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Corrosion problems in tinplate cans for storing contact glues for shoes
Engineering Failure Analysis 26 (2012) 258–265
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Corrosion problems in tinplate cans for storing contact glues for shoes J.I. Martins ⇑ Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Porto, Portugal
a r t i c l e
i n f o
Article history: Received 18 May 2012 Received in revised form 25 June 2012 Accepted 3 August 2012 Available online 8 September 2012 Keywords: Tinplate Corrosion Electroplating Cans Adhesive glue
a b s t r a c t Tinplate steel cans were used to store contact glue for shoes. A few months after its storage, the glue displayed a change in colour to a darker brown. The visual inspection of the interior of the cans showed the presence of corrosion of steel, which was evidenced by the typical colour of the iron oxides on the sheet. SEM/EDS and FTIR tests confirmed that the anomaly was due to the following factors: (a) a bad tin coating, small thickness and lack of homogeneity; (b) a degradation of glue, hydrolysis of chloroprene, which led to its acidification. Some recommendations are made to overcome similar situations within the same use of these packages looking at the specifications of tinplate. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Tinplate is a thin low-carbon mild steel sheet, cold reduced, coated on both sides with pure electrolytic tin, linking the strength and formability of steel with the corrosion resistance and weldability of tin. The resulting coating has the following stratified structure from the bottom to the top [1–3]: an FeSn2 alloy developed from the flow-brightening process that protects the steel against galvanic corrosion by oxidant species; a free tin layer that determines the useful live when acting as a sacrificial anode; a layer of chromium and tin oxides and metal chromium that prevent tin oxide growth and sulphuration processes; and finally a lubricant film covered with a sanitary lacquer as protection barrier between the packed material and the substrates of the container. It is one of the most widely packaging materials in food and drinks cannery. However, there is also another important market in the area of glues, oils, grease, paints, powdered, polishes, waxes, chemicals and many other products. Aerosol containers and caps and closures are also made from tinplates. The container integrity problems reported in the literature [4–10] are mainly related to food products, such as: (a) stress corrosion cracking (SCC); (b) sulphide black corrosion and occasionally discoloration on the product surface; (c) pitting corrosion involving rapid iron dissolution at fractures or pores of coating; (d) filiform corrosion with the formation of rust on the external surface of metal container due to scratch defects in the organic coating. A lot of works have been dedicated on the corrosion of pure tin and tinplate [11,12]. Anodic performances of tin showed active–passive transition in some media, for example, citrate solutions [13–16], Na2CO3 solutions with or without Cl or I ions [17], sodium borate solutions containing halide ions and some inorganic inhibitors [18], and NaOH solutions [11]. Looking to standard potential of the Sn/Sn2+ and Fe/Fe2+, respectively, 0.131 and 0.441 V vs. NHE, it is apparently contradictory to use the tin as sacrificial anode in the galvanic couple with the iron. However, in several media of fruit juices and pure organic acids the tin is more active than the iron. According to the work of Hoar [19] performed with tin and iron elec-
⇑ Tel.: +351 22 508 1643; fax: +351 22 508 1449. E-mail address: [email protected] 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2012.08.003
259
J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265
trodes into dilute acidic media, the tin and iron are covered with oxide-films. After the initial instants, the films dissolve and the potentials of both metals fall to values characteristics of the bare metals. But, the formation of stable tin complexes in those electrolytic media becomes the tin more active than the iron [3,20]. Therefore, the mainly factors determining the polarity of the tin–iron couple are the presence or absence of oxide-films on the metallic surfaces, the composition and structure of the steel, and the ability of the electrolyte to remove stannous ions as complexes. Nevertheless it must be remembered that under other conditions, such as in carbon disulphide [21] and sulphuric acid [19], the iron becomes and remains the anode. This paper deals with the analysis of the failure of tinplate cans used to store contact glue for shoes. 