Evaluation of prolonged exposure of lacquered tinplate cans to a citrate buffer solution using electrochemical techniques

PROGRESS IN ORGANIC COATINGS ELSEVIER

Progress in Organic Coatings 30 (1997) 9-14

Evaluation of prolonged exposure of lacquered tinplate cans to a citrate buffer solution using electrochemical techniques J.M. Bastidasa-*, J.M. Cabaiiesb, R. Catalib hInstituto

“Centro National de Agmquimicu

de Imestigociones Metalrirgiccu, CSIC. Avda. Gregorio y Tecnologia de Alimentos. CSIC, Apartado de Correos

de1 Amo 8. 28040, Madrid, No. 73. 46100-Burjassot.

Spain Vaknciu.

Spain

Received 10 August 1995

Abstract Pour lots of tinplate cans, internally coated with water- and organic solvent-based epoxyphenolic lacquer, were studied. A 0.1 M citriccitrate test buffer solution (pH 3.5) was packed at 90°C sterilised at 12 I “C and stored at room temperature. A full opened can was used as the working electrode and electrolytic cell. Electrochemical a.c. and d.c. experiments were conducted after different time periods up to 280 days. The dissolution of tin and iron was determined by atomic absorption analysis up to 365 days. Lot C, whose lacquering was the thickest and the least porous, showed the greatest corrosion resistance to the citrate solution. Excellent agreement was obtained between the three experimental techniques utilized. The electrochemical impedance spectroscopy method supplies accurate information on the behaviour of the tinplate-lacquer-electrolyte system. 0 1997ElsevierScienceS.A. Keyvtwrds:

Tinplate; Citrate buffer: Electrochemistry

1. Introduction Tinplate cansare widely usedin the canning industry, and account for more than the 80% of the total. The most widely used method of reducing tinplate can corrosion is the application of organic lacquer coatings. Water-based coatings release very little organic solvent into the atmosphere[l]. The protective action of a lacquer tilm is determined by its physical-chemical characteristics.the method of application, and its compatibility with the packed product [2]. Analytical techniquesare required to study the protective action of lacquers. Unfortunately, conventional techniques do not allow for the proper-tiesof the organic coating in the tinplate-lacquer-electrolyte system to be characterized, and it is necessaryto use traditional packing and storage testswhich are expensive, take very long times and provide only limited information. The electrochemical nature of most of the phenomena involved in the protective action of organic coatings calls

* Corresponding author.

0300-9440/97/$ I7.00 % 1997 Elsevier PI1 S0300-944ni9h)00hh9-8

Science

S.A. All right\

reserved

for the application of electrochemical d.c. and a.c. techniques for their study. The advantage of the electrochemical impedancespectroscopy(EIS) technique over d.c. measurements is that it involves a small signal perturbation at the tinplate-lacquer interface [3]. The aim of this paper is to study with the use of electrochemical techniques the protective action of water- and organic solvent-based lacquers, applied on four lots of whole tinplate cans,in the presenceof a citrate buffer solution.

2. Experimental Four lots of lacqueredtinplate cansof commercialquality and size 0.5 kg (RO 425-73) were tested. Table 1 summarizes the characteristics of the cans studied. Porosity wasdeterminedby the application of 6.5 V versusa graphite rod electrode, usinga test solution with 1.5% NaCl and a 1% acetic acid. Thesedeterminationswere carried out in duplicate. The cans were filled with a 0.1 M citric-citrate buffer solution (pH 3.5) at 90°C. After filling (with minimum head

IO Table

J.M.

B C D

in Organic

Cocrrings

30 (1997)

1 Lot of the cans Lacquer

type

Coatmg

Epoxyphenolic water-based (golden pigmented) Epoxyphenolic solvent-based

Porosity (g rn-‘)

(mA)

8

33

7

282

16

(white pigmented) Epoxyphenolic water-based (golden pigmented)

1.105

3.16

SOLVENT-B.

