Environmentally friendly coatings for tinplate cans in contact with synthetic food media.

This article presents the materials, methods, and main results obtained in an experimental study which was aimed at the development of alternative environmentally friendly pretreatments to chromium for tinplate food can applications. A commercial tinplate was used as substrate. Chromium, titanium, zirconium, cerium, and oxalate-based pretreatments were studied. The characterization of the pretreated tinplate specimens was carried out using scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS) techniques. The corrosion behavior of the pretreatments was evaluated using electrochemical measurements and exposure tests in several synthetic media. The results obtained led to the conclusion that, among the different pretreatments studied, the best performance was obtained with titanium and cerium compounds.

Keywords: Adhesion, appearance, application characteristics, chromate replacements, color measurement, color theory, corrosion protection, epoxyphenolic, food cans, friendly pretreatments, packaging, physical properties, stain resistance, tinplate, XPS

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Packaging steels are among the most innovative steel products because the requirements for quality and regularity are very high. To maintain the competitiveness of packaging steels against other materials, several research activities that focus on better understanding and increased knowledge of the processes have been undertaken. Coating of packaging steels is a complex subject, depending upon several specific applications and requirements, such as barrier to moisture, resistance to aggressive products, and so on. As a consequence, surface treatments and coatings and control of surface defects remain essential topics in the food can industry. (1)

Among the different steel cans used (tinplate, tin free steel, and blackplate), tinplate is widely used. It is usually passivated with a chemical or electrochemical treatment, applied on leaving the tinning line. The passivation films are usually composed of tin oxides, trivalent chromium, and, depending on the process used, metallic chromium. (2) In spite of their very low thickness and the minute quantities of matter involved, the passivation films have a marked influence on the chemical behavior of the tinplate, limiting the development of tin oxides, increasing the resistance to rust and sulfide formation, and improving the adhesion of organic films. (3)

However, the use and disposal of chromium compounds give rise to a complex environmental and public health protection problem. As a consequence of their toxicity, and since most of the chromium compounds are confirmed or suspected human carcinogens, there is a trend towards banning their use completely in the future from all industrial sectors where contaminated effluents can be produced. (4)

Moreover, in the case of tinplate for food cans applications, one more potential danger can be foreseen since the chromate layer can be directly in contact with preserved food. Recent studies confirmed that chromium dissolution in aggressive food environments (such as tomato juice or paste) can give rise to high concentrations when compared with the chromium limit for water, proposed by the World Health Organization. (5)

Molybdenum, titanium, and zirconium are, in principle, possible substitutes for chromium compounds as passivation agents. All seem to give similar characteristics, but with lower toxicity and reduced environmental implications. (6) Passivation with molybdenum was widely studied in the literature. (7,8) Coatings with acceptable corrosion resistance have been obtained, although with poor adhesion for the epoxyphenolic lacquers after sterilization and with sulfur staining problems. Passivation with titanium and zirconium compounds was studied in two projects promoted by European Coil and Steel Community. (9,10)

Other possible alternatives proposed for the passivation of tinplate have been the use of silanes or oleic acid, (11) and the possible use of chromium-free pigments commonly used in coatings for aluminum. (4)

In an attempt to gain a better understanding of the possibilities of passivation with titanium and zirconium and other elements with similar characteristics, a new project was started in 1999. The present work summarizes the main results obtained, using titanium, zirconium, cerium, and oxalate-based compounds as passivating agents. A commercial chromium-based passivation treatment was also studied as reference.

EXPERIMENTAL

Unpassivated and passivated tinplate sheets supplied by Aceralia, with the characteristics shown in Table 1, were used as substrates. (12, 13)

Sheets of 200 X 300 X 0.3 mm dimensions of unpassivated tinplate, used in Aceralia to applied alternative passivation treatments, were passivated with four different passivation pretreatments: (1) titanium passivation using 15 g [L.sup.-1] [K.sub.2] Ti[F.sub.6] and 5 g [L.sup.-1] NaN[O.sub.3] electrolyte at 50[degrees]C for 10 sec at the open circuit potential (OCP); (2) zirconium passivation using 20 g [L.sup.-1] Zr(S[O.sub.4])[.sub.2] electrolyte at 65[degrees]C for 5 sec at the OCP; (3) cerium passivation using 10000 ppm Ce(N[O.sub.3])[.sub.3] electrolyte at 50[degrees]C for 60 sec using 1 X [10.sup.-3] A [cm.sup.-2] current density; and (4) oxalate passivation using 15 g [L.sup.-1] [K.sub.2] (TiO([C.sub.2][O.sub.4])[.sub.2]) electrolyte at 65[degrees]C for 5 sec at the OCP.

