surface film formed on a Zn

electrode is stablein an aqueous solution at pH 8.212.1 [37]. Thus,

on immersion in the alkaline solution (3M NaOH) with pH 12 for48h, a thick and uniform hydroxide film was formed on ZE41 alloy

(Fig. 3).The protective performance of any silane coating primar-

ily depends on the successful cross linkage between the silane film

and the metal hydroxide. Plueddamann[11],Franquet et al.[12]

and van Ooij et al.[9]have earlier suggested that the alkaline pre-

treatment facilitates silane deposition, which is consistent withthe

observation of the present study. Formation of a uniform hydrox-

ide film after alkaline pre-treatments indeed aids formation of a

protective coating on the metal surface (as is evidenced from the

electrochemical results).

Among the specimens that were alkaline pre-treated before

BTSE coating, the specimens pre-dipped in hydroxide and then

coated with BTSE showed a superior corrosion resistance to those

subjected to polarization in hydroxide and then coating (as evi-denced from the Nyquist plots (Fig. 8)). This behaviour may be

attributed to the characteristics of the hydroxide films developed

during thetwo pre-treatments. Also,Figs.3and4show formationof

a thicker and relatively uniform hydroxide film upon prior dipping

Table 3

Calculated parameters of the proposed EEC for the coated specimens.

Specification Rsi/cm2 Rc/ cm

2 Qsi/Fcm2 n Cfdl/Fcm

2 Chi squared

value

Total

error/%

V alue Er ro r/ % Value Er ro r/ % Value E rr or /% Value E rr or /% Value Er ro r/ %

Only BTSE coated 3167 1 899 12 1.1105 3 0.9 1 2.1103 12 8.3104 3

Pre-polarized in NaOH + BTSE coated 4620 2 352 11 1.1105 2 0.9 1 7.5104 6 7.5104 3

Pre-dipped in NaOH + BTSE coated 4881 24 997 6 1.3105 8 0.7 2 9.7107 18 2.8103 5


9/9


Fig. 14. Pourbaix diagram[33].

in hydroxide for 48 h. A cursory comparison may not provide any

convincing reason for the priordipping(as opposed to polarization)

to produce a thicker and less defective hydroxide film. However,

the thermodynamic and kinetic factors seem to provide a convinc-

ing explanation. As the Pourbaix diagram (Fig. 14)would suggest,

little thermodynamic preference for the two scenarios since mag-

nesium will form a stable hydroxide film at every potential when

immersed in an aqueous solution of pH 12. Thus, the film deposi-

tion kinetics would be the only governing factor for the properties

of the hydroxide layer. The long dipping time (48 h) in hydroxide

solution, allowed development of a relatively uniform and thicker

film (Figs. 3 and 4) under close to equilibrium condition. On the

other hand, the effective exposure time of the specimen subjected

to prior polarization was only 600 s and the deposition occurred

in a relatively forced condition, compromising the quality of the

hydroxide film.

4. Conclusions

Influence of alkali surface pre-treatment of BTSE coated ZE41

has been analysed using potentiodynamic polarization and elec-

trochemical impedance spectroscopy. A comparative study of the

corrosion resistance offered by BTSE coated (with and without

alkali pre-treatment) and uncoated ZE41 suggests that alkaline

pre-treatment prior to BTSE coating provides superior corrosion

resistance. However, the nature of the alkaline pre-treatments

influences the corrosion resistance of the coated alloy. This

improvement in corrosion resistance was superior in the case

of a pre-treatment that results in the formation of a thick and

uniform hydroxide film on the surface, which facilitates forma-

tion of a protective silane film on the substrate. The EIS results

are in good agreement with the potentiodynamic polarization

results.

Acknowledgements

Authors would like to acknowledge DSTOfor their financialsup-

port for investigation of silane coating for ZE41 alloy. The kind

assistance of Abhishek Saxena is also gratefully acknowledged.

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