2. Experimental details The cans were made with flat cold-rolled low carbon steel (MR, SPCC) product batch annealed, thickness of 0.25 mm, coated on both faces by electrolytic tin (Ferrostan process). Sales specification states tin coating weight 2.8 g/m2 on each side. Table 1 gives the standard chemical composition of MR steel. The bright surface finish is obtained through the melting of the tin coating on the steel smooth surface. A thin and uniform oil film (dioctyl sebacate, DOS) is applied on both surfaces, in order to prevent abrasion and facilitate later processing on automatic machines. This tinplate sheet will be designed as TP1 and another from different provenance as reference by TP2. The glue, Maprene, disposed within the cans in addition to polychloroprene has the following compounds: ethyl acetate (10–25%), rosin (5–10%), naphtha (petroleum), hydrogen-treated lightweight (25–50%) and toluene (5–10%). The glue with and without a brownish colour are referred, respectively, to as Ciprene 2603 and Ciprene 2626. The determination of pH in the glues was performed in two ways: (1) direct reading of the product as such although with some instability; (2) indirect reading on the decanted water, after to have emulsified previously the glue with water at pH = 7. The scanning electron microscopy (SEM) images and the analysis by energy dispersive spectroscopy X-ray (EDS) were made with an equipment FEI Quanta 400FEG, fitted with a probe for micro analysis EDAX Genesis X4M. The pressure inside the chamber was about 6 102 Pa. The distance between the objective lens and the sample, ranged between 6 mm and 15 mm. The SEM filament was operated at variable current and at 15 kV voltage using magnifications 100, 250 and 1000. SEM/EDS analysis were performed in CEMUP (Centro de Materiais da Universidade do Porto). The IR spectra were recorded in diffuse transmittance mode using a FT-IR Nicolet 6700-ThermoElectron Corporation equipment. 3. Initial findings The specifications of tinplate refer a tin thickness coating of 0.38 lm on both sides, without passivation with a finishing oil film. The presence of water inside the cans depends on the purity of the organic compounds of the glue. The ethyl acetate (88– 97%) may contain water up to a maximum value of 5% and the maximum content of water allowed to toluene (99.7–99.97%) is 0.03–0.01%. The glue for shoes in question have the following generic composition: water (0.4–1.0%) from organic solvents, polychloroprene (10–15%), synthetic resins (aliphatic, aromatic, rosin), various organic solvents (toluene, hexane, acetone, ethyl acetate, etc.), and additives such as magnesium or zinc oxide required for the resinate reaction to proceed. The observation of the interior of cans, Fig. 1, shows the presence of rust, which indicates a localized corrosion associated with the steel. Of course, the deformed areas to give rigidity to the can have a higher density of red spots. This suggests that the tin–iron couple has a behaviour predictable from the electrochemical series potential, i.e., the iron is the anode. The medium does not stabilize stannous ion complex or, even if that happens leaves the tin over time to maintain the cathodic protection on the iron in the center of the defective area of tinplate. 4. Results and discussion 4.1. Scanning electron microscopy SEM tests by secondary electrons (SE) or backscattering electrons (BE) were carried out on new sheet (TP1), Fig. 2, and on sheet affected by corrosion after contacting the glue (TP1F), Fig. 3. Table 1 Chemical composition of MR steel (% maximum). Steel type
C
P
Mn
Si
S
Ni
Cr
Cu
Mo
MR
0.13
0.020
0.60
0.02
0.05
–
–
0.2
–
260
J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265
Fig. 1. Interior view of the damaged can; (b) Magnification, detailing the strained zone and the cylindrical surface.
(a)
(b)
Z2 Z1
60 µm
60 µm Fig. 2. SEM 1000 of new sheet TP1: (a) SE; (b) BE.
(b)
(a)
Z4
Z1 200 µm
Z3
Z2
200 µm
(c) Z5F
600 µm Fig. 3. SEM 250 sheet border, TP1F: (a) SE; (b) BE; SEM 100 sheet body, TP1F: (c) BE.
Fig. 2a shows a striated structure associated to its processing and transport, but also defects and a lack of uniformity of the tin coating. The observation by BE, Fig. 2b, clarify this latter aspect: light zones indicate the tin (metal element heavier than iron) and the darker the iron. The EDS analysis in the light, Z1 (8.65% Fe, 91.35% Sn), and dark, Z2 (66.61% Fe, 27.88% Sn) areas show the presence of iron which means a tin thickness of less than 1 lm, taking into consideration the penetration of the electron beam (1–1.5 lm).
261
J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265 Table 2 Chemical composition in different areas of the damaged can. Element
Z1F (%)
Z2F (%)
Z3F (%)
Z4F (%)
Z5F (%)
C O Mg Si Cl Fe Sn Ca Na Al S
69.85 5.89 0.81 0.18 22.80 0.46 n.d. n.d. n.d. n.d. n.d.
48.09 13.99 0.10 0.07 0.41 15.91 n.d. 0.11 n.d. n.d. n.d.
45.78 27.98 1.18 0.48 8.09 n.d. n.d. n.d. 0.56 0.33 0.53
45.95 17.87 1.94 0.31 15.34 16.77 0.84 0.67 n.d n.d. 0.35
21.89 4.26 0.32 n.d. 6.60 19.27 47.67 n.d. n.d. n.d. n.d.
(b)
(a)
Z1 Z2
60 µm
60 µm Fig. 4. SEM 1000 sheet border, TP2: (a) SE; (b) BE.
