Aluminum cathode plates in zinc electrowinning cells: microstructural and failure analysis

The microstructure of aluminum cathode plates used in zinc electrowinning was analyzed using optical microscopy, scanning electron microscopy, and transmission electron microscopy. Three principal phases dominated the microstructure: primary aluminum, uniformly distributed intermetallic particles, and round rosettes. The intermetallics exhibited blade shape morphology, light gray color, and were aligned in the rolling direction. The chemical composition of the intermetallic particles was consistent with FeAl3. Angular particles of elemental silicon were also detected. Failure characteristics of industrial cathode plates were analyzed using optical microscopy, scanning electron microscopy, energy dispersive x-ray analysis, and x-ray diffraction analysis. Three distinct corrosion zones were identified on failed plates: Zone I below the electrolyte/air interface, Zone II 0-40 mm above the electrolyte/air interface, and Zone III 40-140 mm above the electrolyte/air interface. After 24 months in service, the corrosion damage in Zones I and III was equivalent to ca. 10% reduction in plate thickness. Zone II experienced the greatest corrosion damage; the reduction in plate thickness was ca. 80%. Some plates exhibited severe thinning and perforation, which occurred preferentially near the electrical contact edge. Plates often fail in service by fracture in Zone II. (author)