夏大海 宋詩哲,2 王吉會(huì),2,* 畢慧超 韓哲文

(1天津大學(xué)材料科學(xué)與工程學(xué)院,天津300072;2天津市材料復(fù)合與功能化重點(diǎn)實(shí)驗(yàn)室,天津300072)

鍍錫薄鋼板在功能飲料中的腐蝕行為

夏大海1宋詩哲1,2王吉會(huì)1,2,*畢慧超1韓哲文1

(1天津大學(xué)材料科學(xué)與工程學(xué)院,天津300072;2天津市材料復(fù)合與功能化重點(diǎn)實(shí)驗(yàn)室,天津300072)

應(yīng)用電化學(xué)阻抗譜(EIS)技術(shù),結(jié)合掃描電鏡(SEM)、能量散射X射線譜(EDS)、X射線光電子能譜(XPS)、掃描探針顯微鏡(SPM)等表面分析技術(shù),研究了鍍錫薄鋼板在功能飲料中的腐蝕過程并探討了腐蝕機(jī)制.結(jié)果表明:浸泡前期,EIS低頻阻抗模值的增加與前31 h鍍錫薄鋼板表面形成的腐蝕產(chǎn)物膜有關(guān);隨著浸泡時(shí)間的增加,EIS低頻阻抗模值有所下降,這與腐蝕產(chǎn)物膜的部分脫落以及暴露的基底金屬碳鋼的腐蝕有關(guān).鍍錫薄鋼板在功能飲料中浸泡24天后其表面的腐蝕產(chǎn)物膜由外層的富錫層和內(nèi)層的富鐵層組成,XPS結(jié)果表明其成分主要是Sn(II)/Sn(IV)與檸檬酸陰離子及Fe(III)與檸檬酸陰離子組成的化合物,其腐蝕類型主要是功能飲料中的有機(jī)酸對(duì)鍍錫薄鋼板的腐蝕.

鍍錫薄鋼板;功能飲料;腐蝕機(jī)制;電化學(xué)阻抗譜;X射線光電子能譜;掃描探針顯微鏡

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 being used by the canning industry.1-3As 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.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.4In addition,other studies5,6also indicated that high levels of tin in food products may lead to food safety problems.Though tin is not considered to be a poisonous metal,very large doses can produce serious digestive disturbances.5,6The corrosion process of pure tin is relatively simple due to its homogeneity.Numerous studies have been focused on the corrosion behaviors of pure tin.7-14But 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.4,15The 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 functional beverage are still unknown.As we know,functional beverage is a kind of strong corrosive medium.Therefore a study on the corrosion behavior of tinplate in functional beverage has noted significance.16-18The different industries involved in tinplate cans(tinplate producers,can manufactures,food canners,etc.)and health authorities are in need of information about the behavior of this material.

Tinplate is a coating/metal system and its electrochemical signal during corrosion process can be detected by the electrochemical impedance spectroscope(EIS)technique,19-23hence the anticorrosion performance of tinplate can be quantitatively and semi quantitatively evaluated.In this paper,tinplate without organic coatings is taken as the research object in order to investigate the corrosion behavior under organic coatings.The purpose of the present work is to study the corrosion process of tinplate in functional beverage.The corrosion mechanism of tinplate in functional beverage is proposed.

2 Experimental

All the samples were provided by the ORG Canmaking Company,China.The thickness of tin coating on both sides was about 3 μm.The samples(70 mm in length and 70 mm in width)were degreased by ethanol then dried by a hair drier before exposed to functional beverage.The energy spectrum analysis was obtained by an energy dispersive X-ray analyzer (EDAX Genesis,Hitachi,Japan).Scanning electron microscopy(SEM)analyses were carried out on a field-emission scanning electron microscope(FE-SEM S4800,Hitachi,Japan). The surface morphology was characterized by using an AJ-IIIa Scanning Probe Microscopes(Shanghai AJ Nano-science Development Company Limited,China).The XPS spectra were taken using a commercial X-ray photoelectron spectroscopy (PHL1600ESCA XPS).A survey spectrum was first recorded to identify all elements present at the surface,and then high resolution spectra of the following regions were recorded:C 1s, Fe 2p,O ls,and Sn 3d.A Shirley background subtraction was made to obtain the XPS signal intensity.

Fig.1 Experimental device for electrochemical measurement

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 ruthenium-titanium electrode as the counter electrode(CE)was used(Fig.1).The measurements were at the open-circuit potential with a 5 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 cm2.The sample was exposed to functional beverage in contact with air at about 20°C and examined periodically by EIS technique.Fitting was performed with ZSimpWin Software.The pH value of the functional beverage was between 3.0 and 3.2.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 μg·L-1),and other food additives were citric acid,saccharose,essence,benzene sulfonic acid sodium salt,and citric yellow,etc.

