| Aluminum cathodes have long been used for the electrowinning of zinc from zinc sulfate-sulfuric acid electrolyte. Severe corrosion above the electrolyte/air interface limits the life of aluminum cathodes to ca. 18--24 months for a 6 mm thick plate. The failure characteristics of aluminum cathodes were analyzed using optical microscopy, scanning electron microscopy, energy dispersive x-ray analysis, and x-ray diffraction analysis. Three corrosion zones were identified: 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. Zone II experienced maximum corrosion damage in the form of shallow, dense, cusp-like pitting. For a rejected aluminum plate, the corrosion damage in zone II was ca. 80% reduction in plate thickness. The corrosion damage in zones I and III was minor corresponding to ca. 10% reduction in plate thickness. The failure pattern of aluminum cathodes and the factors causing variation in corrosion rate in the different zones were studied.;Corrosion processes occurring under thin electrolyte film conditions limited the life of the cathode plate. The influence of microstructure, chloride, and fluoride on the corrosion susceptibility of aluminum cathodes was investigated in simulated and industrial electrolytes using cyclic polarization, potentiodynamic polarization, potentiostatic, galvanostatic, open circuit, and free immersion techniques. These techniques were coupled with electron metallography and chemical analysis of test solutions. The corrosion rate of the aluminum cathode plate decreased with increased pH and zinc concentration. Titanium addition to the cathode plate undermined its corrosion resistance in zone I by enhancing hydrogen evolution. In the presence of cathodic polarization, fluoride activated zone I corrosion but had minimal effect on zone II corrosion. Chloride had minimal effect on zone I corrosion; however, it strongly activated zone II corrosion and was the primary factor responsible for plate failure. A mechanism through which chloride acted on the aluminum cathode plate was proposed. |
