Pitting Corrosion of Passivated β-Tin Monocrystals Available to Purchase

Anisotropic pitting behavior of tetragonal β-tin (Sn) was studied on low-index surfaces of monocrystals exposed to a near-neutral aqueous solution containing 0.1 M sodium chloride (NaCl) + 0.5 M sodium nitrate (NaNO₃). The critical pitting potentials (E_pit) for the onset of pitting on passivated surfaces were obtained from single-cycle polarization tests conducted at a scan rate of 0.5 mV/s. Pit morphology was studied by scanning electron microscopy, supplemented by light interference microscopy. The orientations of all surfaces and the crystal directions lying in each surface were determined by x-ray techniques. It was found that the (100), (110), (011), and (001) surfaces all exhibited a similar E_pit of –0.288 V_SCE ± 0.005 V_SCE, whereas the (111) surface exhibited a value that was reproducibly lower (less noble) by ~ 20 mV. All pits displayed a crystallographic morphology, with walls and base composed of different variants of the {100}, {011}, and {001} planes. No variants of the {110} or {111} planes were observed inside pits. There was no correlation between the anisotropic pitting behavior and the surface atom packing density. Results were reasonably consistent with a mechanism of pit initiation determined by anodic dissolution processes at the metal-electrolyte interface at the base of existing defects (flaws) in the passive film. Such defects (or film breakdown events) alone were insufficient to cause pitting initiation.