Electrochemistry in Corrosion Control for Food Packaging

Electrochemistry in Food Packaging Corrosion Control – Electrochemical Impedance Spectroscopy of Coated Aluminum Samples The shelf life of food and beverages produced in the food and beverage industries depends largely on the packaging method and the type of packaging material used. Metals stand out as ideal materials for this purpose due to their ability to be coated with different passivating food-grade layers. Metal packaging provides excellent physical protection and barrier properties during both heat treatment and storage stages, while also offering an excellent combination of formability, decorative potential, recyclability, and consumer acceptance. Although many different material combinations exist in metal packaging, steel and aluminum are the most commonly used materials in the food and beverage industries. Aluminum in particular has various forms of application.

Pure Aluminum

Aluminum, typically used for tin cans, foil, laminated paper, or plastic packaging, is a lightweight, silvery-white metal obtained from bauxite ore and is found in the form of a combination of alumina and oxygen. Unlike many metals, aluminum is highly resistant to many types of corrosion; its natural aluminum oxide coating provides highly effective protection against air temperature, humidity, and chemical degradation effects. Aluminum provides an excellent barrier against moisture, air, odor, light, and microorganisms, while also possessing good flexibility and surface resistance, excellent malleability and formability, and exceptional embossing potential. Aluminum is also an ideal material for recycling processes since it is easily recycled and converted into new products. Pure aluminum is typically used for lightweight packaging of beverage cans, pet foods, seafood, and pre-threaded closures.

Aluminum Foil

Aluminum foil is obtained by wrapping pure aluminum metal in very thin layers and then rolling it to give it gauging properties (when a hollow or fold forms on a film, it remains in place). Thus aluminum foil can be folded quite tightly. Aluminum foil exists in a wide range of different thicknesses, from thin layers used for wrapping food to thicker sheets used for laying on trays. As with all aluminum packaging, aluminum foil provides excellent barrier protection against moisture, air, odor, light, and microorganisms, is inert to acidic foods, and requires no polish or protection.

Aluminum Laminates and Metallized Films

Packaging lamination is generally based on the principle of bonding aluminum foil to paper or plastic film and thus improving barrier properties. While lamination to plastic provides thermal insulation, it does not completely prevent moisture and air penetration. Due to the relatively high cost of laminated aluminum, this technique is generally used for packaging high-value foods such as dried soups, spices, and herbs. A more economical alternative to laminated packaging is metallized films. Metallized films are plastics with a thin aluminum metal layer (Fellows and Axtell 2022). These films provide improved barrier properties against moisture, fats, air, and odors, and the highly reflective surface of aluminum is quite attractive to consumers. Metallized films are used predominantly in snack packaging because they are more flexible than laminated films. Although it is technically possible to recycle the components of laminates and metallized films separately, the difficulty in separating these materials makes economically viable recycling difficult.

Corrosion

Corrosion is a general term used to describe the process of metal degradation. A significant portion of corrosion events are electrochemical in nature, with at least two reactions occurring on the metal surface. The first of these reactions is oxidation, also called the anodic partial reaction (e.g., dissolution of iron), while the other is a reduction reaction called the cathodic partial reaction (e.g., reduction of oxygen). The products of electrochemical reactions can also react with each other in a non-electrochemical manner to form the final product. The use of metal in food and beverage packaging processes can be affected by corrosion processes. Corrosion can occur both on the side of the packaging in contact with the atmosphere and on the side exposed to product contents, and therefore metal packaging surfaces are subjected to various coating and/or polishing treatments. One of the most common ways to examine corrosion processes in metal packaging is to evaluate the electrochemical structure of the metal packaging and/or coatings used. In this context, Electrochemical Impedance Spectroscopy (EIS), with its fast results and non-destructive nature, emerges as one of the interesting analysis techniques used in the coating industry. Electrochemical impedance spectroscopy (EIS) is a powerful technique used to investigate the linear response of a system perturbed by an alternating voltage (potentiostatic EIS) or current (galvanostatic EIS). EIS is commonly applied to obtain information about polarization resistance and time constants of corrosion processes. In this review article, we will examine EIS analyses performed on three different aluminum plates with different coatings.

