Arab J Sci Eng (2014) 39:113–122DOI 10.1007/s13369-013-0876-7

RESEARCH ARTICLE - CHEMISTRY

Electrochemical Behaviour of Pure Aluminium and Al–5 %ZnAlloy in 3 % NaCl Solution

S. Lameche-Djeghaba · A. Benchettara ·F. Kellou · V. Ji

Received: 28 January 2012 / Accepted: 15 July 2012 / Published online: 7 November 2013© King Fahd University of Petroleum and Minerals 2013

Abstract The effects of both immersion time and tem-perature were investigated to examine the electrochemicalanode characters of pure aluminium and Al–5 %Zn in 3 %NaCl solution. Open circuit potential (OCP), linear polariza-tion resistance (Rp), potentiodynamic polarization, electro-chemical impedance spectroscopy techniques and Evans dia-grams were used to characterize the electrochemical behav-iour of the studied pure aluminium and Al–5 %Zn alloy. Thematerials surface was observed by means of scanning elec-tron microscope. Whatever the studied temperature and theimmersion time are, the Al–5 %Zn alloy induced a consid-erable activation compared to pure aluminium by shiftingthe OCP toward the negative direction and by a significantincrease of the anodic current densities. Arrhenius relationbetween corrosion current densities and the inverse of tem-perature shows that the anodic oxidation of the Al–5 %Znalloy is more spontaneous than that of pure Al. The Al–5 %Znalloy coupled to the carbon steel leads to a better protectionthan the pure aluminium.

S. Lameche-Djeghaba (B)Centre de Recherche Scientifique et Technique en AnalysesPhysico-Chimiques (C.R.A.P.C), BP 248, Alger RP,16004 Alger, Algeriae-mail: lameche_siham@yahoo.fr

S. Lameche-Djeghaba · A. Benchettara · F. KellouLaboratoire d’Electrochimie Corrosion-Métallurgie et ChimieMinérale, Faculté de chimie, BP 32, El Alia Bab Ezzouar,16069 Alger, Algeria

A. Benchettarae-mail: bbenchettara@hotmail.com

F. Kelloue-mail: kelfarida@yahoo.fr

V. JiICMMO-LEMHE, UMR CNRS 8182,Université Paris-Sud 11, 91405 Orsay, France

Keywords Microstructure · Corrosion · Aluminium ·Al–5 %Zn alloy · Sacrificial anode

1 Introduction

Cathodic protection method is often used for steel com-ponents exposed to seawater against corrosion, such aspipelines, tanks, marine structures including ships hulls andsubmarines [1]. The used process involves the realization ofan electrical connection between steel components and blockof sacrificial metal, which acts as an anode, so that the steelcomponents behave as a cathode and are protected againstelectrolytic corrosion [2].

Aluminium alloys as sacrificial anodes are widely used inthe cathodic protection (CP) of steel structures [3]. Theirinteresting properties, due to their low specific weight,low electrode potential and high current density, are oftenhindered by the presence of a passive oxide film whichcauses several difficulties with regard to their use as sacrifi-cial anodes [3–5]. In order to promote its electrochemical

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114 Arab J Sci Eng (2014) 39:113–122

activation, pure aluminium is usually alloyed with smallquantities of elements such as Zn, Hg, In, Sn, Bi, Ti andMg [6]. The Al–Zn binary-phase equilibrium shows mainlytwo solid solution phases (α and β) [7].

It has been established that small amount of Zn elementin aluminium has an effect mainly, or exclusively, on prop-agation and repassivation of metastable pits. By contrast alarge amount of Zn elements enhances the dissolution kinet-ics facilitating the transition to stable pits [8].

On the other hand, it has been reported that the Zn additionto Aluminium destabilizes the formation of Al2O3 on thesurface which allows a better electrolytic character [9].

Barbucci et al. [6] reported that Al–Zn alloys are consid-ered to be good sacrificial anode materials for the cathodicprotection, having interesting electrode potential and uni-form corrosion due to the activation of aluminium providingminimum zinc content.

In this study, the electrochemical behaviour of pure alu-minium and Al–5 %Zn alloy has been studied in 3 % NaClsolution using an open circuit potential (OCP), linear polar-ization resistance, potentiodynamic polarization, electro-chemical impedance spectroscopy (EIS) measurements andEvans diagrams, to obtain further insights of Aluminium acti-vation. Scanning electron microscopy (SEM) was used toexamine materials surface microstructure. The acceleratingeffect of Zn, as an alloying element, was discussed to obtainadditional informations concerning the actual surface statewhich produces the activation process.

