THE APPLICATION OF AC IMPEDANCE TO STUDY THE PERFORMANCE ...



THE APPLICATION OF AC IMPEDANCE TO STUDY THE PERFORMANCE OF LACQUERED ALUMINIUM

SPECIMENS IN ACETIC ACID SOLUTION

J.D. Scantlebury and * K.Galiæ

Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O.Box 88, Manchester, M60 1QD,

United Kingdom

*Faculty of Food Technology and Biotechnology, Laboratory of Physical Chemistry and Corrosion, University of Zagreb, Pierottijeva 6,HR-10000 Zagreb, Croatia

Abstract

Auminium tubes with single and double coat solvent based and water based lacquers, based on epoxy-phenolic resins, were analysed. To determine the electrochemical parameters to correlate with the actual behaviour of a collapsible tube, impedance spectroscopy was used. The measurements were performed in 3% (v/v) acetic acid, at room temperature. After impedance measurements specimens surface were analysed by Scanning Electron Microscopy (SEM). As the coating degraded from penetration of electrolyte a rapid increase of coating capacitance, Cp, (from 10-10 to 10-8 F cm-2) and a rapid decrease in pore resistance, Rp, (from 108 to 106 (cm2) occurred after 192 hours of immersion. Analysis of data from impedance measurements, such as Rp, Cp, and delaminated area (Adel) , enabled determination of coating performance exposed to an aggressive media.

Key words: Acetic acid, AC impedance, aluminium, delamination, epoxy/phenolic resins, immersion test,

Introduction

Food manufacturers are requiring better performance from food can lacquers especially those aimed as aggressive foodstuff packaging. At the same time there has been a significant change from organic solvents to water based lacquer systems which requires better methods for analysing lacquered aluminium corrosion behaviour. Since interactions between the food products and the cans are essentially electrochemical in nature, electrochemical tests are highly suitable for rapid testing of the behaviour of metal cans (tubes), particularly lacquered ones.

The processes occurring at the metal-electrolyte interface are numerous and different in nature. The presence of an organic coating makes the system even more complex, as the electrical and electrochemical properties of the coating are introduced. As explained by Tait (1( traditional DC measurements cannot fully describe the situation. A system like this may be analysed and measured by impedance techniques. The applications of AC impedance to food packaging materials has shown the importance of electrochemical parameters, enabling both lacquer quality and corrosion evaluation in the presence of aggressive food products, to be analysed (2-4(.

The aim of this work was to investigate the corrosion performance of lacquered aluminium tubes used for tomato pureé packaging. In order to simulate aggressive conditions found in such a food product 3 % acetic acid was used as an electrolyte.

Experimental

Aluminium (purity = 99.7 % ) tubes ( ( 3.5 x 18.5 cm, P = 2.03 dm2) lacquered with epoxy-phenolic resins were obtained directly from a producer (“TOP”, Zagreb). Two sets of specimens were obtained:

a/ aluminium tubes with solvent based and

b/ water based lacquer.

Both sets of specimens were available as a single (5(m) and double coated (10 (m) aluminium tubes.

In order to verify coating thickness, ten measurements were made along a length of the test specimens, before and after removal of lacquers using "cellosolve" as reagent. The thickness specified for the measured specimens represents an average thickness of coating on duplicate samples measured by micrometer.

Specimens:

a/ Al/single-coat solvent based lacquer (S1x)

b/ Al/double-coat solvent based lacquer (S2x)

c/ Al/single-coat water based lacquer (W1x)

d/ Al/double-coat water based lacquer (W2x)

The working electrodes were prepared by cutting the tubes into sheets, masking the edges so the final active surface area was 35 cm2.

The test solution was 3 % v/v acetic acid (pH = 2.8) in distilled water which is, according to EEC rules (5( specified as appropriate to simulate an aggressive products such as tomato paste. Measurements were performed at room temperature.

A two identical electrode system, galvanostatically controlled (the applied potential was set to zero volts, assuming that the potential of the working electrode is its corrosion potential) which is recommended to use in a cases of high resistance systems (6(, was used for impedance measurements (Four Channel Frequency Response Analyser, Solartron Schlumberger 1254, plus potentiostat ACM P325 VI, combined with a computerised system for data analysis and storage).