1

15days

3.105

5.105

7.105 SOdays

days

I

c

8

7

183

space) the cans were closed and sterilised for 15 min at 12 I “C. After cooling, the cans were stored at room temperature (1%22°C). At 12, 15, 20, 50, 90, 120, 150, 200, 250 and 280 days, four cans from each lot were opened and studied using electrochemical techniques. In preparation for the electrochemical experiments, the top end of the filled can was removed and replaced by an acrylic cover hermetically sealed by a silicone O-ring to the can seam and fixed by metallic handles to a base of the same acrylic material placed at the bottom of the can. A conventional three-electrode cell was used. Four apertures were made in the cover to allow for the introduction of a saturated calomel electrode (SCE) as referLot

B:EPOXYPHENOLIC

““wq 20

(golden pigmented) Epoxyphenolic solvent-based

r

9-14

~12doys

studied

weight A

et al. / Prqren

I

Characteristics Lot

Bastidas

A: EPOXYPHENOLIC

REAL

IMPEDANCE,

f-l cm’

WATER-B.

2LrnWZ

Fig. 2. Evolution organic wlvent-based.

with

time

of Nyquist

plots

for

Lot

B. epoxyphenolic

3.105 POK"Z

i;J ,408tlZ

!

1.10'

3.105

5405

7.105

1.10'

3405

5.105

7.105

250days

REAL Fig. 1. Evolution water-baaed.

with

time

IMPEDANCE, of Nyquisl

plots

II

cm2

for

Lot

A. epoxyphenolic

ence. a platinum counter electrode, and two glass tubes for nitrogen inlet and outlet. The filled cans, with a surface area of 250 cm’, operated as the working electrode and electrolytic cell in the electrochemical experiments. An electrical connection was welded to the outside of the can. Prior to electrochemical testing, the citric-citrate buffer solution was deaerated for 2 h by bubbling nitrogen free of oxygen. The nitrogen was purified through a Cr,(SO& solution. Nitrogen was passed through the head space during the whole experiment. A Schlumberger electrochemical interface, model 1286. a Schlumberger frequency response analyzer, model 1253, a HP Plotter, model 7440A, and an IBM computer were connected. Impedance experiments were conducted at the rest potential, E,,,T: applying a sinusoidal signal of 10 mV amplitude, in the frequency range between 20 kHz and 1 mHz. The electrochemical experiments were conducted at room temperature. After the impedance tests, d.c. polarization curves were drawn by applying E,,,, + 300 mV. stabilising at EC,,,-300 mV for 10 min and reducing it progressively at a scanrate of 25 mV s-’ to produce the cathodic curve. When the EC,,, was reached,the polarity was reversedand the.anodic curve was obtained in an identical way. The intersection point of the

anodic and cathodic Tafel behaviour gives the corrosion current density, i,,, [4]. The citrate buffer content of the cans was analyzed for dissolved tin and iron at 20,280 and 365 days. The results of the 365 days test were obtained using unpolarized specimens. These elements were determined by atomic absorption analysis performed on a Perkin-Elmer, model 703.

Lot

D: EPOXYPHENOLIC

WATER-B.

3. Results Figs. l-4 depict a synthesis of the Nyquist plots obtained from the testing of identically prepared lacquered tinplate can samples exposed to a 0.1 M citric-citrate buffer solution, for periods of up to 280 days. Lots A and B (Figs. 1 and 2) depict one arc at high frequencies, accompanied by a straight line diffusion tail at low frequencies. After up to 50 days of experimentation, Lots C and D (Figs. 3 and 4) show similar behaviour to Lots A and B. However, after 90 days experimentation and thereafter Lots C and D show two arcs, at high and intermediate frequencies. accompanied by a straight line diffusion tail in the low frequency range. Fig. 5 shows the variation with time of the length of the chords associated with the high frequency arc (HFA) and the low frequency arc (LFA) for the four lots of lacquered tinplate cans in contact with a 0. I M citric-citrate buffer solution. Since the arcs were incomplete (Figs. I-4) an

1.105

ti

z-10'

C: EPOXYPHENOLIC

I

b;-:. I IO6

1

2-IO6

3.106

210"

3.106

i$~~ El A.10

2 106

organic

3. Evolution solvent-based.

3-106

l,lO'

250days

280 days

REAL Fig,

with

time

1MPEDANCE.n of Nyquist

3.105

IMPEDANCE,

R cm'

SOLVENT-E.

/ IZOdays r

1106

2-105

280doys

Fig. 4. Evolution water-based.