2 Experimental

2.1 Materials Casting

Pure Aluminium (99.90 %) and zinc (99.95 %) were used forcasting Al–5 %Zn alloy anodes.

The Al-Zn alloy was obtained after melting Al and Znelements using a vacuum induction furnace (CELES) undera constant argon flux. The elaborated Al–5 %Zn alloy hasbeen homogenized at 450 ◦C for 24 h and quenched in water.The tested materials were polished successively with met-allographic emery paper of increasing fineness up to 800mesh, then degreased with acetone and thoroughly washedwith running distilled water. The water used throughout theexperiments was distilled and all the solutions have beenprepared from reagents of analytical quality.

2.2 Metallurgical Characteristics of the Materials

After ground pure aluminium and Al–5 %Zn alloy withemery paper (grade 400–800–1,200–2400), they were pol-ished with 3 and 1µm diamond paste, and etched with0.5 % hydrofluoric acid solution. The microstructure was

observed using scanning electron microscope (SEM, Cam-bridge Instrument Stérioscan 260).

2.3 Electrochemical Characterization

2.3.1 Open Circuit Potential

The OCP, the potential difference between the working elec-trode with respect to the saturated calomel electrode (SCE),was continuously monitored during 30 min in the electrolyte(3 % NaCl at 25 ◦C). The solution was stirred slowly usingmagnetic stirring. The measurement precision is better than3 %.

2.3.2 Polarization

Linear polarization resistance and potentiodynamic polariza-tion measurements were carried out using voltalab PGP201(Radiometer) controlled by Volta Master 4 Software.

A standard calomel electrode, a platinum electrode anda coupon having 1.76 cm2 exposed area were used as refer-ence, counter and working electrodes, respectively. In con-ducting linear polarization resistance experiments, the poten-tial was scanned from the cathodic to anodic direction at arate of 0.16 mV/s. The potential range of linear polarizationexperiments is ±25 mV/OCP. Potentiodynamic polarizationcurves were obtained at ±250 mV/OCP using a scan rate of1 mV/s. The measurement precision is better than 8 %.

2.3.3 Electrochemical Impedance Characteristics

Electrochemical impedance spectroscopy was carried outusing voltalab (PGZ 301). The measurements were per-formed at the frequency range (105–10−2 Hz) with referenceto OCP after 30-min exposure of the working electrode in theelectrolyte. The measurement precision is better than 9 %.

2.3.4 Evans Diagrams Galvanic Coupling

The closed circuit potential (CCP) of the anode was moni-tored after coupling with mild steel cathode having a surfacearea in the ratio 1.8:3.92 respectively. The current densitygenerated at the anode surface was maintained constant dur-ing CCP measurements. The measurement precision is betterthan 3 %.

3 Results and Discussion

3.1 Microstructure

Figure 1 shows the typical microstructure of pure aluminium(a) and Al–5 %Zn (b) alloy etched with 0.5 % hydrofluo-

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Arab J Sci Eng (2014) 39:113–122 115

Fig. 1 SEM micrographs for a Al and b Al–5 %Zn alloy

ric acid after homogenization treatment followed by waterquenching [10].

It can be observed that the microstructure of pure Alu-minium and Al–5 %Zn alloy presents a uniform grains size.Similar observation was made previously by Barbucci et al.[6]. Otherwise, Munoz et al. [4] observed an enrichment ofthe grain boundaries by Zn element.

The morphological characterization of Al–5 %Zn (Fig. 1b)alloy shows the presence of matrix mainly composed of alu-minium (α-solid solution) with no second β-phase confirm-ing that the Zn element is in solid solution state.

3.2 The OCP Evolution

The variation of OCP, related to the studied materialsimmersed in 3 % NaCl solution during 30 min, is illustratedin Fig. 2.

The OCP values of aluminium changed continuously(more rapidly during the first seconds) in the anodic directionand reached relatively stationary state after 900 s compared to

0 400 800 1200 1600 2000-1.05

-1.00

-0.95

-0.90

-0.85

-0.80

-0.75 Al Al-Zn

Pote

ntia

l / V

Time /Sec

Fig. 2 Evolution with time of OCP for Al and its alloy in NaCl 3 %solution

Al–5 %Zn alloy whose OCP shifted slowly toward cathodicvalue before stabilization.

The anodic OCP shift for pure Al has been attributed tothe formation of compact adherent aluminium oxide layer[11,12].