Impedance measurements were carried out by applying a sinusoidal signal of 15 mV in amplitude in a frequency range of 10 kHz -10 mHz .

In order to evaluate the impedance changes of the system with time, specimens were left immersed in 3 % acetic acid (v = 500 ml) and analysed at regular intervals. The obtained impedance data represent average of duplicate samples.

Scanning electron microscopy (SEM), coupled to X-ray aluminium mapping was performed after making the impedance measurements.

From the impedance data, the capacitance and resistance of the paint film were determined by fitting a simple RC circuit to the impedance data using the appropriate software. In order to evaluate the impedance changes of an electrochemical cell with frequency, it is reasonable to consider a hypothetical equivalent circuit which is a combination of elements of the system under corrosion. If the circuit consists of a metal coated with an organic film, the equivalent circuit can be represented as in Figures 1a and 1b, where Ro represents the resistance of the electrolyte, Rp is pore resistance due to electrolyte penetration, Cp is the capacitance of the intact coating layer, Rct represents the charge transfer resistance and Cdl is the capacity of the faradaic reaction at the metal/electrolyte interface.

Modeling of impedance data for coating delamination was performed by Haruyama et al (7( who suggested that the decrease of Rp and Rct and the increase of Cdl with exposure time were caused by an increase of delaminated area (Adel) according to:

Rp = Rpo / Adel /1/

Rct = Rcto / Adel /2/

Cdl = Cdlo Adel /3/

where

Rcto = ( d ((cm2) /4/

Rct (( cm2) and Cdlo ((F cm-2) were characteristic values for the corrosion reaction at the coating / metal interface, which was assumed not to change with coating degradation; d was coating thickness; ( was the coating resistivity

Using a value for Cdlo = 20 (F cm-2 (8( and experimentally measured Cdl (F) as a function of exposure time, the delaminated area, as well as delamination ratio (D) were calculated for all investigated samples.

The delamination ration, D, was calculated as:

D = Adel / A /5/

where A was the total exposure area of the polymer coated metal.

Results and Discussion

As can be seen from Figure 2 there was a difference between the samples after two hours of exposure in 3 % acetic acid. On immersion the pore resistance of the specimens, having two coats of epoxy/phenolic lacquer (Figure 3) exceeded value of 1.4 x 108 (cm2. These samples, S2x and W2x, showed lower Cp values (Figure 4) compared to samples which had thinner coating layer. A specimen with a single coat solvent based lacquer, S1x (Figure 2), showed high frequency semicircle which can be attributed to paint film properties as it was associated with a capacitance of 1 x10-9 F cm-2 (Figure 4) while Rp value was 6 x 107 ( cm2 (Figure 3). Low frequency semicircle showed Cdl value of 1.5 x 10-8 F cm-2 (Figure 5) and is related to the degree of delamination and / or corrosion (7-10(.

After 24 hours of immersion (Figure 6) the semicircle diameter decreased for all specimens.

It is widely believed that degradation and loss of adhesion is indicated by a rapid increasing Cp and a rapid decrease in Rp [11,12]. According to many authors [13,14] the systems retain their corrosion protection while the coating resistance remains high (108 to 109 ( cm2 ), but fail when the resistance is low (below 107 ( cm2). Poor coatings are associated with measurements of 106 ( cm2.

Mansfeld et al. [11] suggested that the observed decrease of pore resistance with exposure time is due to damage of the coating and the formation of conductive paths.

The results obtained for all investigated samples, with a long immersion time (timm>96 hours), showed a distinct separation between the semicircles (Figure 7, as all samples showed similar behavior, S2x sample is used as representative), where the low- frequency semicircle decreased indicating corrosion of the substrate through the blistered paint film which was confirmed by SEM analysis (Figures 8 and 9). Similar behaviour has been observed by many authors [15-17].

James [18 ] provided evidence that blistering of the coatings when exposed to water is caused by an osmotic mechanism and that slowly evaporating, hydrophilic solvents retained in the coatings could be responsible. It was shown by Funke [19] that high-boiling hydrophilic, water-miscible solvents, such as glycol ethers, tend to produce this kind of blister formation.

