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Detection of corrosion-induced metal release from tinplate cans using a novel electrochemical sensor and inductively coupled plasma mass spectrometer
Journal of Food Engineering 113 (2012) 11–18
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Detection of corrosion-induced metal release from tinplate cans using a novel electrochemical sensor and inductively coupled plasma mass spectrometer Dahai Xia a, Shizhe Song a, Wenqi Gong b, Yuxuan Jiang c, Zhiming Gao a, Jihui Wang a,⇑ a
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China School of Civil Engineering, Tianjin University, Tianjin 300072, PR China c School of Electronic Information Engineering, Tianjin University, Tianjin 300072, PR China b
a r t i c l e
i n f o
Article history: Received 12 November 2011 Received in revised form 8 May 2012 Accepted 20 May 2012 Available online 1 June 2012 Keywords: Metal release Corrosion Energy drink Electrochemical impedance spectroscopy Electrochemical noise Electrochemical sensor
a b s t r a c t Corrosion-induced tin and iron release were investigated by inductively coupled plasma mass spectrometer (ICP-MS) and a novel electrochemical sensor. The sensor was used for detecting the corrosion extent of energy drink cans by techniques of electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN), and a metal release mechanism was proposed. ICP-MS results showed that tin and iron release increased with the storage time while EIS and EN results showed that coating resistance, charge transfer resistance and noise resistance decreased with the storage time, which indicated that the corrosion beneath the organic coating induced metal release. Consequently, a clear and direct relationship was obtained between the ICP-MS and the electrochemical results. Furthermore, the internal surface morphology of the cans was characterized by scanning probe microscopy (SPM). It was concluded that the novel electrochemical sensor that allowed in situ measurement could be used for the evaluation of corrosion extent and metal release in beverage cans in a more economical and rapid way. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Packaging is the tool that protects and contains goods so that the environmental impact on the food in the package is minimized. Effective packaging is vital to the health of the consumer. Worldwide, the total quantity of cans for food packaging is approximately 80,000 million. Tinplate, as one of the most common packaging materials, is applied in more than 80% of the cases though new alternative materials such as aluminum and chromated steel sheet are increasingly being adopted in the canning industry (Pournaras et al., 2008; Xia et al., 2011; Xia et al., 2012a). Tinplate is also extensively used in the production of beverage cans. As we know, tinplate is a light gauge, cold reduced, low-carbon steel sheet or strip, coated on both sides with commercially pure tin, combining the strength and formability of steel with the corrosion resistance and good appearance of tin. However, there are significant problems related to the use of tinplate cans in contact with corrosive medium, such as corrosion failure, loss of seal integrity, and discoloration that would lead to the consumers’ rejection of the products (Patrick, 1976). In addition, the corrosion of tinplate for food and beverage packaging will result in some tin dissolving into the food content (Xia et al., 2012a,b). Though tin ⇑ Corresponding author. Address: School of Materials Science and Engineering, Tianjin University, Weijin Road 92#, Tianjin 300072, PR China. Tel./fax: +86 22 27890010. E-mail address: [email protected] (J. Wang). 0260-8774/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2012.05.035
is not considered as a poisonous metal, a considerably large dose of intake may cause serious digestive disturbances (Blunden and Wallace, 2003; Boogaard et al., 2003). At present, we have scanty research information available, on the effects of tinplate packaging on food and beverage items. The quality, safety, and nutrition of packaged contents have not been thoroughly researched for certain newly packaged products. The desire for safer products with higher quality and longer shelf life has driven scientists’ attention to the importance of packaging. The migration of food packaging elements to the contained food has received thorough attention over recent years. Test methods of metal release from beverages are various. Although there has been much interest in common analytical methods including atomic absorption spectrometry (AAS) (Ieggli et al., 2011), atomic emission spectrometry (AES) and inductively coupled plasma–optical emission spectrometry (Nardi et al., 2009), and ion chromatography (Rebary et al., 2010), few studies have been dealt with on-line electrochemical detection methods. Besides, traditional chemical analyses are time consuming and provide quite limited information (Catala et al., 1998). It has been documented that metal release is usually caused by corrosion which can be detected by the electrochemical methods. Lacquered tinplate is a coating/metal system and its electrochemical signal during corrosion process can be detected by the electrochemical techniques, such as electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) detections (Liu et al., 2009; Zhang et al., 2009; Rezaei et al., 2010; Zhu et al., 2010; Betova et al.,
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2010). Thus, analyzing the metal release from a corrosion perspective is very important. EIS is very suited to the study of polymercoated metals. Electrochemical noise (EN) is a general term given to fluctuations in the potential and current generated spontaneously by corrosion process. EN occurs naturally in the electrolyte/ electrode interface due to the random ion movements and also originates from the occurrence of cooperative phenomena such as nucleation of pits etc., (Zhao et al., 2010). More deeply, electrochemical noise is associated with all degrees of freedom of the system. It indicates a change in the thermodynamic and kinetic states of the interface and is the only electrochemical technique that does not disturb the system (Garcia and Corvo, 2010; Xia et al., 2012c,d). These methods can be used to fast detect the corrosion extent of the metal-related materials on-line, offering potential way to find out the correlation between corrosion and metal release. The aim of this paper is to identify the corrosion-induced metal release in energy drink by means of electrochemical analyses and inductively coupled plasma mass spectrometer (ICP-MS) analyses. The correlation between electrochemical parameters and ICP-MS results is analyzed. Finally, a metal release mechanism of tin and iron from energy drink cans is proposed.