3.3 Evaluation of Polarization Resistance

The linear polarization resistance curves of pure Al andAl–5 %Zn alloy at different immersion times in NaCl 3 %solution are illustrated in Fig. 3, while the correspondingelectrochemical parameters are given in Table 1.

For pure aluminium anode (Table 1) it can be seen that thepolarization resistance (Rp) increases with the increase of theimmersion time up to 4 h, so we can suggest the formationof a protective layer of aluminium oxide, acting as barrier tooxygen diffusion. The decrease of Rp to a value of 4,850� at16 h can be related to the film formed at the electrode surfacewhich is broken, inducing the corrosion potential (Ecorr) shiftto negative value.

Addition of Zn element to pure Aluminium shifts the cor-rosion potential to more negative values, which is desirablefor the cathodic protection [13]. The Rp values decrease withtime increasing up to 4 h and then raises to reach a value of1,271� at 16 h. We can suggest that after an immersion of16 h, the accumulation of corrosion product formed on elec-trode surface ensures a protection film. However, we note thatthe Al–5 %Zn alloy remains more active than aluminium.

The important Rp values of pure aluminium comparedwith those of Al–5 %Zn alloy indicate the activating effectof Zn element on Aluminium matrix, whatever the immersiontime.

3.4 Evaluation of Potentiodynamic Polarization

Figure 4 shows the potentiodynamic polarization curves ofpure aluminium and Al–5 %Zn Alloy in 3 % NaCl solution.

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116 Arab J Sci Eng (2014) 39:113–122

-0.96 -0.88 -0.80

-3.0x10 -6

0.0

3.0x10 -6

1/2 h1 h 2 h 4 h 16 h

i / A

cm

-2

Potential / V

(a) Aluminium

-1.02 -1.01 -1.00 -0.99-1.0x10-5

0.0

1/2 h 1 h 2 h 4 h 16 h

i /A

cm

-2

Potential / V

(b) Al-5%Zn

Fig. 3 Linear polarization resistance curves for aluminium andAl–5 %Zn alloy in NaCl 3 % at different immersion times

The electrochemical parameters [icorr, Ecorr, anodic Tafelslope (ba) and cathodic Tafel slope (bc)] are gathered inTable 2.

-1.3 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6

-8

-7

-6

-5

-4

-3

-2

-1

log

i(i/A

cm

-2)

Potential / (V)

Aluminium (a)

Al-Zn (b)

Fig. 4 Aluminium and Al–5 %Zn Alloy potentiodynamic polarizationcurves in NaCl 3 % solution at 25 ◦C

The current density icorr is measured by extrapolating theanodic and cathodic Tafel lines at the Ecorr potential.

No active–passive transition was observed on the anodicparts of aluminium and Al–5 %Zn alloy plots (Fig. 4).

Polarization measurements indicated that Al–5 %Zn alloyexhibits an active behaviour whereas it is not observed forpure aluminium. The activation is manifested by a significantshift of icorr from 0.804 to 3.45µA cm−2 and Ecorr to morenegative potentials (Table 2).

It has been reported recently that the variation of thecathodic current densities for aluminium is due to an apparentcatalytic effect of surface element on O2 reduction [14].

Anodic and cathodic processes of aluminium corro-sion in seawater are respectively dissolution of aluminiumand reduction of dissolved oxygen, according to followingreactions:

Table 1 The electrochemical parameters of polarization resistance and EIS of Al, Al–5 % Zn in NaCl 3 % at different times

Samples t Ecorr Rp n (0 ≤ n ≤ 1) Rs Rct CPE1 L1 R1 L2 R2(h) (V) (�cm2) (�cm2) (�cm2) (�−1 cm−2s−1) (H cm2) (�cm2) (H cm2) (�cm2)

Polarization resistance method

Al 1/2 −0.775 4,850 0.74 8.35 5,069 13.1 × 10−5 – – – –

1 −0.860 7,070 0.76 50.76 7,002 9.66 × 10−5 – – – –

2 −0.809 7,990 0.69 8.53 7,323 11.9 × 10−5 – – – –

4 −0.854 9,100 0.72 88.24 9,142 8.44 × 10−5 – – – –

16 −0.982 4,850 0.73 5.713 5,259 10.44 × 10−5 – – – –

Al–Zn 1/2 −0.989 1,006 0.76 7.0 1,243 12.7 × 10−5 1,870 431 – –

1 −0.996 816 0.90 7.78 830 4.714 × 10−5 2,706 244 – –

2 −1.000 660 0.92 9.45 780 2.2 × 10−5 800 300 4,000 1,200

4 −1.000 570 0.89 8.88 670 3.7 × 10−5 130 130 2,600 590

16 −1.010 1,271 0.85 48.59 1,310 4.19 × 10−5 600 203 3,000 678.2

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Arab J Sci Eng (2014) 39:113–122 117

