Electrochemical behavior of organic/inorganic films applied on tinplate in different aggressive media
Introduction
Tinplate is a cold-rolled, low carbon thin steel plate, coated on both sides with pure tin. Tinplate cans are extensively used in the food packaging industry, when mechanical robustness and withstanding at sterilization temperatures are required. Tinplate combines in one material the strength of steel and the corrosion resistance and good appearance of tin [1]. However, tinplate cans in corrosive food media may present problems, such as corrosion failures, loss of seal integrity, and damage in the appearance product that can affect the nutritional value of the canned food. During the corrosion process, foodstuff can be contaminated by tin, even though it is not considered as a poisonous metal, very large dose can produce serious digestive disturbances [1], [2]. Thus, during the manufacturing process, it is necessary to apply passivation treatments that improve corrosion resistance and adhesion of the lacquer coating finishing. The passivation treatments also help in preventing the tin oxide growth. Nowadays, the chromate passivation treatment is widely used in the tinplate industry, due to the excellent protecting behavior provided. Nevertheless, chromate is highly toxic and carcinogenic, therefore, new alternative pre-treatments more environmentally friendly have been proposed recently [3], [4], [5], [6].
In addition, the use of organic coatings applied on tinplate is a generalized procedure employed to improve the corrosion resistance when the cans are in contact with aggressive food products. The lacquers based on epoxy resins are the most widely employed, due to the good barrier properties [7], [8], [9], [10]. One constituent employed in the manufacture of epoxy resins is the monomer BADGE (Bisphenol A diglycidyl ether). BADGE is listed as an IARC (International Agency for Research on Cancer) Group 3 carcinogen, meaning it is “not classifiable as to its carcinogenicity to humans” [11]. Nevertheless, the new legislations are bounded on the use of BADGE precursors. Consequently, the development of new more ecological lacquers formulations is an utmost requirement. The use of inorganic or organically modified inorganic materials (hybrid films) obtained by sol–gel technology represents a compliant alternative. They have been successfully used to improve the corrosion resistance of several metallic substrates [12], [13], [14], [15]. Nevertheless, their use is scarce with tinplate substrates [16], [17], [18], maybe because of the complexity of the metal/coating/food system. The tinplate used in canning industry is a heterogeneous and stratified structure. The material is submitted to a heat treatment to obtain a FeSn2 alloy, which increases the corrosion resistance of the system. On the other hand, the food products play an important role because, depending on their physico-chemical characteristics, they can interact or even dissolve the coating and thus, come into contact with the metallic substrate. Finally, the coating design is important as well. It should be defect free and with good adhesion to the metallic substrate. Besides, it has to be adequately formulated, in order to avoid migration of chemicals into the food [10].
The present work addresses the electrochemical behavior of hybrid sol–gel films applied on a commercial tinplate immersed in two food-simulating solutions: 0.1 M NaCl solution, to reproduce seafood environment, and 0.1 M citric acid/sodium citrate buffer solution, commonly used to simulate fruit juices or pickled sauce. The initial step will be necessarily the characterization of bare tinplate in the same media.
Section snippets
Materials
The hybrid sol–gel solution was made from tetraethyl-orthosilane (TEOS) as the inorganic precursor sol and vinyltrimethoxysilane (VTMS) as the organic one. Fig. 1 displays the structures of TEOS and VTMS, the latter has three hydrolyzable groups (Si–OR) and an additional group (Si–R′) that provides certain flexibility to the film. This compound is included in the “materials and articles intended to come into in contact with food” (RD 866/2008) [19].
The procedure followed for preparing both sols
Surface characterization
Commercial tinplate specimens with a two-sided tin coating weight of 2.8 g m−2 were studied. The specimens had a “stone surface finishing”, which is characterized by directional grindstone patterns and by a mild heat treatment after the tin electroplating to generate through diffusion the layer of intermetallic FeSn2. This type of surface finishing is especially suitable to avoid scratches during the printing and can making processes.
Fig. 2 depicts the X-ray diffractogram obtained for tinplate
Conclusions
The tinplate behavior is strongly dependent on the nature of the electrolyte. Samples immersed in 0.1 M NaCl solution show an equilibrium potential shifted anodically in respect to the pure tin, which is a consequence of the oxide/hydroxide film presence. Samples immersed in 0.1 M citric/citrate buffer solution exhibit a cathodic branch plateau which is due to a partial electrode blockage by adsorption of the citrate ions.
The impedance evolution emphasizes the differential behavior, depending on
Acknowledgements
The authors gratefully acknowledge the financial support for this work by the National Program of Spanish Ministry of Economy and Competitiveness under contract DPI2011-22662.
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