which was made of polytetrafluoroethylene. Two of them were counter electrodes: one was a platinum niobium silk with a diameter of 3 mm and an exposed area of 0.5 cm2, used as the counter electrode for EIS measurement (Fig. 1- component 3); the other was a silicone rubber coated platinum wire with a diameter of 1 mm and an exposed area of 0.3 cm2, used as the counter electrode for EN measurement (Fig. 1- component 5). This method makes the potential remaining basically unchanged before and after coupling working electrode to the silicone rubber coated platinum wire when performing EN measurements (Du et al., 2011). The reference electrode (Fig. 1- component 4) was an antimony one (195 mV vs. saturated calomel electrode) whose potential had been confirmed stable in energy drink in our previous work (Xia et al., 2012a). Three magnets were placed in the sensor, enabling the sensor to adsorb the cans on the top surfaces (Fig. 1component 1). A contactor (Fig. 1- component 2) with a copper bar (Fig. 1- component 8) in the middle and a ring magnet (Fig. 1- component 9) outside the copper bar was exquisitely designed in order to connect the tinplate can (Fig. 1- component 6).
2. Materials and methods
The electrochemical measurements were performed by using VersaSTAT 4 electrochemical workstation (Princeton Applied Research, USA) and VersaStudio control software combined with the electrochemical sensor. EIS measurements were performed at the free corrosion potential with a 10 mV amplitude signal and the applied frequency ranged from 100 kHz to 0.01 Hz. The platinum niobium silk was used as the counter electrode. The results were typically fitted to an equivalent circuit by applying ZSimpWin Software (version 9). When the sensor was performing EIS measurement, the effective area was about 95 cm2, which was previously reported in our paper (Xia et al., 2012d). In the three electrode electrochemical cell, the current flow is between working and auxiliary electrodes. There is a potential difference between the reference electrode and the working electrode, so there is an electrolyte resistance Re. To measure the fluctuating potentials of the specimen through a zero resistance ammeter (ZRA) mode, the silicone rubber coated platinum wire was used as the counter electrode. Potential and current noise signals were measured simultaneously. The EN data were collected with a sampling frequency of 2 Hz. The analyses of the noise data were then performed in the time domain, and noise resistance was calculated.
2.1. Samples Cans were cylindrical (65 mm diameter 90 mm high), made of tinplate internally lacquered (epoxy phenolic), each bearing an easily open top and containing 250 mL energy drink. These three-piece cans are provided by the ORG Canmaking Company, China. The thicknesses of the epoxy phenolic coating and the tin coating were about 6.6 and 3.5 lm, respectively, which were confirmed in our previous work by optical microscopy (Xia et al., 2011; Xia et al., 2012e). The sample number and storage time for each can were listed in Table 1. The pH value of the energy drink was between 3.0 and 3.2, which was measured by a pH meter. The drink contained taurine (0.5 g L1), diaminocaproic acid (0.2 g L1), inositol (0.2 g L1), caffeine (0.2 g L1), nicotinamide (0.04 g L1), vitamin B6 (4 mg L1), vitamin B12 (12 lg L1), and other food additives such as citric acid, saccharose, essence, benzene sulfonic acid sodium salt and citric yellow, and so on. 2.2. The structure of the electrochemical sensor There were two major problems in developing the electrochemical sensors suitable for use in energy drink. The first one was to choose suitable reference electrode whose potential was stable in energy drink. The second one is that the sensor should be small enough to be inserted into the can from the open top. We had employed a number of techniques to solve these problems in the study, and a novel electrochemical sensor was designed to perform EIS and EN measurements. Fig. 1 show the electrochemical sensor (Fig. 1(a)), the corrosion detection of a beverage can (Fig. 1(b)), and a schematic diagram of the electrochemical cell (Fig. 1(c)). The electrochemical sensor used here consisted of three electrodes and one contactor to connect the working electrode (tinplate cans) to the sensor. The three electrodes were placed in a white tube Table 1 Sample number and storage time for energy drink cans. Sample number
Storage time
Resource
s1 s3 s7 s12 S27
1 month 3 months 7 months 12 months 27 months
Supermarket Supermarket ORG company ORG company ORG company
2.3. Electrochemical impedance spectra and electrochemical noise
2.4. Inductively coupled plasma mass spectrometer (ICP-MS) An inductively coupled plasma mass spectrometer (VISTA-MPX, USA) was used to determinate dissolved tin and iron in energy drink. It ran in sequential mode, peak hopping to masses of interest. A cross flow nebuliser served as a sprayer for sample introduction. In an effort to avoid conductive coupling between the load and the plasma, both ends of the load coil were biased with high voltage of equal amplitude but opposite phases. No modification was made to the load coil configuration. Throughout the experiment, the sampling depth between the sampler tip and top coil was fixed at 15.0 and 1.0 l/min, respectively. The radio frequency POWER (1150 W) and the aerosol gas flow rate (1.0 l/min) were optimized through the adoption of a 10 ppb Ce solution for maximum Ce+ signal and the possible minimum CeO+/Ce+ ratio. 2.5. Morphology analysis Morphology analyses were carried out through a scanning probe microscopes(SPM) (AJ-IIIa, China). SPM was operated in contact mode under ambient conditions.
D. Xia et al. / Journal of Food Engineering 113 (2012) 11–18
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Fig. 1. The electrochemical sensor (a), corrosion detection of a beverage can (b), and a schematic diagram of the electrochemical cell (c). 1 – magnet; 2 – contactor of working electrode; 3 – counter electrode (platinized niobium) for EIS measurement; 4 – reference electrode; 5 – counter electrode (silicone rubber coated platinum wire) for EN measurement; 6 – beverage can; 7 – beverage; 8 – copper bar; 9 – magnet.