Fig. 3 displays the SEM observations of the border and body of sheet damaged by corrosion and table 2 the chemical composition by EDS of the areas Z1F, Z2F, Z3F, Z4F, border zone, and Z5F, body zone. The presence of the elements Cl, Si, Ca, Na, S, Al, Mg and C on the damaged areas confirm the generic composition of the glue: polychloroprene (ACH2ACClCHACH2A), toluene (C6H5CH3), methyl acetate (CH3COOCH3), oil, mixture of hydrocarbons (C5AC12), rosin (C20H30O2), and additives containing metallic elements, Mg, Si, Ca, Na, Al, and non-metals, S. The severely attacked areas (Z1F, Z4F and ZF2) do not contain tin or it is much less compared to unaffected areas (Z1 and Z2). The corrosion is more pronounced in the deformed zones of the can. The observations on tinplate sheet from another supplier (TP2) and used as reference for the tinplate sheet in question are in Fig. 4. The photomicrographs show also a striated structure for the sheet, light (96.60% Sn, 3.40% Fe) and dark (26.99% Sn, 73.01% Fe) areas, but the tin coating is more uniform and thicker than TP1. It was not possible to determine the thickness of the tin coating on tinplate from the microscopic observation of a section previously mounted on a resin and polished. This was due to the thinness of the coating together with the action of wear and deformation of the tin. This problem was solved through a bending of the tinplate sheet. The tin coating was broken and separated partially from the base substrate allowing the determination of its thickness by conveyable observation by SEM. The thickness of TP2 sheet is higher than the TP1 sheet as is shown in Fig. 5. 4.2. Hardness The sheets TP1 and TP2 have Vickers hardness (charge 100 g) 107 HV and 134 HV, respectively. The different hardness values are related to the deformation imposed on the rolling operation of the sheets. 4.3. FTIR analysis Figs. 6 and 7 show the FTIR spectra obtained for the Ciprene 2626 and 2603. The bands located in the range 550–750 cm1 and 3200–3500 cm1 are attributed, respectively, to stretching vibration of the CACl and OH (hydrogen bonds) [22]. The FTIR results show that the bands located at 695 cm1 and around 730 cm1 related to CACl bond have a higher transmittance for Ciprene 2603 than CIPRENE 2626, while the band located at 3420 cm1 linked to OH is smaller. This means that CACl bonds are converted to CAOH bonds, i.e., the glue CIPRENE 2603 underwent hydrolysis.
262
J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265
(a)
(b)
290 nm 161 nm
256 nm
267 nm
6 µm
CEMUP SE 1 FeSn x9500 15kV
6 µm
(c)
294 nm
375 nm
3 µm Fig. 5. Determination of thickness in TP1 and TP2 sheets: (a) TP1; (b) and (c) TP2.
40
Ciprene 2626
Transmittance (%)
30
20
10
0 4000
3000
2000
1000
Wave number (cm-1) Fig. 6. FTIR spectrum of Ciprene 2626.
4.4. Determination of pH of the glue Table 3 shows the results of pH of the glues obtained by direct and indirect reading. Ciprene 2603 expressed instability in the direct reading, but in indirect reading clearly indicates a tendency to an acid pH, conversely to Ciprene 2626 that shows pH around 10. Observed acidification in the glue comes from the release of chloride ions from the slow hydrolysis of chloroprene. The allylic carbon–chlorine bonds (C@CACACl) presented in chloroprene units which have been polymerized by 1,2- addition is believed to be particularly susceptible to hydrolysis [23] as shown in the following reaction:
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J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265
Ciprene 2603
Transmittance (%)
25
20
15
4000
3000
2000
1000
Wave number (cm-1) Fig. 7. FTIR spectrum of Ciprene 2603.
Table 3 pH values of the glue obtained by direct and indirect readings. Product
Ciprene 2603 Ciprene 2626
pH
State of cans
‘‘As it is’’
Decanted water
6.30–8.80 10.14
5.49 9.30–9.70
Damaged Not damaged
ð1Þ
Taking into account the observations by SEM/EDS we can say that the CIPRENE 2603 system will be subject to corrosion in acid electrolyte as shown in Fig. 8. In the absence of discontinuities in the tinplate, the tin corrodes due to local-action cells with a current ISn = Iuc,a, Fig. 8a. The cathodic reaction is the reduction of oxygen in acidic medium considering the value of the hydrogen overvoltage on tin.
