Anode Materials and Backfill

For protection of structures in seawater, platinum-clad copper, 2% Ag—Pb, platinized titanium, or platinized niobium have been recommended as corrosion-resistant anodes using impressed current [10-12]. Whereas sacrificial magnesium anodes require replacement approximately every 2 years, the 2% Ag—Pb anodes are estimated to last more than 10 years, and the 90% Pt-10% Ir anodes still 

Whereas auxiliary anodes need not be consumed in order to fulfill their purpose, sacrificial anodes are consumed not less than is required by Faraday s law in order to supply an equivalent electric current. In general, the observed rate of consumption is greater than the theoretical. For zinc the difference is not large, but for magnesium it is appreciable, with the cause being ascribed to local-action currents on the metal surface, to formation of colloidal metal particles [13, 14] or, perhaps more important, to initial formation of univalent magnesium ions [15], The latter ions are unstable and react in part with water in accord with 

in dilute sodium chloride, about half the magnesium corroding anodically appears as Mg(OH)2 and half as MgCl2, accompanied by hydrogen evolution in about the amount expected according to this reaction [15]. Additional lesser side reactions may also take place at the same time. Accordingly, the observed yield of a magnesium anode is only about one-half the lOOOA-h/lb calculated on the basis of Mg + formation. 

---