Steel packaging remains an important mean by which foodstuffs and other products can be stored safely for a prolonged period of time. The industry is being challenged by the dual legislative pressures which require the elimination of Chrome (VI) from the manufacturing process and the elimination of bisphenol A as a component from the lacquer system. Initial indications suggest lower adhesive performance, and it has been postulated that thermal treatment may be a mean of improving adhesion. Three substrates (two current and one future) were physically and chemically characterized prior and post treatment and the resultant impact of adhesion was quantified. The net impact of the thermal treatment is that it increases the adhesion of the lacquer on the surface. As there is minimal change in the physical characteristics of the surface, the authors propose that this is a result of changes in the chemical surface species, particularly the increase in the oxidic nature of each of the substrates which provides additional bonding sites for the organic species in the lacquer. These trends are observed for current substrate materials as well as next generation Chrome VI free substrate. Next generation replacement substrate materials perform better than current materials for dry adhesion while next generation bisphenol A non-intent lacquer materials perform poorer than the current epoxy phenolic materials.
Tinplate surface morphology and chemistry is adjusted during the manufacturing process in order to meet the demands of its subsequent product use, the commonest being visual appearance and food packaging stability. A comprehensive experimental study on an industrial tinning line varied the surface roughness and the tin coating weight with the characterization through X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), white light interferometer (WLI), optical imaging, and lacquer adhesion measurement. Increasing tin weight lowers the adhesion through the production of a thicker disorganized tin oxide layer which has a greater tendency to fracture under shearing forces. There is no evidence that the substrate roughness improves the adhesion of the lacquer. Analysis of the failure location identifies fracture in the tin oxide layer below the passivation layer. The findings have impacts on the next generation of passivation materials for tinplate as it has been clearly demonstrated that growth in tin oxide thickness, particularly when unstructured, has a detrimental impact on lacquer adhesion.
Recent restrictions on industrial usage of hexavalent chromium under new REACH legislation have further sparked the development of hexavalent chromium-free chromium plating processes. An industrially important development in this field is Trivalent Chromium Coating Technology (TCCT®), a chromium electroplating technology for packaging steel developed at Tata Steel. In this process, aqueous trivalent chromium electrolytes rather than hexavalent chromium electrolytes are employed for the electrodeposition of metallic chromium. However, to deposit metallic chromium from a trivalent chromium electrolyte, it is necessary to incorporate a complexing agent given the kinetic inertness of aqueous trivalent chromium complexes. Formate plays this role in the TCCT® process yielding coatings comparable to the conventional hexavalent chromium-based process [1-4].
However, understanding the mechanism and kinetics of chromium electrodeposition from this system is quite limited. Fundamental knowledge of the deposition process is key for industrial process optimization. Essential to determining the reaction mechanism and kinetics is the identification of the chemical species involved in the reaction. Using a hybrid multiscale experimental and computational approach, insights into chromium complexation in the bulk electrolytes and how this speciation influences the composition of the deposit have been gained.
Samples electroplated in electrolytes of varied formate concentrations were characterized using X-ray Fluorescence (XRF) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Results from these analyses show that metallic chromium is only deposited when the electrolyte contains formate ions. In the absence of formate, only oxide and carbide species are deposited. The characterization results also show a current efficiency of the TCCT process of ~ 40%. From observations from surface characterization as well as spectroscopic analysis and density functional theory (DFT) and ab initio molecular dynamics studies (AIMD) of the bulk electrolyte, the coordination and complexation of formate ion in the chromium complex responsible for metallic chromium deposition have also been identified. Voltammetric studies coupled with ex-situ XPS and scanning electron microscopy (SEM) surface characterization also lead to a clear definition of the reaction mechanism of metallic chromium deposition that the incorporation of formate in trivalent chromium electrolytes makes possible.
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