3. Results 3.1. Corrosion detection of beverage cans by EIS Cans containing 250 mL beverage named s1, s3, s7, s12, s27 were detected by the electrochemical sensor. The EIS results can be seen in Fig. 2. With an increase in storage time, EIS characteristics had significant changes, varying from the features of one capacitive loop to two capacitive loops. As we know, electrochemical equivalent circuit (EEC) is a classical analytical method to process EIS data. Coating resistance, coating capacitance, polarized resistance and interfacial capacitance of the substrate metal can be obtained through the analysis of parameters from EEC fitting. Also, there are always some errors when using equivalent circuits to simulate the EIS results for complicated coating system (Xia et al., 2012e). In addition, an equivalent circuit involving three or more circuit elements can often be rearranged in various ways and still yield exactly the same impedance. But EEC method is still an effective one to analyze simple metal/coating system. From the EIS plots of s1 and s3, it is evident that the general shape of the impedance only includes impedance response of a corrosion process of organic coating. Such a shape of impedance dispersion can be characterized by a Re(CcRc) equivalent circuit (Fig. 3(a)) , where Re is electrolyte resistance between the reference electrode and the working electrode, Cc is capacitance of organic coating, and
Rc is resistance of organic coating. If the electrolyte does not penetrate the coating, the parallel interfacial capacitance of the substrate metal Cdl and charge transfer resistance Rct are unnecessary. This is the real situation at the beginning of the tinplate-lacquer system to a beverage solution. The fitting results from the electrochemical equivalent circuit are given in Table 2. One capacitive loop indicated that the coating showed good protective performance and the electrolyte had not permeated through the organic coating, so metal release should be negligible. The coating resistance Rc was approximately 109X cm2. At the beginning of exposure, the organic coating behaved as the dielectric material and showed purely capacitive behavior. Subsequently its resistance decreased due to the penetration of electrolyte through the organic coating. So two capacitive loops were observed in the EIS plots of s7, s12, s27. It is evident that the general shape of the spectra includes impedance response of a coating performance at higher frequencies and a corrosion process of metal substrate at lower frequencies. Such a shape of impedance dispersion can be characterized by a Re(Cc(Rc(CdlRct))) equivalent circuit (Fig. 3 (b)), where Re is electrolyte resistance, Cc is capacitance of organic coating, Rc is resistance of organic coating, Cdl is substrate metal interfacial capacitance, and Rct is charge transfer resistance. Obvious differences were that the capacitive loops at higher frequencies of s12, s27 were much smaller than those of s1, s3, s7, which indicated that the coating resistances of the former were
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Fig. 2. EIS plots of different energy drink cans (a) s1 (b) s3 (c) s7 (d) s12 (e) s27.
indicated that the protective performance deteriorated, and the decreased Rct and increased Cdl indicated that the corrosion on metal substrate increased with time (Zhang et al., 2009; Zhu et al., 2010; Naderi and Attar, 2009). On the other hand, decreased Rc and Rct led to metal release from the tinplate. The Rc and Rct values of s27 were the lowest, which indicated that more serious corrosion and more metal release occurred in s27. 3.2. Corrosion detection of beverage cans by EN
Fig. 3. Electrochemical equivalent circuit for (a) one capacitive loop and (b) two capacitive loops.
much lower than the latter. In this case, the electrolyte had permeated through the pore of organic coating and interfacial capacitance was formed on the coating/metal interface. From Table 2, it can be seen that the sort order of Rc and Rct was s1 > s3 > s7 > s12 > s27. The decreased Rc and increased Cc
Electrochemical noise describes the low level spontaneous fluctuations of potential and current that occurs during an electrochemical process. During a corrosion process, which is predominantly electrochemical in nature, the cathodic and the anodic reactions can cause minute transients in the electrical charges on the electrode. These transients manifest in the form of potential and current noise, which can be exploited to map a corrosion event. Fig. 4 shows the electrochemical potential and current noise of the five cans after a 5th polynomial remove. Not many obvious changes could be observed from the potential noise. Very frequent repetition rates of potential fluctuations of s1, s3, s7, s12 were within amplitude of 1 mV except that s27 was a little higher than 1 mV. Fig. 4 also shows the electrochemical current noise of the five cans after a 5th polynomial remove.
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D. Xia et al. / Journal of Food Engineering 113 (2012) 11–18 Table 2 Fitting results of electrical equivalent circuit. Sample number
s1 s3 s7 s12 s27
Parameters of electrochemical equivalent circuit Cc (F cm2)
Rc (X cm2)
Cdl (F cm2)
Rct (X cm2)
Related coefficient (%)
2.383E-10 5.452E-10 8.783E-10 3.090E-9 5.117E-9
3.157E8 4.879E7 1.008E7 5.184E5 8.692E4
– – 1.341E-7 4.483E-7 6.125E-6
– – 2.562E7 3.467E7 8.332E5
96.23 93.14 97.56 96.77 92.16
Fig. 4. Electrochemical potential noise and electrochemical current noise of different beverage cans (after a dc was removed).