Anodic reaction : Sn ! Sn2þ þ 2e
ð2Þ
Cathodic reaction : 2e þ 1=2O2 þ Hþ ! OH
ð3Þ
The irregularity of the tin coating will cause the areas of lesser thickness corrode quickly, and therefore showing the iron earlier, Fig. 8b. At this point we have a tin–iron couple, the tin is a sacrificial anode and corrodes with the current ISn = Iuc,a + Ic,a. When the exposed area of the iron is raised by the action of the corrosive medium, tin adjacent no longer be able to fully protect the iron, and iron corrodes due to local-action cells [24] according the reactions 4–6 justifying the red spots observed in the damaged cans, Fig. 8c. At that moment, will have a tin–iron couple, and the tin and iron corrode with the current, respectively, ISn = Iuc,a + Ic,a and IFe = Iuc,a1.
Anodic reaction : Fe ! Fe2þ þ 2e
ð4Þ
Cathodic reaction : 2e þ 2Hþ ! H2
ð5Þ
Rust formation reaction : 2Fe þ 3=2O2 þ H2 O ! Fe2 O3 H2 O
ð6Þ
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J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265
(a)
(b)
(c)
Fig. 8. Scheme of the evolution of corrosion in tinplate: (a) local-action cells; (b) tin-iron couple with total cathodic protection of iron; (c) tin-iron couple with corrosion of iron.
Corrosion of both metals is therefore coupled to the reduction reactions of oxygen and hydrogen ions. Charge balance is maintained by summing all anodic and cathodic currents to zero.
Ic;a þ Iuc;a þ Iuc;a1 ¼ Ic;c þ Iuc;c þ Iuc;c1
ð7Þ
5. Conclusions and recommendations According to the experimental results it is concluded the following: 1. Tinplate TP1 sheet has a coating thickness lower than the theoretically predicted (0.38 lm) from its characteristics (weight 2.8 g/m2 of Sn). This irregularity is accentuated in the deformed zones, i.e., in the border area of the can. 2. Tinplate TP2 used as reference showed a better tin coating in comparison with TP1. 3. The Ciprene 2603 suffered a hydrolysis phenomenon that makes the glue more acid, i.e., the medium becomes more corrosive. The tin was quickly consumed by corrosion, and earlier presents the steel substrate. After this moment the corrosion of steel develops the characteristics red products, more sharply in the restricted zone submitted to cold hardening work. As tin is an active metal, it is recommended the following: 1. 2. 3. 4.
Put in tinplate sheet specifications that tin is easier to corrode, especially in acid and complexing media. Use a tin coating thicker, for example, weight 5.6 g/m2 of Sn. Apply 341 passivation treatment on tin and then lacquer to improve corrosion resistance. Elucidate the final consumers that packages are not eternal because the product and the can are susceptible to degradation.
J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265
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References [1] Zumelzu E, Cabezas C. Observations on the influence of microstructure on electrolytic tinplate corrosion. Mater Charact 1995;34:143–8. [2] Do Nascimento GG, Dos Santos JLC, Margarit ICP, Mattos OR. Lacquered tinplate: corrosion resistance in the function of lacquering conditions. Electrochim Acta 2000;46:671–9. [3] Da-hai X, Shi-zhe S, Ji-hui W, Hui-chao B, Yu-xuan J, Zhe-wen H. Corrosion behaviour of tinplate in NaCl solution. Trans Nonferrous Met Soc China 2012;22:717–24. [4] Resnik D, Can corrosion, In: Marsch K, Brody A, (editors.) Wiley’s Encyclopedia; 1997. [5] Charbonneau JE. Recent case histories of food product-metal container interactions using scanning electron microscopy – X-ray microanalysis. Scanning 1997;19:512–8. [6] Huang JG, Li N, Zhou DR. Generation reason and corrosion characteristic of cavity of tinplate alloy layer. J Central South Univ Technol 2004;11:362–6. [7] Huang XQ, Li N, Cao LX, Zheng H. Electrodeposited lanthanum film as chromate replacement for tinplate. Mater Lett 2008;62:466–9. [8] Grassino AN, Grabaric Z, Pezzani A, Fasanaro G, Lo Voi A. Influence of essential onion oil on tin and chromium dissolution from tinplate. Food Chem Toxicol 2009;47:1556–61. [9] Grassino AN, Grabaric Z, Pezzani A, Squitieri G, Berkovic K. Corrosion inhibition with different protective layers in tinplate cans for food preservation. J Sci Food Agric 2010;90:2419–26. [10] Catala R, Alonso M, Gavara R, Almeida E, Bastidas JM, Puente JM, et al. Titanium-passivated tinplate for canning foods. Food Sci Technol Int 2005;11:223–7. [11] Huang BX, Tornatore P, Li YS. IR and Raman spectroelectrochemical studies of corrosion films on tin. Electrochim Acta 2000;46:671–9. [12] Sasaki T, Kanagawa R, Ohtsuka T, Miura K. Corrosion products of tin in humid air containing sulfur dioxide and nitrogen dioxide at room temperature. Corros Sci 2003;45:847–54. [13] Tselesh AS. Anodic behaviour of tin in citrate solutions: the IR and XPS study on the composition of the passive layer. Thin Solid Films 2008;516:6253–60. [14] Jafarian M, Gobal F, Danaee I, Biabani R, Mahjani MG. Electrochemical studies of the pitting corrosion