Corrosion behaviour of 6063 aluminium alloy in acidic and ... · The corrosion rate of 6063 aluminium alloy increased with an increase in the concentration of acid as well as with

Arabian Journal of Chemistry (2014) xxx, xxx–xxx

King Saud University

Arabian Journal of Chemistry

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Corrosion behaviour of 6063 aluminium alloy in acidic

and in alkaline media

Deepa Prabhu, Padmalatha Rao *

Department of Chemistry, Manipal Institute of Technology, Manipal University, 576104 Karnataka, India

Received 21 November 2012; accepted 28 July 2013

*

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KEYWORDS

Corrosion;

6063 aluminium alloy;

Phosphoric acid;

Sodium hydroxide;

Tafel polarization;

EIS

Corresponding author. Tel.-mail addresses: padmalatha

hoo.com (R. Padmalatha).

er review under responsibilit

Production an

78-5352 ª 2014 Production

tp://dx.doi.org/10.1016/j.arab

lease cite this article in preedia. Arabian Journal of C

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ss as: Dhemistr

Abstract The corrosion behaviour of 6063 aluminium alloy was investigated in different concen-

trations of phosphoric acid medium and sodium hydroxide medium at different temperatures. The

study was done by electrochemical method, using Tafel polarization technique and electrochemical

impedance spectroscopy (EIS) technique. The surface morphology was investigated using scanning

electron microscope (SEM) with Energy-dispersive X-ray spectroscopy (EDX). The results showed

that the 6063 aluminium alloy undergoes severe corrosion in sodium hydroxide medium than in

phosphoric acid medium. The corrosion rate of 6063 aluminium alloy increased with an increase

in the concentration of acid as well as with alkali. The corrosion rate was increased with an increase

in temperature. The kinetic parameters and thermodynamic parameters were calculated using

Arrhenius theory and transition state theory. Suitable mechanism was proposed for the corrosion

of 6063 aluminium alloy in phosphoric acid medium and sodium hydroxide medium. The results

obtained by Tafel polarization and electrochemical impedance spectroscopy (EIS) techniques were

in good agreement with each other.ª 2014 Production and hosting by Elsevier B.V. on behalf of King Saud University.

1. Introduction

Corrosion, which is an inevitable problem faced by almost allindustries can be considered as one of the worst technicalcalamities of our time. Besides from its direct costs in dollars,

corrosion is a serious problem because it definitely contributes

80122680.nipal.edu, drpadmalatharao@

Saud University.

g by Elsevier

ng by Elsevier B.V. on behalf of K

7.059

eepa, P., Padmalatha, R. Corrosy (2014), http://dx.doi.org/10.10

to the depletion of our natural resources. Corrosion studieshave also become important due to increasing awareness of

the need to conserve the world’s metal resources (Stansburyand Buchanan, 2000). Now-a-days more attention has beenpaid to control the metallic corrosion, due to increasing use

of metals in all fields of technology.Corrosion studies of aluminium and aluminium alloys have

received considerable attention by researchers because of their

wide industrial applications and economic considerations(Christian Vargel, 2004; Kosting and Heins, 1931; Badawayet al., 1999; Paul and Sigwalt Juniere, 1964). Aluminium and

aluminium alloys have emerged as alternate materials in aero-space and in some chemical processing industries. Due to theirwide applications, they frequently come in contact with acidsor bases during pickling, de-scaling, electrochemical etching

ing Saud University.

ion behaviour of 6063 aluminium alloy in acidic and in alkaline16/j.arabjc.2013.07.059

mailto:[email protected]



http://dx.doi.org/10.1016/j.arabjc.2013.07.059


http://www.sciencedirect.com/science/journal/18785352



-0.8

-0.7

-0.6

-0.5

(V/S

CE)

0.01M 0.1M 1.0M

2 P. Deepa, R. Padmalatha

and extensively used in many chemical process industries.Most of the reported studies were conducted on corrosion ofvarious metals and alloys in HCl and H2SO4 media (Paul

and Sigwalt Juniere, 1964; Ating et al., 2010; Umoren et al.,2009; Obi-Egbedi et al., 2012; Nnanna et al., 2011).

Phosphoric acid medium is widely used for acid cleaning

and elector polishing of aluminium (Christian Vargel, 2004).Even though dissolution rate of aluminium in phosphoric acidmedium is lower, compared to the dissolution of the same in

hydrochloric medium or sulphuric acid medium, it does cor-rode aluminium and its alloys. Phosphoric acid medium is alsoused in pickling delicate, costly components and precisionitems where rerusting after pickling has to be avoided. Sodium

hydroxide is usually used for degreasing purpose. According tothe available literature, not much study has been doneregarding the corrosion behaviour of 6063 aluminium alloy

in phosphoric acid medium as well as with sodium hydroxidemedium. As part of our studies with corrosion behaviour ofaluminium and aluminium alloys in phosphoric acid medium

and sodium hydroxide medium and corrosion control ofthe same using green inhibitors (Deepa and Padmalatha,2013a,b) we report herein the results of corrosion behaviour

of 6063 aluminium alloy in phosphoric acid medium andsodium hydroxide medium of different concentrations atdifferent temperatures.