It was found that current noise showed a substantial difference for various cans. The amplitude of current noise of s1 was smaller than 0.5 nA, indicating that the organic coating basically protected the tinplate from corrosion. The amplitude of current noise of s3 was a little higher than that of s1, indicating that the protective performance of organic coating of s3 degraded but the beverage hadn’t permeated through the coating, which was also confirmed by EIS results. The amplitude of current noise of s7 was 1 nA and EIS plots showed two capacitive loops, which indicated that beverage had permeated through the organic coating and corrosion occurred. With an increase in storage time, more frequent repetition rates of current noise within amplitudes of 6 nA were observed in s27, and the metal under the organic
coating corroded so seriously that the organic coating had lost its protective performance. Furthermore, electrochemical noise data were used to calculate noise resistance Rn. A statistical analysis of the resulting noise-time record provides useful information on the corrosion processes and corrosion rate. The equivalence of noise resistance and polarization resistance Rp was confirmed by the work of Chen and Bogearts (Chen and Bogaerts, 1995). Therefore, the noise resistance represents the corrosion parameter which could be compared directly with corrosion data obtained from impedance measurements, namely with polarization resistance. Noise resistance was calculated as the ratio of standard deviations of the simultaneously measured potential and current noise signals:Rn = r(V)/r(I)
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In Fig. 5, the noise resistance Rn, the coating resistance Rc and the charge transfer resistance Rct which were obtained from EIS measurements are compared. Obviously Rn of s27 was lower ones, indicating the corrosion extents of s27 were more serious. Electrochemical results showed that the anti-corrosion performance of the five beverage cans were different, which was related to the storage time. Can s1 showed good anti-corrosion performance; the other 4 cans had lost their protective performance to some extent, so they did not meet shelf life requirement. 3.3. ICP-MS ICP-MS results of the five different cans are shown in Fig. 5. In all the five samples, there were significant changes in the contents of Sn and Fe. High levels of Sn and Fe in s12 and s27 were caused by the corrosion-induced metal release from the tinplate. In contrast, contents of Fe and Sn were very low in s1 and s3, which indicated that the intact organic coatings prevented tinplate from corrosion. Furthermore, the Sn contents were always higher than the Fe contents, which could be explained by the fact that Sn has a higher electrochemical activity than Fe, and that tin coating is first exposed to the electrolyte where the organic coatings are not intact when being immersed in energy drink without oxygen (Zumelzu and Cabezas, 1995). 3.4. Morphology analysis Scanning probe microscopy (SPM) was performed in order to observe the surface morphology of organic coatings at a micronano level (Fig. 6). SPM images revealed that coating surface of s1 was basically a flat one (Fig. 6(a)). There were numerous hills and valleys on the surfaces of s7 and s27. From small scale SPM images, it could be seen that bump height of coating surface increased with storage time. The bump height of s1 was about 30 nm, but the height increased to 50–100 nm due to swelling or blistering of the organic coating (Fig. 6(b) and (c)). The swelling or blistering of the organic coating intensified the metal release from the cans. 4. Discussion Based on EIS, EN and ICP-MS results, a metal release mechanism of lacquered tinplate in energy drink is proposed, as is illustrated schematically in Fig. 7. For those cans stored for 1 and 3 months, the EIS plots presented one capacitive loop and the amplitude of current noise was lower than 1 nA, which indicated that the organic coatings showed good protective performance and prevented tinplate from corrosion, so the metal release were minimal (stage I).
Fig. 5. Correlation between electrochemical parameters and ICP-MS results.
Fig. 6. SPM images of the internal surface of the cans (a) s1 (b) s7 (c) s27.
With an increase in storage time, energy drink permeated into the organic coating through the pores or defects existing in the coatings, so H+ from the organic acid in the drink (taurine, citric acid, etc.,) also permeated through the pores of the organic coating (Xia et al., 2011; Xia et al., 2012a). In the pores, tinplate was exposed to the electrolyte. Due to a more electrochemical activity of tin, corrosion occurred in the defects because of highly exposed metal. The corrosion formed when the cathodic reaction was hydrogen reduction on the tin coating. Electrons flowed from the tin, which served as the anode (Eqs. (1), and (2), to the hydrogen ions-rich region on the surface, which served as the cathode Eq. (3). At this time, the EIS consisted of two capacitive loops and the amplitudes of current noise were higher than 1 nA, which indicated that the protective performance of the organic coatings deteriorated, so the metal release became larger (stage II).
D. Xia et al. / Journal of Food Engineering 113 (2012) 11–18
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resistance decreased with the storage time, thus indicating that the corrosion beneath the organic coating induced metal release. (3) A clear and direct relationship was obtained between ICPMS and electrochemical data. The novel electrochemical sensor can be used for evaluation of corrosion extent and metal release in beverage cans, offering an inexpenseve and rapid methodology.
Acknowledgements The authors are happy to acknowledge financial support from the National Program on Key Basic Research Project (2011CB610505) and provision of samples by ORG company. We thank Miss Yanhong Liu, who is from School of Foreign Languages in Hebei University of Technology, for the language assistance. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jfoodeng. 2012.05.035. References
Fig. 7. A metal release mechanism of lacquered tinplate in energy drink.
