Electrochemical study of corrosion inaluminium cans in contact with soft drinks

L. Esteves1, E. M. Garcia2, M. das M. R. Castro1 and V. F. C. Lins*1

This work evaluates the corrosion resistance of the inner surface of the aluminium cans in

guarana, cola flavoured and tonic water soft drinks using electrochemical impedance spectro-

scopy (EIS) measurements. The less aggressive electrolyte was tonic water. One time constant

was identified for the aluminium can in contact with guarana and tonic water. In the most

aggressive medium of cola flavoured soft drinks, two time constants were identified and a charge

transfer resistance was obtained associated to a corrosion of aluminium in contact with the

electrolyte.

Keywords: Packaging, Aluminium, Soft drinks, Corrosion, Electrochemical impedance spectroscopy (EIS)

IntroductionThe various sectors of beverage packaging have broughtmany technological developments, aiming to increasethe packing properties, to reach the requirements forprotection of drinking and the conquest of markets andconsumer preference.

The metal, plastic and glass containers are progres-sively lighter. The cans exhibit different formats orreliefs, opening systems, and internal protection. Theplastic containers have shown significant improvementsin barrier properties aimed primarily at meeting therequirements of the protection of the beer. The glasscontainers have shown greater mechanical resistancecaused by a greater control of the thickness distribution,have received external coatings for the protection andimprovement of performance in relation to the break-down and introduction of new colours. Initially, thecontainers for storing carbonated beverages were madeof glass. The beer and the soft drink markets have beenmodified by the introduction of PET (polyethyleneterephthalate), tinplated steel and aluminium cans.

The corrosion process in metal packaging exhibits anelectrochemical mechanism. The electrochemical techni-ques for corrosion resistance evaluation are relativelyrapid, reproducible, reliable, and provide informationabout the corrosion mechanism involved.

The inner lacquer coating is the most widely usedmethod for reducing metal can corrosion.1 Several papersabout the corrosion process of steel and tinplate cans incontact with beverages and various types of food arefound in literature.1–10 Pournaras et al.10 detected flaws inthe organic coating layer using electrochemical impe-dance spectroscopy analysis, which were not identifiedneither with the naked eye or using microscopic analysis.

Detection of corrosion induced metal release fromtinplate cans has also been done using EIS, electroche-mical noise (EN), and inductively coupled plasma massspectrometer.2 According to our knowledge, EIS data ofaluminium cans in soft drinks electrolytes are not foundin literature. A study of evaluation of aluminium contentin beers was found in literature.11

The selected beverages were guarana, cola based andtonic water that are widely consumed soft drinks inBrazil. Cola flavoured soft drinks contain carbonatedwater, sugar, caffeine, extract of the cola nut, caramelcolouring, acidulant (phosphoric acid) and naturalflavouring compounds.12 Guarana soda should manda-torily contain a minimum of two hundredths of a gramof guarana seed (Paullinia gender), or its equivalent inextract, per hundred millilitres of beverage. Thetherapeutic properties of guarana and its extracts havebeen attributed to guaranine, which is a complex ofcaffeine and tannins or to a new xanthine naturalproduct.13 Quinine tonic water must contain three to fivemilligrams of quinine or its salts, expressed in anhydrousquinine per hundred millilitres of beverage.14,15 Tonicwater contains carbonated water, liquid sugar, extract ofquinine, natural aroma, conservatives and acidulants.

Thus, this paper has academic and technologicalrelevance, contributing to the elucidation of the mechan-isms of corrosion of the inner surface of aluminium cansin the midst of carbonated beverages using the electro-chemical impedance spectroscopy technique.

ExperimentalThe CaboQC carbonation measuring equipment, coupledwith the DMA 45 000 Anton Paar was used to measurethe content of carbon dioxide and Brix degree. The DM-32 Digimed conductivity measurement equipment and aDigimed pH meter Model DM-22 were used to measurethe conductivity and the pH of the beverage.