2. Methods

2.1. Material

The experiments were performed with specimens of 6063 alu-minium alloy. The composition of the 6063 aluminium alloy

specimen is given in Table 1.Cylindrical test specimens were sealed with Acrylic resin

material in such a way that the area exposed to the medium

was 1.0 cm2. It was polished with 180, 280, 400, 600, 800,1000, 1500, and 2000 grade emery papers. Further polishingwas done with disc polisher using levigated alumina to get

mirror surface. It was then dried and stored in a desiccatorto avoid moisture before using it for corrosion studies.

2.2. Medium

A stock solution of phosphoric acid medium was preparedusing analytical grade phosphoric acid medium (85%) anddouble distilled water. It was standardized by potentiometric

method. Phosphoric acid medium of concentrations 1.0 M,0.1 M and 0.01 M was prepared by appropriate dilution. Astock solution of sodium hydroxide was prepared by dissolving

analytical grade sodium hydroxide pellets in double distilled

Table 1 Composition of the 6063 aluminium alloy specimen

(% by weight).

Element Composition (%)

Si 0.412

Fe 0.118

Cu 0.0570

Mg 0.492

Al Balance

Please cite this article in press as: Deepa, P., Padmalatha, R. Corrosmedia. Arabian Journal of Chemistry (2014), http://dx.doi.org/10.10

water and standardized by volumetric method using phenol-phthalein indicator. Solutions of required strengths 0.05 M,0.25 M and 0.5 M were prepared by appropriate dilutions as

and when required. Experiments were carried out using acalibrated thermostat at temperatures 30 �C, 35 �C, 40 �C,45 �C and 50 �C (±0.5 �C).

2.3. Electrochemical measurements

Electrochemical measurements were carried out by using an

electrochemical work station, CH600D-series, U.S. Modelwith CH instrument beta software. The electrochemical cellused was a conventional three-electrode compartment having

glass cell with a platinum counter electrode and a saturatedcalomel electrode (SCE) as reference. The working electrodewas made up of 6063 aluminium. All the values of potentialwere measured with reference to the saturated calomel elec-

trode. The polarization studies were done immediately afterthe EIS studies on the same electrode without any further sur-face treatment.

2.3.1. Tafel polarization studies

Finely polished 6063 alloy specimens with 1.0 cm2 surface areawere exposed to corrosion medium of different concentrations

of phosphoric acid (0.01 M, 0.1 M and 1.0 M) and differentconcentrations of sodium hydroxide (0.05 M, 0.25 M and0.5 M) separately at a temperature range of 303–323 K.

The potentiodynamic current–potential curves were recordedby polarizing the specimen to �250 mV cathodicallyand + 250 mV anodically with respect to open circuit poten-

tial (OCP) at a scan rate of 0.01 V/s.

2.3.2. Electrochemical impedance spectroscopy (EIS) studies

Electrochemical impedance spectroscopy (EIS) measurements

were carried out using a small amplitude ac signal of 10 mVover a frequency range of 100 kHz–0.01 Hz.

Both techniques were repeated at least three times. The

average value of best three agreeing values was reported.

-7 -6 -5 -4 -3 -2-1.1

-1.0

-0.9

E

log i (Acm-2)

Figure 1 Tafel polarization curves for 6063 aluminium alloy in

different concentrations of H3PO4 at 30 �C.



-6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

E (V

/SCE

)

log i (A cm-2)

0.05M 0.25M 0.5M

Figure 2 Tafel polarization curves for 6063 aluminium alloy in

different concentrations of NaOH at 30 �C.

Corrosion behaviour of 6063 aluminium alloy 3

2.4. Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX) studies

The scanning electron microscope images were recorded toestablish the interaction of acid medium and alkaline mediumwith the metal surface using EVO 18-15-57 scanning electron

microscope with Energy-dispersive X-ray spectroscopy. Thesurface morphology of 6063 aluminium alloy immersed inphosphoric acid medium and sodium hydroxide medium was

compared with that of polished metal sample.

3. Results and discussion

3.1. Tafel polarization measurements

The effect of phosphoric acid medium and sodium hydroxidemedium on the corrosion rate of 6063 aluminium alloy sample

Table 2 Results of Tafel polarization studies and EIS studies of 60

Tafel polarization

H3PO4 (M) T (K) Ecorr (V vs. SCE) icorr (·10�5 A cm�2) ba (V de

0.01 303 �0.72 2.99 3.73

308 �0.73 3.39 3.56

313 �0.74 3.42 3.50

318 �0.77 3.76 3.69

323 �0.78 4.29 3.78

0.1 303 �0.79 10.8 3.67

308 �0.80 12.8 3.70

313 �0.81 16.3 3.74

318 �0.82 18.1 4.07

323 �0.83 24.5 4.14

1.0 303 �0.76 38.6 3.34

308 �0.77 50.5 3.43

313 �0.78 69.2 3.50

318 �0.80 96.0 3.69

323 �0.81 135.4 3.79


was studied using Tafel polarization technique. Figs. 1 and 2represent the potentiodynamic polarization curves of 6063aluminium alloy in different concentrations of phosphoric acid

medium at 30 �C and in different concentrations of sodiumhydroxide medium at 30 �C respectively. Corrosion parameterssuch as corrosion potential (Ecorr), corrosion current density