3 Results

3.1 EIS analysis

The impedance spectra measured for tinplate in functional beverage with different time are shown in Fig.2,where Zimis the imaginary part of impedance,Zreis the real part of impedance,|Z|is impedance modulus,and θ is phase angle.When the immersion time was 20 min,there were two time constants in the Bode diagram and it degenerated into one time constant with increasing the immersion time.Two time constants indicated that the electrolyte had permeated through the defects or pores in the tin coating and double layer capacitance was formed in it.With the increase of immersion time,the defects or pores were filled with the corrosion product so the EIS plots basically were one time constant.The impedance modulus in low frequency began to increase at the first 31 h and decreased after 31 h,because a layer of corrosion product film with compact structure formed first and then part of the corrosion product film detached,which would be further confirmed by SEM analysis below.

Fig.2 EIS plots of tinplate exposed to functional beverage with different time (a,b)Nyquist plots,(c,d)Bode plots

3.2 Surface and cross section morphologies

Fig.3 shows the SEM images of tinplate after being exposed to functional beverage with different days.No corrosion products were observed on the surface before being exposed to functional beverage(Fig.3(a))while long bar shaped corrosion products with compact structure formed on the surface after 24 h(Fig.3(b)).With increasing time,it was found that some of the corrosion products even detached after being exposed to functional beverage for 24 d(Fig.3(c)).

The component of the corrosion product was investigated using energy dispersive X-ray spectroscopy(EDS)in order to further study the corrosion mechanism.In Fig.4,region 1 was the place where corrosion product detached and region 2 was the place where corrosion product did not detach.C,O,Sn,Fe were detected in the two regions,but their contents showed a big difference.We can see the place where the corrosion products detached(region 1)contained 61.89%(atomic fraction)Fe and 0.93%Sn,while region 2 contained 43.65%Fe and 12.69%Sn.In addition,the detected C element was mainly from the functional beverage.Moreover,scanning probe microscopy(SPM)was used to investigate the surface morphology in an micro-nano level,which would help to understand the corrosion mechanism.From small scale SPM images,it can be seen that defects or pores exist in the surface before immersion (Fig.5(a))while corrosion product with compact structure formed on the surface(Fig.5(b)).The waviness of coating surface was about 20 nm before immersion and increased to 150 nm due to the corrosion product after being exposed to functional beverage for 24 h(Fig.5(b)).As mentioned above,two time constants in EIS plots indicated that the electrolyte had permeated through the defects or pores in the tin coating and double layer capacitance was formed in it at the first couple of hours.With increasing the immersion time,the defects of pores were filled by the corrosion product so the EIS plots basically were one time constant.

Fig.3 SEM surface images of tinplate after being exposed to functional beverage with different time

Fig.4 Compositional analysis and compositional contents of region 1 and region 2 in Fig.3(c)

Fig.5 SPM surface images of tinplate after being exposed to functional beverage for(a)0 h and(b)24 h

The morphology and EDS analysis of the cross section of tinplate before and after being exposed to functional beverage are shown in Fig.6.The tin coating of the tinplate before exposure was about 3 μm(Fig.6(a)).After being exposed to functional beverage for 24 d,two layers of corrosion product films were observed from Fig.6(b).The surface layer was first enriched in Sn and then enriched in Fe.

3.3 XPS study of the corrosion product film

Fig.6 SEM images and compositional line profiles of cross section(a)before immersion;(b)after immersion in functional beverage for 24 d

In order to analyze chemical composition of the corrosion product film,X-ray photoelectron spectroscopy was investigated.Fig.7 shows high-resolution XPS spectra of the C 1s,O 1s, Fe 2p,and Sn 3d regions for the corrosion product film after being exposed in functional beverage for 24 d.The deconvoluted C 1s lines of the spectra were taken with three and four Gaussian functions centred at 284.6,286.2,288.0 eV(Fig.7 (a)).The C 1s peaks at 284.6 eV could be safely attributed to aliphatic－C－C－ bonds.The sub-peaks with maxima at 288.7 eV may be assigned to carbon atoms of－COO?groups.9The intermediate peaks at 286.2 can be ascribed to carbon atoms of C－OH groups of citrate ions.Fig.7(b)shows O 1s region of the corrosion product film.The O 1s region was composed of two peaks.The dominant peaks at 529.5 and 531.0 eV were attributed to O2?.9,24The first one at 529.5 eV could be attributed to SnO or SnO2,9the second one at 531.0 eV can be attributed to tin hydroxide species as well as to the oxygen atoms of the carboxylic groups combined with metal ion(－COOM) or hydrogen(－COOH).Fig.7(c)is Fe 2p region.The dominant peaks at 711.5 and 724.3 eV were attributed to Fe(III).24,25The deconvoluted Sn 3d spectrum shows two components (Fig.7(d))corresponding to tin in form of Sn(II)or Sn(IV).Unfortunately,the obtained XPS spectra do not allow to distinguish unambiguously the Sn(II)oxidation state from that of Sn(IV).9

The XPS spectra of the corrosion product film contain C 1s lines that could be attributed to the carbon atoms of the carboxylic groups of citrate ions.Therefore,the data of the XPS study could give an indication of citrate ions inclusion in the composition of the corrosion product film.The corrosion product film was mainly composed of Sn(II)/Sn(IV)citrate complex or Fe(III)citrate complex.

