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An investigation on the use of indium to increase dissolutionof AlZn anodes in sea water

A. Zazoua *, N. Azzouz

Departement de Chimie Industrielle, Laboratoire Interaction materiaux-Environnement, Universite de Jijel, BP 98, Ouled Aissa, Jijel 18000, Algeria

Available online 26 January 2007

Abstract

This paper presents the production and the electro-chemical characterization (in open and in closed circuits) of a AlZnIn anode. Theaim is to use it as a protective material (cathodic protection) of metallic and immersed (in natural sea water) infrastructures.

The reactivity of this material was examined using two electrochemical techniques: Evans diagrams and I=f(E) curves in surround-ings of natural sea water. The optimised amount of indium (as the main element in the composition) was computed based upon corrosiontenue (the minimal dissolution rate value was aimed) of different samples having various content of this element. 2007 Elsevier Ltd. All rights reserved.

Keywords: Corrosion; Indium; Sea water; Aluminium anode

1. Introduction

In order to face the relative efficiency of mechanicalprotection (protection by surface coating) i.e., obstaclesbetween marine environment and metals scientists havedeveloped a new way of protection, based on considerationof the corrosion problem[1,2]. Why then should we try toprotect steel from corrosion if we can make it impossiblefor the reaction, that causes promotes metal destructions,to take place? In order to achieve this, we could artificiallydisplace the natural corrosion potential, either by makingthe metal react as an anode (anodic protection) or as acathode (cathodic protection)[3,4].

The present study concerns the realization and charac-

terization of a sacrificial anode (of the AlZnIn type, witha new formulation) used for the cathode protection ofmetallic structures that are immersed in natural sea water.The electrochemical study consists in quantifying the reac-tivity of this material in open circuit or closed circuit.

2. Experimental

2.1. Anode elaboration

The composition of our material has been obtained usingthe following raw materials (in predetermined quantities):

1. Pieces of AlZn alloy at 98% aluminium and traces ofiron and silica.

2. ZAMAC alloy with the following composition: 11% Al;1% Cu; 0.025% Mg; 88% Zn.

3. Indium welding alloy with the following composition:32.5% Bi; 16.5% Sn; 51% In.

The addition of sample quantities of calcium carbonateand sodium chloride has been used to decrease the meltingtemperature of the mixture[5].

The cylindrical-shape anode has been elaborated byclassical moulding in green-sand moulds.

2.2. Preparation of samples

The test samples were prepared to make their surfaceelectrochemically active. On the samples, we predeterminedwork area to be 1 cm2 surface.

0261-3069/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.matdes.2007.01.010

* Corresponding author.E-mail address:[email protected](A. Zazoua).

www.elsevier.com/locate/matdes

Available online at www.sciencedirect.com

Materials and Design 29 (2008) 806810

Materials& Design
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2.3. Intrinsic behaviour of the anode in natural sea water

(open circuit)

The reactivity of the samples (six samples from differentmouldings) was tested in natural sea water by plotting curveslog (I) = f(E). The potentiodynamic polarisation experi-ments were conducted with a scan rate of 1 mV. s1. Thestarting potential was 1100 mV/SCE. The experimentaldevice to draw the curves is traditional device with three elec-trodes. To achieve this, a PotentiostatGalvanostat typePGP 201 piloted by the VOLMASTER 4 software was used.

2.4. Behaviour of the galvanic-coupling anode with thestructure to be protected (closed circuit)

The anodesteel coupling in sea water by the Evansmethod was studied. To this purpose, three surface ratiosof the anodecathode were considered. This allows us toaccess the maximum battery current as well as the dualpotential [6]. All of the tests were performed at ambienttemperature without agitation.

3. Results and discussion

3.1. Tafel curves (open circuit)

The dissolving potentials (of corrosion) in open circuit(without galvanic coupling) are located in a spectrumbetween 911.1 and 975.9 mV measured versus the satu-rated calomel electrode (SCE).

The potentials values correspond to 956.1 and1020.9 mV/Ab/AgCl, which means an average potentialof about 988/Ag/AgCl.

This value is electronegative than that of unalloyed alu-minium in sea water, which is 795 mV/Ag/AgCl. That isdue to zinc which lowers the potential towards more elec-tronegative values. According to Laque[7]and Vargel [8]the zinc element confers to aluminium more electronegativepotentials. The resulting average potential is similar to thevalue found by others researchers for zinc-alloyed alumin-ium[8]. According to Vargel, based on the Pourbaix Dia-gram, too electronegative potentials (i.e., potentials thatgo beyond the stability spectrum of water) leads to cath-ode corrosion risks.

3.2. Density of the corrosion current (dissolution in open

circuit)

The extrapolation of the Tafel curves to the corrosion

potential (of dissolution) permits to obtain the corrosion

current densities. The obtained values for the six samplesvary between 5.17 and 6.88 lA cm2, with an average of5.91 lA cm2.

The results obtained by Reboul[9] for aluminium ser-ies 7000 (zinc alloy at 4.5% to 6% without copper) are val-ues of 1.001.50 lA cm2 and potentials of about 1020 to1100 mV (Ag/AgCl) in a sodium chloride solution havingNaCl content similar to that of natural sea water.

Consequently, our material is almost four times morereactive (in open circuit), because of the presence ofindium, which accelerates the dissolution of aluminium.(Tin and mercury were abandoned for reasons of efficiency

and pollution). Let us note that in the preceding figuresthat for the current density values correspond penetrationvalues in mm an1.

