Corrosion process detection of tinplate in deaerated ...

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Trans. Tianjin Univ. 2013, 19: 235-240 DOI 10.1007/s12209-013-2007-7

Corrosion Process Detection of Tinplate in Deaerated Functional Beverage by EIS* Wang Jihui (王吉会)1,Fu Congwei(付丛伟)1,Gao Zhiming(高志明)1, Xia Dahai(夏大海)1,2 (1. Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China; 2. Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4) © Tianjin University and Springer-Verlag Berlin Heidelberg 2013

Abstract:The corrosion process of tinplate in deaerated functional beverage was investigated by using electrochemical impedance spectroscopy (EIS) combined with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques. The results reveal that the uncoated tinplate shows a poor corrosion resistance and the corrosion type is detinning. During the initial stage of immersion, EIS spectrum consisted of two capacitance arcs with obvious time-constant dispersion effect, which was attributed to the two-dimensional and three-dimensional inhomogeneous distribution of the electrode surface. With the increase of immersion time, the capacitance arc of high frequency shrunk and degenerated, due to the corrosion of tin coating. The pore resistance of tin coating and the charger transfer resistance of substrate, which are determined from the electrochemical equivalent circuit, can be used as the indicators of tinplate corrosion process. The decrease of the pore resistance of tin coating indicates that the corrosion degree of tin layer becomes more severe, whereas the decrease of the charger transfer resistance of substrate implies that the corrosion degree of steel substrate also becomes more severe as the immersion time prolongs. Keywords:electrochemical impedance spectroscopy; tinplate; functional beverage; corrosion process; deaerated condition

With the development of modern food packaging, materials with good corrosion resistance and protective performances are desired. Tinplate is widely used in canning industry though new packaging materials such as aluminum and chromated steel sheet are increasingly being used[1]. Tinplate combines the formability and strength of steel with the good appearance and corrosion resistance of tin. However, there are some problems related to the use of this kind of cans, such as the alterations in sensory features and tinplate corrosion that affect food safety[2,3]. In some circumstances, corrosion can damage the appearance of the product and affect the nutritional value and healthiness of the canned food. Many semimacroscopic phenomena related to corrosion, either localized or uniform, are stochastic by nature[4]. Though tin is not considered to be a poisonous metal, the problem of food safety cannot be ignored if a large amount of tin dissolves into the food during corrosion process[5,6]. In

many cases, tinplate is applied to food canning and the corrosion of tinplate in acidic foods can be controlled at a certain rate, because tin is the sacrificial anode if coupled with carbon steel substrate[3]. For example, in a can containing foods which are predominantly acids like citric acid and lactic acid, the role of tin as a sacrificial anode is the basis of the electrochemical protection of carbon steel, making tinplate advantageous for food canning. In recent years, electrochemical techniques such as electrochemical noise (EN) and electrochemical impedance spectroscopy (EIS) have been the primary methods for evaluating the corrosion mechanism of both metal and coating/metal systems because of their in-situ measurement[7-14]. Some researchers used EIS method to evaluate the corrosion resistance of tinplate cans in the presence of both simulated electrolytes and food products. Pournaras et al [15] evaluated the lacquer adhesion failure, discoloration together with side seam steel corrosion in tinplated

Accepted date: 2012-09-13. *Supported by National Key Basic Research Program of China (“973” Program, No. 2011CB610505), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120032110029) and Key Project of Tianjin Natural Science Foundation (No. 13JCZDJC29500). Wang Jihui, born in 1966, male, Dr, Prof. Correspondence to Xia Dahai, E-mail: [email protected], [email protected].

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cans which contained cooked octopus in brine using energy dispersive spectroscopy (EDS) and EIS technique, and the EIS and EDS results could be used for rapid and simple evaluation of corrosion defects in tinplated food cans. Bastidas et al [16] studied the protective performance of lacquered tinplated cans in 0.1 mol/L citric-citrate test buffer solution using EIS technique which provided the in-situ information on the corrosion process of the tinplate/lacquer/electrolyte systems. In our previous paper, the corrosion process of tinplate in aerated NaCl solution[3] and functional beverage[2], and the corrosion process of lacquered tinplate in aerated functional beverage[5,6,17] were investigated by EIS combined with inductively coupled plasma mass spectrometer. Although there has been much interest in the corrosion problems of coated tinplate, there have been few studies on the corrosion type and corrosion process of uncoated tinplate under deaerated condition. Because it is difficult to investigate the corrosion behavior of organic coated tinplate, uncoated tinplate is often taken as the research object herein in order to study the corrosion type and corrosion process under the coating quickly. The aim of this work is to show how EIS technique can be used to determine the corrosion process of tinplate in deaerated functional beverage. By analyzing the EIS spectrum of tinplate, electrochemical parameters could be used as the discrimination indicators of tinplate corrosion process, and thus the corrosion performance of tinplate could be determined.

