Degradation Mechanism of Lacquered Tinplate in Energy Drink by in-situ EIS and EN

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1 Journal of Wuhan University of Technology-Mater. Sci. Ed. Apr DOI /s Degradation Mechanism of Lacquered Tinplate in Energy Drink by in-situ EIS and EN ZHOU Chao 1, WANG Jihui 1, SONG Shizhe 1, XIA Dahai 2*, WANG Ke 1, SHEN Chen 2, LUO Bing 3, SHI Jiangbo 1 (1.School of Materials Science and Engineering, Tianjin University, Tianjin , China; 2.Department of Chemical and Materials Engineering, University of Alberta, Edmonton T6G 2V4, Alberta, Canada; Unit of the PLA, Ji nan , China) Abstract: The corrosion process of phenolic epoxy coated tinplate in energy drink was investigated by in-situ electrochemical impedance spectroscopy(eis) and electrochemical noise(en) techniques. The experimental results indicate that the degradation process of novolac epoxy coated tinplate in energy drink can be divided into three main stages: organic coating wetted by the beverage; corrosion initiation beneath the organic coating; and corrosion extension process. It was proposed that the tin coating and carbon steel were mainly corroded by organic acids in energy drink through the pores of the organic coating. After the tin coating was corroded, the carbon steel started to corrode due to its higher electrochemical activity and became to be the dominated corrosion reaction. Key words: tinplate; energy drink; electrochemical impedance spectroscopy; electrochemical noise; corrosion mechanism 1 Introduction Tinplates, as one of the most common packaging materials, are used in more than 80% of cases though the new alternative materials such as aluminum and chromated steel sheet are increasingly used by the canning industry [1]. 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. Tinplate has become one of the dominant materials for making food cans. However, there are significant problems related to the use of tinplate cans in corrosive medium, such as corrosion failure, loss of seal integrity, or discoloration problems that cause its rejection by the consumer. In addition, other studies also indicated that high Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2013 (Received: July 25, 2012; Accepted: Sep. 3, 2012) ZHOU Chao ( 周超 ): zhouchao@163.com *Corresponding author: XIA Dahai ( 夏大海 ): Ph D; dahai@ ualberta.ca Funded by the National Key Basic Research Program of China (2011CB610505) and the Specialized Research Fund for the Doctoral Program of Higher Education ( ) levels of tin in food products may lead to food safety problems [2, 3]. Though tin is not considered to be a poisonous metal, very large doses can produce serious digestive disturbances [2, 3]. The corrosion process of pure tin is relatively simple due to its homogeneity. Numerous studies have been focused on the corrosion behaviors of pure tin. But the corrosion process of tinplate is complex due to its stratified structure and heterogeneity. In an acid medium without oxygen, it is advantageous that tin is the sacrificial member of the couple, conferring protection to the steel base while corroding at a certain rate itself [4, 5]. The corrosion of tinplate strongly depends on the nature of food matrix, acidity, the presence of complexing agents, duration and temperature of storage. Very few studies on the corrosion behaviors of tinplate have been reported and the corrosion behaviors in energy drink are still unknown. In fact, energy drink is a kind of severe corrosive medium. Therefore the corrosion behavior of tinplate in energy drink has noted significance. The different industries involved in tinplate cans such as tinplate producers, can manufactures, food canners and health authorities are in need of information about the behavior of this material. Since lacquered tinplate is a coating/metal system and its electrochemical signal originated from

2 368 Vol.28 No.2 ZHOU Chao et al: Degradation Mechanism of Lacquered Tinplate in Energy corrosion process can be detected by electrochemical impedance spectroscopy(eis) and electrochemical noise(en) techniques that are widely used to study the corrosion behaviors of metals and coating/metal systems [6-9], hence the corrosion resistance of tinplate can be quantitatively and semi quantitatively evaluated. The purpose of the present lacquered work was to investigate the corrosion process of tinplate in energy drink by electrochemical techniques and surface analysis techniques. After that the corrosion mechanism of tinplate in energy drink was proposed. EIS measurements were performed by VersaSTAT 4 electrochemical workstation (Princeton Applied Research, USA) and VersaStudio control software. A three-electrode cell with tinplate as the working electrode (WE), a high-purity antimony electrode as the reference electrode (RE) and a commercial rutheniumtitanium electrode as the counter electrode (CE) was used (Fig.1). The measurements were at the opencircuit potential with a 10 mv amplitude signal and the applied frequency ranged from 100 khz to 0.01 Hz. The working electrode (WE) was tinplate with an area of 19.6 cm 2. Fitting was performed with ZSimpWin Software. 