6

3105

I-105

REAL Lot

l-105 2IO5

3105

plots

1

cm’ for

Lot

C. epoxyphenolic

with

time

of Nyquist

plots

for Lot

D. epoxyphenolic,

extrapolation was performed. As can be seen the results show some scattering. The HFA and LFA chords for Lot C are around 1 x 1OhQ cm’ and for Lots A, B and D around 1 x 10jQcm’. Fig. 6 displays the evolution with time of capacitance for the four lacquered tinplate cans. Lots A and B show only one capacitance value for each Nyquist plot. Lots C and D depict two capacitance values after 90 days. The capacitance was calculated using the expression C = l/Rw,,,, replacing R with Ri and R, to obtain C, and Cd, respectively (see below), where a,,,, is the frequency at the highest point of the arc [S ]. Fig. 7 depicts the variation with time of the C,,,IC(O, ratio, where C,,, is the coating capacitance at the time t and C,,, the capacitance determined at the beginning of exposure, both C,,, and C,o, were obtained from the arc at high frequencies. The impedance of the coating at the beginning was higher than is permitted by the measuring equipment used. In this way, a period of lo- 12 days seems to be enough to obtain results: C,o, refers to 12 days exposure. Fig. 8 displays the variation with time of the Warburg coefficient 0. The straight line diffusion tails of Figs. l-4 made it possible to obtain the value of g (a = Z’,(w)“‘) from the real part of Warburg impedance: Z, = a( 1 -j)(w)-” [6]. It can be seen that Lot C shows u values in the range

12

J.M.

I

I

I

lo4

!

40

1

I

80

120

I,

Bastidas

I 240

1

160

et al. /Progress

2;O

,

in Organtc

Coatings

30 (1997)

9-14

TI ME,days

1 280

TIME,days

Fig. 7. Evolution Fig. 5. Evolution quency arc (LFA)

with time of high frequency arc (HFA) chords for the four can lots studied.

and low fre-

3 x lo’- 1.2 x lo6 Q s-l’* and Lots A, B and D in the range (2-8) x lo4 Q sP2. Table 2 shows the corrosion current density, i,,, results obtained by d.c. polarization curves. Table 3 summarizesthe chemical analysisof tin and iron. Theseanalyseswere carried out in duplicate.

4. Discussion The time dependenceof the Nyquist plots of the lac-

with time of C,,JC,,,, ratio for the four can lots studied.

quered tinplate cans allowed for damageto the coating to be monitored. In Lots A and B one arc chord (0.5-3) x lo5 s2cm’ (Fig. 5) and a tail remainedthroughout the 280 days of experimentation (Figs. 1 and 2). Lot D alsoshowssimilar behaviour during the first 50 days (Fig. 4). The behaviour of Lot C during the first 50 days of experimentation consisted of one arc chord, higher than Lots A, B and D at around (l5) x lo6 Q cm’ (Fig. 5), and a straight line tail (Fig. 3). After 90 days experimentation two arcs and a straight line diffusiontail are drawn for Lots C andD. The high frequency arc representsthe bulk properties of the coating and the low frequency arc and tail representthe interfacial properties. The equivalent circuit of Fig. 9a may explain the behaviour of the tinplate-lacquer-electrolyte system[ 1,3,5,7,8]. In this figure, R, is the ionic resistanceof the electrolyte between the reference electrode and the working electrode, C, is the capacitanceof the coating, R, is the ionic resistance of the coating impregnated with the electrolyte, Cd is the capacitance of the electrochemical double layer, R, is the charge transfer resistance of the corrosion process at the lacquer-metal interface, and Z, is the Warburg diffusion impedance.If the electrolyte doesnot penetrate the coating the parallel electrochemical double layer capacitance and charge transfer resistanceis unnecessary.This is the real situation at the beginning of exposure of the tinplate-lac-

1

I 40

Fig. 6. Evolution

I,

1

80

I

I,

120 160 TIME,days

with time of capacitance

I

I

200

,

,

240

,

lo’+

,

280

10

T I ME, days

for the four can lots studied.

Fig. 8. Evolution lots studied.

with time of the Warburg

coefficient

o for the four can

J.M.