Table 2 The electrochemical parameters of potentiodynamic polarization of Al and Al–5 % Zn in NaCl 3 % solution at 25 ◦C

Samples t (h) EOCP (V) Ecorr (V) icorr (µA cm−2) −bc (V dec−1) ba (V dec−1)

Al 1/2 −0.814 −0.886 0.804 0.107 0.128

Al–Zn 1/2 −0.988 −0.990 3.450 0.205 0.010

4Al → 4Al3+ + 12e− (1)

3O2 + 6H2O + 12 e− → 12OH− (2)

Hence, Al3+ reacts with OH− to form aluminium hydroxidenear the aluminium surface as below

4Al + 3O2 + 6 H2O → 4Al(OH)3 (3)

This hydroxide precipitates on the surface due to its low-solubility product. Aluminium hydroxide changes graduallyto aluminium oxide, resulting in the formation of passive film[15]:

2 Al(OH)3 → (Al2O3) + 3 H2O (4)

However, this kind of oxide film does not offer sufficientprotection against aggressive anions inducing substrate alu-minium dissolution when it is exposed to corrosion solution.The solubility of the aluminium oxide film increased [16] inthe following pH range [4.0–8.5].

Seawater predominantly consists of ∼3.5 % NaCl andmany other ions. Chloride ions are very small and could eas-ily penetrate the passive film. Thus, dissolution of aluminiumsubstrate occurs and results in local corrosion [17].

Pit morphology of aluminium and Al–5 %Zn after poten-tiodynamic polarization in 3 % NaCl is presented in Fig. 5.

It can be observed that the Al–5 %Zn micrography(Fig. 5b) presents numerous hemispherical isolated pitswhich are deeper and larger than those formed on pure Al(Fig. 5a). On the other hand, Al–5 %Zn alloy revealed numer-ous pits in accordance with the results of Ezuber et al. [18].

Zinc has an effect on propagation and repassivation of themetastable pit in Al–5 %Zn alloy. High amount of Zn inducedstable pitting, while low amount results in poor aluminiumactivation [19].

3.5 Effect of Immersion Time on the ElectrochemicalImpedance Diagrams

AC impedance spectroscopic studies were carried out toget further information about both the electrochemical andphysic-chemical phenomena associated with the electrodereactions.

Figure 6 shows the EIS spectra obtained for pure alu-minium and Al–5 %Zn alloy at OCP in NaCl (3 %) solutionas a function of immersion time.

The EIS diagrams varying with the immersion time canbe analyzed generally using equivalent electrical circuit as

Fig. 5 Optical photographs (×200) of the surface morphology for aaluminium and b Al–5 %Zn alloy after potentiodynamic polarization inNaCl 3 % solution at 25 ◦C

shown in Fig. 7. The fitting values obtained by ZView soft-ware for the equivalent elements are listed in Table 1.

To obtain more precious fitting results, the double-layercapacitance (C) is replaced by constant phase element (CPE).The impedance of CPE is defined by the following equation[20,21]:

Z( jw) = (Y0)−1( jw)−n (5)

where Y0 is the CPE constant, j the imaginary unit, n theCPE power (0 ≤ n ≤ 1) and ω the angular frequency(ω = 2π f, f is the frequency). Obviously, the CPE is apure capacitance when n is equal to 1. The deviation of nfrom the unit is due to the heterogeneous effect.

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118 Arab J Sci Eng (2014) 39:113–122

0 5000 10000 15000

0

2000

4000

6000

8000

10000

12000

14000 (a) Aluminium 1/2 h 1 h 2 h 4 h 16 h

-Zi /o

hm c

m²

Zr / ohm cm²

0 600 1200

-400

0

400

800

1200

1/2 h1 h2 h4 h16 h

- Zi /

ohm

cm

2

Zr/ohm cm2

(b) Al-5%Zn

Fig. 6 EIS diagrams for a aluminium and b Al–5 %Zn alloy at theOCP in NaCl 3 % solution at different immersion times

Fig. 7 EIS equivalent electrical circuit for aluminium and Al–5 %Zn:a aluminium at different immersion times and temperatures, b Al–5 %Zn at immersion time of 1/2, 1 h and at different temperatures andc Al–5 %Zn at immersion times of 2, 4 and 16 h

The EIS parameters are listed in Table 1, where Rs

represents the solution resistance, Rct and CPE1 are respec-tively the charge-transfer resistance and the double-layercapacitance.