Anodic reaction (Jafarian et al., 2008; Rehim et al., 2003):
Sn ! Sn2þ þ 2e
ð1Þ
Sn2þ ! Sn4þ þ 2e
ð2Þ
Cathodic reaction: þ
2H þ 2e ! H2
ð3Þ
When part of tin coating was corroded away, carbon steel was exposed to the electrolyte and began to corrode (stage III). In addition to (1) and (2), another anodic reaction was:
Fe ! Fe3þ þ 3e
ð4Þ
On the other hand, it was found that tin is the sacrificial member of the tin/carbon steel couple, conferring protection to the steel base while corroding at a certain rate itself in an acid medium without oxygen (Zumelzu and Cabezas, 1995; Patrick, 1976). So it can be seen the total Fe release was lower than the Sn release (see Fig. 5). 5. Conclusions The corrosion-induced metal release from lacquered tinplate cans was investigated by using a novel electrochemical sensor and inductively coupled plasma mass spectrometry, and the results permit the following conclusions: (1) A novel electrochemical sensor was designed to detect the corrosion extent of beverage cans by techniques of electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN), which allowed in situ measurement. (2) ICP-MS results showed that tin and iron release increased with storage time while EIS and EN results showed that coating resistance, charge transfer resistance and noise
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Detection of corrosion-induced metal release from tinplate cans using a novel electrochemical sensor and inductively coupled plasma mass spectrometer Dahai Xia a, Shizhe Song a, Wenqi Gong b, Yuxuan Jiang c, Zhiming Gao a, Jihui Wang a,⇑ a
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China School of Civil Engineering, Tianjin University, Tianjin 300072, PR China c School of Electronic Information Engineering, Tianjin University, Tianjin 300072, PR China b
a r t i c l e
i n f o
Article history: Received 12 November 2011 Received in revised form 8 May 2012 Accepted 20 May 2012 Available online 1 June 2012 Keywords: Metal release Corrosion Energy drink Electrochemical impedance spectroscopy Electrochemical noise Electrochemical sensor
a b s t r a c t Corrosion-induced tin and iron release were investigated by inductively coupled plasma mass spectrometer (ICP-MS) and a novel electrochemical sensor. The sensor was used for detecting the corrosion extent of energy drink cans by techniques of electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN), and a metal release mechanism was proposed. ICP-MS results showed that tin and iron release increased with the storage time while EIS and EN results showed that coating resistance, charge transfer resistance and noise resistance decreased with the storage time, which indicated that the corrosion beneath the organic coating induced metal release. Consequently, a clear and direct relationship was obtained between the ICP-MS and the electrochemical results. Furthermore, the internal surface morphology of the cans was characterized by scanning probe microscopy (SPM). It was concluded that the novel electrochemical sensor that allowed in situ measurement could be used for the evaluation of corrosion extent and metal release in beverage cans in a more economical and rapid way. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Packaging is the tool that protects and contains goods so that the environmental impact on the food in the package is minimized. Effective packaging is vital to the health of the consumer. Worldwide, the total quantity of cans for food packaging is approximately 80,000 million. Tinplate, as one of the most common packaging materials, is applied in more than 80% of the cases though new alternative materials such as aluminum and chromated steel sheet are increasingly being adopted in the canning industry (Pournaras et al., 2008; Xia et al., 2011; Xia et al., 2012a). Tinplate is also extensively used in the production of beverage cans. As we know, tinplate is a light gauge, cold reduced, low-carbon steel sheet or strip, coated on both sides with commercially pure tin, combining the strength and formability of steel with the corrosion resistance and good appearance of tin. However, there are significant problems related to the use of tinplate cans in contact with corrosive medium, such as corrosion failure, loss of seal integrity, and discoloration that would lead to the consumers’ rejection of the products (Patrick, 1976). In addition, the corrosion of tinplate for food and beverage packaging will result in some tin dissolving into the food content (Xia et al., 2012a,b). Though tin ⇑ Corresponding author. Address: School of Materials Science and Engineering, Tianjin University, Weijin Road 92#, Tianjin 300072, PR China. Tel./fax: +86 22 27890010. E-mail address: [email protected] (J. Wang). 0260-8774/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2012.05.035
is not considered as a poisonous metal, a considerably large dose of intake may cause serious digestive disturbances (Blunden and Wallace, 2003; Boogaard et al., 2003). At present, we have scanty research information available, on the effects of tinplate packaging on food and beverage items. The quality, safety, and nutrition of packaged contents have not been thoroughly researched for certain newly packaged products. The desire for safer products with higher quality and longer shelf life has driven scientists’ attention to the importance of packaging. The migration of food packaging elements to the contained food has received thorough attention over recent years. Test methods of metal release from beverages are various. Although there has been much interest in common analytical methods including atomic absorption spectrometry (AAS) (Ieggli et al., 2011), atomic emission spectrometry (AES) and inductively coupled plasma–optical emission spectrometry (Nardi et al., 2009), and ion chromatography (Rebary et al., 2010), few studies have been dealt with on-line electrochemical detection methods. Besides, traditional chemical analyses are time consuming and provide quite limited information (Catala et al., 1998). It has been documented that metal release is usually caused by corrosion which can be detected by the electrochemical methods. Lacquered tinplate is a coating/metal system and its electrochemical signal during corrosion process can be detected by the electrochemical techniques, such as electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) detections (Liu et al., 2009; Zhang et al., 2009; Rezaei et al., 2010; Zhu et al., 2010; Betova et al.,
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2010). Thus, analyzing the metal release from a corrosion perspective is very important. EIS is very suited to the study of polymercoated metals. Electrochemical noise (EN) is a general term given to fluctuations in the potential and current generated spontaneously by corrosion process. EN occurs naturally in the electrolyte/ electrode interface due to the random ion movements and also originates from the occurrence of cooperative phenomena such as nucleation of pits etc., (Zhao et al., 2010). More deeply, electrochemical noise is associated with all degrees of freedom of the system. It indicates a change in the thermodynamic and kinetic states of the interface and is the only electrochemical technique that does not disturb the system (Garcia and Corvo, 2010; Xia et al., 2012c,d). These methods can be used to fast detect the corrosion extent of the metal-related materials on-line, offering potential way to find out the correlation between corrosion and metal release. The aim of this paper is to identify the corrosion-induced metal release in energy drink by means of electrochemical analyses and inductively coupled plasma mass spectrometer (ICP-MS) analyses. The correlation between electrochemical parameters and ICP-MS results is analyzed. Finally, a metal release mechanism of tin and iron from energy drink cans is proposed.
