Electrochemical Behaviour of Four Lacquers for Tomato Cans

Abstract

20% of the overall production of Italian canmakers is intended for tomato products. In recent years the use of internal lacquered cans has continued to increase, both for commercial reasons and for greater guarantee of quality. The study of the interaction between the product and the container (which couples a metal with an organic coating) is of immense practical interest, because of the effect this interaction has on the shelf-life of canned tomatoes. In our experimental work we examined four types of commercial lacquer covering five different metal substrata, four tinplates and one tin free steel. In the first stage of our study we examined the corrosion resistance of each of the lacquering systems by placing them in a solution simulating tomato products (made with citric acid) and using electrochemical impedance spectroscopy. The corrosion resistance of each system was significantly different: the electrochemical parameters of the organosol lacquer (lowest coating capacitance (Cc) and highest pore resistance (Rp) denote that this lacquer has good barrier properties and indicate that corrosion occurs upon small areas (high value of Rp) and slowly. The white lacquer showed the lowest resistance to lacquer delamination (high value of double layer capacitance (Cdl) immediately after 500 hours of contact), while the epoxyphenolic lacquer gave an intermediate performance. In the second stage of the work we studied the influence of the mechanical stress resulting from the formation of the body beads upon the corrosion resistance of the four lacquering systems. In each case, proceeding from the sheet material to the can body, the impedance value decreased two-fold; with the organosol lacquer continuing to show better barrier properties. The degree of corrosion of the can body is influenced by the type of can end to which the body is galvanically coupled. The higher the impedance of the can ends, the lower the corrosion of the can body.