Tinplate with conventional chromium passivation (cathodic treatment with sodium dichromate, CDC) pretreatment, known as 311 code, performed in an industrial line of Aceralia and widely used by the canning food sector, was also tested as a reference. (2,3)

Morphological and chemical characterizations were performed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) using a JEOL JSM 35C unit with a Noran Voyager spectrometer for EDX measurements. The specimens were analyzed after being cleaned with ethanol, using secondary and backscattering electron images and an X-ray map (Fe K[alpha]). An acceleration voltage of 10 kV was selected to increase sensitivity at the coating surface.

X-ray photoelectron spectroscopy (XPS) analysis was performed to characterize the passive films formed. The spectra were obtained utilizing a VG ESCALAB 200A electron spectrometer, using an Mg K[[alpha].sub.1.2] anode X-ray source (hv=1253.6 eV), with a primary beam energy of 15 keV and an electron current of 20 mA. The pressure in the analysis chamber was maintained at 1 X [10.sup.-9] Torr throughout the measurements. The regions of interest were C 1s, O 1s, Sn 3[d.sub.5/2], Cr 2[p.sub.3/2], Ti 2[p.sub.3/2], Zr 3d, K 2[p.sub.3/2], and Ce 2[p.sub.3/2]. The instrumentation was calibrated periodically using Ag 3[d.sub.5/2] (368.3 eV binding energy, BE) and Au 4[f.sub.7/2] (84.0 eV, BE) substrates. The concentration of each element was estimated by measuring the respective peak area in the high resolution spectrum and using elemental sensitivity factors. Spectral analysis was performed using peak fitting with Gaussian-Lorentzian peak shape and Shirley types background subtraction.

Corrosion behavior was evaluated using electrochemical measurements and several exposure tests in synthetic food media. Polarization curves were performed at a scan rate of 0.2 mV [s.sup.-1]. A 1287 Solartron electrochemical interface was used. Experiments were carried out using a three-electrode set up. The counter electrode was platinum foil. A saturated calomel electrode (SCE) was used as reference. The working electrode with a surface area of 1 [cm.sup.2] was the tinplate. The electrolyte was a citric-citrate buffer solution pH 4.3. Anaerobic conditions were obtained with a continuous supply of nitrogen (99.9998%) through the solution. In some cases an NaCl solution was also used. Experiments were performed after one hour at 25[degrees]C.

Considering that lacquer defects, such as holidays and pinholings, allow food media to come into direct contact with passivated tinplate, some special tests were performed to evaluate stability to food processing and sterilization for comparison of different passivated tinplate specimens (PTS). Corrosion resistance in synthetic food media, staining resistance including sulfur resistance, and atmospheric corrosion resistance, were evaluated.

Corrosion resistance in synthetic food media involves the use of solutions containing organic acids present in foods, such as fruits and vegetables. This evaluation was carried out by immersing the specimens in the referred solutions. The five solutions selected were: (1) 1% citric acid and 0.5% tartaric acid (14); (2) 2% citric acid and 10% sucrose (14); (3) 1% NaCl and 3% acetic acid (14,15); (4) 3% NaCl and 1% citric acid (14); and (5) 1% lactic acid. (14) Distilled water was also utilized for comparative purposes and as reference. All solutions were prepared with demineralized water. Test assemblies were generally exposed at 121[degrees]C for 60 min. After cooling, examination of the surface for staining and darkening, gloss reduction, and blistering, was used to evaluate the ability of the coating to withstand food processing conditions. Gloss reduction was measured with Rhopoint T[TM] Novo-Curve at 60[degrees].

