Chemical and morphological characterization of commercial tinplate for food packaging

The structure of tinplate is usually described as a sequence of layers involving an external tin layer (passivated by a chromate conversion coating), an intermediate FeSn2 layer, and the steel substrate. This structure description applies well to tinplate produced in the past by hot dipping but not necessarily to modern materials fabricated through tin electrodeposition and subsequent heat treatment. In this work, the chemical composition and the structure of 5 commercial tinplate materials have been investigated using Auger electron spectroscopy, scanning electron microscopy, 3D white light interferometry, and electrochemical chrono-potentiometry. The study revealed that the intermediate layer consists indeed in a wide interface where tin, steel, and the FeSn2 alloy coexist. Moreover, the thickness of the Sn film varied locally significantly. A new tinplate structure was proposed in order to reliably appraise corrosion relevant features.