(icorr), anodic slope (ba) and cathodic slope (bc) are obtainedfrom the Tafel polarization curves. Results are tabulated inTables 2 and 3. The corrosion rate was calculated using Eq. (1).

tcorrðmm y�1Þ ¼ 3270 � M � icorrq � Z

; ð1Þ

where 3270 is a constant that defines the unit of corrosion rate,

icorr is the corrosion current density in A cm�2, q is the densityof the corroding material (g cm�3), M is the atomic mass of themetal, and Z is the number of electrons transferred per atom

(Fontana, 1987). The results of Tafel polarization measure-ments are reported in Tables 2 and 3 respectively.

The results indicate the increase in the corrosion rate (tcorr)with an increase in the concentration of phosphoric acid med-

ium and sodium hydroxide medium. The positive shift in thecorrosion potential (Ecorr) with the increase in the concentra-tion of phosphoric acid medium and sodium hydroxide med-

ium indicates that the anodic process is much more affectedthan the cathodic process (El-Sayed, 1997). This observationis in accordance with Muralidharan (Muralidharan and Rajag-

opalan, 1979), who proposed dependence of Ecorr and icorr onsolution parameters. Tafel slopes remained almost unchangedindicating that acid strength does not change the mechanism ofthe corrosion process. It is evident from data of Tables 2 and 3

that corrosion rate is more in case of sodium hydroxide med-ium than in phosphoric acid medium.

3.2. Electrochemical impedance spectroscopy (EIS)measurements

The corrosion behaviour of 6063 aluminium alloy specimens

was also investigated by EIS techniques in various concentra-tions of phosphoric acid medium and sodium hydroxidemedium. The impedance spectra were recorded and displayed

63 aluminium alloy in H3PO4.

EIS

c�1) � bc (V dec�1) tcorr (mm y�1) Rp (X cm2) CPE (l Fcm�2)

5.40 0.32 1922.6 45

5.76 0.36 1767.8 57

6.04 0.37 1512.1 63

5.97 0.40 1324.8 74

5.94 0.46 993.5 83

6.57 1.17 407.3 58

6.46 1.39 339.5 65

6.01 1.77 280.2 77

5.32 1.97 234.5 86

5.71 2.67 172.7 98

6.72 4.20 101.70 69

6.58 5.50 75.90 78

6.43 7.54 54.00 90

5.97 10.46 36.80 117

5.60 14.75 24.20 129



0 200 400 600 800 1000 1200

0

200

400

600

800

1000

1200

-ZII (o

hm c

m2 )

Zl (ohm cm2)

0.01M 0.1M 1.0M

Figure 3 Nyquist plot for 6063 aluminium alloy in different

concentrations of H3PO4 at 30 �C.

0 2 4 6 8 10 12 14 16

0

2

4

6

8

10

12

14

16

-Z" (

ohm

cm

2 )

Z' (ohm cm2)

0.05M 0.25M 0.5M

Figure 4 Nyquist plot for 6063 aluminium alloy in different

concentrations of NaOH at 30 �C.

Table 3 Results of Tafel polarization studies and EIS studies of 6063 aluminium alloy in NaOH.

Tafel polarization EIS

NaOH (M) T (K) Ecorr (V vs. SCE) icorr (·10�5A cm�2) ba (V dec�1) � bc (V dec�1) tcorr (mm y�1) Rp (X cm2) CPE (l F cm�2)

0.05 303 �1.55 40.4 4.84 4.86 4.40 101.8 66

308 �1.56 46.9 4.87 4.83 5.11 82.5 72

313 �1.57 52.6 5.21 5.31 5.73 80.2 75

318 �1.58 57.2 4.83 5.33 6.23 77.3 79

323 �1.59 59.8 4.45 5.44 6.51 75.6 85

0.25 303 �1.57 209 5.04 4.84 22.78 24.9 76

308 �1.59 244 4.90 4.92 26.59 18.3 84

313 �1.60 281 5.11 4.47 30.62 17.3 96

318 �1.61 307 4.80 4.70 33.46 15.0 107

323 �1.62 331 4.93 4.73 36.07 14.4 114

0.5 303 �1.59 326 4.92 4.71 35.53 12.2 83

308 �1.60 374 4.92 4.91 40.76 11.5 95

313 �1.61 424 5.03 4.88 46.21 9.9 101

318 �1.62 437 4.67 4.84 47.63 9.7 111

323 �1.63 455 4.92 4.96 49.59 8.3 123


as Nyquist plots as a function of acid strength. Nyquistplots for different concentrations of phosphoric acid medium

and sodium hydroxide medium at 30 �C are shown in Figs. 3and 4 respectively. The EIS results are reported in Tables 2and 3.