Sulfur resistance was evaluated by the exposure of specimens to a 3 g [L.sup.-1] L-cystein chlorhydrate neutralized with a 1% [Na.sub.2]C[O.sub.3] solution. (16) This solution was selected because L-cystein chlorhydrate is considered the sulfur-containing compound to give the most reliable results, (14) regarding the effect of sulfur-compounds existing in food media (fish and meat). It was gently boiled for one hour and had to be prepared just before using, since L-cystein is not very stable. Specimens were cut in the form of approximately 50 mm diameter discs, with a central hole of 4-5 mm, fixed in along a glass rod, and separated from each other by 15 mm. The assembly, enclosed in an appropriate container, (16) was exposed at 110[degrees]C for 30 min. (14) Good gloss and color retention were taken as indication of high sulfur resistance. (14)

Atmospheric corrosion resistance was evaluated in a Liebisch KSE-300 cabinet chamber (Kesternich type), where passivated specimens were submitted to humid atmosphere containing sulfur dioxide according to the DIN EN ISO 6988 standard. (17) The test cycle used exposed specimens to S[O.sub.2] at 40 [+ or -] 3[degrees]C for eight hours, followed by sulfur dioxide removal and further humid atmosphere exposure at room temperature for 16 hr. For each cycle of 24 hr, a volume of 0.2 L of S[O.sub.2] was introduced into the cabinet chamber.

For comparison of different PTS, an Elecktrotest method (14,18,20) was also performed. In this method 6.5 V potential was applied versus a graphite rod counter electrode for 10 min, using a 1.5% NaCl and 1% acetic acid test solution, and measuring the current (A) after polarization. (6, 18-20) This current, measured at the end of the test, is due to the oxidation of exposed iron, since tin is passive at that potential (6.5 V). (18) In the experimental set up, a continuous adjustable digital power supply Peaktechn was used. This equipment allows either an input voltage of 0-30 V or an output current of 0-5 A, DC.

[FIGURE 1 OMITTED]

All the passivated tinplate specimens were lacquered in an industrial plant of Crown Cork Spain SA, applying a conventional not modified epoxy-phenolic lacquer (GGV164344), 7-8 g [m.sup.-2], cured at 205[degrees]C for 10 min. (7)

RESULTS AND DISCUSSION

Surface Analysis and Composition of Passivated Tinplate Specimens (PTS)

Figure 1 shows SEM and EDX results obtained using different PTS. Figure 1a shows that chromium-based PTS presents a striated structure, characteristic of tinplate, (2) which was more visible in backscattering electron observations. The EDX results verify tin as the main component, iron also being present. By selection of experimental conditions (10 keV), the iron emission from the substrate, near the coating interface, was eliminated. The iron emission being observed seems to be a result of iron from the visible grooves observed in the atomic number contrast images, as proved by X-ray Fe K[alpha] maps of this region. The presence of chromium, confirmed by the XPS method, was only detected on specimens under special observation conditions (inclination of the specimen until extinction of the emission).

All alternative PTS (Figure 1b) revealed the striated structure associated with a relatively weak localized relief and constituted of lower atomic number elements. The EDX spectra of all these surfaces verify that tin is their main component. However, in all cases, due to the low tin thickness, it was possible to detect iron, visible in the atomic number contrast images. The presence of the main passivation element was only clearly detected by SEM and EDX on titanium-based PTS, which also contained potassium. These results may possibly be due to residues of passivation bath on those surfaces.

[FIGURE 2 OMITTED]

On titanium-based PTS (Figure 1b1), it was also possible to observe, associated with the oriented striated structure, a significant number of incisions of low size (some micrometers) and polygonal contour, dispersed on the surface and sometimes, significant scribes, possibly already existing on the substrate, which suggest local or partial dissolution of tin inside them. Zirconium-based PTS (Figure 1b2) associated with a striated structure revealed a relatively homogeneous structure, without significant open areas. Oxalate-based PTS (Figure 1b3) showed, associated with the striated structure characteristic of the substrate, some pitting of some micrometers size, essentially associated to some defects, suggesting a local dissolution of tin coating and revealing the defects previously existing. Finally, cerium-based PTS (Figure 1b4) revealed identical striated structure and some regions of near elliptical shape with an average diameter of 15-30 [micro]m, whose inner part shows a reticulated structure, possibly a consequence of localized dissolution-precipitation phenomenon of tin during the passivation process.