which was made of polytetrafluoroethylene. Two of them were counter electrodes: one was a platinum niobium silk with a diameter of 3 mm and an exposed area of 0.5 cm2, used as the counter electrode for EIS measurement (Fig. 1- component 3); the other was a silicone rubber coated platinum wire with a diameter of 1 mm and an exposed area of 0.3 cm2, used as the counter electrode for EN measurement (Fig. 1- component 5). This method makes the potential remaining basically unchanged before and after coupling working electrode to the silicone rubber coated platinum wire when performing EN measurements (Du et al., 2011). The reference electrode (Fig. 1- component 4) was an antimony one (195 mV vs. saturated calomel electrode) whose potential had been confirmed stable in energy drink in our previous work (Xia et al., 2012a). Three magnets were placed in the sensor, enabling the sensor to adsorb the cans on the top surfaces (Fig. 1component 1). A contactor (Fig. 1- component 2) with a copper bar (Fig. 1- component 8) in the middle and a ring magnet (Fig. 1- component 9) outside the copper bar was exquisitely designed in order to connect the tinplate can (Fig. 1- component 6).
2. Materials and methods
The electrochemical measurements were performed by using VersaSTAT 4 electrochemical workstation (Princeton Applied Research, USA) and VersaStudio control software combined with the electrochemical sensor. EIS measurements were performed at the free corrosion potential with a 10 mV amplitude signal and the applied frequency ranged from 100 kHz to 0.01 Hz. The platinum niobium silk was used as the counter electrode. The results were typically fitted to an equivalent circuit by applying ZSimpWin Software (version 9). When the sensor was performing EIS measurement, the effective area was about 95 cm2, which was previously reported in our paper (Xia et al., 2012d). In the three electrode electrochemical cell, the current flow is between working and auxiliary electrodes. There is a potential difference between the reference electrode and the working electrode, so there is an electrolyte resistance Re. To measure the fluctuating potentials of the specimen through a zero resistance ammeter (ZRA) mode, the silicone rubber coated platinum wire was used as the counter electrode. Potential and current noise signals were measured simultaneously. The EN data were collected with a sampling frequency of 2 Hz. The analyses of the noise data were then performed in the time domain, and noise resistance was calculated.
2.1. Samples Cans were cylindrical (65 mm diameter 90 mm high), made of tinplate internally lacquered (epoxy phenolic), each bearing an easily open top and containing 250 mL energy drink. These three-piece cans are provided by the ORG Canmaking Company, China. The thicknesses of the epoxy phenolic coating and the tin coating were about 6.6 and 3.5 lm, respectively, which were confirmed in our previous work by optical microscopy (Xia et al., 2011; Xia et al., 2012e). The sample number and storage time for each can were listed in Table 1. The pH value of the energy drink was between 3.0 and 3.2, which was measured by a pH meter. The drink contained taurine (0.5 g L1), diaminocaproic acid (0.2 g L1), inositol (0.2 g L1), caffeine (0.2 g L1), nicotinamide (0.04 g L1), vitamin B6 (4 mg L1), vitamin B12 (12 lg L1), and other food additives such as citric acid, saccharose, essence, benzene sulfonic acid sodium salt and citric yellow, and so on. 2.2. The structure of the electrochemical sensor There were two major problems in developing the electrochemical sensors suitable for use in energy drink. The first one was to choose suitable reference electrode whose potential was stable in energy drink. The second one is that the sensor should be small enough to be inserted into the can from the open top. We had employed a number of techniques to solve these problems in the study, and a novel electrochemical sensor was designed to perform EIS and EN measurements. Fig. 1 show the electrochemical sensor (Fig. 1(a)), the corrosion detection of a beverage can (Fig. 1(b)), and a schematic diagram of the electrochemical cell (Fig. 1(c)). The electrochemical sensor used here consisted of three electrodes and one contactor to connect the working electrode (tinplate cans) to the sensor. The three electrodes were placed in a white tube Table 1 Sample number and storage time for energy drink cans. Sample number
Storage time
Resource
s1 s3 s7 s12 S27
1 month 3 months 7 months 12 months 27 months
Supermarket Supermarket ORG company ORG company ORG company
2.3. Electrochemical impedance spectra and electrochemical noise
2.4. Inductively coupled plasma mass spectrometer (ICP-MS) An inductively coupled plasma mass spectrometer (VISTA-MPX, USA) was used to determinate dissolved tin and iron in energy drink. It ran in sequential mode, peak hopping to masses of interest. A cross flow nebuliser served as a sprayer for sample introduction. In an effort to avoid conductive coupling between the load and the plasma, both ends of the load coil were biased with high voltage of equal amplitude but opposite phases. No modification was made to the load coil configuration. Throughout the experiment, the sampling depth between the sampler tip and top coil was fixed at 15.0 and 1.0 l/min, respectively. The radio frequency POWER (1150 W) and the aerosol gas flow rate (1.0 l/min) were optimized through the adoption of a 10 ppb Ce solution for maximum Ce+ signal and the possible minimum CeO+/Ce+ ratio. 2.5. Morphology analysis Morphology analyses were carried out through a scanning probe microscopes(SPM) (AJ-IIIa, China). SPM was operated in contact mode under ambient conditions.
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Fig. 1. The electrochemical sensor (a), corrosion detection of a beverage can (b), and a schematic diagram of the electrochemical cell (c). 1 – magnet; 2 – contactor of working electrode; 3 – counter electrode (platinized niobium) for EIS measurement; 4 – reference electrode; 5 – counter electrode (silicone rubber coated platinum wire) for EN measurement; 6 – beverage can; 7 – beverage; 8 – copper bar; 9 – magnet.