2.3 EN measurement 2 Experimental 2.1 Materials All the samples were provided by the ORG Canmaking Company, China. The thicknesses of organic coating and tin coating on both sides were about 6 m and 3 m, respetively. The samples (70 mm in length and 70 mm in width) were degreased by ethanol then dried by a hair drier before exposed to energy drink. The sample was exposed to energy drink in contact with air at about 25 ± 3 and examined periodically by EIS and EN techniques. The ph value of energy drink was between The beverage contained taurine (0.5 g L 1 ), diaminocaproic acid (0.2 g L 1 ), inositol (0.2 g L 1 ), caffeine (0.2 g L 1 ), nicotinamide (0.04 g L 1 ), vitamin B6 (4 mg L 1 ), vitamin B12 (12 g L 1 ), and other food additives were citric acid, saccharose, essence, benzene sulfonic acid sodium salt and citric yellow etc. 2.2 EIS measurement A three-electrode configuration was used for the EN measurement (Fig.2). Two nominally identical organic coating coated tinplate were used as the working electrodes. The reference electrode was antimony electrode whose potential was proved to be stable in energy drink [10]. The two working electrodes with an area of 5 cm 2 were immersed in an energy drink. The electrochemical current noise (ECN) was measured between the two working electrode, and the electrochemical potential noise (EPN) was measured between one of the working electrodes and the reference electrode through a zero resistance ammeter (ZRA) mode at about room temperature (25±3 ). The sampling frequency was 2 Hz. 3 Results and discussion 3.1 EIS results Fig.3 is the measured Nyquist plots together with the established electrochemical equivalent circuit: where, R e is the electrolyte resistance; Q c is the capacitance of organic coating; R c is the resistance of organic coating; Q 1 is the interfacial capacitance of tin coating; Q 2 is the interfacial capacitance of carbon steel; R t is charge transfer resistance of tin coating; R ct is charge transfer resistance of carbon steel. This circuit is used for fitting the EIS results, and the obtained data are shown in Fig.4. In Nyquist plots, Z im is the imaginary part of impedance and Z re is the real part of impedance. According to the EIS characteristics, the degradation process of novolac epoxy coated tinplate in energy drink can be divided into three main stages. Stage I is the wetting of the organic coating; stage II is

3 Journal of Wuhan University of Technology-Mater. Sci. Ed. Apr the corrosion initiation beneath the coating; stage III is the corrosion extension. Stage I: Wetting of the organic coating The wetting of solid by liquid is defined as an extension phenomena when liquid contact with solid. At the first three days, the Bode plots containing one time-constant are observed, indicating that the coating has a good protective performance; there is no electrolyte permeation through the organic coating, characterized by the Z 0.01Hz closing to 10 9 cm 2 (Fig.3 (a)). This is the real situation at the beginning of the tinplate-lacquer system to a beverage solution. At the beginning of exposure, the organic coating behaves as the dielectric material and shows purely capacitive behavior. After 5 days, the R c decreases by one magnitude (see Fig.4), indicating the organic coating has been wetted by the bevarage. Stage II: Corrosion initiation beneath the coating After 28 days, the EIS plots with two timeconstants are presented (Fig.3(b)). The time constant at high frequency is a result of interface capacitance and surface pore resistance of organic coating; the time constant at low frequency is caused by interfacial capacitance of tin coating and charge transfer resistance of tinplate, reflecting the corrosion rate of metal substrate. At this time, the electrolyte has permeated through the pore of coating, and interfacial capacitance is formed at the substrate metal/coating interface. From Fig.4, It can be seen that the R c decreases to 10 8 cm 2 and the R t is about 10 7 cm 2. Stage:III Corrosion extension As immersion time prolonged, after 220 d, the EIS plot with three-constant is observed (Fig.3(c)), which reflects the complicated interface status under the coating. The first time-constant is attributed to the R c and Q c, the second time-constant stems from R t and Q 1, and the third time-constant comes from the contribution of Q 2 and R ct. At this time, carbon steel starts to be corroded. From Fig.4, It also can be seen that the R c decreases to 10 7 cm 2 and the R t is about