The reference electrode (RE) was a high-purity antimony electrode and the counter electrode (CE) was a commercial ruthenium-titanium electrode. The working electrode (WE) was tinplate with an area of 33.2 cm2. The EIS results were fitted via ZSimpWin Software. Tinplate was immersed in deaerated functional beverage at ambient temperature and examined regularly by EIS technique. The electrolytic cell was put in a sealed experimental device that was vacuumized by a vacuum pump and filled with N2 until air was taken out entirely before testing (Fig.1(b)). The pH value of the functional beverage was between 3.0—3.2 and the content of acid was between 0.30%—0.32%. The beverage contained organic acids such as citric acid and taurine; other ingredients were saccharose, essence, benzene sulfonic acid, sodium salt and citric yellow. The components of the beverage in detail can be found in Ref.[2]. 3

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1—Reference electrode; 2—Teflon plate; 3—Counter electrode; 4—Beverage; 5—Bolt. (a) Electrolytic cell for EIS measurement 1

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Experiment

The tinplate sheets were obtained from ORG Can Making Company. The thickness of tin layer was about 3 μm. The samples (70 mm × 70 mm) were degreased by ethanol and then dried by a hair drier. Scanning electron microscopy (SEM) was conducted to observe the changes in cross section morphology. Energy dispersive spectroscopy (EDS) was used to detect the element distribution. The EIS measurements were carried out by using a VersaSTAT 4 electrochemical workstation and VersaStudio software in a self-made electrolytic cell. The experimental device is shown in Fig.1. The EIS measurements were performed under the open circuit potential. A 5 mV alternating-current signal was imposed with frequencies stepped from 100 kHz to 10 mHz. EIS was performed by using the three-electrode method (Fig.1(a)). —236—

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1—Two holes for reference electrode and counter electrode; 2—Pipe for N2 inlet; 3—Pipe for N2 outlet; 4—Electrolytic cell for EIS measurement; 5— Polyvinyl chloride tank.

Fig.1

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Experimental device for EIS test

Results

2.1 EIS characteristics The impedance spectra measured for tinplate in deaerated functional beverage with various immersion time are plotted in Fig.2. After immersing for 10 min, the Bode plot shows two time constants ( Fig.2(a ) and

Wang Jihui et al: Corrosion Process Detection of Tinplate in Deaerated Functional Beverage by EIS

Fig.2(d)), indicating that the electrolyte has permeated through the pore of tin coating and the double-layer capacitance is formed on the carbon steel/tin interface. With the immersion time being prolonged, the radius of the capacitive reactance loop decreases and the low fre-

quency impedance at 0.01 Hz decreases from 14 960 Ω·cm2 to 4 320 Ω·cm2 (Fig.2(b) and Fig.2(c)), indicating the bad protective performance of tinplate. 2.2 EDS and SEM results Fig.3 is the SEM and EDS results of tinplate during

2 1 10 m m

(a) Nyquist plots of tinplate after immersing in functional beverage for 10 min, 5 h, and 2 d (a) Cross section image before immersion

(b) Nyquist plots of tinplate after immersing in functional beverage for 10 d and 15 d

(b) EDS result of tinplate before immersion

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(c) Bode plots (impedance module) of tinplate after immersing in functional beverage for 10 min, 5 h, 2 d, 10 d and 15 d -75

(c) Cross section image after 15 d immersion in functional beverage

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(d) Bode plots (phase angle) of tinplate after immersing in functional beverage for 10 min, 5 h, 2 d, 10 d and 15 d

Fig.2 EIS spectra of tinplate after immersing in deaerated functional beverage for different time

(d) EDS result of tinplate after 15 d immersion in functional beverage

Fig.3 Cross section images and EDS results of tinplate during corrosion process in functional beverage

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the corrosion process before and after immersing in deaerated functional beverage for 15 d. As mentioned above, the tinplate is made up of low carbon steel sheet coated with pure tin on both sides, so a tin layer with the thickness of 3 μm is observed in Fig.3(a). The EDS line scanning analysis on the cross section of tinplate shows that Fe content is higher in Region 1 but Sn content is lower; Fe content is lower in Region 2 but Sn content is higher (Fig.3(b)), indicating that Region 1 is the substrate metal (carbon steel) and Region 2 is the tin layer. Fig.3(c) is the SEM image of the cross section of tinplate after 15 d immersion in functional beverage. Because of corrosion and detachment, tin layer almost can not be observed. Only the substrate metal (Region 1) is observed. The EDS line scanning analysis on the cross section shows that Fe content is still higher in Region 1 but Sn content is lower; Region 2 almost disappears (Fig.3(d)), indicating that tin is corroded away.