As can be seen from Figs. 3 and 4, the impedance diagramsshow semicircles, indicating that the corrosion process ismainly charge transfer controlled (Wit and Lenderink, 1996;

Lenderink et al., 1993).The depressed semicircles of the Ny-quist plots suggest the distribution of capacitance due to inho-mogeneous surface associated with the metal surface (Morad,1999). The shapes of the curves are identical and it consists of a

large capacitive loop at higher frequencies (HF), inductiveloop at intermediate frequencies (IF), followed by a secondcapacitive loop at lower frequency (LF) values (Brett, 1990).

Similar impedance plots have been reported in the literaturefor the corrosion of aluminium and aluminium alloys invarious electrolytes such as, sodium sulphate (Brett, 1992),


sulphuric acid (Poornima et al., 2010), and sodium hydroxide(Shao et al., 2003; Abdel-Gaber et al., 2008; Reena Kumariet al., 2012). The HF capacitive loop is attributed to the

presence of a protective oxide film covering the surface ofthe metal. According to Brett (Brett, 1992), the capacitive loopis corresponding to the interfacial reactions, particularly, the

reaction of aluminium oxidation at the metal/oxide/electrolyteinterface. The process includes the formation of Al+ ions atthe metal/oxide interface, and their migration through the

oxide/solution interface where they are oxidized to Al3+. Atthe oxide/solution interface, OH� or O2� ions are also formed.The fact that all the three processes are represented by onlyone loop could be attributed either to the overlapping of the

loops of processes, or to the assumption that one process dom-inates and, therefore, excludes the other processes (Morad,1999). The other explanation offered to the high frequency

capacitive loop is the oxide film itself. The origin of the



Figure 5 Equivalent circuit fit for 6063 Al alloy in 1.0 M H3PO4 at 30 �C.

-6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

-0.5

E(V/

SCE)

log i(Acm-2)

30oC 35oC 40oC 45oC 50oC

Figure 6 Tafel polarization curves for 6063 aluminium alloy at

different temperatures in 1 M H3PO4.

-5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5

-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3E

(V/S

CE)

log i (A cm-2)


Figure 7 Tafel polarization curves for 6063 aluminium at

different temperatures in 0.5 M NaOH.


inductive loop has often been attributed to the surface or bulkrelaxation of species in the oxide layer (Morad, 1999). The LF

inductive loop may be related to the relaxation process ob-tained by adsorption and incorporation of phosphate ionsand hydroxide ions into the oxide film (Lorenz and Mansfeld,

1981).An equivalent circuit of nine elements depicted in Fig. 5a

was used to simulate the measured impedance data of the

6063 aluminium alloy, as shown in Fig. 5b. In this equivalentcircuit, Rs is the solution resistance and Rt is the charge trans-fer resistance. RL and L represent the inductive elements. Thisalso consists of constant phase element; CPE (Q) in parallel to

the series capacitors C1 and C2 and series resistors R1, R2, RL

and Rt. RL is parallel with the inductor L. The polarizationresistance Rp and double layer capacitance Cdl are calculated

from Eqs. (2) and (3):

Rp ¼ RL þ Rt þ R1 þ R2; ð2Þ

Cdl ¼ C1 þ C2: ð3Þ


The measured value of polarization resistance decreasedwith increase while the CPE value increased with increasing

concentration of the phosphoric acid medium and sodiumhydroxide medium, indicating that the rate of corrosion in-creased with increasing concentration of the phosphoric acidmedium and sodium hydroxide medium. This is in agreement

with the results obtained from potentiodynamic polarizationdata.

3.3. Effect of temperature

The effect of temperature on the corrosion rate of the 6063 al-loy was studied by measuring the corrosion rate at the temper-

ature range of 303–323 K. Figs. 6 and 7 represent thepotentiodynamic polarization curves of 6063 aluminium alloyin 1.0 M phosphoric acid medium and 0.5 M sodium hydride

medium at temperature range of 303–323 K. Figs. 8 and 9represent Nyquist plots for the same. From both figures it is



0 20 40 60 80 100 120

0

20

40

60

80

100

120-Z

II (ohm

cm

2 )

ZI (ohm cm2)


Figure 8 Nyquist plots for the corrosion of 6063 aluminium

alloy at different temperatures in 1.0 M H3PO4.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Z' (ohm cm2)

-Z" (

ohm

cm

2 )


Figure 9 Nyquist plots for the corrosion of 6063 aluminium

alloy at different temperatures in 0.5 M NaOH.