4 370 Vol.28 No.2 ZHOU Chao et al: Degradation Mechanism of Lacquered Tinplate in Energy 10 6 cm 2. The EIS spectrum presents one timeconstant after 364 days, illustrating the organic coating has degraded seriously and the electrode process is determined by the substrate metal. Fig.5 shows the evolution of corrosion potential as a function of immersion time. It can be seen obviously that the corrosion potential decreases as the immersion time prolongs, and finally it close to the potential of uncoated tinplate. From the fitting results, it is found that E and log(t) meet the linear relationship in E = log(t). 3.2 EN results

5 Journal of Wuhan University of Technology-Mater. Sci. Ed. Apr 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. Both the measured EPN and ECN of phenolic epoxy coated tinplate with different immersion time are presented in Fig. 6. The direct current (dc) component was first removed from original EN data by using a 5-order polynomial fitting. Fig. 6(a1) shows the EPN after being immersed in energy drink for 1 day. The signals are similar to white noise and show very high repetition rates of potential fluctuations within amplitudes of 0.75 mv. Fig. 6(b1) shows the EPN after being immersed in energy drink for 49 days. Very high repetition rates of potential fluctuations within amplitudes of 0.75 mv are observed. Fig. 6(d1) shows EPN after being immersed in energy drink for 186 days. Some transient peaks are observed with amplitudes more than 1 mv. From the ECN in Fig. 6 (a2, b2, c2, d2)), the amplitudes of ECN increase with the immersion time, varying from 0.3 na to 0.8 na. The increased amplitudes of ECN indicate the degradation of the organic coatings and the corrosion of tinplate beneath the organic coatings. From the probability distribution of current noise in Fig.7, it is found that the distribution of current becomes wider as the day prolongs, indicating the corrosion degree becomes more severe. The distribution of current for 1d mainly concentrates around zero while the extension of that distribution for 13 d and 186 d increase to 9 na and 12 na, respectively. 3.3 Discussion Based on EIS, and EN and results, the corrosion mechanism of lacquered tinplate in energy drink is discussed. At the initial immersion days, the EIS plots present one capacitive loop and the amplitude of current noise is lower than 0.5 na, which indicates that the organic coating shows good protective performance and prevents tinplate from corrosion (stage I). With an increase in immersion time, energy drink permeates into the organic coating through the pores or defects existing in the coatings, so H + ions from the organic acid in the drink (taurine, citric acid, etc) also permeate through the pores of the organic coating. In the pores, tinplate is exposed to the electrolyte. Due to a more electrochemical activity of tin, corrosion occurs in the defects because of the exposed tin. The corrosion formed when the cathodic reaction is hydrogen reduction on the tin coating. Electrons flow from the tin, which serves as the anode (Eqs.1-2), to the hydrogen ions-rich region on the surface, which serves as the cathode (Eq. 3). At this time, the EIS consists of two capacitive loops and the amplitudes of current noise are higher than 0.7 na, which indicates that the protective performance of the organic coatings degrades (stage II). Anodic reaction: Cathodic reaction: Sn Sn 2+ +2e (1) Sn 2+ Sn 4+ +2e (2) 2H + +2e H 2 (3) When part of tin coating is corroded away, carbon steel is exposed to the electrolyte and began to corrode (stage III). In addition to equations (1) and (2), another anodic reaction was: Fe Fe 3+ +3e (4) On the other hand, after the tin coating was corroded, the carbon steel began to corrode due to its higher electrochemical activity and it becomes to be the dominated corrosion reaction (stage III). For the corrosion mechanism of uncoated tinplate in energy drink, it was found that the tin coating and carbon steel are mainly corroded by the organic acids in energy drink through the pores of the organic coating. After the tin coating is corroded, the carbon steel begins to be corroded due to its higher electrochemical activity and become to be the dominated corrosion reaction [11]. Therefore, the corrosion mechnism of lacquered tinplate in energy drink, after the organic coating degrades, is similar to that of the uncoated tinplate.