Electrochemical measurements were performed usingan AUTOLAB PGSTAT 302 with an impedancemodule, FRA and GPES software. The working

1Federal University of Minas Gerais, Antonio Carlos Avenue 6627, 13565-905, Brazil2Federal University of Sao Joao Del Rey, Brazil

*Corresponding author, email vlins@deq.ufmg.br

� 2014 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 8 February 2014; accepted 29 May 2014DOI 10.1179/1743278214Y.0000000197 Corrosion Engineering, Science and Technology 2014 VOL 49 NO 7 665

electrode used was a can of aluminium in a square5?065?0 cm form. An electrochemical cell with a nozzlesize of the area being examined of 2?2 cm2 was used.Measurements were made in triplicate, analysing differ-ent regions of the same can. This cell is pressed against asheet of aluminium can to avoid the appearance ofcrevices. The portion of the work electrode that was inelectric contact with the potentiostat was polished withsandpaper number 600 and washed with distilled waterbefore each experiment. The counter electrode was madeof platinum, with area equal to 3?75 cm2, and thereference electrode was Ag/AgCl. All experiments wereperformed without agitation of the electrolyte at a roomtemperature. The electrolytes used were guarana, colaand tonic water soft drinks. The following parameterswere selected after initial experiments for optimisation ofexperimental conditions: measurements with 200 points,the frequency range of 1 MHz to 50 mHz. The immersiontime of the aluminium can in the electrolyte was 20 days.The open circuit potential (OCP) was monitored for 1 hor up to stabilisation. Nine replicates of each test wereperformed, and the mean resistance value was calculated.

Results and discussion

Characterisation of electrolytesTable 1 shows the results of Brix degree, pH, conduc-tivity and CO2 content of the soft drinks studied. Colaflavoured soft drinks had the lowest pH and the highestvalues of conductivity, Brix degree and CO2 contentamong the electrolytes.

Open circuit potential measurementFigure 1 shows the measurement of the open circuitpotential versus time for tonic water beverages. Theopen circuit potential remained approximately constantor increased and then decreased as the time increased. Inthe last behaviour, a corrosion product layer wasproduced and then dissolved as the immersion timeincreased. Table 2 shows the average values of corrosionpotential of aluminium cans in the soft drinks studied.The highest corrosion potential of aluminium cans wasobserved for the tonic water beverage, highlighting thenoblest behaviour of aluminium in tonic water. Thelowest corrosion potential was observed for aluminiumin cola flavoured soft drinks.

Measurements were performed with in-nature sam-ples, without removal of CO2 to try to reproduce theservice life of the product. The CO2 can be degas duringthe experiment and can be replaced by O2, acceleratingcorrosion by contributing to the cathodic reaction ofoxygen reduction in acidic media.

Electrochemical impedance spectroscopy studyOptimisation

EIS studies of can electrodes were conducted initiallyusing a frequency range of 50 mHz to 104 Hz withamplitude of 10 mV. Data dispersion was observed for

the low frequency region of 50 mHz. Using potentialamplitude of 50 mV, the dispersion of data was reducedas shown in Fig. 2. Comparing values of impedanceusing potential amplitude of 10 and 50 mV at lowfrequency region similarity was observed, which indi-cated that the electrochemical impedance spectra meetsthe condition of linearity. On the other hand, Kramers–Kronig transforms could have been used for impedancedata validation because the K-K relation links the realand imaginary parts of the impedance.16 The amounts ofpoints were also increased from 100 to 200 points perfrequency decade to improve definition of diagrams.

Model developmentThe EIS data were fitted to different equivalentelectrochemical circuits. Tables 3–5 show the EIS resultsfor one replicate of the aluminium cans in contact withtonic water, cola flavoured soft drinks, and guarana softdrinks, respectively. A pure capacitor was used if n51and a constant phase element was utilised if n would beless than one. Nyquist and Bode diagrams of aluminiumcans in tonic water are shown in Fig. 3a and b. Bodediagram (Fig. 3b) indicates the presence of one timeconstant. Figure 4 shows the equivalent circuit pro-posed, considering that in some testing the constantphase element could be replaced by a capacitor. In thisless aggressive electrolyte, the resistance R2 is associatedto the ionic resistance through the polymer coatingimpregnated with the electrolyte.17 The phase angleassociated with the maximum peak is close to 90u,indicating the capacitive character of the process.

Another equivalent electrochemical circuit possibleshowed two time constants (Fig. 5) for the corrosion ofaluminium cans in cola flavoured soft drinks. TheNyquist diagram is shown in Fig. 6a and the BodeDiagrams (Fig. 6b) is shown in Fig. 6b. R2 is the ionicresistance through the polymeric coating and R3 is the

Table 1 Physicochemical parameters of soft drinks

Soft drinks Brix/uBx Content of CO2 pH Conductivity/mS

Cola flavoured 10.75¡0.20 3.6¡0.2 2.50¡0.1 1048.9Guarana 10.00¡0.20 3.1¡0.2 3.10¡0.2 439.3Tonic water 7.60¡0.20 3.3¡0.2 2.80¡0.2 831.1

1 Open circuit potential (OCP) of can electrodes versus

time in tonic water beverage

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666 Corrosion Engineering, Science and Technology 2014 VOL 49 NO 7

charge transfer resistance of aluminium in contact withthe electrolyte due to defects in polymeric coating andpores of aluminium oxide. Flaws in the organic layerand in the oxide aligned and in the same position, exposethe aluminium to the solution. The cathodic reactionsare the hydrogen reduction and the oxygen reduction inacidic media generating water as the carbon dioxide issubstituted by oxygen in solution during the analysis.