4 Discussion

Based on the morphology and electrochemical analysis,the corrosion process of tinplate in functional beverage is discussed.It is proposed that tinplate is mainly corroded by organic acid existed in functional beverage and the process can be divided into three main stages which are shown as follows.Stage I was the corrosion of tin coating and corrosion product film began to form on the surface.Stage II was the corrosion of carbon steel.Stage III was the detachment of corrosion product of tin.The corrosion process of tinplate in functional beverage is illustrated schematically in Fig.8.

4.1 Corrosion of tin coating

Fig.7 XPS spectra of the corrosion product film of the tinplate after being exposed in functional beverage for 24 d

When the tinplate was exposed to functional beverage containing much organic acid,tin was directly exposed to the electrolyte and tin was not stable in organic acid solution.26So tin was easily corroded in the earlier stage.The corrosion product film was formed on the surface.From the EIS results,it was found that the radius of the semi-circle began to increase in the first 31 h;from the SEM observations,it was found that a corrosion product film with compact structure was formed on the surface.From the XPS analysis,it was found that the corrosion product film was mainly composed of Sn(II)/Sn(IV)citrate complex and Fe(III)citrate complex.

Fig.8 Schematic diagrams of the corrosion mechanism of tinplate in functional beverage

4.2 Corrosion of carbon steel

When some part of the tin coating corroded away,the Sn-Fe alloyed layer(FeSn2)was exposed to aggressive solution but the layer was so thin and carbon steel was soon exposed to the electrolyte.Due to the potential difference between tin and carbon steel,and the latterhas higher electrochemicalactivity,so preferential corrosion occurred on the exposed carbon steel.The corrosion product of carbon steel was under the corrosion product of tin,whichwas confirmedby SEM andEDS analyses.

4.3 Detachment of the corrosion product of tin

Due to the corrosion of the carbon steel,the corrosion product of carbon steel became thicker and thicker,which made the corrosion product of tin become looser and looser,and it detached at last.From the EIS results,it was found that the radius of the semi-circle began to decrease after 31 h,indicating the compact corrosionproductfilmbecamelooseandultimatelydetached.

5 Conclusions

The corrosion behavior of tinplate in functional beverage was investigatedandtheresultspermitthefollowingconclusions:

After tinplate exposed to functional beverage for 24 d,the corrosion product film on tinplate was mainly composed of outer long bar shaped product of Sn and product of Fe in the inner layer.The corrosion product film was mainly composed of Sn(II)/Sn(IV)citrate complex and Fe(III)citrate complex.It is concluded that tinplate is mainly corroded by organic acid existed in functional beverage and the corrosion process can be divided into three main stages shown as follows.Stage I was the corrosion of tin coating,a layer of corrosion product film formed on the surface.Stage II was the corrosion of carbon steel,the corrosion product of carbon steel was formed under the corrosion product of Sn.Stage III was the detachment of corrosion product.

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Corrosion Behavior of Tinplates in a Functional Beverage

XIADa-Hai1SONG Shi-Zhe1,2WANG Ji-Hui1,2,*BI Hui-Chao1HAN Zhe-Wen1
(1School of Materials Science and Engineering,Tianjin University,Tianjin 300072,P.R.China;
2Tianjin Key Laboratory of Composite and Functional Materials,Tianjin 300072,P.R.China)

In this paper,the corrosion process of a tinplate in a functional beverage was investigated using electrochemical impedance spectroscope(EIS),scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS),scanning probe microscopy(SPM),and X-ray photoelectron spectroscopy(XPS),and a corrosion mechanism is proposed.We conclude that an increase in the impedance modulus at low frequency is due to the corrosion product forming on the surface of the tinplate over the first 31 h.With an increase in the immersion time a decrease in the impedance modulus at low frequency is due to the detachment of the corrosion product and the corrosion of the carbon steel substrate.X-ray photoelectron spectroscopy(XPS)results show that the corrosion product is mainly composed of a Sn(II)/Sn(IV)citrate complex or an Fe(III)citrate complex.Furthermore,the corrosion product film is first enriched with Sn and then enriched with Fe after immersion in functional beverage for 24 d.We propose that the tinplate is mainly corroded by the organic acids that exist in functional beverages.

Tinplate;Functional beverage;Corrosion mechanism;Electrochemical impedance spectroscope;X-ray photoelectron spectroscopy;Scanning probe microscopy

10.3866/PKU.WHXB201228121

*Corresponding author.Email:jhwang@tju.edu.cn;Tel:+86-22-27890010.

The project was supported by the National Key Basic Research Program of China(973)(2011CB610500).

國家重點(diǎn)基礎(chǔ)研究發(fā)展規(guī)劃項(xiàng)目(973)(2011CB610500)資助

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