The characteristics of a typical halogen and a patented methanesulfonic-acid-based electrotinning process were studied, using laboratory tests. The stability of the bath and its corrosiveness toward the steel substrate were evaluatedsimultaneously by a "spinning disc" experimental technique. The results show that the MSA bath is very stable, with a low oxidation rate of stannous ions, but is much more corrosive to the steel substrate. The effects of operating conditions, such as line speed and current density, on the plating efficiency, coating morphology and preferred crystal orientation were investigated, using a rotating cylinder cathode. Comparison of the plating efficiencies obtained from the rotating cylinder cathode andproduction identified a window of test conditions that is representative of production conditions. Within this window, the MSA process offers a higher plating efficiency at high current densities, but different microstructure and preferred crystalorientation compared with those of the halogen process. The coating morphology in the MSA bath also exhibits a greater variation at low current densities, resulting in a narrower operating window. In both processes, the grain size increases in thedirections parallel as well as normal to the steel substrate. This three-dimensional grain growth is accompanied by a continuing change in crystal orientation, suggesting that recrystallization takes place during these two electrotinning processes.
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