Multi-scale characterization of a chromium-free passivation layer for tinplate food-packaging

The ban on the use of hexavalent chromium in the protection process for food-grade tinplate has led to the development of a new passivation process called CFPA (Chromium-Free Passivation Alternative). Nevertheless, its corrosion performance is lower than that of the original process. The objective of this work is to thoroughly characterize the CFPA layer in order to identify the origin of the adhesion and corrosion weaknesses observed. Thus XPS spectroscopy in conventional and image mode and STEM HAADF imaging combined with EELS spectroscopy were used. A "ridge-valley" nano-roughness following the rolling patterns was observed. The thickness of the CFPA layer can be either single (~8 nm) or double (~18 nm), depending on where it is. In the valleys, the passivation consists of a gradient of transition metal oxides or even (oxy)fluorides and a polymer on the surface, arranged on a protective layer of tin oxides. On the ridges, the finer passivation layer consists of a single oxides nano-layer. Its lateral distribution is very heterogeneous, even leading to the formation of nano-clusters at the extreme surface. Indeed, the chemical heterogeneity in tinplate leads to variations in chemical potential at the surface, resulting in different responses to corrosion and weakened steel protection.