The EIS diagrams of Al–5 %Zn is characterized by twotime constants, one at high frequencies and the second atlow frequencies, this latter corresponds to the inductive loop(Fig. 6b). In the case of pure aluminium anode (Fig. 6a), noinductive loop is observed.

R1, R2, L1 and L2 are the corresponding parameters forAl–5 %Zn.

The semicircle in the high-frequency regions is associatedwith charge transfer resistance Rct.

The Rct is more intimately correlated to the corrosionrate than Rp, especially in the case of non-uniform corrosionmodel because Rct is determined only by faradaic process ofthe charge transfer controlled corrosion. Note that when theimpedance diagram has more than one time constant, valuesof corrosion rates are flawed, resulting from the calculationof Rp [22].

The size of aluminium electrochemical impedance spectraincreases slightly with the immersion time duration from 0.5to 4 h, then decreases for the longest periods.

The decrease of the polarization resistance Rp and the raiseof the CPE values during 16 h of immersion can be related tothe degradation of the adsorbed layer on the metal surface.

The size of the electrochemical impedance spectra of Al–5 %Zn alloy continuously decreases with the raise of immer-sion time up to 4 h, then increases for the longest durations.This increase with immersion time indicates a thickening ofthe passive film, whereas the inductive loop is indicative oflocal corrosion in the pitting model of Al alloys [23,24]. Baiet al. [25] have attributed the appearance of the inductive loopat low frequencies to the formation of intermediates when thealloy undergoes active dissolution [25]. The observed induc-tive loop could be related to the slow relaxation process ofhydrogen adsorption, as well as aluminium dissolution in theform of Al3+ ion [3]. According to the works of some authors[26,27], the origin of inductive response of aluminium alloyrevealed poor layer homogeneity, which could be due to thepore structure, and possible adsorption relaxation processresulting in pitting corrosion.

The EIS results are in good agreement with thoseobtained by the polarization resistance and potentiodynamicpolarization.

3.6 Temperature Effect

To examine the effect of the temperature on the reactionalmechanism it is interesting to systematically study the tem-perature effect for the selected materials. The temperatureeffect at 15, 25, 35 and 45 ◦C, on aluminium and Al–5 %Znalloy in NaCl 3 % solution, is determined using potentio-dynamic polarization and EIS. The corresponding plots aregiven in Figs. 8 and 9, respectively. Tafel and impedance para-meters are given in Tables 3 and 4.

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Arab J Sci Eng (2014) 39:113–122 119

-1.2 -1.0 -0.8 -0.6-8

-7

-6

-5

-4

-3

-2

-1

Potential / V

log

i (i/

A c

m-2)

15 °C 25 °C 35 °C 45 °C

(a) Aluminium

-1.3 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7-7

-6

-5

-4

-3

-2

-1

log

i(i/

A c

m-2

)

Potential /(V)

15 °C 25 °C 35 °C 45 °C

(b) Al-Zn

Fig. 8 Potentiodynamic polarization curves for a aluminium and bAl–5 %Zn Alloy in 3 % NaCl solution at different temperatures

It can be seen from Fig. 8a, as temperature increases, thecurves are translated towards the more negative potentialsand the domain of the anodic activation slowdown is moreextensive without however appearance of a passivation. Itis also apparent from Table 3 that the values of Ecorr wereslightly shifted in the negative direction (for Al). The valuesof icorr, however, increase as temperature increases.

The shape of polarization curves (Fig. 8b) of the binaryalloy is not affected by the temperature increase, this meansthat the electrode processes are not modified. Otherwise, noactive–passive transition was observed on the anodic parts ofthe plots, so the activation continues on the anodic branch.

An increase in test solution temperature is expected tofacilitate oxygen diffusion to aluminium and Al–5 %Zn alloysurface and, thereby, creating higher concentration of corro-sion cells [28].

Figure 9a shows the EIS diagrams (Nyquist plots) ofpure aluminium which are represented by one depressedsemicircle.