6 372 Vol.28 No.2 ZHOU Chao et al: Degradation Mechanism of Lacquered Tinplate in Energy 4 Conclusions The degradation mechanism of lacquered tinplate was investigated by using EIS and EN methods, and the following conclusions are drawn: a) The corrosion process was judged by the EIS characteristics. EN results showed that the amplitude of ECN increased as the immersion time increased, which indicated that the corrosion degree of the system was more severe. b) The entire degradation process of phenolic epoxy coated tinplate in energy drink can be divided into three main stages. Stage I is the wetting of the organic coating; stage II is the corrosion initiation beneath the coating; stage III is the corrosion extension. c) The tin coating and carbon steel were mainly corroded by the organic acids in energy drink through the pores of the organic coating. After the tin coating was corroded, the carbon steel started to be corroded due to its higher electrochemical activity and became to be the dominated corrosion reaction. References [1] Rammelt U, Köhler S, Reinhard G. EIS Characterization of the Inhibition of Mild Steel Corrosion with Carboxylates in Neutral Aqueous Solution[J]. Electrochim. Acta, 2008, 53(23): [2] Blunden S, and Wallace T. Tin in Canned Food: A Review and Understanding of Occurrence and Effect[J]. Food Chem. Toxicol., 2003, 41(12): [3] Boogaard PJ, Boisset M, Blunden S, et al. Comparative Assessment of Gastrointestinal Irritant Potency in Mman of Tin(II) Chloride and Tin Migrated from Packaging[J]. Food Chem. Toxicol., 2003, 41(12): [4] Patrick GW. Internal Corrosion of Tinplate Food Containers[J]. Anti- Corros. Method. M., 1976, 23(6): 9-11 [5] Zumelzua E, and Cabezasb C. Observations on the Influence of Microstructure on Electrolytic Tinplate Corrosion[J]. Mater. Charact., 1995, 34(2): [6] Xia DH, Shi JB, Gong WQ, et al. The Significance of Correlation Dimension Obtained from Electrochemical Noise[J]. Electrochemistry, 2012, 80(11): [7] Xia DH, Song SZ, Wang JH, et al. Determination of Corrosion Types from Electrochemical Noise by Phase Space Reconstruction Theory[J]. Electrochem. Commun., 2012, 15(1): [8] Lou XY, Singh PM, Phase Angle Analysis for Stress Corrosion Cracking of Carbon Steel in Fuel-grade Ethanol: Experiments and Simulation[J]. Electrochim. Acta, 2011, 56(4): [9] Cheng XQ, Li XG, Yang LX, et al. Corrosion Resistance of 316L Stainless Steel in Acetic Acid by EIS and Mott-Schottky[J]. J. Wuhan Univ. Technol., 2008, 23(4): [10] Xia DH, Song SZ, Gong WQ, et al. Detection of Corrosion-induced Metal Release from Tinplate Cans Using a Novel Electrochemical Sensor and Inductively Coupled Plasma Mass Spectrometer[J]. J. Food Eng., 2012, 113(1): [11] Xia DH, Song SZ, Wang JH, et al. Corrosion Behavior of Tinplates in A Functional Beverage[J]. Acta Phys-Chim. Sin., 2012, 28(1):