Experimental Content

In this study, three different aluminum plate samples coated with Al2O3 were tested. Two of the samples have additional coating layers. Due to the lack of detailed information about the exact composition of the additional coatings, these two coatings will be referred to as "pink" and "gold" coatings. [caption id="attachment_129606" align="aligncenter"] Table 1: Equipment and software used[/caption] Each plate was cut into 1.5 cm diameter discs to fit into the sample holder of the 1 L Metrohm Autolab corrosion cell compliant with ASTM standards shown in Figure 1. The area exposed to measurement was set at 1 cm2. One side of each sample was polished with 600 grit sandpaper to remove the coating layers and provide good electrical contact with the sample holder. All measurements were performed in artificial seawater composition obtained by dissolving 33 g NaCl in one liter of ultrapure water. [caption id="attachment_129607" align="aligncenter"] Figure 1: 1 L corrosion cell and PGSTAT204 with integrated FRA32M module[/caption] A three-electrode system was completed using a stainless steel counter electrode and an Ag/AgCl reference electrode filled with 3 M KCl electrolyte. All potential values in this study are reported with respect to the potential of the reference electrode. Electrodes were connected to a Metrohm Autolab PGSTAT204 potentiostat/galvanostat system with an integrated FRA32M impedance module shown in Figure 1 for measurements.

Procedure

Stepwise dissolution measurement (SDM) is an electrochemical technique used to test the corrosion properties of aluminum solder plates. The SDM technique briefly includes the following steps: • Open circuit potential (OCP) is recorded for five minutes. • The potential is increased by 20 mV versus OCP and the system is polarized for 30 minutes. • OCP is recorded for five minutes. The last two commands are repeated 12 times over a period of approximately 7 hours. A plot of cycle number versus OCP is created. In this application, EIS measurements were taken before and after SDM. A 10 mV (rms) amplitude signal was applied to the open circuit potential (OCP) in the frequency range of 100 kHz to 10 Hz, with logarithmic frequency steps and 10 frequencies per decade. Data before and after SDM were compared and fitted with appropriate equivalent circuits. SDM, EIS procedures, and data processing steps were performed using NOVA software.

Equivalent Circuits

In Electrochemical Impedance Spectroscopy, the structural properties of metal substrates and coatings on them are represented by electrical equivalent circuits following "Fit & Simulation" processing of the measurement results obtained and their interpretation. The identification of an appropriate equivalent circuit for the sample under investigation requires more in-depth knowledge about the system. Some basic descriptive information about different coating types and their corresponding equivalent circuits is presented below. Ideally, a non-conductive coating bonds perfectly to a metal substrate and a perfect coating interface without any surface defects is obtained. This situation results in a blocking electrode described by the equivalent circuit shown in Figure 2, where the coating layer has a capacitance CL and a resistance RΩ not compensated in series. [caption id="attachment_129608" align="aligncenter"] Figure 2: Ideal, perfect non-conductive coating on a metal substrate and corresponding RΩCL equivalent circuit.[/caption] In reality, however, a coating with surface defects is a more common condition. Such defects can appear as an imperfect coating where the coated area, shown in Figure 3, coexists with an uncoated surface (modeled with a double layer capacitance Cdl in parallel with a general faradaic impedance Zf). The resulting equivalent circuit is also shown in Figure 3. [caption id="attachment_129609" align="aligncenter"] Figure 3: Metal substrate with partial coating and its equivalent circuit.[/caption] Pores on the surface are also among the defects of the coating layer. A pore can be evaluated as an uncoated area of the substrate, as in Figure 3, but it is small enough to prevent electrolyte exchange between itself and the bulk. Therefore, the ion concentration inside the pores may differ from the bulk. This results in a resistance RL different from RΩ bulk electrolyte resistance. Figure 4 shows the presence of pores and the corresponding equivalent circuit. [caption id="attachment_129610" align="aligncenter"] Figure 4: Non-conductive porous coating and its equivalent circuit.[/caption] Finally, the presence of an outer coating applied over the first layer must also be considered. This top layer adds a second interface to the system and is modeled with an additional RL2 resistance and parallel CL2 double layer capacitance. The resulting final system and equivalent circuit are shown in Figure 5. [caption id="attachment_129611" align="aligncenter"] Figure 5: Two non-conductive porous coatings and their equivalent circuit.[/caption] With respect to general impedance Zf, it is important to know the composition and behavior of coating layers exposed to the electrolyte in order to select the appropriate circuit element. Typical examples of such circuit elements are constant phase and Warburg diffusion elements.

Measurement Results

All measurements assumed the presence of pores. The equivalent circuits mentioned above were evaluated during the fitting of data obtained before SDM. While the equivalent circuit shown in Figure 4 was selected for the oxide-coated sample plate, the equivalent circuit shown in Figure 5 was used for the "gold" and "pink" coated samples. In the following sections, Nyquist plots of the three samples before and after SDM are shown. For each Nyquist plot, the blue dots represent the experimental data and the black line represents the fit obtained with the equivalent circuit drawn within the plot. In this review, equivalent circuits selected in relation to the models mentioned above are evaluated solely in light of experimental data, and numerical values of circuit elements are omitted from discussion.