In our previous paper [20] it was shown that the amount of residual solvents was higher for the solvent based specimens (1.877 mg m-2) compared with water based specimens (0.904 mg m-2) with ethylene-glycol-monoethyl-ether concentration being higher for the solvent based specimens. This could be one of the possible explanation for the larger number of blisters formed on the surface of the solvent based lacquer (Figure 8) compared with water based lacquer (Figure 9) having two coats of epoxy/phenolic lacquer.

Kojima and Watanabe (21( found that water based and solvent based epoxy/phenolic coatings show different mechanism of adhesion to the substrate although both coatings possess excellent adhesion performance.

Samples with single coat of epoxy/phenolic lacquer showed no blisters formed after 528 hours of exposure which could be due to better adhesion of thinner coating to the substrate.

As our earlier investigations showed [20] both sets of specimens showed certain porosity, expressed either in number of pores having different sizes (chemical method) or in current density values (electrochemical method). The consequence of external defects on the coatings was a rapid decrease of pore resistance (Figure 3) due to entry of acetic acid solution into the capillaries.

The laws of permeation suggest that when a penetrant molecule passes through a polymer, three steps (22( are involved: the adsorption on and dissolution in the polymer, followed by diffusion through the polymer and finally desorption at the other side. The first step depends on polarity, i.e. a polar penetrant molecule dissolves best in a polar polymer. Molecular movement, the second step, depends on size and shape of the penetrant and the concentration gradient. In this case water permeated through the polymer together with the carboxylic acid and interfered with the adhesive forces between the two layers, producing H3O+. At the low pH (2.5) the protective aluminium oxide layer is dissolved under reaction with acetic acid (23( which induce adhesion losses for the coating and electrochemical underfilm corrosion. According to Sotoudeh et al. (24( the corrosion rate of aluminium alloy in acetic solution is low but appreciable.

The result of water uptake by the coating (11, 25,26( was the increase of Cp (Figure 4). For epoxy coating, an increase in Cp values, after immersion in an electrolyte, reaching a constant value after some hours, was found by many authors [27-29]. At the end of exposure, Cp was lowest for S1x followed by W2x while S2x and W1x showed identical and the highest values.

A continuous increase of the double layer capacitance, Cdl, (Figure 5) was evidence that the area at which delamination occurred increased (Figure 10). This increase although occurred first for S1x sample (Figure 10), after 192 hours remained unchanged and the lowest, while at the same time Rp value increased (Figure 3). This increase was probably due to aluminium acetate formation, which is slightly soluble, (24( causing "blockage" of exposed substrate through the pores in the coating. In addition aluminum acetate forms hydrates and binds 0.5 - 2.5 molecules of water per molecule (30(. At this point, some type of adsorbed intermediate could be responsible for the inductive loops appearance (31(, which can be seen in Figure 7.

Therefore, the magnitude of change in Cp as a function of time is a qualitative measure of water uptake. A large water uptake often is an indicator that a coating is porous and has a high probability for early deterioration.

Further studies would be necessary to establish mechanism of adhesion to the substrate and species of lacquer composition which are likely to entrance corrosion.

Conclusions

Impedance spectroscopy can be applied to characterise coatings on aluminium in an acidic solution.

Pore resistance values decreased during immersion, indicating constant reduction of the protective properties. The change of coating capacitance and pore resistance with exposure time, to a corrosive medium, gives information on electrolyte uptake of different coating systems.

Based on the highest Rp and the lowest Cp values as well as the lowest delamination ratio, single coat solvent based, S1x, sample provide the best corrosion protection properties of an epoxy / phenolic coated aluminium samples in an acetic acid.

Acknowledgments

K.Galiæ would like to thank The Royal Society of Chemistry for provision of a J.W.T.Jones Travelling Fellowship.

The same author would also like to thank the Corrosion and Protection Centre, UMIST for provision of research facilities as well as to colleagues who provided invaluable advice and assistance with the analysis of the results.

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Fig.1. Equivalent circuit for a/ dry organic coating and b/ defective organic coating

a) b)

Cp

Cp

Cdl Ro

Rp

Rp

Rct

Fig.8.

Scanning electron micrograph of aluminium sample having

double coat solvent based lacquer, after impedance measurements. Exposure time: 528 hours in 3% acetic acid.

Fig 9.

Scanning electron micrograph of aluminium sample having

double coat water based lacquer, after impedance

measurements. Exposure time: 528 hours in 3% acetic acid.

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