Corrosion of lead anodes in base metals electrowinning

Lead-based alloys are the primary anodes used in electrowinning from sulphate-based aqueous systems.Lead anode technology has evolved over the years, migrating from pure lead and lead-antimonial alloys to the presentday lead-calcium-tin alloys for copper electrowinning and lead-silver alloys for zinc electrowinning (Prengaman, Ellis, and Mirza, 2010).Anode technology has also migrated from cast microstructure to mechanically deformed, rolled microstructures in search of improved mechanical properties and higher corrosion resistance that results from the rapid production of a protective oxide coating (Prengaman, 1987; Prengaman and Morgan, 1992, 2001).Although great strides have been made in the development of new alloys and production processes (Prengaman, 2000), the industry still has unresolved issues related to untimely corrosion which limits anode life and may lead to higher contaminant levels in the metal being produced.The comments in this work, although directed towards copper electrowinning, can be extended to similar phenomena in other metals electrowon from sulphate media (i.e., zinc, cobalt, nickel, manganese).The fundamental efficacy of leadbased anodes is dependent on the properties of PbO 2 (Prengaman and McDonald, 1980).In fact, lead anodes are used predominantly in sulphate-based electrolyte systems because of the protective ability of PbO 2 .In particular, the protective adherent -phase, which is the brown PbO 2 layer closest to the anode surface, isolates the lead from the corrosive electrolyte.It is composed of large, rhombic, closely packed crystals.The -phase of PbO 2 , which is black in colour, is the interface between the protective -PbO 2 and the electrolyte, as shown in Figure 1.It is composed of tetragonal, fine, loosely adhering needleshaped crystals.The transformation of -PbO 2 to -PbO 2 is marked by formation of PbO, Pb(OH) 2 , PbSO 4 , and other complex sulphates as reaction intermediates.The mechanism of electrochemical oxidation of lead to lead dioxide in sulphuric acid is discussed in detail by Pavlov and Dinev (1980).The oxidation of a lead anode is therefore determined not by the reaction rate of lead with sulphuric acid but by mass transport of the ions participating in the reaction through