3.1x10-3 3.1x10-3 3.2x10-3 3.2x10-3 3.3x10-3 3.3x10-33.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

ln(C

R) (m

m y

-1)

1/T (K-1)

0.01M 0.05M 0.1M

Figure 10 Arrhenius plots for the corrosion of 6063 aluminium

alloy in H3PO4.


obvious that the corrosion rate increased with an increase intemperature. This results in the rapid dissolution of the oxide

film, thereby enhancing the corrosion rate. This is attributableto the enhanced chemical film dissolution due to enrichedphosphate ions, OH� ions and depleted AlðOHÞ�4 ions at the

film/solution interface. Thus with the increasing temperaturethe oxide passive film becomes thin, porous and less protectiveas a result of dissolution of the film by the electrolyte (Foley

and Nguyen, 1982).The energy of activation of corrosion of 6063 aluminium

alloy was calculated from Arrhenius Eq. (4). The Arrhenius

plot (lnCR vs. 1/T) at all the concentrations of phosphoricacid medium and sodium hydroxide medium is representedin Figs. 10 and 11.

lnðtcorrÞ ¼ B� Ea

RT; ð4Þ


where B is a constant, R is the universal gas constant, and T isthe absolute temperature.

Enthalpy of activation (DHa) and entropy of activation

(DSa) were calculated from the transition state theory Eq. (5),

tcorr ¼RT

Nhexp

DSa

R

� �exp

DHa

RT

� �ð5Þ

where h is Plank’s constant and N is Avagadro’s number.A plot of ln(tcorr/T) vs 1/T gives a straight line withslope = �DHa/T and intercept = ln(R/Nh) + DSa/R. Theplots of ln(tcorr/T) vs. 1/T for the corrosion of 6063 aluminium

alloy in the presence of different concentrations of phosphoricacid medium and sodium hydroxide medium are shown inFigs. 12 and 13.

The values of energies of activation, enthalpy of activationand entropy of activation are listed in Tables 4 and 5.

As indicated in Table 4, except for 0.01 M phosphoric acid

medium, value of energy of activation is greater than20 kJ mol�1.

This suggested that at higher strengths of acid, the whole

process is controlled by surface reaction (El-Neami et al.,1995). This phenomenon may be due to the fact that theactivation energy for the corrosion reaction is independent ofconcentration polarization (Oguzie, 2007) and mainly due to

activation polarization. However, in case of sodium hydroxidemedium, energy of activation at all studied concentrationswas less than 20 kJ mol�1 and Ea decreased with an

increase in sodium hydroxide medium concentration, whichis suggestive of concentration polarization as a main reactionat electrode–electrolyte interface.

The entropy of activation is large and negative. This impliesthat the activated complex in the rate determining step repre-sents association rather than dissociation, indicating that a de-crease in disorder takes place, in going from reactant to the

activated complex (Trowsdale et al., 1996). The value of DSa

decreased with an increase in acid concentration reveals thatdissolution of alloy is facilitated as the concentration of the

acid medium increases.



3.1x10-3 3.1x10-3 3.2x10-3 3.2x10-3 3.3x10-3 3.3x10-3

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2ln

(CR

/T) (

mm

y-1 K

-1)

1/T (K-1)

0.01M 0.05M 0.1M

Figure 11 Plots of ln(CR/T) vs. 1/T for the corrosion of 6063

aluminium alloy in H3PO4.

3.1x10-3 3.1x10-3 3.2x10-3 3.2x10-3 3.3x10-3 3.3x10-36.0

6.5

7.0

7.5

8.0

8.5

ln (C

R) (

mm

y-1)

1/T (K-1)

0.05M 0.25M 0.5M

Figure 12 Arrhenius plots for the corrosion of 6063 aluminium

alloy in NaOH.

3.1x10-3 3.1x10-3 3.2x10-3 3.2x10-3 3.3x10-3 3.3x10-3

0.5

1.0

1.5

2.0

2.5

3.0

ln (C

R/T)

(mm

y-1 K

-1)

1/T (K-1)

0.05M 0.25M 0.5M

Figure 13 Plots of ln(CR/T) vs. 1/T for the corrosion of 6063

aluminium alloy in NaOH.

Table 5 Activation parameters for the corrosion of 6063

aluminium alloy in NaOH.

NaOH (M) Ea (kJ mol�1) DHa (kJ mol�1) DSa (J mol�1 K�1)

0.05 18.43 15.88 �150.540.25 15.79 13.24 �141.490.5 13.25 10.70 �128.21

Figure 14 SEM image of freshly polished surface of the 6063

aluminium alloy.

Table 4 Activation parameters for the corrosion of 6063

aluminium alloy in H3PO4.