of tin in citric acid solution containing Cl. Electrochim Acta 2008;53:4528–36. [15] Almeida CMVB, Giannetti BF. Protective film growth on tin in perchlorate and citric acid electrolytes. Mater Chem Phys 2001;69:261–6. [16] Gervasi CA, Palacios PA, Fioribimb MV, Alvarez PE. Electrochemical studies on the anodic behaviour of tin in citrate buffer solutions. J Electroanal Chem 2010;639:141–6. [17] Refaey SAM, Schwitzgebel G. Electrochemical impedance spectroscopic investigation of dissolution, passivation and pitting corrosion of tin in Na2CO3 solution and the effect of Cl and I ions. Appl Surf Sci 1998;135:243–53. [18] El-Sherbini EEF, Abd-El-Wahab SM, Amin MA, Deyab MA. Electrochemical behaviour of tin in sodium borate solutions and the effect of halide ions and some inorganic inhibitors. Corros Sci 2006;48:1885–98. [19] Hoar TP. The electrochemical behaviour of the tin–iron couple in dilute acid media. Trans Faraday Soc 1934;30:472–82. [20] Patrick GW. Internal corrosion of tinplate food containers. Anti-Corros Methods Mater 1976;23:9–11. [21] Board PW, Holland RV, Britz D. Dithiocarbamates, carbon disulphide and the corrosion of tinplate. Brit Corr J 1968;3:238–41. [22] Lin-Vien D, Colthup NB, Fateley WG, Grasselli JG. The handbook of infrared and raman characteristic frequencies of organic molecules. USA: Academic Press; 1991. [23] Blackley DC. Polymer latices: type of latices. 2nd ed. UK: Chapman & Hall; 1997. [24] Martins JI, Nunes MC. Reconsidering differential aeration cells. Electrochim Acta 2006;52:552–9.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Corrosion problems in tinplate cans for storing contact glues for shoes J.I. Martins ⇑ Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Porto, Portugal
a r t i c l e
i n f o
Article history: Received 18 May 2012 Received in revised form 25 June 2012 Accepted 3 August 2012 Available online 8 September 2012 Keywords: Tinplate Corrosion Electroplating Cans Adhesive glue
a b s t r a c t Tinplate steel cans were used to store contact glue for shoes. A few months after its storage, the glue displayed a change in colour to a darker brown. The visual inspection of the interior of the cans showed the presence of corrosion of steel, which was evidenced by the typical colour of the iron oxides on the sheet. SEM/EDS and FTIR tests confirmed that the anomaly was due to the following factors: (a) a bad tin coating, small thickness and lack of homogeneity; (b) a degradation of glue, hydrolysis of chloroprene, which led to its acidification. Some recommendations are made to overcome similar situations within the same use of these packages looking at the specifications of tinplate. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Tinplate is a thin low-carbon mild steel sheet, cold reduced, coated on both sides with pure electrolytic tin, linking the strength and formability of steel with the corrosion resistance and weldability of tin. The resulting coating has the following stratified structure from the bottom to the top [1–3]: an FeSn2 alloy developed from the flow-brightening process that protects the steel against galvanic corrosion by oxidant species; a free tin layer that determines the useful live when acting as a sacrificial anode; a layer of chromium and tin oxides and metal chromium that prevent tin oxide growth and sulphuration processes; and finally a lubricant film covered with a sanitary lacquer as protection barrier between the packed material and the substrates of the container. It is one of the most widely packaging materials in food and drinks cannery. However, there is also another important market in the area of glues, oils, grease, paints, powdered, polishes, waxes, chemicals and many other products. Aerosol containers and caps and closures are also made from tinplates. The container integrity problems reported in the literature [4–10] are mainly related to food products, such as: (a) stress corrosion cracking (SCC); (b) sulphide black corrosion and occasionally discoloration on the product surface; (c) pitting corrosion involving rapid iron dissolution at fractures or pores of coating; (d) filiform corrosion with the formation of rust on the external surface of metal container due to scratch defects in the organic coating. A lot of works have been dedicated on the corrosion of pure tin and tinplate [11,12]. Anodic performances of tin showed active–passive transition in some media, for example, citrate solutions [13–16], Na2CO3 solutions with or without Cl or I ions [17], sodium borate solutions containing halide ions and some inorganic inhibitors [18], and NaOH solutions [11]. Looking to standard potential of the Sn/Sn2+ and Fe/Fe2+, respectively, 0.131 and 0.441 V vs. NHE, it is apparently contradictory to use the tin as sacrificial anode in the galvanic couple with the iron. However, in several media of fruit juices and pure organic acids the tin is more active than the iron. According to the work of Hoar [19] performed with tin and iron elec-