As temperature increases, the values of Rct decrease andCPE1 increases (Table 4), this is in accordance with the

0 3000 6000 9000

0

1000

2000

3000(a) Aluminium 15°C

25°C 35°C 45°C

-Zi /o

hm c

m²

-Zi /o

hm c

m²

Zr / ohm cm²

Zr / ohm cm²-1000 0 1000 2000 3000 4000 5000 6000

-2000

-1000

0

1000

2000 15 °C 25 °C 35 °C 45 °C

(b) Al-Zn

Fig. 9 EIS diagrams of a aluminium and b Al–5 %Zn alloy at the OCPin NaCl 3 % solution at different temperatures

Table 3 Electrochemical parameters of potentiodynamic polarizationof Al and Al–5 % Zn in NaCl 3 % solution at different temperatures

Samples T Ecorr icorr −bc ba

(◦C) (V) (µA cm−2) (V dec−1) (V dec−1)

Tafel technique

Al 15 −0.844 0.656 0.087 0.088

25 −0.886 0.804 0.107 0.128

35 −0.917 1.980 0.165 0.119

45 −0.901 2.870 0.141 0.136

Al–Zn 15 −1.028 3.00 0.238 0.009

25 −0.990 3.45 0.205 0.010

35 −0.995 4.78 0.209 0.0136

45 −1.001 6.30 0.201 0.0124

results of Amin et al. [29]. All the Rct values are smallerin presence of Zn in Alloy compared with pure Aluminium,which confirms the polarization results. The impedance spec-tra of Al–5 %Zn alloy (Fig. 9b) exhibited a capacitive semicir-cle in the high-frequency range and inductive loop in the low-frequency ones, whose semicircle diameter has decreased astemperature increases. The values of Rct decrease as temper-ature increases whereas the double layer capacitances CPE1increase.

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120 Arab J Sci Eng (2014) 39:113–122

Table 4 Electrochemicalparameters of EIS of Al, Al–5 %Zn in NaCl 3 % solution atdifferent temperatures

Samples T (◦C) Rs (�cm2) n Rct (�cm2) CPE1 (�−1 cm−2 s−1) L R1

Impedance method

Al 15 13.44 0.72 13,837 13.8 × 10−5 – –

25 8.35 0.74 5,069 13.1 × 10−5 – –

35 4.98 0.74 3,508 18.8 × 10−5 – –

45 3.92 0,68 3,098 31.9 × 10−5 – –

Al–Zn 15 5.84 0.82 4,000 4.49 × 10−5 16,581 1,181

25 7.00 0.76 1,243 12.7 × 10−5 1,870 431.1

35 6.18 0.73 974 12.9 × 10−5 4,021 150

45 3.40 0.74 854 23.5 × 10−5 4,396 204.6

3.1 3.2 3.3 3.4 3.5

-5

-4

-3ln i = -2.328 103 T -1 + 4.526r2= 0.955

ln i = -4.682 103 T -1 + 11.16r2= 0.930

Al Al-Zn

lni(

A.m

-2)

1000/T(K-1)

Fig. 10 Arrhenius plots for a aluminium and b Al–5 %Zn alloy in 3 %NaCl solution

For an electrochemical oxidation under activation control,the corrosion current is related to reciprocal temperature byArrhenius equation:

icorr = nFkOxexp (αana F Ecorr/RT )

ln icorr = ln(nFkOx) + αana F Ecorr103/RT

ln icorr = ln(nFkOx) + (αana F Ecorr/R) × (103T −1)

The graph of ln icorr versus 103 T −1 (Fig. 10) is a straightline with ln (nFkOx) as the intercept and αanaFEcorr/Rthe slope. The activation energy of the dissolution process�E = F Ecorr is calculated from the slope, the obtainedvalues are �E (Al) = 38.95 kJ mol−1 and �E (Al–5 %Zn) =19.37 kJ mol−1. These results reveal that the oxidation of theAl–5 %Zn alloy is more spontaneous compared to Al.

3.7 Evans Diagrams

The CCP of aluminium or Al–5 %Zn alloy anode using car-bon steel as cathode, under a current (0.01 mA), were com-pared (Fig. 11).

Evans diagram built for a current of 0.01 mA shows thatthe CCP of steel coupled to the binary is more cathodic than

0.0 0.3 0.6

-800

-720

-640

E max

Imax

Ia = 0.01 mA

Aluminium Carbon steel

Imax= 0.574 mA

E mixte= -749 mV

Pote

ntia

l / m

V

I/ mA

0.0 0.6 1.2-1000

-800

-600

Imax

Imax= 1.363 mA

Emixte= - 968 mV

Ia= 0.01 mA

Pote

ntia

l / m

V

I / mA

AlZn Carbon steel

Emax

Fig. 11 Evans diagram for aluminium and Al–5 %Zn alloy couplingwith Carbon steel Ia = 0.01 mA

that of carbon steel coupled to aluminium (Table 5). Thisobservation leads us to say that the binary is used as a sacri-ficial anode is more efficient than pure aluminium.