H3PO4 Ea (kJ mol�1) DHa (kJ mol�1) DSa (J mol�1 K�1)

0.01 13.1958 10.644 �180.990.1 31.6439 29.0923 �109.6521.0 50.2442 47.6926 �37.8584


3.4. Scanning electron microscopic study

The SEM images of freshly polished surface of 6063 alumin-ium alloy are given in Fig. 14 which shows polished surface

with few scratches due to polishing. The surface morphologyof the 6063 aluminium alloy sample was examined by SEMimmediately after corrosion tests in 1.0 M phosphoric acid

medium and 0.5 M sodium hydroxide medium. The SEMimage of corroded sample given in Figs. 15 and 16 shows thedegradation of alloy, with more or less uniform attack in

phosphoric acid medium and sodium hydroxide medium.Fig. 17 represents EDX spectrum of the polished sample ofthe 6063 aluminium alloy. The spectrum shows peaks for

aluminium and oxygen suggesting the presence of aluminiumoxide/hydroxide. Fig. 18 depicts the EDX spectrum for thecorroded sample in the presence of phosphorous indicatingthe attack of phosphoric acid medium.

Please cite this article in press as: Deepa, P., Padmalatha, R. Corrosion behaviour of 6063 aluminium alloy in acidic and in alkalinemedia. Arabian Journal of Chemistry (2014), http://dx.doi.org/10.1016/j.arabjc.2013.07.059


Figure 15 SEM image of 6063 aluminium alloy after immersion

in 1.0 M H3PO4.

Figure 16 SEM image of 6063 aluminium alloy after immersion

in 0.5 M NaOH.

Figure 17 EDX spectrum of the polished



For aluminium there is deposition of aluminium phosphateprecipitate on the surface and for 6063 aluminium alloy due tothe alloying elements, it may be attributed to deposition of the

phase Mg5Al8 which makes an anodic phase in the aluminiummatrix and Si acts as cathode and contributes to the intergran-ular corrosion resulting in uniform attack and making it an

optimum region of dissolution (Scamans et al., 1987). Closeobservation of SEM metal images indicates the depositionof precipitates of aluminium phosphate and magnesium

phosphate on 6063 aluminium alloy.Fig. 19 depicts the EDX spectrum for the corroded sample

in the presence of sodium hydroxide medium. The spectrum re-veals the presence of sodium hydroxide medium, the increase

in the amount of oxygen can be attributed to the presence ofaluminium hydroxide film formed due to the attack of hydrox-ide ion on the surface of metal.

3.5. Mechanism of corrosion process

Aluminium and its alloys have air formed oxide film of

amorphous c alumina which initially thickens on exposure toneutral aqueous solution with the formation of a layer ofcrystalline hydrated alumina (Hart, 1957).

In acid solution the mechanism of dissolution of aluminiumis as follows (Obot et al., 2010):

AlþH2O! AlOHðadsÞ þHþ þ e�; ð6Þ

AlOHðadsÞ þ 5H2OþHþ ! Al3þ þ 6H2Oþ 2e�; ð7Þ

Al3þ þH2O! ½AlOH�2þ þHþ; ð8Þ

½AlOH�2þ þX� ! ½AlOHX�þ: ð9Þ

Thus soluble complex ion formed leads to the dissolution ofthe metal.

In case of sodium hydroxide medium corrosion of metals isknown to proceed by the action of local cells, which areestablished by metals, comprising a partial anodic reaction

and a partial cathodic reaction occurring simultaneously onthe metal surface (Uhlig and Revie, 1991). So, in order to

sample of the 6063 aluminium alloy.



Figure 19 EDX spectrum of 6063 aluminium alloy after immersion in 0.5 M NaOH.

Figure 18 EDX spectrum of 6063 aluminium alloy after immersion in 1.0 M H3PO4.


follow the mechanism underlying the corrosion of metals, it isnecessary to explore which partial anodic reactions and partial

cathodic reactions involve and to determine which of them pre-vails in the gross corrosion reaction to a greater extent. Inaddition, the presence of the native surface oxide film should

be considered in studying the corrosion mechanism ofaluminium in alkaline solution. The anodic aluminiumdissolution reaction in the presence of the oxide film can be

classified into a direct metal dissolution reaction by the move-ment of aluminium ions through the film and an indirect metaldissolution reaction by consecutive oxide film formation and

dissolution (Moon and Pyun, 1998; Moon and Pyun, 1999).It is generally accepted (Wernick et al., 1987; Brock and

Wood, 1967) that the film formation proceeds electrochemi-cally by the incorporation of hydroxide ions into the film

and migration through the film towards the aluminium/filminterface in the presence of a thick oxide film on the surfaceof aluminium as (11):

Alþ 3OH� ! AlðOHÞ3 þ 3e� ð10Þ


The aluminium hydroxide film formed electrochemically willbe dissolved chemically by OH- attack at the film/solutioninterface:

AlðOHÞ3 þOH� ! AlðOHÞ�4 ð11Þ

where AlðOHÞ�4 represents the aluminate ions. A partial ano-dic dissolution reaction of aluminium in alkaline solutioncan be obtained by combining electrochemical film formation

(10) and chemical film dissolution (11), which can be writtenas:

Alþ 4OH� ! AlðOHÞ�4 þ 3e� ð12Þ

The electrons produced by the partial anodic Reaction (12)

will be consumed immediately by such partial cathodic reac-tions as the oxygen reduction reaction:

3=4O2 þ 3=2H2Oþ 3e� ! 3OH� ð13Þ

and/or the water reduction reaction:

3H2Oþ 3e� ! 3=2H2 þ 3OH� ð14Þ




The short-circuited corrosion reaction of pure aluminium in

alkaline solution can be obtained by combining Reactions(12) and (13):

Alþ OH� þ 3=4 O2 þ 3=2 H2O ! AlðOHÞ�4 ð15Þ

and/or by combining Reactions (12) and (14):

Alþ 3H2Oþ OH� ! 3=2 H2 þ AlðOHÞ�4 ð16Þ

As a consequence of the overall corrosion Reaction (15), alu-minium metal dissolution occurs. On the other hand, hydrogenevolution takes place simultaneously with the dissolution of

aluminium metal as a result of the overall corrosion Reaction(16). The observation of gas bubbles on the corrodingaluminium surface shows that the corrosion of aluminium in

alkaline solution proceeds mainly by water reduction (Pyunet al., 1999) according to Reaction (16).

Therefore, it is concluded that the overall corrosion reac-tion of aluminium in alkaline solution can be divided into

two substeps of a partial anodic reaction comprising the elec-trochemical formation and chemical dissolution reactions ofthe film, and a partial cathodic reaction of water reduction

reaction.

4. Conclusions

� 6063 aluminium alloy undergoes corrosion both in phos-

phoric acid medium and in sodium hydroxide medium.� The corrosion rate increases with an increase in the concen-tration of phosphoric acid medium as well as sodium

hydroxide medium.� The corrosion rate increases with an increase intemperature.

� The corrosion of 6063 aluminium alloy is more severe insodium hydroxide medium than in phosphoric acidmedium.

References

Abdel-Gaber, A.M., Khamis, E., Abo-EIDahab, Sh., Adeel, H., 2008.

Inhibition of aluminum corrosion in alkaline solutions using

natural compound. Mater. Chem. Phys. 109, 297–305.

Ating, E.I., Umoren, S.A., Udousoro, I.I., Ebenso, E.E., Udoh, A.P.,

2010. Leaves extract of Ananas sativum as green corrosion inhibitor

for aluminium in hydrochloric acid solutions. Green Chem. Lett.

Rev. 3, 61–68.

Badaway, W.A., AlKharafi, F.M., El-Azab, A.S., 1999. Electrochem-

ical behavior and corrosion inhibition of Al, Al-6061 and Al–Cu in

neutral aqueous solutions. Corros. Sci. 41, 709–727.

Brett, C.M.A., 1990. The application of electrochemical impedance

techniques to aluminium corrosion in acidic chloride solution. J.

Appl. Electrochem. 20, 1000–1003.

Brett, C.M.A., 1992. On the electrochemical behaviour of aluminium

in acidic chloride solution. Corros. Sci. 33, 203–210.

Brock, A.J., Wood, G.C., 1967. Hydroxyl ion and proton mobility

during anodic oxidation of aluminium. Electrochem. Acta 12, 395–

412.

Christian Vargel, 2004. Corrosion of Aluminium. Elsevier Ltd.,

NewYork.

Deepa, Prabhu, Padmalatha, Rao, 2013a. Corrosion inhibition of 6063

aluminum alloy by Coriandrum sativum L. seed extract in

phosphoric acid medium. J. Mater. Environ. Sci. 4, 732–743.


Deepa, Prabhu, Padmalatha, Rao, 2013b. Coriandrum sativum L. – a

novel green inhibitor for the corrosion inhibition of aluminium in

1.0 M phosphoric acid solution. J. Environ. Chem. Eng. (in press).

http://dx.doi.org/10.1016/j.jece.2013.07.004.

El-Neami, K.K.H., Mohamed, A.K., Kenawy, A.S., Fouda, I.N.,

1995. Inhibition of the corrosion of iron by oxygen and nitrogen

containing compounds. Monatsh. Chem. J. 126, 369–376.

El-Sayed, A., 1997. Phenothiazine as inhibitor of the corrosion of

cadmium in acidic solutions. J. Appl. Electrochem. 27, 193–200.

Foley, R.T., Nguyen, T.H., 1982. The chemical nature of aluminum

corrosion. J. Electrochem. Soc. 129, 464–467.

Fontana, M.G., 1987. Corrosion Engineering, third ed. McGraw-Hill,

Singapore.

Hart, R.K., 1957. The formation of films on aluminium immersed in

water. Trans. Faraday Soc. 53, 1020–1027.

Kosting, P.R., Heins, C., 1931. Corrosion of metals by phosphoric

acid. Ind. Eng. Chem. 23, 140–150.

Lenderink, H.J.W., Linden, M.V.D., De Wit, J.H.W., 1993. Corrosion

of aluminium in acidic and neutral solutions. Electrochim. Acta 38,

1989–1992.

Lorenz, W.J., Mansfeld, F., 1981. Determination of corrosion rates by

electrochemical DC and AC methods. Corros. Sci. 21, 647–

672.