3. Results 3.1. Corrosion detection of beverage cans by EIS Cans containing 250 mL beverage named s1, s3, s7, s12, s27 were detected by the electrochemical sensor. The EIS results can be seen in Fig. 2. With an increase in storage time, EIS characteristics had significant changes, varying from the features of one capacitive loop to two capacitive loops. As we know, electrochemical equivalent circuit (EEC) is a classical analytical method to process EIS data. Coating resistance, coating capacitance, polarized resistance and interfacial capacitance of the substrate metal can be obtained through the analysis of parameters from EEC fitting. Also, there are always some errors when using equivalent circuits to simulate the EIS results for complicated coating system (Xia et al., 2012e). In addition, an equivalent circuit involving three or more circuit elements can often be rearranged in various ways and still yield exactly the same impedance. But EEC method is still an effective one to analyze simple metal/coating system. From the EIS plots of s1 and s3, it is evident that the general shape of the impedance only includes impedance response of a corrosion process of organic coating. Such a shape of impedance dispersion can be characterized by a Re(CcRc) equivalent circuit (Fig. 3(a)) , where Re is electrolyte resistance between the reference electrode and the working electrode, Cc is capacitance of organic coating, and
Rc is resistance of organic coating. If the electrolyte does not penetrate the coating, the parallel interfacial capacitance of the substrate metal Cdl and charge transfer resistance Rct are unnecessary. This is the real situation at the beginning of the tinplate-lacquer system to a beverage solution. The fitting results from the electrochemical equivalent circuit are given in Table 2. One capacitive loop indicated that the coating showed good protective performance and the electrolyte had not permeated through the organic coating, so metal release should be negligible. The coating resistance Rc was approximately 109X cm2. At the beginning of exposure, the organic coating behaved as the dielectric material and showed purely capacitive behavior. Subsequently its resistance decreased due to the penetration of electrolyte through the organic coating. So two capacitive loops were observed in the EIS plots of s7, s12, s27. It is evident that the general shape of the spectra includes impedance response of a coating performance at higher frequencies and a corrosion process of metal substrate at lower frequencies. Such a shape of impedance dispersion can be characterized by a Re(Cc(Rc(CdlRct))) equivalent circuit (Fig. 3 (b)), where Re is electrolyte resistance, Cc is capacitance of organic coating, Rc is resistance of organic coating, Cdl is substrate metal interfacial capacitance, and Rct is charge transfer resistance. Obvious differences were that the capacitive loops at higher frequencies of s12, s27 were much smaller than those of s1, s3, s7, which indicated that the coating resistances of the former were
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Fig. 2. EIS plots of different energy drink cans (a) s1 (b) s3 (c) s7 (d) s12 (e) s27.
indicated that the protective performance deteriorated, and the decreased Rct and increased Cdl indicated that the corrosion on metal substrate increased with time (Zhang et al., 2009; Zhu et al., 2010; Naderi and Attar, 2009). On the other hand, decreased Rc and Rct led to metal release from the tinplate. The Rc and Rct values of s27 were the lowest, which indicated that more serious corrosion and more metal release occurred in s27. 3.2. Corrosion detection of beverage cans by EN
Fig. 3. Electrochemical equivalent circuit for (a) one capacitive loop and (b) two capacitive loops.
much lower than the latter. In this case, the electrolyte had permeated through the pore of organic coating and interfacial capacitance was formed on the coating/metal interface. From Table 2, it can be seen that the sort order of Rc and Rct was s1 > s3 > s7 > s12 > s27. The decreased Rc and increased Cc
Electrochemical noise describes the low level spontaneous fluctuations of potential and current that occurs during an electrochemical process. During a corrosion process, which is predominantly electrochemical in nature, the cathodic and the anodic reactions can cause minute transients in the electrical charges on the electrode. These transients manifest in the form of potential and current noise, which can be exploited to map a corrosion event. Fig. 4 shows the electrochemical potential and current noise of the five cans after a 5th polynomial remove. Not many obvious changes could be observed from the potential noise. Very frequent repetition rates of potential fluctuations of s1, s3, s7, s12 were within amplitude of 1 mV except that s27 was a little higher than 1 mV. Fig. 4 also shows the electrochemical current noise of the five cans after a 5th polynomial remove.
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D. Xia et al. / Journal of Food Engineering 113 (2012) 11–18 Table 2 Fitting results of electrical equivalent circuit. Sample number
s1 s3 s7 s12 s27
Parameters of electrochemical equivalent circuit Cc (F cm2)
Rc (X cm2)
Cdl (F cm2)
Rct (X cm2)
Related coefficient (%)
2.383E-10 5.452E-10 8.783E-10 3.090E-9 5.117E-9
3.157E8 4.879E7 1.008E7 5.184E5 8.692E4
– – 1.341E-7 4.483E-7 6.125E-6
– – 2.562E7 3.467E7 8.332E5
96.23 93.14 97.56 96.77 92.16
Fig. 4. Electrochemical potential noise and electrochemical current noise of different beverage cans (after a dc was removed).