Basridas

et al. /Progress

in Organic

Coarings

30 (1997)

13

9-14

Table 2 Corrosion

current

Lot

density,

i,,,

i,,,. results obtained

by d.c. polarization

curves

(PA cm-‘)

15 days

20 days

50 days

90 days

120 days

150 days

200

0.013

0.055 0.055 0.00 I

0.067 0.036 0.002

0.037 0.047 0.011

0.045 0.032 0.016

0.023 0.057

0.020 0.038

C

0.035 0.001

0.01 I

D

0.036

0.030

0.035

0.032

0.045

0.070

A B

quer system to an ionically conducting aqueous solution. As a consequence of the nature of the lacquer film and the handling of the can during the manufacturing process, it presents pores and defects through which the electrolyte can come into direct contact with the metallic substrate (Fig. 9b), increasing the current density of the corrosion process [ 91. At longer times (120- 150 days, Fig. 5) there is a fluctuation in Ri which may be attributed to the continuous blocking and unblocking of pores by corrosion products. By comparing the data for HFA and LFA chords (Fig. 5) with the capacitanceresults (Fig. 6), it can be observedthat the capacitance was significantly lower and the resistance was higher for Lot C than for Lots A, B and D. Consequently, better protection was offered by the Lot C lacquer in presenceof the aggressivecitrate buffer solution. On the other hand, the i,,, data obtained by d.c. polarization curves (Table 2) and chemical analysis (Table 3) agreeswell with the a.c. results of Fig. 5. At the sametime. theseresults are in agreementwith porosity data (Table 1). In general, the capacitance value increasesat the beginning followed by a more or less steady-state value and a further increase (Fig. 6). Similar results have been interpreted in the literature as water uptake in organic coatings

1101. The C(,JC,o,parameter has been related with the wetted surface area beneaththe paint [ 1,111.Fig. 7 showsthat the least water diffusion under the lacquer film occurred in Lots C and D. In Lot A some scattering was produced. It is possible that the high value of u (Fig. 8) masksthe high frequency arc. Fig. 8 showsthe Warburg coefficient results. It is known that u is inversely related to the diffusion coefficient D of the electroactive speciesthrough the pores of the coating film

days

days

250

280

0.008 0.012

0.004 0.025

0.011

0.011

0.012

0.032

0.010

0.012

days

[ 111.The value of D per the unit of coating surfacedepends on the size and numberof the pores.Consequently,the value of u meansthe barrier effect of the coating. At the end of the experiments somedetinning spotswere found beneaththe coating. As is shownby chemical analysis results (Table 3).

5. Conclusions A.c. and d.c. electrochemical resultsobtained with whole lacquered tinplate cans are similar to those obtained with lacqueredtinplate sheets.However, the whole can electrode takes into account all can circumstances:lacquer film porosity, protection of the seam,defects originated in the manufacturing process,etc., as well as the aggressivity of the test solution. Electrochemical impedance spectroscopy, d.c. polarization curves and chemical analysis (storage test) techniques yielded results with excellent agreement. Lot C, with the lowest porosity (8 mA) and the highest coating weight (16 g rn-‘), presentsthe best behaviour of the four lots of tinplate cans studied. The most representative parameter of the tinplate-lacquer-electrolyte system was the capacitance.The Warburg coefficient, u, is a promising parameterbecauseit permits the calculation of the diffusion coefficient, D. Consequently, the solubility, S, (S = P/D) of an electrolyte in an organic

Table 3 Electrolyte

Tin and iron storage results obtained by chemical analysis: 20 and 280 days results after polarization curves, 365 days results unpolarized cans Lot

20days (ppm)

280 days (ppm)

365 days (ppm)

Sn

Fe

Sn

Fe

Sn

Fe

14.35

2.3 5.15

27.75 62.8

2.0 9.75

66.6 90.75

5.6 II.05

0.55 3.9

22.15 53.85

6.5 6.65

26.15 61.15

Lacquer

(b)

A B C

D

18.4 2.08

21.6

9.05 8.9

Tin-free Tin alloy Steel

Fig. 9. (a) Equivalent circuit for the tinplate-lacquer-electrolyte (b) Damage of the tinplate-lacquer interface.

system.

coating can be calculated, once the permeability coefficient, P, is known. Further data will be forthcoming.

Acknowledgements The authors express their gratitude to the CICYT of Spain for financial support to this project No. ALI90-0740.

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