Moon, S.M., Pyun, S.I., 1998. Growth mechanism of anodic oxide

films on pure aluminium in aqueous acidic and alkaline solutions. J.

Solid State Electrochem. 2, 156–161.

Moon, S.M., Pyun, S.I., 1999. The formation and dissolution of

anodic oxide films on pure aluminium in alkaline solution.

Electrochim. Acta 44, 2445–2454.

Morad, M.S., 1999. Influence of propargyl alcohol on the corrosion

behaviour of mild steel in H3PO4 solutions. Mater. Chem. Phys. 60,

188–195.

Muralidharan, V.S., Rajagopalan, K.S., 1979. Kinetics and mecha-

nism of corrosion of iron in phosphoric acid. Corros. Sci. 19, 205–

207.

Nnanna, L.A., Anozie, I.U., Avoaja, A.G.I., Akoma, C.S., Eti, E.P.,

2011. Comparative study of corrosion inhibition of aluminium

alloy of type AA3003 in acidic and alkaline media by Euphorbia

hirta extract. Afr. J. Pure Appl. Chem. 5, 265–271.

Obi-Egbedi, N.O., Obot, I.B., Umoren, S.A., 2012. Spondias mombin

L. as a green corrosion inhibitor for aluminium in sulphuric acid:

Correlation between inhibitive effect and electronic properties of

extracts major constituents using density functional theory. Ara-

bian Journal of Chemistry 5, 361–373.

Obot, I.B., Obi-Egbedi, N.O., Umoren, S.A., Ebenso, E.E., 2010.

Synergistic and antagonistic effects of anions and Ipomoea invulc-

rata as green corrosion inhibitor for aluminium dissolution in

acidic medium. Int. J. Electrochem. Sci. 5, 994–1007.

Oguzie, E.E., 2007. Corrosion inhibition of aluminum in acidic and

alkaline media by Sansevieria trisfasciata extract. Corros. Sci. 49,

1527–1539.

Paul, Sigwalt Juniere, M., 1964. Aluminium in the Chemistry and

Food Industries. British Aluminium Co. Ltd., London, p. 46.

Poornima, T., Nayak, J., Shetty, A.N., 2010. Corrosion of aged and

annealed 18 Ni 250 grade maraging steel in phosphoric acid

medium. Int. J. Electrochem. Sci. 5, 56–71.

Pyun, S.I., Moon, S.M., Ahn, S.H., Kim, S.H., 1999. Effects of Cl�,

NO�3 and SO2�4 ions on anodic dissolution of pure aluminum in

alkaline solution. Corros. Sci. 41, 653–667.

Reena Kumari, P.D., Jagannath Nayak, Nityananda Shetty, A., 2012.

Corrosion behavior of 6061/Al-15 vol. pct. SiC(p) composite and

the base alloy in sodium hydroxide solution. Arab. J. Chem. (in

press). http://dx.doi.org/10.1016/j.arabjc.2011.12.003.

Scamans, G.M., Jholroyd, N., Tuck, C.D., 1987. The role of

magnesium segregation in the intergranular stress corrosion crack-

ing of aluminium alloys. Corros. Sci. 27, 329–347.

Shao, H.B., Wang, J.M., Zhang, Z., Zhang, J.Q., Cao, C.N., 2003. The

cooperative effect of calcium ions and tartarate ions on the


http://refhub.elsevier.com/S1878-5352(13)00259-1/h0005























http://dx.doi.org/10.1016/j.jece.2013.07.004




































































corrosion inhibition of pure aluminum in an alkaline solution.

Mater. Chem. Phys. 77, 305–309.

Stansbury, E.E., Buchanan, R.A., 2000. Fundamentals of Electro-

chemical Corrosion. ASM International, Materials Park, USA.

Trowsdale, A.J., Noble, B., Harris, S.J., Gibbins, I.S.R., Thompson,

G.E., Wood, G.C., 1996. The influence of silicon carbide rein-

forcement on the pitting behaviour of aluminium. Corros. Sci. 2,

177–191.

Uhlig, H.H., Revie, R.W., 1991. Corrosion and its Control, third ed.

Wiley, Singapore.


Umoren, S.A., Obot, I.B., Ebenso, E.E., Obi-Egbedi, N.O., 2009. The

Inhibition of aluminum corrosion in hydrochloric acid solution by

exudate gum from Raphia hookeri. Desalination 247, 561–572.

Wernick, S., Pinner, R., Sheasby, P.G., 1987. The Surface Treatment

and Finishing of Aluminum and its Alloys. ASM International,

Ohio 1, 935.

Wit, J.H., Lenderink, H.J.W., 1996. Electrochemical impedance

spectroscopy as a tool to obtain mechanistic information on the

passive behaviour of aluminium. Electrochim. Acta 41, 1111–

1119.























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Corrosion behaviour of 6063 aluminium alloy in acidic and ... · The corrosion rate of 6063 aluminium alloy increased with an increase in the concentration of acid as well as with