refers to the “semicircle” rotating downwards. The timeconstant dispersion can be attributed to the distribution of time constants along either the axis normal to the electrode surface (involving a three-dimensional aspect of the electrode) or the area of the electrode (involving only a two-dimensional surface). The two-dimensional distribution can be attributed to the change along the tinplate surface of reactivity or of potential and current. The timeconstant dispersion can also be induced by a distribution of time constants that reflect a local property of the tinplate surface, resulting in a three-dimensional distribution. EIS response typically reflects a distribution of reactivity that commonly exhibits in equivalent electrical circuits (EEC) as a constant phase element (CPE) which is not a simple capacitance. The expression corresponding to the impedance of CPE is shown in Eq.(1)[18]: 1 n Z CPE   j  , 0<n<1 (1) Y0 where Y0 is a positive value and its unit is Ω·cm2·s n or S·cm2·s n; ω is the angular frequency; j is the imaginary symbol; the parameter n is a dimensionless parameter, indicating the dispersion degree of CPE from a capacitance. When n = 1, CPE represents a capacitance element; when 0<n<1, CPE presents behaviors that have been attributed to the two-dimensional and threedimensional distribution. Both Cc and Cdl are CPEs in this paper. The two-dimensional distribution can stem from the surface heterogeneity of tinplate surface, for example, SnO film formed on the electrode surface in atmospheric air or other factors in surface properties. This CPE behavior may also come from a variation of properties in the direction that is normal to the electrode surface, and this variation may be attributed to the changes in the conductivity of SnO oxidation film or the surface roughness or the porosity of electrode. From the equivalent circuit fitting results, the n value of Cdl increases from 0.608 3 to 0.972, indicating that the interface heterogeneity caused by the two-dimensional and three-dimensional distribution is reduced [19,20]. After immersing in functional beverage for 5 h, the corrosion products were observed on the surface of tinplate, indicating the poor protective performance of tin layer. In Bode plot, |Z|-f curve moves towards lowfrequency region (Fig.2(c)). After 2 d, the radius of semi-circle decreases, indicating the continuous corrosion process of tin. After 10 d, the capacitance arc of high frequency shrinks and degenerates, indicating that tin has lost protective performance, most tin layer was corroded -

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Discussion

The electrochemical equivalent circuit in Fig.4 was applied to explain the behavior of the tinplate-electrolyte system[2]. In Fig.4, Re is the electrolyte resistance; Cc is the capacitance of tin coating; Rc is the pore resistance of tin coating; Cdl is the double-layer capacitance of the substrate metal; Zf is the charge transfer resistance of the substrate metal. This circuit is used for the fitting of the EIS results. The criterion adopted for the fitting admits a relative error of less than 5% for the real and imaginary parts of the impedance. After immersing in functional beverage for 10 min, the Bode plot has the characteristic of two time constants (Fig.2(d)). The time constant at higher frequency region corresponds to Cc and Rc; while the time constant at lower frequency region corresponds to Cdl and Zf , and Zf reflects the corrosion rate of the substrate metal.

Fig.4 Electrochemical equivalent circuit used for corrosion of tinplate

According to Fig.2(a) and Fig.2(b), it can be seen that the time-constant dispersion is obvious in the EIS plots of 10 min, 5 h and 2 d. This dispersion phenomenon —238—

Wang Jihui et al: Corrosion Process Detection of Tinplate in Deaerated Functional Beverage by EIS

and corrosion products fell off. It can be observed that uncoated tinplate shows poor anti-corrosion performance and the typical corrosion type is detinning corrosion. From the EDS line scanning results(Fig.3(d)), no tin is observed after immersing in functional beverage for 15 d. The Rc and Zf values obtained from the electrochemical equivalent circuit are shown in Fig.5. Because tinplate shows bad anti-corrosion performance in acid medium, the Rc value decreases along with the corrosion of tin, indicating that the pore resistance of tin layer decreases fast. Rc value decreases to 0.015 5 kΩ·cm2 after 15 d. So the Rc value can characterize the process of tin corrosion and detachment of corrosion products. In addition, the Zf value also decreases fast from 12.78 kΩ·cm2 to 4.192 kΩ·cm2, indicating the corrosion degree of the substrate metal becomes more serious.

tance arc of high frequency shrinks and degenerates, indicating that the tin coating is corroded away. The decrease of Rc value indicates the severe corrosion degree of tin layer, whereas the decrease of Zf value implies the lower corrosion resistance of steel substrate. The electrochemical parameters Rc and Zf can be used as the indicators of tinplate corrosion process. References [1] Zheng X, Xia D H, Wang H H et al. Detection of the corrosion degree of beverage cans using a novel electrochemical sensor [J]. Anti-Corrosion Methods and Materials, 2013, 60 (3): 153-159. [2] Xia D H, Song S Z, Wang J H et al. Corrosion behavior of tinplates in a functional beverage[J]. Acta PhysicoChimica Sinica, 2012, 28(1): 121-126. [3] Xia D H, Song S Z, Wang J H et al. Corrosion behavior of tinplate in NaCl solution[J]. Transactions of Nonferrous Metals Society of China, 2012, 22(3): 717-724. [4] Zhang X G, Wang J, Zhou Y W. Analytical modeling for corrosion-induced cover cracking of corrosive reinforced concrete structures[J]. Transactions of Tianjin University, 2012, 18(4): 285-290. [5] Xia D H, Song S Z, Gong W Q et al. Detection of

Fig.5 Evolution of Rc and Zf values of electrochemical equivalent circuit as a function of immersion time

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