Corrosion mechanisms and durability models for historical tinplate food cans

Since the early of the 19th century, canning is one the most efficient ways to preserve foods. Tinplated steel has been used since the beginning for this purpose and it is still one of the most used materials for cans production because it combines the good mechanical strength of steel with the corrosion resistance of tin. Nowadays, historically relevant cans enrich the collections of several museums around the world. However, these collections are threatened by severe corrosion phenomena, which may lead to perforation, swelling and bursting of the container. The aim of this thesis is to study the corrosion mechanism of tinplate and to propose a long-term durability model that can be used by museums conservators in order to define an intervention timeline for historical and new cans collections. The strategy of the thesis has been to investigate separately the material structure and the corrosion mechanisms of the tinplate constituents in model electrolytes and then to combine the two outcomes in a predictive model. Tinplate is a composite material constituted by several metallic layers, which interacts with the complex food matrix. The sandwich like structure of tinplate, constituted by an external tin layer, an intermediate intermetallic compound layer and a core of mild steel, was characterized via a multi-technique approach combining chrono-potentiometry, chrono-amperometry, Auger Electron Spectroscopy, X-ray Fluorescence Spectroscopy, Scanning Electron Microscopy and 3D White Light Interferometry. The analysis pointed out that the relevant features that need to be considered in order to make predictions on the corrosion mechanism and lifetime of tinplate are the thickness of the tin layer, its porosity, the base steel roughness and the structure of the tin-steel interface. Canned foods cover a wide range of pH, from neutrality to pH 3 and they contain several molecules and ions known to impact tinplate corrosion, such as polyprotic complexing acids, protons, oxygen, nitrates, sulphur dioxide, natural dyes etc. The mechanisms and the kinetics of tin corrosion and tin-iron galvanic coupling were investigated in model electrolytes constituted by buffer solutions of citric, malic and oxalic acids at constant oxygen concentration, mimicking the typical environment of canned fruits and vegetables. The study was carried out by means of an electrochemical approach based on potentiodynamic polarization technique in static conditions and under convection (Rotating Disk Electrode). X-ray Photoelectron Spectroscopy was used as additional technique for the investigation of the tin-iron galvanic coupling mechanism. The results showed that, depending on the potential, different mechanisms control tin corrosion: charge transfer reduction of protons, diffusion-limited reduction of oxygen, active dissolution of tin and its passivation. Interestingly, all these reaction are affected by parameters such as pH, oxygen content and complexing strength of electrolytes but to a different extent. Furthermore, the study of tin-iron galvanic coupling has shown an increase of the kinetic of tin dissolution but at the same time, a reduction of the reactivity of iron towards acid corrosion due to of a self-healing mechanism. The rationalization of the corrosion mechanisms through kinetic equations depending by well-defined parameters has allowed to develop a durability model able to predict the lifetime of tinplated cans based on the material structure.