On the other hand, one can notice the coupling functionsunder cathodic control which is in accordance with resultsreported by Zazoua et al. [30].

The slope of cathodic branches (Evans diagrams) corre-sponding to Carbon steel coupling with aluminium is higherthan that of Al–5 %Zn alloy (Fig. 12). This result means thatthe binary coupled to the steel is more active than aluminium.

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Arab J Sci Eng (2014) 39:113–122 121

Table 5 Evans parameters of Al and Al–5 %Zn; Ia = 0.01 mA

Samples Emax Imax Slope (mV/mA)(mV) (mA) (carbon steel)

Al −749 0.574 −245.5

Al–Zn −968 1.363 −172.4

0.0 0.2 0.4 0.6

-720

-660

-600

E = -245.5 I - 610.8

r2= 0.990

Al

Al-Zn

E /

mV

I / mA

E = -172.46 I - 623.2

r2= 0.992

Fig. 12 Cathodic branches plots of Evans diagrams relative to carbonsteel coupling with aluminium and Al–5 %Zn alloy

We can conclude that Zinc activate the anodic process of purealuminium. Comparing the results of the present study, it canbe concluded that the good correlation was obtained betweenthe different electrochemical methods.

4 Conclusion

The effects of Zn addition on the electrochemical behaviourof aluminium in NaCl 3 % solution at different temperaturesand immersion times were studied using SEM analysis, linearpolarization resistance, potentiodynamic polarization, EIStechniques and Evans diagrams. The presence of zinc in thealuminium matrix has accelerated the activation process ofthe Al–5 %Zn alloy by removing the current plateau locatedin the anodic branch of the Al polarization curve. We noticedthat the polarization resistance of pure Al is higher than thatof Al–5 %Zn alloy. The temperature effect study revealed thatthe ratio icor(45 ◦C)/ icorr(15 ◦C) of Al–5 %Zn is smaller thanthat of pure aluminium. This conclusion is very significant,in the sense that, although the binary is more active than alu-minium, however it corrodes less than aluminium with therise in temperature between 15 and 45 ◦C. Consequently, thelifespan of a sacrificial Al–5 %Zn anode used in the range oftemperature 15–45 ◦C is longer than that of Aluminium. TheAl–5 %Zn alloy EIS diagram is characterized by a capaci-

tive loop at high frequencies and an inductive loop at lowfrequencies compared to that of pure Al where no inductiveloop is observed. The activation energies of the dissolutionprocess show that the anodic oxidation of the binary is morespontaneous than that of Al. The Al–5 %Zn alloy coupledto the carbon steel leads to a better protection than the pureAluminium. The data obtained from the following methods:linear polarization resistance, potentiodynamic polarization,EIS techniques and Evans diagrams are in good agreement.

References

1. Leleyter, L.; Rousseau, C.; Gil, O.; Baraud, F.: Répartition desmétaux lourds dans les différentes fractions des sediments marins:influence de la protection cathodique. CR Geosci. 339, 31–39(2007)

2. Gurrappa, I.: Cathodic protection of cooling water systems andselection of appropriate materials. J. Mater. Process. Technol. 166,256–267 (2005)

3. Shibli, S.M.A.; George, S.: Electrochemical impedance spectro-scopic analysis of activation of Al–Zn alloy sacrificial anode byRuO2 catalytic coating. Appl. Surf. Sci. 253, 7510–7515 (2007)

4. Munoz, A.G.; Saidman, S.B.; Bessone, J.B.: Corrosion of an Al–Zn–In alloy in chloride media. Corros. Sci. 44, 2171–2182 (2002)

5. Shibli, S.M.A.; Jabeera, B.; Manu, R.: Development of high per-formance aluminium alloy sacrificial anodes reinforced with metaloxides. Mater. Lett. 61, 3000–3004 (2007)

6. Barbucci, A.; Cerisola, G.; Bruzzone, G.; Saccone, A.: Activationof aluminium anodes by the presence of intermetallic compounds.Electrochim. Acta 42, 2369–2380 (1997)

7. Hawkins, D.T.; Hultgren, R.: Metals Handbook, 8th edn. Am SocMet, Ohio (1973)

8. Sato, F.; Newman, R.C.: Mechanism of activation of aluminum bylow-melting point elements: Part 2—effect of zinc on activation ofaluminum in pitting corrosion. Corrosion 55, 3–9 (1999)

9. Shibli, S.M.A.; Gireesh, V.S.: Activation of aluminium alloy sac-rificial anodes by selenium. Corros. Sci. 47, 2091–2097 (2005)