Table 2 shows the main XPS results obtained on different PTS, including peak range and atomic percentage of the elements analyzed. The presence of carbon and oxygen in significant amount can be observed on all surface specimens. The presence of chromium, as expected, was only significant on chromium-based PTS. On the other pretreated surfaces, the presence of chromium can only be explained, as arising from residual chromium contamination left after washing the industrial line. The presence of metallic tin ([Sn.sup.0]) 485 eV, BE and oxidized state (~487 eV, BE) was clearly observed on all specimens. Notwithstanding, the data bases are not clarified if the oxidation state is Sn(II) or Sn(IV), because the two forms have very similar BE: 486.9 eV and 486.6 eV, respectively. (2,21)

The detection of Ti 2[p.sub.3/2] peak in titanium-based PTS, corresponding to Ti[O.sub.2] (456.5 to 461.5 eV, BE), (21) confirms the presence of a titanium-based passivation layer on these surfaces. Zirconium and oxalate-based PTS, showing the lower values of atomic percentage of Sn 3[d.sub.5/2], seem to suggest relatively compact passivation layers. Significant atomic percentage of Zr 3d on zirconium PTS, and C 1s on both PTS types, was also observed. Finally, the presence of Ce 3[d.sub.5/2] peak in cerium-based PTS also was confirmed the presence of a thin passivated layer on them.

Pretreatment Evaluation

Figure 2 shows polarization curves performed in the citric-citrate buffer solution pH 4.3. The behavior of the titanium-based PTS:Rp = 1.2k[OMEGA] [cm.sup.2] and [i.sub.corr] = 19 [micro]A/[cm.sup.2], was very similar to that of chromium-based PTS. Also, zirconium-based PTS seems to improve the corrosion resistance of tinplate, while oxalate-based PTS showed worse corrosion behavior than unpassivated specimens: Rp = 1.1k [OMEGA] [cm.sup.2] and [i.sub.corr] = 21[micro]A/[cm.sup.2]. Finally, cerium-based PTS (not included) showed relatively good corrosion properties among all alternative PTS:Rp = 1.3k[OMEGA] [cm.sup.2] and [i.sub.corr] = 17[micro]A/[cm.sup.2]. It is possible to observe a small effect of the different passivation layers on the anodic branch of the polarization curves. In fact, the curve slop is steeper (~75 mV/decade) for specimens treated in the chromate or titanium baths, with respect to that treated in the oxalate or zirconium baths (~50 mV/decade).

Cathodic branches of the polarization curves also show some differences. All specimens treated in chromium-free baths show a limiting current in the range of 6-8 X [10.sup.-6] A [cm.sup.-2]. On the contrary, the cathodic polarization curve of the chromate-treated specimens show a lower limiting cathodic current in the range of 3 X [10.sup.-6] A [cm.sup.-2]. The trend of the polarization curves shows that the chromate treatment has a better effect on the corrosion behavior of tinplate in the citric-citrate buffer solution pH 4.3, decreasing both anodic and cathodic currents. They were immediately followed by titanium and cerium-based PTS.

Table 3 summarizes the main results obtained in exposure tests in synthetic food media. These results show a significant loss of gloss (see footnote of Table 3) of all tested specimens in all media and, especially, in 2% citric acid and 10% sucrose, followed by 1% lactic acid medium. Slight to intense staining and darkening of some specimens were also observed. From a practical point of view, iron corrosion was not observed. Among all PTS studied, the better results were yielded by chromium, titanium, and cerium-based PTS.

Table 4 shows the main results obtained for the sulfur resistance test in L-cystein chlorhydrate neutralized with a 1% [Na.sub.2]C[O.sub.3] solution at 110[degrees]C for 30 min. A significant corrosion level of zirconium and oxalate-based PTS can be observed. The chromium, titanium, and cerium-based PTS clearly showed the highest corrosion resistance.

Table 5 shows the results of the atmospheric corrosion resistance test, which shows relatively similar corrosion behavior of chromium and cerium-based PTS, with a similar classification according to Champion scale. (22) However, as can be seen in Table 3 (gloss value before test), the chromium-based PTS have higher gloss before test and, consequently, also show higher gloss after test, than do cerium-based PTS. This influence of the initial gloss of PTS on different test results can be clearly observed in Table 4. In this specific case, it can be observed that, notwithstanding the good behavior of titanium and cerium-based PTS in the test conditions described, the glossy appearance of chromium-based PTS gave a nicer appearance to these surfaces.

