arabian corroson

aand in alkaline media

Deepa Prabhu, Padmal

Department of Chemistry, Manipal

R ; accep

Phosphoric acid;

Sodium hydroxide;pedance spectroscopy (EIS) technique. The surface morphology was investigated using scanning

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

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

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

industries can be considered as one of the worst technical and Buchanan, 2000). Now-a-days more attention has been

Corrosion studies of aluminium and aluminium alloys havereceived 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

* Corresponding author. Tel.: +91 9880122680.E-mail addresses: [email protected], drpadmalatharao@

yahoo.com (R. Padmalatha).

Peer review under responsibility of King Saud University.

Production and hosting by Elsevier

Arabian Journal of Chemistry (2014) xxx, xxxxxx

King Saud

Arabian Journa

www.ksuwww.sciencecalamities of our time. Besides from its direct costs in dollars,

corrosion is a serious problem because it denitely contributes

paid to control the metallic corrosion, due to increasing use

of metals in all elds of technology.1. Introduction

Corrosion, which is an inevitable problem faced by almost all

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

the need to conserve the worlds metal resources (Stansburyin good agreement with each other. 2014 Production and hosting by Elsevier B.V. on behalf of King Saud University.Arrhenius theory and transition state theory. Suitable mechanism was proposed for the corrosionTafel polarization;

EISthat 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 increase18

ht

Pmeceived 21 November 2012

KEYWORDS

Corrosion;

6063 aluminium alloy;78-5352 2014 Productiontp://dx.doi.org/10.1016/j.arab

lease cite this article in preedia. Arabian Journal of Cs

imand hosti

jc.2013.0

ss as: Dhemistratha Rao *

Institute of Technology, Manipal University, 576104 Karnataka, India

ted 28 July 2013

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

tudy was done by electrochemical method, using Tafel polarization technique and electrochemicalORIGINAL ARTICLE

Corrosion behaviour of 6063ng by Elsevier B.V. on behalf of K

7.059

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

University

l of Chemistry

.edu.sadirect.coming Saud University.

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

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 also

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 area

2 P. Deepa, R. Padmalathaused 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 BalancePlease cite this article in press as: Deepa, P., Padmalatha, R. Corrosmedia. Arabian Journal of Chemistry (2014), http://dx.doi.org/10.10were 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 303323 K.

The potentiodynamic currentpotential 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 kHz0.01 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

-0.8

-0.7

-0.6

-0.5

E(V/

SCE)

log i (Acm-2)

0.01M 0.1M 1.0M

Figure 1 Tafel polarization curves for 6063 aluminium alloy in

different concentrations of H3PO4 at 30 C.ion behaviour of 6063 aluminium alloy in acidic and in alkaline16/j.arabjc.2013.07.059

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).

tcorrmm y1 3270 M icorrq Z ; 1

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

icorr is the corrosion current density in A cm2, q is the density

of the corroding material (g cm3), 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-

-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 32.4. 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 sampleTable 2 Results of Tafel polarization studies and EIS studies of 60

Tafel polarization

H3PO4 (M) T (K) Ecorr (V vs. SCE) icorr (105 A cm2) ba (V de

0.01 303 0.72 2.99 3.73308 0.73 3.39 3.56313 0.74 3.42 3.50318 0.77 3.76 3.69323 0.78 4.29 3.78

0.1 303 0.79 10.8 3.67308 0.80 12.8 3.70313 0.81 16.3 3.74318 0.82 18.1 4.07323 0.83 24.5 4.14

1.0 303 0.76 38.6 3.34308 0.77 50.5 3.43313 0.78 69.2 3.50318 0.80 96.0 3.69323 0.81 135.4 3.79

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

c1) bc (V dec1) tcorr (mm y1) Rp (X cm2) CPE (l Fcm2)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 129ion behaviour of 6063 aluminium alloy in acidic and in alkaline16/j.arabjc.2013.07.059

f 60

dec

4.96 49.59 8.3 123

4 P. Deepa, R. PadmalathaTable 3 Results of Tafel polarization studies and EIS studies o

Tafel polarization

NaOH (M) T (K) Ecorr (V vs. SCE) icorr (105A cm2) ba (V

0.05 303 1.55 40.4 4.84308 1.56 46.9 4.87313 1.57 52.6 5.21318 1.58 57.2 4.83323 1.59 59.8 4.45

0.25 303 1.57 209 5.04308 1.59 244 4.90313 1.60 281 5.11318 1.61 307 4.80323 1.62 331 4.93

0.5 303 1.59 326 4.92308 1.60 374 4.92313 1.61 424 5.03318 1.62 437 4.67323 1.63 455 4.92as 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),

0 200 400 600 800 1000 1200

0

200

400

600

800

1000

1200

-ZII (

ohm

cm

2 )

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.

Please cite this article in press as: Deepa, P., Padmalatha, R. Corrosmedia. Arabian Journal of Chemistry (2014), http://dx.doi.org/10.1063 aluminium alloy in NaOH.

EIS

1) bc (V dec1) tcorr (mm y1) Rp (X cm2) CPE (l F cm2)4.86 4.40 101.8 66

4.83 5.11 82.5 72

5.31 5.73 80.2 75

5.33 6.23 77.3 79

5.44 6.51 75.6 85

4.84 22.78 24.9 76

4.92 26.59 18.3 84

4.47 30.62 17.3 96

4.70 33.46 15.0 107

4.73 36.07 14.4 114

4.71 35.53 12.2 83

4.91 40.76 11.5 95

4.88 46.21 9.9 101

4.84 47.63 9.7 111sulphuric 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 lm 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 lm itself. The origin of the

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.