A diffusive control of the corrosive process was notobserved. The degassing of the solution during theexperiment can contribute to agitate the medium andminimise the diffusive control.

In measurements in SEM/EDS the aluminium con-centration decreases and the carbon concentrationincreases near the can surface.18 On the inner surfaceof this aluminium can there is a polymeric layer ofy3 mm.18

Table 2 shows the mean values of polarizationresistance of aluminium cans in contact with guaranaand cola based soft drinks, and in tonic water. Thealuminium cans in cola flavoured soft drinks showed thelowest corrosion potential and the lowest polarisationresistance. The aluminium cans exhibited the highestcorrosion resistance in the medium of tonic water.

In its composition, cola carbonated beverages havephosphoric acid as the acidulant and therefore has thelowest pH value (2?50) among other soft drinks studiedas shown in Table 1, and also have the highestconductivity (1048?0 mS), a higher level of dissolvedCO2 (3?6¡0?2) and a higher soluble solid content – Brix

Table 2 Average values of electrolyte resistance, andcorrosion potential and polarisation resistance ofaluminium cans in soft drinks

Soft drinks

Corrosion potentialof aluminiumcan mV(Ag/AgCl)

Polarisation resistanceof aluminium can/kV cm2

Cola flavoured 2683¡54 134¡9Guarana 2555¡43 8070¡564Tonic water 2493¡29 18 900¡1134

2 Nyquist diagram using frequency range of 50 mHz–

104 Hz with potential amplitude of 10 and 50 mV in

guarana beverage

Table 3 EIS parameters obtained for aluminium cans intonic water

Parameter Value % Error

R1/V cm2 2460 1.43CPE1-T/F sn cm22 1.0661028 0.37CPE1-P (n) 0.92 0.06R2/V cm2 1.046108 0.64x2 261023 …

Table 4 EIS parameters obtained for aluminium cans incola flavoured soft drinks

Parameter Value % Error

R1/V cm2 2172 2.05C (F cm22) 3.8961029 1.17R2 (V cm2) 16 594 2.23R3 (V cm2) 6.086106 0.34CPE2-T/F sn cm22 1.5261028 0.96CPE2-P (n) 0.89 0.17x2 1.0061023 …

3 Electrochemical impedance spectra: a Nyquist diagram

and b Bode diagram using frequency range of 50 mHz–

104 Hz with potential amplitude of 50 mV in tonic water

electrolyte

Table 5 EIS obtained for aluminium cans in guarana softdrinks

Parameter Value % Error

R1/V cm2 4628 5.12CPE1-T/F sn cm22 2.6261028 1.33CPE1-P (n) 0.86 0.24R2/V cm2 8.206106 0.99x2 9.1023 …

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(10?75¡0?20uBx), therefore the highest aggressiveness ofthe cola-flavoured soft drinks is justified. Tonic water iscomposed of herbal extract of quinine and exhibits alower level of Brix grade (7?60¡0?20uBx), showing thelowest aggressiveness among the beverages studied.Aluminium cans in tonic water showed the highestcorrosion potential and the highest polarisation resistance.

The electrochemical impedance spectroscopy is pro-posed as an efficient method of the quality evaluation ofthe polymer coated aluminium cans.

ConclusionsThe technique of electrochemical impedance spectro-scopy was able to distinguish the efficiency of organiccoating applied to the aluminium can, with regard to thebarrier property of the coating, and to distinguish theaggressiveness among the soft drinks. EIS results showedthe conditions of linearity, causality and stability in thebeverage electrolytes.

The less aggressive electrolyte was tonic water. Onetime constant is observed for the aluminium in tonicwater and guarana beverage. In the most aggressivemedium of cola based soft drinks, two time constantswere identified and in the equivalent circuit proposed tofit results, a charge transfer resistance is associated withthe Al/electrolyte interface.

Acknowledgements

The authors are grateful to the Brazilian governmentagencies – CNPQ, CAPES and FAPEMIG and Dr Johndi Fiore for revision of the manuscript.

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4 Equivalent circuit proposed for corrosion of aluminium

cans in tonic water

5 Equivalent circuit proposed for aluminium can in cola

flavoured soft drinks

6 a Nyquist diagram and b Bode diagram of aluminium

cans in cola based beverages

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