Table 6 shows Elecktrotest method results, confirming a similar corrosion behavior of chromium and cerium-based PTS, immediately followed by titanium-based specimens. In this case, the glossy aspect of chromium-based PTS also affects the final appearance of the specimens.

CONCLUSIONS

All PTS involved in the study showed an orientated striated structure in SEM observations, more visible in backscattering images, associated with a slight local relief with lower atomic number. This corresponds to a reduction of tin coating thickness and an eventual better local detection of iron from substrate. This structure, characteristic of the tinplate substrate used, confirms that the studied passivated tinplate layers are very thin, with insufficient thickness to completely cover the initial profile of the substrate.

Some of the pretreatments, such as the titanium-based tinplate specimens, show a significant number of incisions of small size (some micrometers) with polygonal contour. The incisions are dispersed at the surface and some are significant scribes which may possibly already exist on the substrate. Local or partial dissolution of tin inside the incisions seems to reveal the initial substrate defects.

Cerium-based PTS showed relatively better coverage but also a large number of regions showing a near elliptical shape, oriented in the groove direction, with an average diameter of 15-30 [micro]m. The presence of iron can clearly be detected in these regions, suggesting a local partial dissolution of tin coatings, as confirmed by EDX spectra.

XPS results obtained with the different PTS clearly confirm the presence of chromium, zirconium, and oxalate-based passivation layers on tinplate. Oxalate-based specimens showed the highest atomic percentage of carbon on them and a very low atomic percentage of potassium. The results also verify the presence of tin in the metallic state and in the oxidation states Sn(II) and Sn(IV) on all PTS studied. Finally, the detection of the different passivation elements on the PTS confirmed the presence of passivation layers on them.

Concerning corrosion behavior of the studied PTS, all results obtained from polarization curves, exposure tests in several synthetic food media, sulfur, atmospheric corrosion resistance, and Elecktrotest suggest a better behavior for titanium and cerium-based PTS, among the studied alternative environmental friendly PTS.

Table 1 -- Characteristics of Tinplate Specimens (12, 13)

                                 Tinplate
Characteristics       Unpassivated      Passivated (CDC)

Coating type          E 2.8/2.8         E 2.8/2.8
Chemical treatment    None              311
Oil                   DOS               DOS
Reduction             Double            Single
Base steel            MR (ASTM A 623)   MR (ASTM A 623)
Steel thickness, mm   0.16              0.22
Temper grade          DR550 (EN 10202)  T57 (EN 10202)
Annealing             Continuous        Batch

Table 2 -- XPS Results Obtained on Passivated Tinplate Specimens

                                 Passivated Specimen, Atomic
                                   Percentage, %
Element          Peak range, eV  Chromium  Titanium  Zirconium  Oxalate

C 1s             280.5-292.7     34.53     37.28     50.61      59.59
O 1s             526.5-538.7     46.12     38.17     35.27      29.06
Sn 3[d.sub.5/2]  482.3-492.2     11.54     23.45     10.91      10.54
Cr 2[p.sub.3/2]  573.8-582.2      7.80      0.70      --         0.67
Ti 2[p.sub.3/2]  456.5-461.5      --        0.40      --         --
Zr 3d            178.8-189.3      --        --       13.3        --
K 2[p.sub.3/2]   292.1-295.2      --        --        --         0.14
Ce 2[p.sub.3/2]  876.0-894.3      --        --        --         --

                 Passivated Specimen, Atomic
                   Percentage, %
Element          Cerium

C 1s             47.73
O 1s             34.03
Sn 3[d.sub.5/2]  16.65
Cr 2[p.sub.3/2]   0.58
Ti 2[p.sub.3/2]   --
Zr 3d             --
K 2[p.sub.3/2]    --
Ce 2[p.sub.3/2]   1.01

Table 3 -- Corrosion Resistance Test of Passivated Tinplate Specimens to
Synthetic Food Media

Specimen                        Synthetic Food Media
and Gloss                         1% Citric Acid       2% Citric Acid
Before Test (a)  Distilled Water  0.5% Tartaric Acid   10% Sucrose