⇑ Tel.: +351 22 508 1643; fax: +351 22 508 1449. E-mail address: [email protected] 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2012.08.003
259
J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265
trodes into dilute acidic media, the tin and iron are covered with oxide-films. After the initial instants, the films dissolve and the potentials of both metals fall to values characteristics of the bare metals. But, the formation of stable tin complexes in those electrolytic media becomes the tin more active than the iron [3,20]. Therefore, the mainly factors determining the polarity of the tin–iron couple are the presence or absence of oxide-films on the metallic surfaces, the composition and structure of the steel, and the ability of the electrolyte to remove stannous ions as complexes. Nevertheless it must be remembered that under other conditions, such as in carbon disulphide [21] and sulphuric acid [19], the iron becomes and remains the anode. This paper deals with the analysis of the failure of tinplate cans used to store contact glue for shoes. 2. Experimental details The cans were made with flat cold-rolled low carbon steel (MR, SPCC) product batch annealed, thickness of 0.25 mm, coated on both faces by electrolytic tin (Ferrostan process). Sales specification states tin coating weight 2.8 g/m2 on each side. Table 1 gives the standard chemical composition of MR steel. The bright surface finish is obtained through the melting of the tin coating on the steel smooth surface. A thin and uniform oil film (dioctyl sebacate, DOS) is applied on both surfaces, in order to prevent abrasion and facilitate later processing on automatic machines. This tinplate sheet will be designed as TP1 and another from different provenance as reference by TP2. The glue, Maprene, disposed within the cans in addition to polychloroprene has the following compounds: ethyl acetate (10–25%), rosin (5–10%), naphtha (petroleum), hydrogen-treated lightweight (25–50%) and toluene (5–10%). The glue with and without a brownish colour are referred, respectively, to as Ciprene 2603 and Ciprene 2626. The determination of pH in the glues was performed in two ways: (1) direct reading of the product as such although with some instability; (2) indirect reading on the decanted water, after to have emulsified previously the glue with water at pH = 7. The scanning electron microscopy (SEM) images and the analysis by energy dispersive spectroscopy X-ray (EDS) were made with an equipment FEI Quanta 400FEG, fitted with a probe for micro analysis EDAX Genesis X4M. The pressure inside the chamber was about 6 102 Pa. The distance between the objective lens and the sample, ranged between 6 mm and 15 mm. The SEM filament was operated at variable current and at 15 kV voltage using magnifications 100, 250 and 1000. SEM/EDS analysis were performed in CEMUP (Centro de Materiais da Universidade do Porto). The IR spectra were recorded in diffuse transmittance mode using a FT-IR Nicolet 6700-ThermoElectron Corporation equipment. 3. Initial findings The specifications of tinplate refer a tin thickness coating of 0.38 lm on both sides, without passivation with a finishing oil film. The presence of water inside the cans depends on the purity of the organic compounds of the glue. The ethyl acetate (88– 97%) may contain water up to a maximum value of 5% and the maximum content of water allowed to toluene (99.7–99.97%) is 0.03–0.01%. The glue for shoes in question have the following generic composition: water (0.4–1.0%) from organic solvents, polychloroprene (10–15%), synthetic resins (aliphatic, aromatic, rosin), various organic solvents (toluene, hexane, acetone, ethyl acetate, etc.), and additives such as magnesium or zinc oxide required for the resinate reaction to proceed. The observation of the interior of cans, Fig. 1, shows the presence of rust, which indicates a localized corrosion associated with the steel. Of course, the deformed areas to give rigidity to the can have a higher density of red spots. This suggests that the tin–iron couple has a behaviour predictable from the electrochemical series potential, i.e., the iron is the anode. The medium does not stabilize stannous ion complex or, even if that happens leaves the tin over time to maintain the cathodic protection on the iron in the center of the defective area of tinplate. 4. Results and discussion 4.1. Scanning electron microscopy SEM tests by secondary electrons (SE) or backscattering electrons (BE) were carried out on new sheet (TP1), Fig. 2, and on sheet affected by corrosion after contacting the glue (TP1F), Fig. 3. Table 1 Chemical composition of MR steel (% maximum). Steel type
C
P
Mn
Si
S
Ni
Cr
Cu
Mo
MR
0.13
0.020
0.60
0.02
0.05
–
–
0.2
–
260
J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265
Fig. 1. Interior view of the damaged can; (b) Magnification, detailing the strained zone and the cylindrical surface.