10. Brandes, E.A.; Brook, G.B.: Smithells Metals Reference Book, 7thedn. Butterworth-Heinemann, Oxford (1992)

11. Shams El Din, A.M.; Paul, N.J.: Oxide film thickening on the sur-face of metals in aqueous solutions: a critique of the theory ofopen-circuit potential transients. Thin Solid Films 189, 205–216(1990)

12. Shams El Din, A.M.; Hammoud, A.A.: Oxide film formation andthickening on titanium in water. Thin Solid Films 167, 269–280(1988)

13. Ma, J.; Wen, J.: The effects of lanthanum on microstructure andelectrochemical properties of Al–Zn–In based sacrificial anodealloy. Corros. Sci. 51, 2115–2119 (2009)

14. Sina, H.; Emamy, M.; Saremi, M.; Keyvani, A.; Mahta, M.; Camp-bell, J.: The influence of Ti and Zr on electrochemical propertiesof aluminium sacrificial anodes. Mater. Sci. Eng. A 431, 263–276(2006)

15. Gao, B.; Zhang, X.; Sheng, Y.: Studies on preparing and corrosioninhibition behavior of quaternized for low carbon steel in sulfuricacid. Mater. Chem. Phys. 108, 375–381 (2008)

16. Yurt, A.; Ulutas, S. Dal, H.: Electrochemical and theoretical inves-tigation on the corrosion of aluminium in acidic solution containingsome Schiff bases. Appl. Surf. Sci. 253, 919–925 (2006)

17. Rosliza, R.; Wan Nik, W.B.: Improvement of corrosion resistanceof AA6061 alloy by tapioca starch in seawater. Curr. Appl. Phys.10, 221–229 (2010)

123

122 Arab J Sci Eng (2014) 39:113–122

18. Ezuber, H.; El-Houd, A.; El-Shawesh, F.: A study on the corrosionbehavior of aluminium alloys in seawater. Mater. Des. 29, 801–805(2008)

19. Shibli, S.M.A.; Gireesh, V.S.; George, S.: Surface catalysis basedon ruthenium dioxide for effective activation of aluminium sacri-ficial anodes. Corros. Sci. 46, 819–830 (2004)

20. Souto, R.M.; Fernández-Mérida, L.; González, S.; Scantlebury,D.J.: Comparative EIS study of different Zn-based intermediatemetallic layers in coil-coated steels. Corros. Sci. 48, 1182–1192(2006)

21. Zhang, S.S.; Jow, T.R.: aluminium corrosion in electrolyte of Li-ionbattery. J. Power Sources 109, 458–465 (2002)

22. Lorenz, W.J.; Mansfeld, F.: Corros. Sci. 21, 641–672 (1981)23. Mansfeld, F.: Models for the impedance behavior of protective

coatings and cases of localized corrosion. Electrochim. Acta 38,1891–1897 (1993)

24. Zhang, G.A.; Cheng, Y.F.: Corrosion of X65 steel in CO2-saturatedoilfield formation water in the absence and presence of acetic acid.Corros. Sci. 51, 1589–1595 (2009)

25. Bai, L.; Conway, B.E.: Three-dimensional impedance spectroscopydiagrams for processes involving electrosorbed intermediates,

introducing the third electrode-potential variable—examination ofconditions leading to pseudo-inductive behavior. Electrochim. Acta38, 1803–1815 (1993)

26. Bessone, J.B.; Salinas, D.R.; Mayer, C.E.; Ebert, M.; Lorenz, W.J.:An EIS study of aluminium barrier-type oxide films formed indifferent media. Electrochim. Acta 37, 2283–2290 (1992)

27. de Wit, J.H.W.; Lenderink, H.J.W.: Electrochemical impedancespectroscopy as a tool to obtain mechanistic information on thepassive behaviour of aluminium. Electrochim. Acta 41, 1111–1119(1996)

28. Melchers, R.E.: Influence of temperature on seawater immersioncarrion of aluminium (UNS A95086). Br. Corros. J. 36, 201–204(2001)

29. Amin, M.A.; Abd El-Rehim, S.S.; El-Sherbini, E.E.F.; Mahmoud,S.R.; Abbas, M.N.: Pitting corrosion studies on Al and Al-Zn alloysin SCN− solutions. Electrochim. Acta 54, 4288–4296 (2009)

30. Zazoua, A.; Azzouz, N.: An investigation on the use of indium toincrease dissolution of AlZn anodes in sea water. Mater. Des. 29,806–810 (2008)

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