PITTING CORROSION OF PASSIVATED ZINC MONOCRYSTALS

The pitting and polarization characteristics of low-index surfaces of zinc (Zn) monocrystals were studied in weakly alkaline (pH 9.2) solutions containing 0.1 M sodium chloride (NaCl), using single-cycle potentiodynamic polarization tests and a scan rate of 0.5 mV/s. Tests were conducted in the presence and absence of a bicarbonate (HCO3-)/carbonate (CO32-) buffer. Electrode surface orientations and crystallographic directions lying in these surfaces were determined using x-ray techniques. The surfaces were close to the (0001), (10 ($) over bar 10), and (11<($)over bar >20) planes. Pit morphology was studied by scanning electron microscopy (SEM). Passivity was observed only in the buffered solution, and pitting of the passivated surface occurred near -0.85 V-SCE. The (0001) basal plane appeared to have a slightly better pitting resistance and a lower passive current density (i(p)) than the other orientations. Basal plane surfaces prepared by cleavage exhibited superior resistance to anodic dissolution in the unbuffered solution. Crystallographic pitting geometries observed on passivated crystals were attributed to the enhanced resistance of the (0001) plane to active dissolution. Events leading to pit initiation were reviewed in terms of localized changes in solution chemistry that resulted in a decreased pH and formation of soluble zinc chloride ion complexes (ZnClx[2-x]+).