It was found that current noise showed a substantial difference for various cans. The amplitude of current noise of s1 was smaller than 0.5 nA, indicating that the organic coating basically protected the tinplate from corrosion. The amplitude of current noise of s3 was a little higher than that of s1, indicating that the protective performance of organic coating of s3 degraded but the beverage hadn’t permeated through the coating, which was also confirmed by EIS results. The amplitude of current noise of s7 was 1 nA and EIS plots showed two capacitive loops, which indicated that beverage had permeated through the organic coating and corrosion occurred. With an increase in storage time, more frequent repetition rates of current noise within amplitudes of 6 nA were observed in s27, and the metal under the organic
coating corroded so seriously that the organic coating had lost its protective performance. Furthermore, electrochemical noise data were used to calculate noise resistance Rn. A statistical analysis of the resulting noise-time record provides useful information on the corrosion processes and corrosion rate. The equivalence of noise resistance and polarization resistance Rp was confirmed by the work of Chen and Bogearts (Chen and Bogaerts, 1995). Therefore, the noise resistance represents the corrosion parameter which could be compared directly with corrosion data obtained from impedance measurements, namely with polarization resistance. Noise resistance was calculated as the ratio of standard deviations of the simultaneously measured potential and current noise signals:Rn = r(V)/r(I)
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In Fig. 5, the noise resistance Rn, the coating resistance Rc and the charge transfer resistance Rct which were obtained from EIS measurements are compared. Obviously Rn of s27 was lower ones, indicating the corrosion extents of s27 were more serious. Electrochemical results showed that the anti-corrosion performance of the five beverage cans were different, which was related to the storage time. Can s1 showed good anti-corrosion performance; the other 4 cans had lost their protective performance to some extent, so they did not meet shelf life requirement. 3.3. ICP-MS ICP-MS results of the five different cans are shown in Fig. 5. In all the five samples, there were significant changes in the contents of Sn and Fe. High levels of Sn and Fe in s12 and s27 were caused by the corrosion-induced metal release from the tinplate. In contrast, contents of Fe and Sn were very low in s1 and s3, which indicated that the intact organic coatings prevented tinplate from corrosion. Furthermore, the Sn contents were always higher than the Fe contents, which could be explained by the fact that Sn has a higher electrochemical activity than Fe, and that tin coating is first exposed to the electrolyte where the organic coatings are not intact when being immersed in energy drink without oxygen (Zumelzu and Cabezas, 1995). 3.4. Morphology analysis Scanning probe microscopy (SPM) was performed in order to observe the surface morphology of organic coatings at a micronano level (Fig. 6). SPM images revealed that coating surface of s1 was basically a flat one (Fig. 6(a)). There were numerous hills and valleys on the surfaces of s7 and s27. From small scale SPM images, it could be seen that bump height of coating surface increased with storage time. The bump height of s1 was about 30 nm, but the height increased to 50–100 nm due to swelling or blistering of the organic coating (Fig. 6(b) and (c)). The swelling or blistering of the organic coating intensified the metal release from the cans. 4. Discussion Based on EIS, EN and ICP-MS results, a metal release mechanism of lacquered tinplate in energy drink is proposed, as is illustrated schematically in Fig. 7. For those cans stored for 1 and 3 months, the EIS plots presented one capacitive loop and the amplitude of current noise was lower than 1 nA, which indicated that the organic coatings showed good protective performance and prevented tinplate from corrosion, so the metal release were minimal (stage I).
Fig. 5. Correlation between electrochemical parameters and ICP-MS results.
Fig. 6. SPM images of the internal surface of the cans (a) s1 (b) s7 (c) s27.
With an increase in storage time, energy drink permeated into the organic coating through the pores or defects existing in the coatings, so H+ from the organic acid in the drink (taurine, citric acid, etc.,) also permeated through the pores of the organic coating (Xia et al., 2011; Xia et al., 2012a). In the pores, tinplate was exposed to the electrolyte. Due to a more electrochemical activity of tin, corrosion occurred in the defects because of highly exposed metal. The corrosion formed when the cathodic reaction was hydrogen reduction on the tin coating. Electrons flowed from the tin, which served as the anode (Eqs. (1), and (2), to the hydrogen ions-rich region on the surface, which served as the cathode Eq. (3). At this time, the EIS consisted of two capacitive loops and the amplitudes of current noise were higher than 1 nA, which indicated that the protective performance of the organic coatings deteriorated, so the metal release became larger (stage II).
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resistance decreased with the storage time, thus indicating that the corrosion beneath the organic coating induced metal release. (3) A clear and direct relationship was obtained between ICPMS and electrochemical data. The novel electrochemical sensor can be used for evaluation of corrosion extent and metal release in beverage cans, offering an inexpenseve and rapid methodology.
Acknowledgements The authors are happy to acknowledge financial support from the National Program on Key Basic Research Project (2011CB610505) and provision of samples by ORG company. We thank Miss Yanhong Liu, who is from School of Foreign Languages in Hebei University of Technology, for the language assistance. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jfoodeng. 2012.05.035. References
Fig. 7. A metal release mechanism of lacquered tinplate in energy drink.
Anodic reaction (Jafarian et al., 2008; Rehim et al., 2003):
Sn ! Sn2þ þ 2e
ð1Þ
Sn2þ ! Sn4þ þ 2e
ð2Þ
Cathodic reaction: þ
2H þ 2e ! H2
ð3Þ
When part of tin coating was corroded away, carbon steel was exposed to the electrolyte and began to corrode (stage III). In addition to (1) and (2), another anodic reaction was:
Fe ! Fe3þ þ 3e
ð4Þ
On the other hand, it was found that tin is the sacrificial member of the tin/carbon steel couple, conferring protection to the steel base while corroding at a certain rate itself in an acid medium without oxygen (Zumelzu and Cabezas, 1995; Patrick, 1976). So it can be seen the total Fe release was lower than the Sn release (see Fig. 5). 5. Conclusions The corrosion-induced metal release from lacquered tinplate cans was investigated by using a novel electrochemical sensor and inductively coupled plasma mass spectrometry, and the results permit the following conclusions: (1) A novel electrochemical sensor was designed to detect the corrosion extent of beverage cans by techniques of electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN), which allowed in situ measurement. (2) ICP-MS results showed that tin and iron release increased with storage time while EIS and EN results showed that coating resistance, charge transfer resistance and noise
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