(a)
(b)
Z2 Z1
60 µm
60 µm Fig. 2. SEM 1000 of new sheet TP1: (a) SE; (b) BE.
(b)
(a)
Z4
Z1 200 µm
Z3
Z2
200 µm
(c) Z5F
600 µm Fig. 3. SEM 250 sheet border, TP1F: (a) SE; (b) BE; SEM 100 sheet body, TP1F: (c) BE.
Fig. 2a shows a striated structure associated to its processing and transport, but also defects and a lack of uniformity of the tin coating. The observation by BE, Fig. 2b, clarify this latter aspect: light zones indicate the tin (metal element heavier than iron) and the darker the iron. The EDS analysis in the light, Z1 (8.65% Fe, 91.35% Sn), and dark, Z2 (66.61% Fe, 27.88% Sn) areas show the presence of iron which means a tin thickness of less than 1 lm, taking into consideration the penetration of the electron beam (1–1.5 lm).
261
J.I. Martins / Engineering Failure Analysis 26 (2012) 258–265 Table 2 Chemical composition in different areas of the damaged can. Element
Z1F (%)
Z2F (%)
Z3F (%)
Z4F (%)
Z5F (%)
C O Mg Si Cl Fe Sn Ca Na Al S
69.85 5.89 0.81 0.18 22.80 0.46 n.d. n.d. n.d. n.d. n.d.
48.09 13.99 0.10 0.07 0.41 15.91 n.d. 0.11 n.d. n.d. n.d.
45.78 27.98 1.18 0.48 8.09 n.d. n.d. n.d. 0.56 0.33 0.53
45.95 17.87 1.94 0.31 15.34 16.77 0.84 0.67 n.d n.d. 0.35
21.89 4.26 0.32 n.d. 6.60 19.27 47.67 n.d. n.d. n.d. n.d.
(b)
(a)
Z1 Z2
60 µm
60 µm Fig. 4. SEM 1000 sheet border, TP2: (a) SE; (b) BE.
Fig. 3 displays the SEM observations of the border and body of sheet damaged by corrosion and table 2 the chemical composition by EDS of the areas Z1F, Z2F, Z3F, Z4F, border zone, and Z5F, body zone. The presence of the elements Cl, Si, Ca, Na, S, Al, Mg and C on the damaged areas confirm the generic composition of the glue: polychloroprene (ACH2ACClCHACH2A), toluene (C6H5CH3), methyl acetate (CH3COOCH3), oil, mixture of hydrocarbons (C5AC12), rosin (C20H30O2), and additives containing metallic elements, Mg, Si, Ca, Na, Al, and non-metals, S. The severely attacked areas (Z1F, Z4F and ZF2) do not contain tin or it is much less compared to unaffected areas (Z1 and Z2). The corrosion is more pronounced in the deformed zones of the can. The observations on tinplate sheet from another supplier (TP2) and used as reference for the tinplate sheet in question are in Fig. 4. The photomicrographs show also a striated structure for the sheet, light (96.60% Sn, 3.40% Fe) and dark (26.99% Sn, 73.01% Fe) areas, but the tin coating is more uniform and thicker than TP1. It was not possible to determine the thickness of the tin coating on tinplate from the microscopic observation of a section previously mounted on a resin and polished. This was due to the thinness of the coating together with the action of wear and deformation of the tin. This problem was solved through a bending of the tinplate sheet. The tin coating was broken and separated partially from the base substrate allowing the determination of its thickness by conveyable observation by SEM. The thickness of TP2 sheet is higher than the TP1 sheet as is shown in Fig. 5. 4.2. Hardness The sheets TP1 and TP2 have Vickers hardness (charge 100 g) 107 HV and 134 HV, respectively. The different hardness values are related to the deformation imposed on the rolling operation of the sheets. 4.3. FTIR analysis Figs. 6 and 7 show the FTIR spectra obtained for the Ciprene 2626 and 2603. The bands located in the range 550–750 cm1 and 3200–3500 cm1 are attributed, respectively, to stretching vibration of the CACl and OH (hydrogen bonds) [22]. The FTIR results show that the bands located at 695 cm1 and around 730 cm1 related to CACl bond have a higher transmittance for Ciprene 2603 than CIPRENE 2626, while the band located at 3420 cm1 linked to OH is smaller. This means that CACl bonds are converted to CAOH bonds, i.e., the glue CIPRENE 2603 underwent hydrolysis.