                                  Tin Corros
                                  S in ~30% of          Tin Corros
                                  the specimen         Intense S in
                                  with dark gray       ~90% of
Chromium         Very slight S    products             specimen
594                  413               188                 2
                                                       Tin Corros
                                                       Intense S in
                                  No Corros            100% of
Titanium        Almost no S       Very slight S        specimen
230                   69                76                 5.3
                                                       Tin Corros
                                                       M to I, D in
                 Low Tin Corros                        100% of
                 M, S, and D                           specimen
                 Very low iron                         Low iron
                 Corros in the    Low Tin Corros       Corros in the
                 lower area of    Slight               lower area of
Zirconium        specimen         heterogeneous S      specimen
209                  251               242                 2.1
                                                       Tin Corros
                                  No Tin Corros        I, S, and D in
Oxalate          Slight S         Very slight S and D  specimen
593                  198               177                11
                                                       Tin Corros
                                                       I, S, and D in
                 M, S in ~10%                           ~60% of
Cerium           of specimen      No S                 specimen
186                   76               106                 7.0

Specimen                     Synthetic Food Media
and Gloss        1% NaCl          3% NaCl
Before Test (a)  3% Acetic Acid   1% Citric Acid     1% Lactic Acid

                 Tin Corros
                 Several S and
                 D, ~70% of        Tin Corros         Tin Corros
                 the area dark    Severe D in        Intense S and
                 and 30% M        ~80% of             D in ~60% of
Chromium         gray             specimen           specimen
594                  138                49                38
                 Tin Corros
                 M, S and D       Low Tin Corros     Low Tin Corros
                 ~15% of           Slight             Slight
Titanium         specimen         homogeneous S      heterogeneous S
230                   67                72                60
                 Low Tin Corros   Low Tin Corros     Low Stain Corros
                 Slight to M, S,  Slight S and D     Slight S and D
                 and D in ~10%     in ~2% of the       in ~5% of
Zirconium        of specimen      specimen           the specimen
209                  111               224                56.8
                 Low Tin Corros
                 Slight S and D                      Low Tin Corros
                 in ~2% of         No Tin Corros      Heterogeneous
Oxalate          specimen         Almost no S and D  S and D
593                  145               336               101
                 Low Tin Corros
                 M, S, and D in
                 ~5% of the        Low Tin Corros     Low Tin Corros
Cerium           specimen         Slight S and D     Slight S and D
186                   50                46                42

(a) Measured with Rhopoint T[TM] Novo-Curve at 60[degrees],
Corros=Corrosion; M=Medium; I=Intense; S=Staining; D=Darkening.

Table 4 -- Sulfur Resistance Test of Passivated Tinplate Specimens Using
L-Cystein Solution at 100[degrees]C for 30 min.

                 Visual Aspect
Specimen   Before  After Test  Observations

Chromium                       Very slight gray coloring after test.
                               Bright metallic gloss can still be seen.
                               The best behavior among all studied
                               passivated tinplate specimens.
Titanium                       Similar behavior to chromium-based
                               passivated tinplate specimens. Only very
                               slight to medium gray color after test in
                               ~20% of the area (loss of gloss).
Zirconium                      All specimen's surfaces became dark gray
                               with intense formation of tin corrosion
                               products.
Oxalate                        All specimen's surfaces became very dark
                               gray with intense formation of tin
                               corrosion products.
Cerium                         Similar behavior to titanium-based
                               passivated tinplate specimens. Only
                               slight to medium gray color, as a
                               consequence of loss of gloss.

Table 5 -- Atmospheric Corrosion Resistance Test of Passivated Tinplate
Specimens; One Cycle of Exposure to Humid Atmosphere Containing Sulfur
Dioxide Using a Liebisch KSE-300 Chamber According to DIN EN ISO 6988
Standard, (17) and Champion Scale (22)

Test Conditions     Chromium            Titanium

Image
                    Metallic gloss can
                    still be seen.      Moderate corrosion
Visual              Very moderate       products spread all
observation         corrosion products  over the surface
Champion         A  4                   5/6
Scale            B  2                   1/2