The application of Faradays Law to the average currentdensity gives the mass loss:

M 1=96500 27=3 i t

Knowing that i= 5.91 lA cm2, t = 1 year, we get a massof 173.8 g.

This result is in accordance with those mentioned in thebibliography [10] for a more aggressive environment (seeFigs. 17).

3.3. The evans diagrams (closed circuit)

The set structure to be protected anode works like abattery in the presence of a conducting electrolyte: It is gal-vanic corrosion. The Faradays Law applies to it. Thus, itis necessary to experimentally determine the current densityof this battery. We can also access its dual potential. Thecouplings current density, which gives the importance ofgalvanic corrosion, depends on several factors [7] amongwhich are:

Nomenclature

i currents (A)E potential (mV)

m mass (kg)t time (year)

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0

E (V)/CSE

LogI(A/cm)

Fig. 1. log I=f(E) curve (sample 01).

A. Zazoua, N. Azzouz / Materials and Design 29 (2008) 806810 807


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(a) nature of the electrolyte,(b) polarization phenomena,(c) surface ratio of the two metals,(d) distance,(e) temperature.

A comparison of the dissolution potentials of the alu-minium alloys can lead to some aberrations[8]. For exam-ple, we prefer alloys of 2000 family (copper alloys) whosedissolution potential is a lot more electronegative (about650 mV) than those of 5000 family (magnesium alloys)

which have more electronegative potentials (around

800 mV). The reason is that the latter ones have an excel-lent behaviour (resistance) to corrosion, whereas the first

ones are very sensitive to corrosion in natural environment.

-1

-0.5

0

0.5

1

1.5

2

2.5

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0

E (V)/CSE

LogI(A/cm)

Fig. 2. logI=f(E) curve (sample 02).

-1

-0.5

0

0.5

1

1.5

2

2.5

3

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0

E (V)/CSE

LogI(A

/cm)


-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0

E (V)/CSE

LogI(A/cm)


-1

-0.5

0

0.5

1

1.5

2

2.5

3

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0

E (V)/CSE

LogI

(A/cm)


-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0

E (V)/CSE

LogI(A/cm)


Fig. 7. Variation of corrosion speed of the Al type 1050 in NaOHsolutions.

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Consequently, it is more efficient to measure the densityof the galvanic current.

The results obtained are shown inFig. 8.From these results, one notices that the battery func-

tions under cathode control (cathodic polarization). Onthe diagram, shows a dual potential of916.7 mV/SCE(that is 961.7 mV/Ag/AgCl) and a density of the dissolu-tion current of 0.237 mA cm2. As to the current density0.237 mA cm2 or 2.37 A m2, the application of Fara-days Law gives an annual mass loss of 6.97 kg (that isthe equivalent of 2.94 kg per each Ampere).

As mentioned above, the surface ratio anodecathodeplays an important role in the anode reactivity. As the bat-tery functions under cathodic control, we have voluntarilyincreased the cathode surface up to 13.5 and 27 times the

anode surface in order to check this dependence. Theobtained results are shown inFigs. 9 and 10:

Indeed, the current densities have increased with anincrease of the cathode surface by from 0.854 to2.243 mA cm2. This lead to an increase of mass loss of,respectively 25.11 and 65.97 kg (that is annual amountsof 2.940 and 2.941 kg for an Ampere).

3.4. RDX characterization

The material has been studied by ray diffraction X in

order to determine impurities or additions elements suscep-tible to be present in the matrix. The sample presents twotypes of structures: in majority phase, it is the alloy, Alcubic centered phase which can be substituted by Zn or

Fig. 8. Evans diagram galvanic coupling (surface ratio 1/1).

Fig. 9. Evans diagram galvanic coupling (surface ratio 1/13.5).

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Cu. The rays of BiO3are in fact the rays of indium shifted(seeFig. 11).

4. Conclusions

The study aimed at the reactivity of an AlZnIn anodeused for the cathodic protection of a metallic installationthat is immersed in natural sea water. To this purpose,two electrochemical techniques have been used to charac-terize this material in natural sea water solutions that arepicked up regularly to achieve our tests.

The obtained results have revealed that indium favourssignificantly the reactivity of aluminium in natural seawater, which gives anode a better yield in the cathodic pro-tection of immersed marine installations.

References

[1] Shibli SMA, Gireesh VS, George S. Corros Sci 2004;46:81930.

[2] Shibli SMA, Gireesh VS. Appl Surf Sci 2003;219:20310.[3] Liou YM, Chung SC, Tsai WJ, Shih SC. Corros Prevent Contr

2000;47:62.[4] Schaffer GB, Huo SH, Drennan J, Auchterlonie GJ. Acta Mater

2001;49:2671.[5] Venugopal A, Raja VS. Corros Sci 1997;39:2053.[6] Baptistia W, Jose CM. Mater Perform 1999;38:34.[7] Laque FL. Marine corrosion causes and prevention. New York: John

Willey; 1975.[8] Vargel C. Ingenieur Conseil en Corrosion de laluminium. Corrosion

de laluminium. Dunod; 1998.[9] Reboul M. Methodes de mesure de la corrosion generalisee de

laluminium et de ses alliages. Revue de lAluminium1973;419:399412.

[10] Oldfield JW. Electrochemical theory of galvanic corrosion. ASTM

STP 1988;978:522.

Fig. 11. Determination of impurities and additions elements by ray diffraction X.

Fig. 10. Evans diagram galvanic coupling (surface ratio 1/27).

810 A. Zazoua, N. Azzouz / Materials and Design 29 (2008) 806810

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