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(a)
(b)
290 nm 161 nm
256 nm
267 nm
6 µm
CEMUP SE 1 FeSn x9500 15kV
6 µm
(c)
294 nm
375 nm
3 µm Fig. 5. Determination of thickness in TP1 and TP2 sheets: (a) TP1; (b) and (c) TP2.
40
Ciprene 2626
Transmittance (%)
30
20
10
0 4000
3000
2000
1000
Wave number (cm-1) Fig. 6. FTIR spectrum of Ciprene 2626.
4.4. Determination of pH of the glue Table 3 shows the results of pH of the glues obtained by direct and indirect reading. Ciprene 2603 expressed instability in the direct reading, but in indirect reading clearly indicates a tendency to an acid pH, conversely to Ciprene 2626 that shows pH around 10. Observed acidification in the glue comes from the release of chloride ions from the slow hydrolysis of chloroprene. The allylic carbon–chlorine bonds (C@CACACl) presented in chloroprene units which have been polymerized by 1,2- addition is believed to be particularly susceptible to hydrolysis [23] as shown in the following reaction:
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Ciprene 2603
Transmittance (%)
25
20
15
4000
3000
2000
1000
Wave number (cm-1) Fig. 7. FTIR spectrum of Ciprene 2603.
Table 3 pH values of the glue obtained by direct and indirect readings. Product
Ciprene 2603 Ciprene 2626
pH
State of cans
‘‘As it is’’
Decanted water
6.30–8.80 10.14
5.49 9.30–9.70
Damaged Not damaged
ð1Þ
Taking into account the observations by SEM/EDS we can say that the CIPRENE 2603 system will be subject to corrosion in acid electrolyte as shown in Fig. 8. In the absence of discontinuities in the tinplate, the tin corrodes due to local-action cells with a current ISn = Iuc,a, Fig. 8a. The cathodic reaction is the reduction of oxygen in acidic medium considering the value of the hydrogen overvoltage on tin.
Anodic reaction : Sn ! Sn2þ þ 2e
ð2Þ
Cathodic reaction : 2e þ 1=2O2 þ Hþ ! OH
ð3Þ
The irregularity of the tin coating will cause the areas of lesser thickness corrode quickly, and therefore showing the iron earlier, Fig. 8b. At this point we have a tin–iron couple, the tin is a sacrificial anode and corrodes with the current ISn = Iuc,a + Ic,a. When the exposed area of the iron is raised by the action of the corrosive medium, tin adjacent no longer be able to fully protect the iron, and iron corrodes due to local-action cells [24] according the reactions 4–6 justifying the red spots observed in the damaged cans, Fig. 8c. At that moment, will have a tin–iron couple, and the tin and iron corrode with the current, respectively, ISn = Iuc,a + Ic,a and IFe = Iuc,a1.
Anodic reaction : Fe ! Fe2þ þ 2e
ð4Þ
Cathodic reaction : 2e þ 2Hþ ! H2
ð5Þ
Rust formation reaction : 2Fe þ 3=2O2 þ H2 O ! Fe2 O3 H2 O
ð6Þ
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(a)
(b)
(c)
Fig. 8. Scheme of the evolution of corrosion in tinplate: (a) local-action cells; (b) tin-iron couple with total cathodic protection of iron; (c) tin-iron couple with corrosion of iron.
Corrosion of both metals is therefore coupled to the reduction reactions of oxygen and hydrogen ions. Charge balance is maintained by summing all anodic and cathodic currents to zero.
Ic;a þ Iuc;a þ Iuc;a1 ¼ Ic;c þ Iuc;c þ Iuc;c1
ð7Þ
5. Conclusions and recommendations According to the experimental results it is concluded the following: 1. Tinplate TP1 sheet has a coating thickness lower than the theoretically predicted (0.38 lm) from its characteristics (weight 2.8 g/m2 of Sn). This irregularity is accentuated in the deformed zones, i.e., in the border area of the can. 2. Tinplate TP2 used as reference showed a better tin coating in comparison with TP1. 3. The Ciprene 2603 suffered a hydrolysis phenomenon that makes the glue more acid, i.e., the medium becomes more corrosive. The tin was quickly consumed by corrosion, and earlier presents the steel substrate. After this moment the corrosion of steel develops the characteristics red products, more sharply in the restricted zone submitted to cold hardening work. As tin is an active metal, it is recommended the following: 1. 2. 3. 4.
Put in tinplate sheet specifications that tin is easier to corrode, especially in acid and complexing media. Use a tin coating thicker, for example, weight 5.6 g/m2 of Sn. Apply 341 passivation treatment on tin and then lacquer to improve corrosion resistance. Elucidate the final consumers that packages are not eternal because the product and the can are susceptible to degradation.
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