063

Corrosion behaviour of 6063 aluminium alloy 5Figure 5 Equivalent circuit t for 6inductive 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 lm (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, RLand Rt. RL is parallel with the inductor L. The polarizationresistance Rp and double layer capacitance Cdl are calculated

from Eqs. (2) and (3):

Rp RL Rt R1 R2; 2

Cdl C1 C2: 3

-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.

Please cite this article in press as: Deepa, P., Padmalatha, R. Corrosmedia. Arabian Journal of Chemistry (2014), http://dx.doi.org/10.10Al alloy in 1.0 M H3PO4 at 30 C.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 303323 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 303323 K. Figs. 8 and 9represent Nyquist plots for the same. From both gures it is

-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.


120 30oC 5.6 0.01M

6 P. Deepa, R. Padmalatha0 20 40 60 80 100 120

0

20

40

60

80

100

-ZII (

ohm

cm

2 )

ZI (ohm cm2)

35oC 40oC 45oC 50oCobvious that the corrosion rate increased with an increase intemperature. This results in the rapid dissolution of the oxide

lm, thereby enhancing the corrosion rate. This is attributableto the enhanced chemical lm dissolution due to enrichedphosphate ions, OH ions and depleted AlOH4 ions at thelm/solution interface. Thus with the increasing temperaturethe oxide passive lm becomes thin, porous and less protectiveas a result of dissolution of the lm 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.

lntcorr B EaRT

; 4

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.

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

expDSaR

exp

DHaRT

5

where h is Planks constant and N is Avagadros 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 aluminiumalloy 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

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

ln(C

R) (m

m y

-1)

1/T (K-1)

0.05M 0.1M

Figure 10 Arrhenius plots for the corrosion of 6063 aluminium

alloy in H3PO4.medium, value of energy of activation is greater than20 kJ mol1.

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 mol1 and Ea decreased with anincrease in sodium hydroxide medium concentration, whichis suggestive of concentration polarization as a main reactionat electrodeelectrolyte 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 DSadecreased 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

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

Corrosion behaviour of 6063 aluminium alloy 73.1x10-3 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 0.1M3.4. 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.

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.

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 mol1) DHa (kJ mol

1) DSa (J mol1 K1)

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 mol1) DHa (kJ mol

1) DSa (J mol1 K1)

0.01 13.1958 10.644 180.990.1 31.6439 29.0923 109.6521.0 50.2442 47.6926 37.8584

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

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 lm 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 lm of

amorphous c alumina which initially thickens on exposure to

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

8 P. Deepa, R. Padmalatha

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

AlH2O! AlOHads H e; 6

AlOHads 5H2OH ! Al3 6H2O 2e; 7

Al3 H2O! AlOH2 H; 8

AlOH2 X ! AlOHX: 9Thus 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.ion behaviour of 6063 aluminium alloy in acidic and in alkaline16/j.arabjc.2013.07.059

nium

Corrosion behaviour of 6063 aluminium alloy 9Figure 18 EDX spectrum of 6063 alumifollow 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 lm should

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

classied into a direct metal dissolution reaction by the move-ment of aluminium ions through the lm and an indirect metaldissolution reaction by consecutive oxide lm 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 lm formation proceeds electrochemi-cally by the incorporation of hydroxide ions into the lm

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

Al 3OH ! AlOH3 3e 10

Figure 19 EDX spectrum of 6063 aluminiu

Please cite this article in press as: Deepa, P., Padmalatha, R. Corrosmedia. Arabian Journal of Chemistry (2014), http://dx.doi.org/10.10alloy after immersion in 1.0 M H3PO4.The aluminium hydroxide lm formed electrochemically willbe dissolved chemically by OH- attack at the lm/solutioninterface:

AlOH3 OH ! AlOH4 11where AlOH4 represents the aluminate ions. A partial ano-dic dissolution reaction of aluminium in alkaline solutioncan be obtained by combining electrochemical lm formation

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

Al 4OH ! AlOH4 3e 12The 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 3=2H2O 3e ! 3OH 13and/or the water reduction reaction:

3H2O 3e ! 3=2H2 3OH 14

m alloy after immersion in 0.5 M NaOH.


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Corrosion behaviour of 6063 aluminium alloy 11Please cite this article in press as: Deepa, P., Padmalatha, R. Corrosmedia. Arabian Journal of Chemistry (2014), http://dx.doi.org/10.10ion behaviour of 6063 aluminium alloy in acidic and in alkaline16/j.arabjc.2013.07.059

Corrosion behaviour of 6063 aluminium alloy in acidic and in alkaline media1 Introduction2 Methods2.1 Material2.2 Medium2.3 Electrochemical measurements2.3.1 Tafel polarization studies2.3.2 Electrochemical impedance spectroscopy (EIS) studies

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

3 Results and discussion3.1 Tafel polarization measurements3.2 Electrochemical impedance spectroscopy (EIS) measurements3.3 Effect of temperature3.4 Scanning electron microscopic study3.5 Mechanism of corrosion process

4 ConclusionsReferences

arabian corroson

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