Test Conditions  Zirconium            Oxalate             Cerium

Image
                 Very intense
                 corrosion products
Visual           spread all over the  Considerable        Moderate
observation      surface              corrosion products  corrosion
                                                          products
Champion         7                    5                   4
Scale            2                    2/3                 2

Table 6 -- Elecktrotest Method of Passivated Tinplate Specimens

                                     Specimen
Test Conditions     Chromium         Titanium         Zirconium

1.5% NaCl and
1% acetic acid at
6.5 V for 10 min
Mean tin thickness
after testing
([micro]m)            0.8              0.8              0.2
                                     Very slight
                    Very slight      staining and     Almost complete
                    dissolution of   dissolution of   dissolution of
Type of             passivation and  passivation and  passivation and
damaged area        tin layers       tin layers       tin layers
Elecktrotest,
current (mA)        130              140              200

                            Specimen
Test Conditions     Oxalate          Cerium

1.5% NaCl and
1% acetic acid at
6.5 V for 10 min
Mean tin thickness
after testing
([micro]m)            0.2              0.6
                                     Very slight
                    Significant      staining and
                    dissolution of   dissolution of
Type of             passivation and  passivation and
damaged area        tin layers       tin layers
Elecktrotest,
current (mA)        210              170

ACKNOWLEDGMENT

The authors express their gratitude to the Commission of the European Communities for financial support under agreements 7210-PA/193, 7210-PB/193, 7210-PC/193, and 7210-PD/193.

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(15) Do Nascimento, G.G., Mattos, O.R., Dos Santos, J.L.C., and Margarit, I.C.P., "Impedance Measurements on Lacquered Tinplate: Fitting with Equivalent Circuits," J. Appl. Electrochem., 29, 383-392 (1999).

(16) ECSC Research, "Environmentally Friendly Coated Tinplate for Food Cans," Final Report, Agreement 7210-PA/193, 7210-PB/193, 7210-PC/193, and 7210-PD/193, European Commission of Carbon and Steel, Brussels, Belgium, 2002.

(17) DIN EN ISO 6988 Standard, "Metallic and Other Non-organic Coatings. Sulphur Dioxide Test with General Condensation of Moisture," DIN, Berlin, 1995.

(18) Montanari, A., Pezzani, A., Cassara, A., Quaranta, A., and Lupi, R., "Quality of Organic Coatings for Food Cans: Evaluation Techniques and Projects of Improvement," Prog. Org. Coat., 29, 159-165 (1996).

(19) Bastidas, J.M., Cabanes, J.M., and Catala, R., "Effect of Passivation Treatment and Storing on Adhesion and Protective Properties of Lacquered Tinplate Cans," JOURNAL OF COATINGS TECHNOLOGY, 69, No. 871, 67 (1997).

(20) Bastidas, J.M., Cabanes, J.M., and Catala, R, "Evaluation of Prolonged Exposure of Lacquered Tinplate Cans to a Citrate Buffer Solution Using Electrochemical Techniques," Prog. Org. Coat., 30, 9-14 (1997).

(21) McIntyre, N.S. and Chan, T.C., in Practical Surface Analysis. Auger and X-ray Photoelectron Spectroscopy, Briggs, D. and Seah, M.P. (Eds.), p. 485-529, John Wiley, New York, 1990.

(22) Champion, F.A., in Corrosion Testing Procedures, Chapman and Hall, London, p. 201, 1952.

E. Almeida*** and M.R. Costa -- INETI-Instituto Nacional de Engenharia e Tecnologia Industrial*

N. De Cristofaro -- CSM-Centro Sviluppo Materiali ([dagger])

N. Mora and J.M. Bastidas -- CENIM-Centro Nacional de Investigaciones Metalurgicas**

J.M. Puente -- Aceralia, Grupo Acerlor ([double dagger])

* DMTP/LTR, Estrada do Paco do Lumiar, 1649-038 Lisboa, Portugal.

([dagger]) Via di Castel Romano 100, 00128 Roma, Italy.

** CSIC, Avda. Gregorio del Amo 8, 28040 Madrid, Spain.

([double dagger]) Apartado de Correos 90, 33480 Aviles, Asturias, Spain.

*** Author to whom correspondence should be addressed. Voice: 351.21.7165141; fax: [phone]; email: [e-mail].