Aluminum Oxide

Figure 6 shows the Nyquist plot of the oxide sample before SDM. The selected equivalent circuit is similar to the equivalent circuit used to model the presence of pores in the coating (Figure 4). [caption id="attachment_129615" align="aligncenter"] Figure 6: Nyquist curve of oxide sample before SDM[/caption] The CL double layer capacitance of the oxide layer was replaced with a constant phase element (CPEL) to account for surface roughness. The general impedance Zf was found to be close to the Rct charge transfer resistance. [caption id="attachment_129617" align="aligncenter"] Figure 7: Nyquist curve of oxide sample after SDM[/caption] After SDM, that is after the corrosion process, the Nyquist plot of the oxide sample (Figure 7) shows a 45° line at low frequencies; this can be associated with ion diffusion within the pores and can be matched with a Warburg element. The ions are evaluated as potentially being a product of the corrosion process.

Gold-Coated Aluminum

In the gold-coated sample, a two-layer coating model was selected because the gold coating was evaluated to have been applied over the oxide layer. This is represented by the Nyquist plot shown in Figure 8. In the equivalent circuit used for data fitting, capacitors were also replaced with constant phase elements. The general impedance Zf was selected as an interface with an Rct charge transfer resistance in parallel with a CPEdl constant phase element. [caption id="attachment_129620" align="aligncenter"] Figure 8: Nyquist curve of gold-coated sample before SDM

[/caption] The corrosion process alters the properties of the coatings, as can be seen from the Nyquist plot in Figure 9. [caption id="attachment_129622" align="aligncenter"] Figure 9: Nyquist curve of gold-coated sample after SDM[/caption] Pink-Coated Aluminum In the final sample, the pink layer was applied over the oxide layer, as in the gold-coated sample. However, Figure 10 shows that before SDM, the Nyquist plot is almost a vertical line at low frequencies. The best fit was achieved by neglecting the faradaic impedance Zf and adding a series constant phase element (CPEdl) representing the interface between the metal and oxide surface. This can be an indication of a high-quality coating. [caption id="attachment_129623" align="aligncenter"] Figure 10: Nyquist curve of pink-coated sample before SDM[/caption] After SDM, the best fit is obtained by replacing CPEdl with double layer capacitance Cdl and is shown in Figure 11. This means that the coating is less affected by the corrosion process induced by SDM compared to the other two samples, and no faradaic processes occur at the aluminum-pore interface. [caption id="attachment_129625" align="aligncenter"] Figure 11: Nyquist curve of pink-coated sample after SDM[/caption] Figure 12 shows the three Nyquist plots after SDM overlaid, illustrating the comparison between the impedances of the pink-coated sample, oxide, and gold-coated samples. [caption id="attachment_129626" align="aligncenter"] Figure 12: Nyquist curves of three samples after SDM[/caption] Based on this information, it can be seen that the pink coating exhibits the best performance against corrosion, at least during SDM.

Overall Conclusion

Electrochemical Impedance Spectroscopy (EIS) is a fast, non-destructive, and powerful technique commonly used in corrosion science. EIS can provide important information about the quality of coatings against corrosion when detailed information about the system under investigation is provided. The application detailed above demonstrates how EIS can be performed on aluminum samples coated with different materials when using an Autolab PGSTAT204 system with NOVA software and an Autolab 1 L corrosion cell. As the Metrohm electrochemistry group, we offer with our Metrohm Autolab and Metrohm DropSens brands a wide range of instruments, software, and accessories to meet the requirements of all types of electrochemical research, from basic compact and economical systems to multi-channel project stations and fully integrated spectroelectrochemical platforms (UV-Vis, VisNIR, and Raman) that allow the combination of electrochemistry with spectroscopic measurements.

References 1. Metrohm Electrochemistry White Paper, Electrochemistry in the quality control of food and beverage packaging 2. Metrohm Electrochemistry Application Note AN-COR-009, Electrochemical impedance spectroscopy of three coated aluminum samples 3. Metrohm Electrochemistry Application Note AN-COR-008, Stepwise dissolution measurement 4. K. Marsh, and B. Bugusu, "Food Packaging—Roles, Materials, and Environmental Issues", Journal of Food Science, Vol. 72, Nr. 3, R39–R55, 2007.

  Yeliz Yavuz Çevik Product Manager Metrohm Turkey